diff --git a/open-sse/services/combo.ts b/open-sse/services/combo.ts
index 82e4e36d74..8b23ad3d33 100644
--- a/open-sse/services/combo.ts
+++ b/open-sse/services/combo.ts
@@ -220,6 +220,11 @@ import {
 } from "./combo/quotaExhaustionCutoff.ts";
 import { expandTargetsByFingerprints } from "./combo/fingerprintExpansion.ts";
 import { resolveComboTargetPipeline } from "./combo/targetResolution.ts";
+import {
+  isQuotaExhaustionResponse,
+  recordQuotaExhaustionClassification,
+  withQuotaExhaustionClassification,
+} from "./combo/quotaExhaustion.ts";
 
 export { RESET_WINDOW_NAMES, QUOTA_SOFT_DEPRIORITIZE_FACTOR, setCandidateQuotaSoftPenalty };
 export { scoreAutoTargets, expandAutoComboCandidatePool };
@@ -817,6 +822,26 @@ export async function handleComboChat({
   // Accumulator for per-model error details across targets in the current set try.
   // Reset at the start of each set retry (same lifecycle as lastError/recordedAttempts).
   let comboErrors: Array<{ model: string; status: number; error: string }> = [];
+  // Quota trust spans set retries and recursive cooldown re-dispatches. Once any
+  // failure is non-quota, a nested caller must never treat this dispatch as quota-only.
+  let observedFailure = false;
+  let allObservedFailuresQuota = true;
+  const targetFailureTrust = new Map<
+    string,
+    { observedFailure: boolean; allObservedFailuresQuota: boolean }
+  >();
+  const observeFailure = (quotaExhausted: boolean, targetExecutionKey?: string) => {
+    observedFailure = true;
+    allObservedFailuresQuota &&= quotaExhausted;
+    if (!targetExecutionKey) return;
+    const trust = targetFailureTrust.get(targetExecutionKey) ?? {
+      observedFailure: false,
+      allObservedFailuresQuota: true,
+    };
+    trust.observedFailure = true;
+    trust.allObservedFailuresQuota &&= quotaExhausted;
+    targetFailureTrust.set(targetExecutionKey, trust);
+  };
 
   // FASE 2.1: per-connection concurrency limit for quota-share. The gating in
   // selectQuotaShareTarget is fail-open and cannot hard-limit a single-connection
@@ -904,6 +929,9 @@ export async function handleComboChat({
       let anySuccess = false;
       const abortControllers = new Map<number, AbortController>();
       const zeroLatencyOptimizationsEnabled = config.zeroLatencyOptimizationsEnabled === true;
+      const hasProtectedPriorityTarget =
+        strategy === "priority" &&
+        orderedTargets.some((target) => target.fallbackOnlyOnQuotaExhaustion === true);
 
       const executeTarget = async (
         i: number
@@ -912,12 +940,20 @@ export async function handleComboChat({
         const modelStr = target.modelStr;
         const rawModel = parseModel(modelStr).model || modelStr;
         const provider = target.provider;
+        const protectedPriorityTarget =
+          strategy === "priority" && target.fallbackOnlyOnQuotaExhaustion === true;
+        const stopProtectedPriorityTarget = (message: string) => {
+          observeFailure(false, target.executionKey);
+          return protectedPriorityTarget
+            ? { ok: false, response: errorResponse(503, message) }
+            : null;
+        };
 
         const cb = getCircuitBreaker(provider);
         if (cb.getStatus().state === "OPEN") {
           log.info("COMBO", `Skipping ${modelStr} — circuit breaker OPEN for ${provider}`);
           if (i > 0) fallbackCount++;
-          return null;
+          return stopProtectedPriorityTarget(`Provider ${provider} circuit breaker is open`);
         }
 
         if (
@@ -927,7 +963,7 @@ export async function handleComboChat({
         ) {
           log.info("COMBO", `Skipping ${modelStr} — provider ${provider} in global cooldown`);
           if (i > 0) fallbackCount++;
-          return null;
+          return stopProtectedPriorityTarget(`Provider ${provider} is in cooldown`);
         }
 
         // Use pre-screened profile if available, otherwise fetch on demand
@@ -954,14 +990,14 @@ export async function handleComboChat({
         if (exhaustedSkip) {
           log.info("COMBO", exhaustedSkip);
           if (i > 0) fallbackCount++;
-          return null;
+          return stopProtectedPriorityTarget(`Target ${modelStr} is unavailable`);
         }
 
         // Pre-check: skip models locked by the resilience system (model-level lockout)
         if (provider && rawModel && isModelLocked(provider, target.connectionId || "", rawModel)) {
           log.info("COMBO", `Skipping ${modelStr} — model locked by resilience (cooldown active)`);
           if (i > 0) fallbackCount++;
-          return null;
+          return stopProtectedPriorityTarget(`Model ${modelStr} is locked`);
         }
 
         // #5923 (Finding #4) — honor the same opt-in quota-exhaustion cutoff the
@@ -987,6 +1023,16 @@ export async function handleComboChat({
               `Skipping ${modelStr} — quota exhaustion cutoff (${quotaCutoff.reason || "quota_exhausted"})`
             );
             if (i > 0) fallbackCount++;
+            observeFailure(true, target.executionKey);
+            if (protectedPriorityTarget) {
+              const protectedTargetTrust = targetFailureTrust.get(target.executionKey);
+              if (!protectedTargetTrust?.allObservedFailuresQuota) {
+                return {
+                  ok: false,
+                  response: errorResponse(503, `Target ${modelStr} is unavailable`),
+                };
+              }
+            }
             return null;
           }
         }
@@ -1004,7 +1050,7 @@ export async function handleComboChat({
               `Skipping ${modelStr} — no credentials available or model excluded`
             );
             if (i > 0) fallbackCount++;
-            return null;
+            return stopProtectedPriorityTarget(`Model ${modelStr} is unavailable`);
           }
         }
 
@@ -1015,7 +1061,7 @@ export async function handleComboChat({
           if (gateResult.allowed === false) {
             logCredentialSkip(log, modelStr, gateResult.reason || "Credential gate blocked");
             if (i > 0) fallbackCount++;
-            return null;
+            return stopProtectedPriorityTarget(`Credential gate blocked ${modelStr}`);
           }
 
           // Concurrency gate: fail-fast skip when connection is at max_concurrent capacity (e.g. Featherless 1/1)
@@ -1029,7 +1075,7 @@ export async function handleComboChat({
               `Skipping ${modelStr} — connection ${connectionId} is at max concurrency cap (${maxConcurrentCap})`
             );
             if (i > 0) fallbackCount++;
-            return null;
+            return stopProtectedPriorityTarget(`Connection capacity reached for ${modelStr}`);
           }
         }
 
@@ -1084,7 +1130,7 @@ export async function handleComboChat({
                   "COMBO",
                   `Predictive TTFT Circuit Breaker: skipping ${modelStr} (avg ${m.avgLatencyMs}ms > max ${config.predictiveTtftMs}ms)`
                 );
-                return null;
+                return stopProtectedPriorityTarget(`Predictive latency check rejected ${modelStr}`);
               }
             }
           }
@@ -1294,7 +1340,13 @@ export async function handleComboChat({
                 error: `Quality: ${quality.reason}`,
                 latencyMs: Date.now() - startTime,
               });
-              return null;
+              observeFailure(false, target.executionKey);
+              return protectedPriorityTarget
+                ? {
+                    ok: false,
+                    response: errorResponse(502, "Upstream response failed quality validation"),
+                  }
+                : null;
             }
 
             // Success decay: a healthy response walks the model's lockout failure
@@ -1645,7 +1697,7 @@ export async function handleComboChat({
             result.status,
             errorText,
             0,
-            null,
+            protectedPriorityTarget ? rawModel : null,
             provider,
             result.headers,
             profile,
@@ -1776,6 +1828,15 @@ export async function handleComboChat({
             recordProviderFailure(provider, log, targetWithConnection.connectionId, profile);
           }
 
+          const quotaExhausted = await isQuotaExhaustionResponse(
+            result,
+            provider,
+            rawModel,
+            profile
+          );
+          recordQuotaExhaustionClassification(result, quotaExhausted);
+          observeFailure(quotaExhausted, target.executionKey);
+
           // Check if this is a transient error worth retrying on same model.
           // A token-limit 429 is terminal for the client — never retry it.
           const isTransient =
@@ -1785,6 +1846,7 @@ export async function handleComboChat({
             [408, 429, 500, 502, 503, 504].includes(result.status);
           if (retry < maxRetries && isTransient && !providerExhausted) {
             if (
+              !protectedPriorityTarget &&
               provider &&
               rawModel &&
               isModelLocked(provider, targetWithConnection.connectionId || "", rawModel)
@@ -1807,7 +1869,7 @@ export async function handleComboChat({
             // once the model is cooling down, retrying it would waste an upstream
             // call and extend the cooldown via exponential backoff.
             let lockoutRecorded = false;
-            if (provider && rawModel && retry === 0 && !scopedFailure) {
+            if (!protectedPriorityTarget && provider && rawModel && retry === 0 && !scopedFailure) {
               const mlSettings = resolveModelLockoutSettings(settings);
               if (mlSettings.enabled && mlSettings.errorCodes.includes(result.status)) {
                 recordModelLockoutFailure(
@@ -1847,6 +1909,22 @@ export async function handleComboChat({
           }
 
           // Done retrying this model
+          const protectedTargetTrust = targetFailureTrust.get(target.executionKey);
+          if (
+            protectedPriorityTarget &&
+            (!protectedTargetTrust?.observedFailure ||
+              !protectedTargetTrust.allObservedFailuresQuota)
+          ) {
+            recordComboRequest(combo.name, modelStr, {
+              success: false,
+              latencyMs: Date.now() - startTime,
+              fallbackCount,
+              strategy,
+              target: toRecordedTarget(target),
+            });
+            recordedAttempts++;
+            return { ok: false, response: result };
+          }
           recordComboRequest(combo.name, modelStr, {
             success: false,
             latencyMs: Date.now() - startTime,
@@ -1975,7 +2053,12 @@ export async function handleComboChat({
         runningTasks.add(task);
         task.finally(() => runningTasks.delete(task));
 
-        if (zeroLatencyOptimizationsEnabled && config.hedging && i + 1 < orderedTargets.length) {
+        if (
+          zeroLatencyOptimizationsEnabled &&
+          config.hedging &&
+          !hasProtectedPriorityTarget &&
+          i + 1 < orderedTargets.length
+        ) {
           const hedgeDelay = resolveDelayMs(config.hedgeDelayMs, 500);
           let timeoutResolve: () => void;
           const timeoutPromise = new Promise<void>((r) => {
@@ -2062,11 +2145,14 @@ export async function handleComboChat({
             latencyMs,
             fallbackCount,
           });
-          return errorResponseWithComboDiagnostics(
-            503,
-            "Service temporarily unavailable: all targets were skipped by pre-dispatch filters",
-            buildComboDiag("all_targets_skipped"),
-            { code: "ALL_TARGETS_SKIPPED", type: "service_unavailable" }
+          return withQuotaExhaustionClassification(
+            errorResponseWithComboDiagnostics(
+              503,
+              "Service temporarily unavailable: all targets were skipped by pre-dispatch filters",
+              buildComboDiag("all_targets_skipped"),
+              { code: "ALL_TARGETS_SKIPPED", type: "service_unavailable" }
+            ),
+            observedFailure ? allObservedFailuresQuota : null
           );
         }
         notifyWebhookEvent("request.failed", {
@@ -2157,7 +2243,10 @@ export async function handleComboChat({
       if (earliestRetryAfter && isRetryAfterEligibleStatus(status)) {
         const retryHuman = formatRetryAfter(toRetryAfterDisplayValue(earliestRetryAfter));
         log.warn("COMBO", `All models failed | ${msg} (${retryHuman})`);
-        return unavailableResponse(status, msg, earliestRetryAfter, retryHuman);
+        return withQuotaExhaustionClassification(
+          unavailableResponse(status, msg, earliestRetryAfter, retryHuman),
+          observedFailure ? allObservedFailuresQuota : null
+        );
       }
 
       // Silent-stop fix: bump the failure counter (pin clears on 3rd consecutive) and emit
@@ -2173,10 +2262,13 @@ export async function handleComboChat({
         );
       }
       const retryAfterSeconds = undefined;
-      return errorResponseWithComboDiagnostics(
-        status,
-        msg,
-        buildComboDiag(lastError ?? "all_models_failed", retryAfterSeconds)
+      return withQuotaExhaustionClassification(
+        errorResponseWithComboDiagnostics(
+          status,
+          msg,
+          buildComboDiag(lastError ?? "all_models_failed", retryAfterSeconds)
+        ),
+        observedFailure ? allObservedFailuresQuota : null
       );
     }
 
diff --git a/open-sse/services/combo/comboStructure.ts b/open-sse/services/combo/comboStructure.ts
index 0430f03482..c125bc9052 100644
--- a/open-sse/services/combo/comboStructure.ts
+++ b/open-sse/services/combo/comboStructure.ts
@@ -115,6 +115,7 @@ function normalizeRuntimeStep(
       comboName: step.comboName,
       weight,
       label,
+      ...(step.fallbackOnlyOnQuotaExhaustion ? { fallbackOnlyOnQuotaExhaustion: true } : {}),
     };
   }
 
@@ -138,7 +139,12 @@ function normalizeRuntimeStep(
       : {}),
     weight,
     label,
-    prompt: step.kind === "model" ? step.prompt || null : null,
+    // `prompt` is a per-step pipeline input and only exists on a model step —
+    // #8894 widened the union with ComboProviderWildcardStep, which has no prompt.
+    prompt: (step.kind === "model" ? step.prompt : null) || null,
+    ...(step.kind === "model" && step.fallbackOnlyOnQuotaExhaustion
+      ? { fallbackOnlyOnQuotaExhaustion: true }
+      : {}),
   } satisfies ResolvedComboTarget;
 }
 
diff --git a/open-sse/services/combo/quotaExhaustion.ts b/open-sse/services/combo/quotaExhaustion.ts
new file mode 100644
index 0000000000..50e2d71863
--- /dev/null
+++ b/open-sse/services/combo/quotaExhaustion.ts
@@ -0,0 +1,107 @@
+import { checkFallbackError, type ProviderProfile } from "../accountFallback.ts";
+import { classifyGeminiQuotaMetricFromText } from "../geminiRateLimitTracker.ts";
+
+const TERMINAL_QUOTA_CODES = new Set([
+  "billing_hard_limit_reached",
+  "credits_exhausted",
+  "insufficient_quota",
+  "quota_exhausted",
+]);
+
+const trustedClassifications = new WeakMap<Response, boolean>();
+
+type ParsedError = {
+  text: string;
+  structuredError: { code?: string; type?: string } | null;
+};
+
+async function parseError(response: Response): Promise<ParsedError> {
+  let text = response.statusText;
+  let structuredError: ParsedError["structuredError"] = null;
+  try {
+    const body = (await response.clone().json()) as {
+      error?: string | { message?: unknown; code?: unknown; type?: unknown };
+      message?: unknown;
+    };
+    if (typeof body.error === "string") text = body.error;
+    else if (body.error && typeof body.error === "object") {
+      if (typeof body.error.message === "string") text = body.error.message;
+      structuredError = {
+        ...(body.error.code == null ? {} : { code: String(body.error.code) }),
+        ...(body.error.type == null ? {} : { type: String(body.error.type) }),
+      };
+    } else if (typeof body.message === "string") text = body.message;
+  } catch {
+    try {
+      text = await response.clone().text();
+    } catch {
+      // The status and trusted in-process classification remain available.
+    }
+  }
+  return { text, structuredError };
+}
+
+export function recordQuotaExhaustionClassification(response: Response, exhausted: boolean): void {
+  trustedClassifications.set(response, exhausted);
+}
+
+export function withQuotaExhaustionClassification(
+  response: Response,
+  exhausted: boolean | null
+): Response {
+  if (exhausted !== null) recordQuotaExhaustionClassification(response, exhausted);
+  return response;
+}
+
+export async function isQuotaExhaustionResponse(
+  response: Response,
+  provider: string | null,
+  model: string | null,
+  profile: ProviderProfile | null = null
+): Promise<boolean> {
+  const trusted = trustedClassifications.get(response);
+  if (trusted !== undefined) return trusted;
+
+  if (response.status !== 402 && response.status !== 429) return false;
+
+  const { text, structuredError } = await parseError(response);
+  if (provider === "gemini" && response.status === 429) {
+    const metric = classifyGeminiQuotaMetricFromText(text);
+    if (metric === "rpm" || metric === "tpm") return false;
+    if (metric === "rpd") return true;
+  }
+  const normalizedCode = structuredError?.code?.toLowerCase();
+  const normalizedType = structuredError?.type?.toLowerCase();
+  if (
+    (normalizedCode && TERMINAL_QUOTA_CODES.has(normalizedCode)) ||
+    (normalizedType && TERMINAL_QUOTA_CODES.has(normalizedType))
+  ) {
+    return true;
+  }
+
+  if (
+    /\b(?:billing hard limit reached|credits? exhausted|subscription quota exhausted)\b/i.test(text)
+  ) {
+    return true;
+  }
+
+  if (
+    provider?.startsWith("openai-compatible-") ||
+    provider?.startsWith("openai-compatible-chat-")
+  ) {
+    return false;
+  }
+
+  return (
+    checkFallbackError(
+      response.status,
+      text,
+      0,
+      model,
+      provider,
+      response.headers,
+      profile,
+      structuredError
+    ).reason === "quota_exhausted"
+  );
+}
diff --git a/open-sse/services/combo/runtimeUnits.ts b/open-sse/services/combo/runtimeUnits.ts
index 90ad1c2cac..453ce38116 100644
--- a/open-sse/services/combo/runtimeUnits.ts
+++ b/open-sse/services/combo/runtimeUnits.ts
@@ -9,6 +9,7 @@ import { errorResponse } from "../../utils/error.ts";
 import { recordComboRequest } from "../comboMetrics.ts";
 import { resolveDelayMs } from "./comboPredicates.ts";
 import { isRuntimeUnitAtConcurrencyCap } from "./runtimeUnitCapacity.ts";
+import { isQuotaExhaustionResponse, withQuotaExhaustionClassification } from "./quotaExhaustion.ts";
 import { validateResponseQuality, releaseQualityClone } from "./validateQuality.ts";
 import type { ResponseValidationConfig } from "./responseValidation.ts";
 import type {
@@ -197,8 +198,28 @@ export async function executeRuntimeUnitCombo(args: {
   const effectiveStrategy = args.effectiveComboStrategy ?? args.strategy;
   let lastResponse: Response | null = null;
   let fallbackCount = 0;
+  let observedFailure = false;
+  let allObservedFailuresQuota = true;
+  const targetFailureTrust = new Map<
+    string,
+    { observedFailure: boolean; allObservedFailuresQuota: boolean }
+  >();
+  const observeFailure = async (response: Response, unit: ResolvedComboUnit): Promise<boolean> => {
+    const quotaExhausted = await isQuotaExhaustionResponse(
+      response,
+      unit.kind === "model" ? unit.provider : null,
+      unit.kind === "model" ? unit.modelStr : null
+    );
+    observedFailure = true;
+    allObservedFailuresQuota &&= quotaExhausted;
+    return quotaExhausted;
+  };
+  const finalFailure = (response: Response): Response =>
+    withQuotaExhaustionClassification(response, observedFailure ? allObservedFailuresQuota : null);
 
   for (const unit of orderedUnits) {
+    const protectedPriorityUnit =
+      effectiveStrategy === "priority" && unit.fallbackOnlyOnQuotaExhaustion === true;
     if (
       await isRuntimeUnitAtConcurrencyCap(
         unit,
@@ -211,16 +232,24 @@ export async function executeRuntimeUnitCombo(args: {
         "COMBO",
         `Skipping ${unit.kind} ${unitDisplayName(unit)} — concurrency cap reached`
       );
+      lastResponse = errorResponse(503, `${unitDisplayName(unit)} is at concurrency capacity`);
+      await observeFailure(lastResponse, unit);
+      if (protectedPriorityUnit) return { response: finalFailure(lastResponse), unit };
       fallbackCount += 1;
       continue;
     }
 
     for (let retry = 0; retry <= maxRetries; retry += 1) {
-      if (args.signal?.aborted)
-        return { response: errorResponse(499, "Client disconnected"), unit };
+      if (args.signal?.aborted) {
+        lastResponse = errorResponse(499, "Client disconnected");
+        await observeFailure(lastResponse, unit);
+        return { response: finalFailure(lastResponse), unit };
+      }
       args.nesting.attemptBudget.count += 1;
       if (args.nesting.attemptBudget.count > args.nesting.attemptBudget.limit) {
-        return { response: errorResponse(503, "Maximum combo retry limit reached"), unit };
+        lastResponse = errorResponse(503, "Maximum combo retry limit reached");
+        await observeFailure(lastResponse, unit);
+        return { response: finalFailure(lastResponse), unit };
       }
       if (retry > 0) {
         await new Promise((resolve) => setTimeout(resolve, retryDelayMs));
@@ -276,9 +305,31 @@ export async function executeRuntimeUnitCombo(args: {
           });
           return { response, unit };
         }
+        lastResponse = errorResponse(502, "Upstream response failed quality validation");
+      }
+      if (lastResponse) {
+        const quotaExhausted = await observeFailure(lastResponse, unit);
+        if (protectedPriorityUnit) {
+          const trust = targetFailureTrust.get(unit.executionKey) ?? {
+            observedFailure: false,
+            allObservedFailuresQuota: true,
+          };
+          trust.observedFailure = true;
+          trust.allObservedFailuresQuota &&= quotaExhausted;
+          targetFailureTrust.set(unit.executionKey, trust);
+        }
       }
       if (![408, 429, 500, 502, 503, 504].includes(response.status)) break;
     }
+    const protectedTargetTrust = targetFailureTrust.get(unit.executionKey);
+    if (
+      protectedPriorityUnit &&
+      protectedTargetTrust?.observedFailure &&
+      !protectedTargetTrust.allObservedFailuresQuota &&
+      lastResponse
+    ) {
+      return { response: finalFailure(lastResponse), unit };
+    }
     fallbackCount += 1;
   }
   recordComboRequest(args.combo.name, null, {
@@ -288,7 +339,9 @@ export async function executeRuntimeUnitCombo(args: {
     strategy: effectiveStrategy,
   });
   return {
-    response: lastResponse || errorResponse(503, "All nested combo units unavailable"),
+    response: finalFailure(
+      lastResponse || errorResponse(503, "All nested combo units unavailable")
+    ),
     unit: null,
   };
 }
diff --git a/open-sse/services/combo/types.ts b/open-sse/services/combo/types.ts
index 1252a87c90..71b4e849ec 100644
--- a/open-sse/services/combo/types.ts
+++ b/open-sse/services/combo/types.ts
@@ -174,6 +174,7 @@ export type ResolvedComboTarget = {
   label: string | null;
   prompt?: string | null;
   failoverBeforeRetry?: unknown;
+  fallbackOnlyOnQuotaExhaustion?: boolean;
   trafficType?: "production" | "shadow";
   /**
    * Fingerprint-based account pin resolved from a combo builder composite
@@ -200,6 +201,7 @@ export type ResolvedComboRefTarget = {
   comboName: string;
   weight: number;
   label: string | null;
+  fallbackOnlyOnQuotaExhaustion?: boolean;
 };
 
 export type ResolvedComboUnit = ResolvedComboTarget | ResolvedComboRefTarget;
diff --git a/src/app/(dashboard)/dashboard/combos/ComboQuotaOnlyFallbackToggle.tsx b/src/app/(dashboard)/dashboard/combos/ComboQuotaOnlyFallbackToggle.tsx
new file mode 100644
index 0000000000..e1464f6ee6
--- /dev/null
+++ b/src/app/(dashboard)/dashboard/combos/ComboQuotaOnlyFallbackToggle.tsx
@@ -0,0 +1,60 @@
+import type { ComboStep } from "@/lib/combos/steps";
+
+type Translate = (key: string, fallback: string) => string;
+
+type Props = {
+  strategy: string;
+  entry: ComboStep;
+  index: number;
+  hasPricing: boolean;
+  translate: Translate;
+  onCheckedChange: (index: number, enabled: boolean) => void;
+};
+
+export function ComboTargetOptions({
+  strategy,
+  entry,
+  index,
+  hasPricing,
+  translate,
+  onCheckedChange,
+}: Props) {
+  return (
+    <>
+      {strategy === "cost-optimized" && (
+        <span
+          className={`text-[9px] px-1.5 py-0.5 rounded-full uppercase font-semibold ${
+            hasPricing
+              ? "bg-emerald-500/15 text-emerald-600 dark:text-emerald-400"
+              : "bg-amber-500/15 text-amber-600 dark:text-amber-400"
+          }`}
+          title={translate(
+            hasPricing ? "pricingAvailable" : "pricingMissing",
+            hasPricing ? "Pricing available" : "No pricing"
+          )}
+        >
+          {translate(
+            hasPricing ? "pricingAvailableShort" : "pricingMissingShort",
+            hasPricing ? "priced" : "no-price"
+          )}
+        </span>
+      )}
+      {strategy === "priority" && (entry.kind === "model" || entry.kind === "combo-ref") && (
+        <label className="flex items-center gap-1 text-[10px] text-text-muted shrink-0">
+          <input
+            type="checkbox"
+            checked={entry.fallbackOnlyOnQuotaExhaustion === true}
+            onChange={(event) => onCheckedChange(index, event.target.checked)}
+            aria-label={translate(
+              "fallbackOnlyOnQuotaExhaustion",
+              "Only advance on quota exhaustion"
+            )}
+          />
+          <span className="hidden lg:inline">
+            {translate("fallbackOnlyOnQuotaExhaustionShort", "quota-only fallback")}
+          </span>
+        </label>
+      )}
+    </>
+  );
+}
diff --git a/src/app/(dashboard)/dashboard/combos/comboQuotaOnlyFallback.ts b/src/app/(dashboard)/dashboard/combos/comboQuotaOnlyFallback.ts
new file mode 100644
index 0000000000..8f879b3160
--- /dev/null
+++ b/src/app/(dashboard)/dashboard/combos/comboQuotaOnlyFallback.ts
@@ -0,0 +1,33 @@
+import type { ComboStep } from "@/lib/combos/steps";
+
+export function setQuotaOnlyFallback(
+  entries: ComboStep[],
+  index: number,
+  enabled: boolean
+): ComboStep[] {
+  const selected = entries[index];
+  if (!selected || (selected.kind !== "model" && selected.kind !== "combo-ref")) return entries;
+  const next = [...entries];
+  const { fallbackOnlyOnQuotaExhaustion: _removed, ...entry } = selected;
+  next[index] = enabled ? { ...entry, fallbackOnlyOnQuotaExhaustion: true } : entry;
+  return next;
+}
+
+export function requiresExecuteMode(strategy: string, entries: ComboStep[]): boolean {
+  return (
+    strategy === "priority" &&
+    entries.some(
+      (entry) => entry.kind === "combo-ref" && entry.fallbackOnlyOnQuotaExhaustion === true
+    )
+  );
+}
+
+export function applyQuotaOnlyFallbackConfig(
+  strategy: string,
+  entries: ComboStep[],
+  config: Record<string, unknown>
+): Record<string, unknown> {
+  return requiresExecuteMode(strategy, entries)
+    ? { ...config, nestedComboMode: "execute" }
+    : config;
+}
diff --git a/src/app/(dashboard)/dashboard/combos/page.tsx b/src/app/(dashboard)/dashboard/combos/page.tsx
index 93f77723f9..8d36151319 100644
--- a/src/app/(dashboard)/dashboard/combos/page.tsx
+++ b/src/app/(dashboard)/dashboard/combos/page.tsx
@@ -16,6 +16,8 @@ import Tooltip from "@/shared/components/Tooltip";
 import { ComboCompressionModeSelect } from "@/shared/components/compression/ComboCompressionModeSelect";
 import { useCopyToClipboard } from "@/shared/hooks/useCopyToClipboard";
 import { FieldLabelWithHelp, WeightTotalBar } from "./parts";
+import { ComboTargetOptions } from "./ComboQuotaOnlyFallbackToggle";
+import { applyQuotaOnlyFallbackConfig, setQuotaOnlyFallback } from "./comboQuotaOnlyFallback";
 import { useComboProxyAssignments } from "./useComboProxyAssignments";
 import { ResponseValidationEditor, type ResponseValidationValue } from "./ResponseValidationEditor";
 import ReasoningTokenBufferToggle from "./ReasoningTokenBufferToggle";
@@ -2793,7 +2795,11 @@ function ComboFormModal({ isOpen, combo, onClose, onSave, activeProviders, combo
       saveData.description = null;
     }
 
-    const configToSave = sanitizeComboRuntimeConfig(config);
+    const configToSave = applyQuotaOnlyFallbackConfig(
+      strategy,
+      models,
+      sanitizeComboRuntimeConfig(config)
+    );
     if (strategy === "round-robin") {
       if (config.concurrencyPerModel !== undefined)
         configToSave.concurrencyPerModel = config.concurrencyPerModel;
@@ -3477,24 +3483,16 @@ function ComboFormModal({ isOpen, combo, onClose, onSave, activeProviders, combo
                         </div>
                       </div>
 
-                      {strategy === "cost-optimized" && (
-                        <span
-                          className={`text-[9px] px-1.5 py-0.5 rounded-full uppercase font-semibold ${
-                            hasPricingForModel(entry.model)
-                              ? "bg-emerald-500/15 text-emerald-600 dark:text-emerald-400"
-                              : "bg-amber-500/15 text-amber-600 dark:text-amber-400"
-                          }`}
-                          title={
-                            hasPricingForModel(entry.model)
-                              ? getI18nOrFallback(t, "pricingAvailable", "Pricing available")
-                              : getI18nOrFallback(t, "pricingMissing", "No pricing")
-                          }
-                        >
-                          {hasPricingForModel(entry.model)
-                            ? getI18nOrFallback(t, "pricingAvailableShort", "priced")
-                            : getI18nOrFallback(t, "pricingMissingShort", "no-price")}
-                        </span>
-                      )}
+                      <ComboTargetOptions
+                        strategy={strategy}
+                        entry={entry}
+                        index={index}
+                        onCheckedChange={(stepIndex, enabled) =>
+                          setModels(setQuotaOnlyFallback(models, stepIndex, enabled))
+                        }
+                        hasPricing={hasPricingForModel(entry.model)}
+                        translate={(key, fallback) => getI18nOrFallback(t, key, fallback)}
+                      />
 
                       {/* Weight input (weighted mode only) */}
                       {strategy === "weighted" && (
diff --git a/src/app/api/combos/[id]/route.ts b/src/app/api/combos/[id]/route.ts
index a1ba03ee1b..869114486f 100644
--- a/src/app/api/combos/[id]/route.ts
+++ b/src/app/api/combos/[id]/route.ts
@@ -1,18 +1,13 @@
 import { NextResponse } from "next/server";
-import {
-  getComboById,
-  updateCombo,
-  deleteCombo,
-  getComboByName,
-  getCombos,
-  isCloudEnabled,
-} from "@/lib/localDb";
+import { getComboById, updateCombo, deleteCombo, getComboByName, getCombos } from "@/lib/db/combos";
+import { isCloudEnabled } from "@/lib/db/settings";
 import { getConsistentMachineId } from "@/shared/utils/machineId";
 import { syncToCloud } from "@/lib/cloudSync";
 import { validateCompositeTiersConfig } from "@/lib/combos/compositeTiers";
 import { normalizeComboModels } from "@/lib/combos/steps";
 import { validateComboDAG, clampComboDepth } from "@omniroute/open-sse/services/combo.ts";
 import { updateComboSchema } from "@/shared/validation/schemas";
+import { requiresQuotaOnlyComboRefExecute } from "@/shared/validation/schemas/combo";
 import { isValidationFailure, validateBody } from "@/shared/validation/helpers";
 import { requireManagementAuth } from "@/lib/api/requireManagementAuth";
 import { QUOTA_MODEL_PREFIX } from "@/lib/quota/quotaModelNaming";
@@ -151,17 +146,17 @@ export async function PUT(request, { params }) {
     const normalizedUpdate = { ...validation.data };
     if (normalizedUpdate.compressionOverride !== undefined) {
       const legacyCompressionOverride = normalizedUpdate.compressionOverride;
-    const nextConfig: Record<string, unknown> =
-      currentCombo.config &&
-      typeof currentCombo.config === "object" &&
-      !Array.isArray(currentCombo.config)
-        ? { ...(currentCombo.config as Record<string, unknown>) }
-        : {};
-    if (legacyCompressionOverride) {
-      nextConfig.compressionMode = legacyCompressionOverride;
-    } else {
-      delete nextConfig.compressionMode;
-    }
+      const nextConfig: Record<string, unknown> =
+        currentCombo.config &&
+        typeof currentCombo.config === "object" &&
+        !Array.isArray(currentCombo.config)
+          ? { ...(currentCombo.config as Record<string, unknown>) }
+          : {};
+      if (legacyCompressionOverride) {
+        nextConfig.compressionMode = legacyCompressionOverride;
+      } else {
+        delete nextConfig.compressionMode;
+      }
       normalizedUpdate.config = nextConfig;
       delete normalizedUpdate.compressionOverride;
     }
@@ -187,6 +182,17 @@ export async function PUT(request, { params }) {
       ...body,
       name: comboName,
     };
+    if (requiresQuotaOnlyComboRefExecute(nextComboState as never)) {
+      return comboErrorResponse(
+        "COMBO_002",
+        400,
+        {
+          firstField: "config.nestedComboMode",
+          firstMessage: "Quota-only combo references require nestedComboMode execute",
+        },
+        request
+      );
+    }
     const compositeValidation = validateCompositeTiersConfig(nextComboState);
     if (compositeValidation.success === false) {
       const failure = compositeValidation as {
@@ -235,12 +241,7 @@ export async function PUT(request, { params }) {
               : dagError instanceof Error && /depth/i.test(dagError.message)
                 ? "max-depth-exceeded"
                 : "invalid-graph";
-          return comboErrorResponse(
-            "COMBO_005",
-            400,
-            { comboName, reason },
-            request
-          );
+          return comboErrorResponse("COMBO_005", 400, { comboName, reason }, request);
         }
       }
     }
@@ -253,9 +254,7 @@ export async function PUT(request, { params }) {
     // #8530: a combo renamed to a real model id is a supported pattern
     // (#6940 — bare-model-id provider fallback), so it is never rejected.
     // Surface it as a non-blocking warning instead of silently shadowing it.
-    const warning = comboName
-      ? buildComboNameCollisionWarning(String(comboName))
-      : null;
+    const warning = comboName ? buildComboNameCollisionWarning(String(comboName)) : null;
     return NextResponse.json(warning ? { ...combo, warning } : combo);
   } catch (error) {
     if (error instanceof ComboInvariantError) {
diff --git a/src/i18n/messages/en.json b/src/i18n/messages/en.json
index e154bc29d7..8af252a53f 100644
--- a/src/i18n/messages/en.json
+++ b/src/i18n/messages/en.json
@@ -3248,6 +3248,8 @@
     "pricingMissing": "No pricing",
     "pricingAvailableShort": "priced",
     "pricingMissingShort": "no-price",
+    "fallbackOnlyOnQuotaExhaustion": "Only advance on quota exhaustion",
+    "fallbackOnlyOnQuotaExhaustionShort": "quota-only fallback",
     "warningRoundRobinSingleModel": "Round-robin is most useful with at least 2 models.",
     "warningCostOptimizedPartialPricing": "Only {priced} of {total} models have pricing. Routing may be partially cost-aware.",
     "warningCostOptimizedNoPricing": "No pricing data found for this combo. Cost-optimized may route unexpectedly.",
diff --git a/src/i18n/messages/zh-CN.json b/src/i18n/messages/zh-CN.json
index d522262829..4666bb4183 100644
--- a/src/i18n/messages/zh-CN.json
+++ b/src/i18n/messages/zh-CN.json
@@ -3273,6 +3273,8 @@
     "pricingMissing": "无定价",
     "pricingAvailableShort": "已定价",
     "pricingMissingShort": "无定价",
+    "fallbackOnlyOnQuotaExhaustion": "仅配额/额度耗尽时进入下一目标",
+    "fallbackOnlyOnQuotaExhaustionShort": "仅配额耗尽回退",
     "warningRoundRobinSingleModel": "轮询策略至少有 2 个模型时才最有价值。",
     "warningCostOptimizedPartialPricing": "在 {total} 个模型中，只有 {priced} 个有定价信息。路由可能只具备部分成本感知能力。",
     "warningCostOptimizedNoPricing": "该组合未找到任何定价数据。成本优化策略的路由结果可能不符合预期。",
diff --git a/src/lib/combos/steps.ts b/src/lib/combos/steps.ts
index d9f4ea7673..ed10e0ad76 100644
--- a/src/lib/combos/steps.ts
+++ b/src/lib/combos/steps.ts
@@ -25,6 +25,7 @@ export interface ComboModelStep {
   label?: string;
   prompt?: string | null;
   tags?: string[];
+  fallbackOnlyOnQuotaExhaustion?: boolean;
 }
 
 export interface ComboRefStep {
@@ -33,6 +34,7 @@ export interface ComboRefStep {
   comboName: string;
   weight: number;
   label?: string;
+  fallbackOnlyOnQuotaExhaustion?: boolean;
 }
 
 export interface ComboProviderWildcardStep {
@@ -287,6 +289,7 @@ export function normalizeComboStep(
   const weight = toWeight(value.weight);
   const label = toTrimmedString(value.label);
   const prompt = toTrimmedString(value.prompt);
+  const fallbackOnlyOnQuotaExhaustion = value.fallbackOnlyOnQuotaExhaustion === true;
 
   if (value.kind === "combo-ref") {
     const comboRefName = toTrimmedString(value.comboName);
@@ -297,6 +300,7 @@ export function normalizeComboStep(
       comboName: comboRefName,
       weight,
       ...(label ? { label } : {}),
+      ...(fallbackOnlyOnQuotaExhaustion ? { fallbackOnlyOnQuotaExhaustion: true } : {}),
     };
   }
 
@@ -409,6 +413,7 @@ export function normalizeComboStep(
     ...(prompt ? { prompt } : {}),
     ...(tags && tags.length > 0 ? { tags } : {}),
     ...(allowedConnectionIds && allowedConnectionIds.length > 0 ? { allowedConnectionIds } : {}),
+    ...(fallbackOnlyOnQuotaExhaustion ? { fallbackOnlyOnQuotaExhaustion: true } : {}),
   };
 }
 
diff --git a/src/shared/validation/schemas/combo.ts b/src/shared/validation/schemas/combo.ts
index df9f9335fb..71d2a992de 100644
--- a/src/shared/validation/schemas/combo.ts
+++ b/src/shared/validation/schemas/combo.ts
@@ -20,6 +20,7 @@ export const comboStepMetaSchema = {
   id: z.string().trim().min(1).max(200).optional(),
   weight: z.number().min(0).max(100).optional().default(0),
   label: z.string().trim().min(1).max(200).optional(),
+  fallbackOnlyOnQuotaExhaustion: z.boolean().optional(),
 };
 
 export const comboModelStepInputSchema = z.object({
@@ -284,29 +285,61 @@ export const comboNameSchema = z
     "Name can only contain letters, numbers, spaces, -, _, /, ., [ and ]."
   );
 
-export const createComboSchema = z.object({
-  name: comboNameSchema,
-  description: z.string().max(2000).optional(),
-  models: z.array(comboModelEntry).optional().default([]),
-  strategy: comboStrategySchema.optional().default("priority"),
-  config: comboRuntimeConfigSchema.optional(),
-  allowedProviders: z.array(z.string().trim().min(1).max(200)).max(100).optional(),
-  allowedModelFamilies: z.array(z.string().trim().min(1).max(100)).max(100).optional(),
-  system_message: z.string().max(50000).optional(),
-  tool_filter_regex: z.string().max(1000).optional(),
-  context_cache_protection: z.boolean().optional(),
-  context_length: z.number().int().min(1000).max(2000000).optional(),
-  // Optional embedding dimensions override for embedding combos.
-  // When set, the value is injected into every upstream embedding request as
-  // the `dimensions` field (and translated to `outputDimensionality` for Gemini).
-  // Stored as a string to match the OpenAI API convention; coerced to number
-  // by the embedding handler. Leave unset to use each model's default.
-  dimensions: z
-    .string()
-    .regex(/^\d+$/, "dimensions must be a positive integer string")
-    .optional()
-    .nullable(),
-});
+type QuotaOnlyComboRefState = {
+  models?: Array<z.infer<typeof comboModelEntry>>;
+  strategy?: string;
+  config?: z.infer<typeof comboRuntimeConfigSchema>;
+};
+
+export function requiresQuotaOnlyComboRefExecute(value: QuotaOnlyComboRefState): boolean {
+  const hasProtectedComboRef = value.models?.some(
+    (step) =>
+      typeof step === "object" &&
+      step.kind === "combo-ref" &&
+      step.fallbackOnlyOnQuotaExhaustion === true
+  );
+  return (
+    (value.strategy === undefined || value.strategy === "priority") &&
+    hasProtectedComboRef === true &&
+    value.config?.nestedComboMode !== "execute"
+  );
+}
+
+function validateQuotaOnlyComboRefs(value: QuotaOnlyComboRefState, ctx: z.RefinementCtx): void {
+  if (requiresQuotaOnlyComboRefExecute(value)) {
+    ctx.addIssue({
+      code: z.ZodIssueCode.custom,
+      message: "Quota-only combo references require nestedComboMode execute",
+      path: ["config", "nestedComboMode"],
+    });
+  }
+}
+
+export const createComboSchema = z
+  .object({
+    name: comboNameSchema,
+    description: z.string().max(2000).optional(),
+    models: z.array(comboModelEntry).optional().default([]),
+    strategy: comboStrategySchema.optional().default("priority"),
+    config: comboRuntimeConfigSchema.optional(),
+    allowedProviders: z.array(z.string().trim().min(1).max(200)).max(100).optional(),
+    allowedModelFamilies: z.array(z.string().trim().min(1).max(100)).max(100).optional(),
+    system_message: z.string().max(50000).optional(),
+    tool_filter_regex: z.string().max(1000).optional(),
+    context_cache_protection: z.boolean().optional(),
+    context_length: z.number().int().min(1000).max(2000000).optional(),
+    // Optional embedding dimensions override for embedding combos.
+    // When set, the value is injected into every upstream embedding request as
+    // the `dimensions` field (and translated to `outputDimensionality` for Gemini).
+    // Stored as a string to match the OpenAI API convention; coerced to number
+    // by the embedding handler. Leave unset to use each model's default.
+    dimensions: z
+      .string()
+      .regex(/^\d+$/, "dimensions must be a positive integer string")
+      .optional()
+      .nullable(),
+  })
+  .superRefine(validateQuotaOnlyComboRefs);
 
 export const updateComboDefaultsSchema = z
   .object({
diff --git a/tests/unit/chat-route-coverage.test.ts b/tests/unit/chat-route-coverage.test.ts
index e249e5247a..b7174d6358 100644
--- a/tests/unit/chat-route-coverage.test.ts
+++ b/tests/unit/chat-route-coverage.test.ts
@@ -233,34 +233,48 @@ test("handleChat applies task-aware routing when a semantic override is enabled"
   assert.equal(json.choices[0].message.content, "Task-routed response");
 });
 
-test("handleChat routes exact combo names and can recover via global fallback", async () => {
+test("handleChat keeps protected combo fallback separate from Global Fallback Model", async () => {
   await seedConnection("openai", { apiKey: "sk-openai-combo-route" });
+  const ordinaryTarget = await seedConnection("deepseek", {
+    apiKey: "sk-deepseek-combo-backup",
+  });
   await seedConnection("claude", { apiKey: "sk-claude-global-fallback" });
   await combosDb.createCombo({
     name: "router-global-fallback",
     strategy: "priority",
     config: { maxRetries: 0, retryDelayMs: 0 },
-    models: ["openai/gpt-4.1"],
+    models: [
+      {
+        kind: "model",
+        model: "openai/gpt-4.1",
+        fallbackOnlyOnQuotaExhaustion: true,
+      },
+      {
+        kind: "model",
+        model: "deepseek/deepseek-v4-flash",
+        connectionId: ordinaryTarget.id,
+      },
+    ],
   });
   await settingsDb.updateSettings({
     globalFallbackModel: "claude/claude-3-5-sonnet-20241022",
   });
 
-  let attempts = 0;
-  globalThis.fetch = async (_url, init = {}) => {
-    attempts += 1;
-    const headers = toPlainHeaders(init.headers);
-    if (attempts === 1) {
-      assert.equal(headers.Authorization ?? headers.authorization, "Bearer sk-openai-combo-route");
+  const attemptedKeys: string[] = [];
+  globalThis.fetch = async (_url, init) => {
+    const headers = toPlainHeaders(init?.headers);
+    const key = headers["x-api-key"] ?? headers.Authorization ?? headers.authorization ?? "";
+    attemptedKeys.push(key);
+    if (key === "sk-deepseek-combo-backup") {
+      assert.fail("protected primary must not invoke the ordinary combo target");
+    }
+    if (key === "Bearer sk-openai-combo-route") {
       return new Response(JSON.stringify({ error: { message: "primary combo failed" } }), {
         status: 503,
         headers: { "Content-Type": "application/json" },
       });
     }
-    assert.equal(
-      headers["x-api-key"] ?? headers.Authorization ?? headers.authorization,
-      "sk-claude-global-fallback"
-    );
+    assert.equal(key, "sk-claude-global-fallback");
     return buildClaudeResponse("Global fallback answered");
   };
 
@@ -276,7 +290,7 @@ test("handleChat routes exact combo names and can recover via global fallback",
   const json = (await response.json()) as any;
 
   assert.equal(response.status, 200);
-  assert.equal(attempts, 2);
+  assert.deepEqual(attemptedKeys, ["Bearer sk-openai-combo-route", "sk-claude-global-fallback"]);
   assert.equal(json.choices[0].message.content, "Global fallback answered");
 });
 
diff --git a/tests/unit/combo-dispatch-prelude.test.ts b/tests/unit/combo-dispatch-prelude.test.ts
index 688abc8b26..87d35ef25d 100644
--- a/tests/unit/combo-dispatch-prelude.test.ts
+++ b/tests/unit/combo-dispatch-prelude.test.ts
@@ -606,6 +606,82 @@ test("tryRuntimeUnitDispatch: round-robin rotates across successive dispatches",
   );
 });
 
+test("tryRuntimeUnitDispatch: sticky weighted keeps quota-only fallback dormant", async () => {
+  const combo = {
+    ...twoRefCombo("weighted-quota-dormant", "weighted", { stickyWeightedLimit: 5 }),
+    models: [
+      { kind: "combo-ref", comboName: "leafA", fallbackOnlyOnQuotaExhaustion: true },
+      { kind: "combo-ref", comboName: "leafB" },
+    ],
+  };
+  const ctx = setup(combo);
+  const units = resolveComboRuntimeUnits(ctx.combo, [ctx.combo, LEAF_A, LEAF_B], "execute", 3);
+  const protectedUnit = units.find(
+    (unit) => unit.kind === "combo-ref" && unit.fallbackOnlyOnQuotaExhaustion === true
+  );
+  assert.ok(protectedUnit);
+  recordStickyWeightedSuccess(combo.name, protectedUnit.executionKey, 5);
+
+  const recursedInto: string[] = [];
+  const result = await tryRuntimeUnitDispatch({
+    body: ctx.body,
+    combo: ctx.combo,
+    config: ctx.config,
+    strategy: "weighted",
+    allCombos: [ctx.combo, LEAF_A, LEAF_B],
+    handleSingleModel: async () => okResponse("raw"),
+    handleSingleModelWithTimeout: async () => okResponse("wrapped"),
+    log: ctx.log,
+    settings: {},
+    runCombo: async (options) => {
+      recursedInto.push(options.combo.name);
+      return options.combo.name === "leafA"
+        ? new Response("unavailable", { status: 503 })
+        : okResponse("backup");
+    },
+  });
+  assert.equal(result?.status, 200);
+  assert.deepEqual(recursedInto, [protectedUnit.comboName, protectedUnit.comboName, "leafB"]);
+});
+
+test("tryRuntimeUnitDispatch: protected execute response validation returns local 502", async () => {
+  const combo = {
+    name: "protected-quality-execute",
+    strategy: "priority",
+    models: [
+      { kind: "model", model: "p/invalid", fallbackOnlyOnQuotaExhaustion: true },
+      COMBO_REF_STEP,
+    ],
+    config: {
+      nestedComboMode: "execute",
+      maxRetries: 0,
+      responseValidation: { minContentLength: 100 },
+    },
+  };
+  const ctx = setup(combo);
+  const calls: string[] = [];
+  const result = await tryRuntimeUnitDispatch({
+    body: ctx.body,
+    combo: ctx.combo,
+    config: ctx.config,
+    strategy: "priority",
+    allCombos: [ctx.combo, LEAF_COMBO],
+    handleSingleModel: async () => okResponse("raw"),
+    handleSingleModelWithTimeout: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return okResponse("short");
+    },
+    log: ctx.log,
+    settings: {},
+    runCombo: async () => {
+      calls.push("backup");
+      return okResponse("backup");
+    },
+  });
+  assert.equal(result?.status, 502);
+  assert.deepEqual(calls, ["p/invalid"]);
+});
+
 test("tryRuntimeUnitDispatch: weighted honors a previously recorded sticky unit", async () => {
   const combo = twoRefCombo("weighted-sticky", "weighted", { stickyWeightedLimit: 5 });
   const ctx = setup(combo);
diff --git a/tests/unit/combo-quota-exhaustion-only-fallback.test.ts b/tests/unit/combo-quota-exhaustion-only-fallback.test.ts
new file mode 100644
index 0000000000..eb4a7baaff
--- /dev/null
+++ b/tests/unit/combo-quota-exhaustion-only-fallback.test.ts
@@ -0,0 +1,696 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-quota-only-"));
+const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
+process.env.DATA_DIR = TEST_DATA_DIR;
+const { handleComboChat } = await import("../../open-sse/services/combo.ts");
+const { isQuotaExhaustionResponse } =
+  await import("../../open-sse/services/combo/quotaExhaustion.ts");
+const { clearAllModelLockouts, lockModel } =
+  await import("../../open-sse/services/accountFallback.ts");
+const { recordComboRequest, resetComboMetrics } =
+  await import("../../open-sse/services/comboMetrics.ts");
+const { resetDbInstance } = await import("../../src/lib/db/core.ts");
+const { resetAllCircuitBreakers } = await import("../../src/shared/utils/circuitBreaker.ts");
+
+test.afterEach(() => {
+  clearAllModelLockouts();
+  resetAllCircuitBreakers();
+});
+
+test.after(() => {
+  resetDbInstance();
+  if (ORIGINAL_DATA_DIR === undefined) delete process.env.DATA_DIR;
+  else process.env.DATA_DIR = ORIGINAL_DATA_DIR;
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+function log() {
+  return { info() {}, warn() {}, debug() {}, error() {} };
+}
+
+function response(
+  status: number,
+  message: string,
+  options: { code?: string; type?: string; headers?: Record<string, string> } = {}
+) {
+  return Response.json(
+    {
+      error: {
+        message,
+        ...(options.code ? { code: options.code } : {}),
+        ...(options.type ? { type: options.type } : {}),
+      },
+    },
+    { status, headers: options.headers }
+  );
+}
+
+function ok(model: string) {
+  return Response.json({ model });
+}
+
+function model(model: string, fallbackOnlyOnQuotaExhaustion?: boolean) {
+  return {
+    kind: "model" as const,
+    model,
+    providerId: model.split("/")[0],
+    ...(fallbackOnlyOnQuotaExhaustion === undefined ? {} : { fallbackOnlyOnQuotaExhaustion }),
+  };
+}
+
+async function run(
+  first: Response | Response[],
+  options: {
+    enabled?: boolean;
+    maxRetries?: number;
+    strategy?: string;
+    primary?: string;
+    connectionId?: string;
+    config?: Record<string, unknown>;
+    settings?: Record<string, unknown> | null;
+    available?: (modelStr: string) => Promise<boolean> | boolean;
+  } = {}
+) {
+  const calls: string[] = [];
+  const primaryResponses = Array.isArray(first) ? first : [first];
+  let primaryAttempt = 0;
+  const combo = {
+    name: `quota-only-${Math.random()}`,
+    strategy: options.strategy ?? "priority",
+    models: [
+      {
+        ...model(options.primary ?? "openai/primary", options.enabled),
+        ...(options.connectionId ? { connectionId: options.connectionId } : {}),
+      },
+      model("anthropic/backup"),
+    ],
+    config: { maxRetries: options.maxRetries ?? 0, retryDelayMs: 0, ...options.config },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo,
+    allCombos: [combo],
+    settings: options.settings ?? null,
+    log: log(),
+    isModelAvailable: options.available ?? (async () => true),
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr !== (options.primary ?? "openai/primary")) return ok(modelStr);
+      const result = primaryResponses[Math.min(primaryAttempt, primaryResponses.length - 1)];
+      primaryAttempt += 1;
+      return result.clone();
+    },
+  });
+  return { result, calls };
+}
+
+test("quota classifier rejects terminal-looking evidence on ineligible statuses", async () => {
+  for (const status of [400, 401, 403, 404, 408, 409, 422, 500, 502, 503, 504]) {
+    for (const terminal of ["insufficient_quota", "quota_exhausted", "credits_exhausted"]) {
+      assert.equal(
+        await isQuotaExhaustionResponse(
+          response(status, `${terminal}: billing hard limit reached`, {
+            code: terminal,
+            type: terminal,
+          }),
+          "openai",
+          "gpt-4o"
+        ),
+        false,
+        `${status} with ${terminal} must not classify as terminal depletion`
+      );
+    }
+  }
+});
+
+test("quota classifier accepts explicit terminal depletion only on eligible statuses", async () => {
+  for (const status of [402, 429]) {
+    assert.equal(
+      await isQuotaExhaustionResponse(
+        response(status, "Payment required", { code: "insufficient_quota" }),
+        "openai",
+        "gpt-4o"
+      ),
+      true
+    );
+  }
+});
+
+test("priority target advances on explicit quota exhaustion", async () => {
+  const { result, calls } = await run(response(429, "You have exceeded your current quota"), {
+    enabled: true,
+  });
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/primary", "anthropic/backup"]);
+});
+
+test("priority target retries plain 429 then returns the last response without advancing", async () => {
+  const { result, calls } = await run(response(429, "Rate limit exceeded; retry later"), {
+    enabled: true,
+    maxRetries: 1,
+  });
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["openai/primary", "openai/primary"]);
+});
+
+test("priority target keeps a non-quota retry monotonic through terminal quota exhaustion", async () => {
+  const { result, calls } = await run(
+    [
+      response(503, "Service unavailable"),
+      response(429, "Payment required", { code: "insufficient_quota" }),
+    ],
+    { enabled: true, maxRetries: 1 }
+  );
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["openai/primary", "openai/primary"]);
+});
+
+test("priority target returns a later successful retry", async () => {
+  const { result, calls } = await run(
+    [response(503, "Service unavailable"), ok("openai/primary")],
+    { enabled: true, maxRetries: 1 }
+  );
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/primary", "openai/primary"]);
+});
+
+test("priority target advances when every retry is quota exhaustion", async () => {
+  const { result, calls } = await run(
+    [
+      response(429, "Payment required", { code: "quota_exhausted" }),
+      response(429, "Billing hard limit reached", { code: "quota_exhausted" }),
+    ],
+    { enabled: true, maxRetries: 1 }
+  );
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/primary", "openai/primary", "anthropic/backup"]);
+});
+
+test("failures of another target do not contaminate protected target trust", async () => {
+  const calls: string[] = [];
+  const combo = {
+    name: `target-local-${Math.random()}`,
+    strategy: "priority",
+    models: [model("openai/unprotected"), model("anthropic/protected", true), model("paid/backup")],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo,
+    allCombos: [combo],
+    settings: null,
+    log: log(),
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return modelStr === "openai/unprotected"
+        ? response(503, "Service unavailable")
+        : modelStr === "anthropic/protected"
+          ? response(429, "Payment required", { code: "quota_exhausted" })
+          : ok(modelStr);
+    },
+  });
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/unprotected", "anthropic/protected", "paid/backup"]);
+});
+
+for (const [status, message] of [
+  [408, "Request timeout"],
+  [502, "Bad gateway"],
+  [503, "Service unavailable"],
+  [503, "Local concurrency capacity reached"],
+  [504, "Gateway timeout"],
+  [401, "Invalid API key"],
+] as const) {
+  test(`priority target does not advance on non-quota ${status} (${message})`, async () => {
+    const { result, calls } = await run(response(status, message), { enabled: true });
+    assert.equal(result.status, status);
+    assert.deepEqual(calls, ["openai/primary"]);
+  });
+}
+
+test("protected non-quota response preserves status, body, and headers", async () => {
+  const first = response(502, "upstream detail", { headers: { "x-upstream": "kept" } });
+  const { result } = await run(first, { enabled: true });
+  assert.equal(result.status, 502);
+  assert.equal(result.headers.get("x-upstream"), "kept");
+  assert.deepEqual(await result.json(), { error: { message: "upstream detail" } });
+});
+
+for (const [metric, advances] of [
+  ["generate_content_free_tier_requests", false],
+  ["generate_content_free_tier_input_token_count", false],
+  ["generate_content_free_tier_requests_per_day", true],
+] as const) {
+  test(`Gemini ${metric} classification ${advances ? "advances" : "stops"} direct routing`, async () => {
+    const message = `You exceeded your current quota for metric generativelanguage.googleapis.com/${metric}`;
+    const { result, calls } = await run(response(429, message), {
+      enabled: true,
+      primary: "gemini/gemini-2.5-flash",
+    });
+    assert.equal(result.ok, advances);
+    assert.deepEqual(
+      calls,
+      advances ? ["gemini/gemini-2.5-flash", "anthropic/backup"] : ["gemini/gemini-2.5-flash"]
+    );
+  });
+}
+
+test("compatible provider advances only on explicit structured terminal depletion", async () => {
+  const plain = await run(response(402, "billing service unavailable"), {
+    enabled: true,
+    primary: "openai-compatible-chat-test/primary",
+  });
+  assert.equal(plain.result.status, 402);
+  assert.deepEqual(plain.calls, ["openai-compatible-chat-test/primary"]);
+
+  const terminal = await run(response(402, "Payment required", { code: "insufficient_quota" }), {
+    enabled: true,
+    primary: "openai-compatible-chat-test/primary",
+  });
+  assert.equal(terminal.result.ok, true);
+  assert.deepEqual(terminal.calls, ["openai-compatible-chat-test/primary", "anthropic/backup"]);
+});
+
+test("unset Gemini RPM response retains baseline fallback", async () => {
+  const { result, calls } = await run(
+    response(
+      429,
+      "You exceeded your current quota for metric generativelanguage.googleapis.com/generate_content_free_tier_requests"
+    ),
+    { primary: "gemini/gemini-2.5-flash" }
+  );
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["gemini/gemini-2.5-flash", "anthropic/backup"]);
+});
+
+test("unset target retains generic fallback", async () => {
+  const { result, calls } = await run(response(503, "Service unavailable"));
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/primary", "anthropic/backup"]);
+});
+
+test("option is dormant outside priority", async () => {
+  const { result, calls } = await run(response(503, "Service unavailable"), {
+    enabled: true,
+    strategy: "fill-first",
+  });
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["openai/primary", "anthropic/backup"]);
+});
+
+test("authoritative local quota cutoff advances an opted-in priority target", async () => {
+  const calls: string[] = [];
+  const combo = {
+    name: `local-quota-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { ...model("openai/primary", true), connectionId: "quota-empty" },
+      model("anthropic/backup"),
+    ],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const quotaPreflight = await import("../../open-sse/services/quotaPreflight.ts");
+  quotaPreflight.registerQuotaFetcher("openai", async () => ({
+    used: 100,
+    total: 100,
+    percentUsed: 1,
+  }));
+  const result = await handleComboChat({
+    body: {},
+    combo,
+    allCombos: [combo],
+    log: log(),
+    settings: {
+      resilienceSettings: {
+        quotaPreflight: { enabled: true, defaultThresholdPercent: 2, warnThresholdPercent: 20 },
+      },
+    },
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return ok(modelStr);
+    },
+  });
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["anthropic/backup"]);
+});
+
+test("protected pre-existing model lockout stops routing with a local 503", async () => {
+  lockModel("openai", "locked-connection", "primary", "rate_limited", 60_000);
+  const { result, calls } = await run(response(503, "unused"), {
+    enabled: true,
+    connectionId: "locked-connection",
+  });
+  assert.equal(result.status, 503);
+  assert.deepEqual(calls, []);
+});
+
+test("protected target keeps configured retries after recording a transient model lockout", async () => {
+  const { result, calls } = await run(response(503, "Service unavailable"), {
+    enabled: true,
+    maxRetries: 1,
+    connectionId: "retry-connection",
+    settings: {
+      resilienceSettings: {
+        modelLockout: { enabled: true, errorCodes: [503], baseCooldownMs: 60_000 },
+      },
+    },
+  });
+  assert.equal(result.status, 503);
+  assert.deepEqual(calls, ["openai/primary", "openai/primary"]);
+});
+
+test("protected response-quality rejection returns a sanitized 502 without advancing", async () => {
+  const { result, calls } = await run(ok("openai/primary"), {
+    enabled: true,
+    config: { responseValidation: { minContentLength: 100 } },
+  });
+  assert.equal(result.status, 502);
+  assert.deepEqual(await result.json(), {
+    error: {
+      message: "Upstream response failed quality validation",
+      type: "server_error",
+      code: "bad_gateway",
+    },
+  });
+  assert.deepEqual(calls, ["openai/primary"]);
+});
+
+test("protected predictive TTFT skip returns local 503 without advancing", async () => {
+  const name = `predictive-${Math.random()}`;
+  resetComboMetrics(name);
+  for (let index = 0; index < 5; index += 1) {
+    recordComboRequest(name, "openai/primary", { success: true, latencyMs: 10_000 });
+  }
+  const calls: string[] = [];
+  const combo = {
+    name,
+    strategy: "priority",
+    models: [model("openai/primary", true), model("anthropic/backup")],
+    config: { maxRetries: 0, zeroLatencyOptimizationsEnabled: true, predictiveTtftMs: 1 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo,
+    allCombos: [combo],
+    log: log(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return ok(modelStr);
+    },
+  });
+  assert.equal(result.status, 503);
+  assert.deepEqual(calls, []);
+});
+
+test("opted-in combo ref remains a black box and only quota exhaustion advances parent", async () => {
+  for (const childError of [
+    response(429, "Billing hard limit reached"),
+    response(503, "Service unavailable"),
+  ]) {
+    const calls: string[] = [];
+    const child = {
+      name: `child-${childError.status}-${Math.random()}`,
+      strategy: "priority",
+      models: [model("openai/child")],
+      config: { maxRetries: 0, retryDelayMs: 0 },
+    };
+    const outer = {
+      name: `outer-${Math.random()}`,
+      strategy: "priority",
+      models: [
+        {
+          kind: "combo-ref" as const,
+          comboName: child.name,
+          fallbackOnlyOnQuotaExhaustion: true,
+        },
+        model("anthropic/backup"),
+      ],
+      config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+    };
+    const result = await handleComboChat({
+      body: {},
+      combo: outer,
+      allCombos: [outer, child],
+      settings: null,
+      log: log(),
+      isModelAvailable: async () => true,
+      handleSingleModel: async (_body, modelStr) => {
+        calls.push(modelStr);
+        return modelStr === "openai/child" ? childError.clone() : ok(modelStr);
+      },
+    });
+    if (childError.status === 429) {
+      assert.equal(result.ok, true);
+      assert.deepEqual(calls, ["openai/child", "anthropic/backup"]);
+    } else {
+      assert.equal(result.status, 503);
+      assert.deepEqual(calls, ["openai/child"]);
+    }
+  }
+});
+
+test("protected parent combo-ref stops after normal child quality rejection then quota exhaustion", async () => {
+  const calls: string[] = [];
+  const child = {
+    name: `quality-child-${Math.random()}`,
+    strategy: "priority",
+    models: [model("openai/quality-invalid"), model("anthropic/quota")],
+    config: {
+      maxRetries: 0,
+      retryDelayMs: 0,
+      flattenSingleTargetNested: true,
+      responseValidation: { minContentLength: 100 },
+    },
+  };
+  const outer = {
+    name: `quality-outer-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      model("paid/backup"),
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, child],
+    settings: null,
+    log: log(),
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return modelStr === "openai/quality-invalid"
+        ? Response.json({ choices: [{ message: {} }] })
+        : modelStr === "anthropic/quota"
+          ? response(429, "Payment required", { code: "insufficient_quota" })
+          : ok(modelStr);
+    },
+  });
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["openai/quality-invalid", "anthropic/quota"]);
+});
+
+test("protected parent combo-ref stops when a child has mixed quota and non-quota failures", async () => {
+  const calls: string[] = [];
+  const child = {
+    name: `mixed-child-${Math.random()}`,
+    strategy: "priority",
+    models: [model("openai/quota"), model("anthropic/unavailable")],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: `mixed-outer-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      model("anthropic/paid-backup"),
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, child],
+    settings: null,
+    log: log(),
+    isModelAvailable: async (modelStr) => modelStr !== "anthropic/unavailable",
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return modelStr === "openai/quota"
+        ? response(429, "Billing hard limit reached")
+        : ok(modelStr);
+    },
+  });
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["openai/quota"]);
+});
+
+test("protected parent combo-ref keeps a child's non-quota target retry monotonic", async () => {
+  const calls: string[] = [];
+  const child = {
+    name: `target-retry-child-${Math.random()}`,
+    strategy: "priority",
+    models: [model("retry-provider/child")],
+    config: { maxRetries: 1, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: `target-retry-outer-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      model("anthropic/paid-backup"),
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, child],
+    settings: null,
+    log: log(),
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return calls.length === 1
+        ? response(503, "Service unavailable")
+        : response(429, "Payment required", { code: "insufficient_quota" });
+    },
+  });
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["retry-provider/child", "retry-provider/child"]);
+});
+
+test("protected parent combo-ref keeps a child's non-quota set failure monotonic", async () => {
+  const calls: string[] = [];
+  const child = {
+    name: `set-retry-child-${Math.random()}`,
+    strategy: "priority",
+    models: [model("openai/child")],
+    config: { maxRetries: 0, retryDelayMs: 0, maxSetRetries: 1, setRetryDelayMs: 0 },
+  };
+  const outer = {
+    name: `set-retry-outer-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      model("anthropic/paid-backup"),
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, child],
+    settings: null,
+    log: log(),
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return calls.length === 1
+        ? response(503, "Service unavailable")
+        : response(429, "Payment required", { code: "insufficient_quota" });
+    },
+  });
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, ["openai/child", "openai/child"]);
+});
+
+test("protected parent combo-ref advances on a single authoritative child quota cutoff", async () => {
+  const calls: string[] = [];
+  const child = {
+    name: `cutoff-child-${Math.random()}`,
+    strategy: "priority",
+    models: [{ ...model("openai/cutoff"), connectionId: "nested-quota-empty" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: `cutoff-outer-${Math.random()}`,
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      model("anthropic/paid-backup"),
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const quotaPreflight = await import("../../open-sse/services/quotaPreflight.ts");
+  quotaPreflight.registerQuotaFetcher("openai", async () => ({
+    used: 100,
+    total: 100,
+    percentUsed: 1,
+  }));
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, child],
+    settings: {
+      resilienceSettings: {
+        quotaPreflight: { enabled: true, defaultThresholdPercent: 2, warnThresholdPercent: 20 },
+      },
+    },
+    log: log(),
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return ok(modelStr);
+    },
+  });
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, ["anthropic/paid-backup"]);
+});
+
+for (const [metric, advances] of [
+  ["generate_content_free_tier_requests", false],
+  ["generate_content_free_tier_input_token_count", false],
+  ["generate_content_free_tier_requests_per_day", true],
+] as const) {
+  test(`nested Gemini ${metric} ${advances ? "advances" : "stops"} parent routing`, async () => {
+    const calls: string[] = [];
+    const child = {
+      name: `gemini-child-${Math.random()}`,
+      strategy: "priority",
+      models: [model("gemini/gemini-2.5-flash")],
+      config: { maxRetries: 0, retryDelayMs: 0 },
+    };
+    const outer = {
+      name: `gemini-outer-${Math.random()}`,
+      strategy: "priority",
+      models: [
+        { kind: "combo-ref" as const, comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+        model("anthropic/backup"),
+      ],
+      config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+    };
+    const result = await handleComboChat({
+      body: {},
+      combo: outer,
+      allCombos: [outer, child],
+      settings: null,
+      log: log(),
+      isModelAvailable: async () => true,
+      handleSingleModel: async (_body, modelStr) => {
+        calls.push(modelStr);
+        return modelStr.startsWith("gemini/")
+          ? response(
+              429,
+              `You exceeded your current quota for metric generativelanguage.googleapis.com/${metric}`
+            )
+          : ok(modelStr);
+      },
+    });
+    assert.equal(result.ok, advances);
+    assert.deepEqual(
+      calls,
+      advances ? ["gemini/gemini-2.5-flash", "anthropic/backup"] : ["gemini/gemini-2.5-flash"]
+    );
+  });
+}
diff --git a/tests/unit/combo-quota-exhaustion-option-schema.test.ts b/tests/unit/combo-quota-exhaustion-option-schema.test.ts
new file mode 100644
index 0000000000..f2a4cb8ca4
--- /dev/null
+++ b/tests/unit/combo-quota-exhaustion-option-schema.test.ts
@@ -0,0 +1,113 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import {
+  createComboSchema,
+  requiresQuotaOnlyComboRefExecute,
+  updateComboSchema,
+} from "../../src/shared/validation/schemas/combo.ts";
+import { normalizeComboStep } from "../../src/lib/combos/steps.ts";
+
+const modelStep = {
+  kind: "model" as const,
+  model: "openai/gpt-4o",
+  fallbackOnlyOnQuotaExhaustion: true,
+};
+const comboRefStep = {
+  kind: "combo-ref" as const,
+  comboName: "child",
+  fallbackOnlyOnQuotaExhaustion: true,
+};
+
+test("create/update accept and preserve strict quota-only booleans on model and combo-ref steps", () => {
+  const payload = {
+    name: "quota-only",
+    strategy: "priority" as const,
+    models: [modelStep, comboRefStep],
+    config: { nestedComboMode: "execute" as const },
+  };
+  const created = createComboSchema.parse(payload);
+  assert.equal(created.models[0].fallbackOnlyOnQuotaExhaustion, true);
+  assert.equal(created.models[1].fallbackOnlyOnQuotaExhaustion, true);
+
+  const updated = updateComboSchema.parse(payload);
+  assert.equal(updated.models?.[0].fallbackOnlyOnQuotaExhaustion, true);
+  assert.equal(updated.models?.[1].fallbackOnlyOnQuotaExhaustion, true);
+
+  for (const bad of ["true", 1, null]) {
+    assert.equal(
+      createComboSchema.safeParse({
+        ...payload,
+        models: [{ ...modelStep, fallbackOnlyOnQuotaExhaustion: bad }],
+      }).success,
+      false
+    );
+  }
+});
+
+test("non-priority combos preserve the dormant option", () => {
+  const parsed = createComboSchema.parse({
+    name: "dormant",
+    strategy: "weighted",
+    models: [modelStep],
+  });
+  assert.equal(parsed.models[0].fallbackOnlyOnQuotaExhaustion, true);
+  assert.equal(normalizeComboStep(parsed.models[0])?.fallbackOnlyOnQuotaExhaustion, true);
+});
+
+test("only active priority combo refs require execute mode", () => {
+  assert.equal(
+    createComboSchema.safeParse({ name: "invalid-ref", models: [comboRefStep] }).success,
+    false
+  );
+  assert.equal(
+    createComboSchema.safeParse({
+      name: "valid-ref",
+      models: [comboRefStep],
+      config: { nestedComboMode: "execute" },
+    }).success,
+    true
+  );
+  assert.equal(
+    createComboSchema.safeParse({
+      name: "dormant-ref",
+      strategy: "weighted",
+      models: [comboRefStep],
+    }).success,
+    true
+  );
+  assert.equal(
+    createComboSchema.safeParse({
+      name: "ordinary-ref",
+      models: [{ kind: "combo-ref", comboName: "child" }],
+    }).success,
+    true
+  );
+});
+
+test("partial updates defer active combo-ref validation to the merged route state", () => {
+  assert.equal(updateComboSchema.safeParse({ models: [comboRefStep] }).success, true);
+  assert.equal(updateComboSchema.safeParse({ strategy: "priority" }).success, true);
+  assert.equal(
+    requiresQuotaOnlyComboRefExecute({
+      strategy: "priority",
+      models: [comboRefStep],
+      config: {},
+    }),
+    true
+  );
+  assert.equal(
+    requiresQuotaOnlyComboRefExecute({
+      strategy: "weighted",
+      models: [comboRefStep],
+      config: {},
+    }),
+    false
+  );
+});
+
+test("normalization omits false and preserves true", () => {
+  const disabled = normalizeComboStep({ ...modelStep, fallbackOnlyOnQuotaExhaustion: false });
+  const enabled = normalizeComboStep(modelStep);
+  assert.equal("fallbackOnlyOnQuotaExhaustion" in (disabled || {}), false);
+  assert.equal(enabled?.fallbackOnlyOnQuotaExhaustion, true);
+});
diff --git a/tests/unit/combo-runtime-unit-concurrency.test.ts b/tests/unit/combo-runtime-unit-concurrency.test.ts
index d496bb7c2a..36a14a59ce 100644
--- a/tests/unit/combo-runtime-unit-concurrency.test.ts
+++ b/tests/unit/combo-runtime-unit-concurrency.test.ts
@@ -10,20 +10,28 @@ import assert from "node:assert/strict";
 import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
-import {
-  acquire,
-  buildAccountSemaphoreKey,
-  resetAll,
-} from "../../open-sse/services/accountSemaphore.ts";
-import { isRuntimeUnitAtConcurrencyCap } from "../../open-sse/services/combo/runtimeUnitCapacity.ts";
-import type { ResolvedComboUnit } from "../../open-sse/services/combo/types.ts";
 
 const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-runtime-unit-cap-"));
 const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
 process.env.DATA_DIR = TEST_DATA_DIR;
 
+const { acquire, buildAccountSemaphoreKey, resetAll } =
+  await import("../../open-sse/services/accountSemaphore.ts");
+const { isRuntimeUnitAtConcurrencyCap } =
+  await import("../../open-sse/services/combo/runtimeUnitCapacity.ts");
 const { handleComboChat } = await import("../../open-sse/services/combo.ts");
 const { getDbInstance, resetDbInstance } = await import("../../src/lib/db/core.ts");
+type ResolvedComboUnit = import("../../open-sse/services/combo/types.ts").ResolvedComboUnit;
+
+const db = getDbInstance();
+const databases = db.pragma("database_list") as Array<{ file?: string; name?: string }>;
+const activeDbPath = databases.find((database) => database.name === "main")?.file;
+assert.ok(activeDbPath, "test requires a file-backed main SQLite database");
+assert.equal(
+  path.dirname(path.resolve(activeDbPath)),
+  path.resolve(TEST_DATA_DIR),
+  `active test database must be under TEST_DATA_DIR before inserts: ${activeDbPath}`
+);
 
 function createLog() {
   return {
@@ -44,7 +52,7 @@ function okResponse() {
 function seedConnection(id: string, provider: string, maxConcurrent: number) {
   const db = getDbInstance();
   db.prepare(
-    `INSERT INTO provider_connections
+    `INSERT OR REPLACE INTO provider_connections
       (id, provider, auth_type, is_active, max_concurrent, created_at, updated_at)
      VALUES (?, ?, 'apikey', 1, ?, datetime('now'), datetime('now'))`
   ).run(id, provider, maxConcurrent);
@@ -62,7 +70,7 @@ test("isRuntimeUnitAtConcurrencyCap returns true for a model unit at cap", async
   const connectionId = "conn-feather-cap";
   const provider = "featherless-ai";
   const key = buildAccountSemaphoreKey({ provider, accountKey: connectionId });
-  const release1 = await acquire(key, { maxConcurrency: 2 });
+  let release1 = await acquire(key, { maxConcurrency: 2 });
   const release2 = await acquire(key, { maxConcurrency: 2 });
 
   const unit: ResolvedComboUnit = {
@@ -77,19 +85,409 @@ test("isRuntimeUnitAtConcurrencyCap returns true for a model unit at cap", async
     label: null,
   };
 
-  assert.equal(
-    await isRuntimeUnitAtConcurrencyCap(unit, [], async () => 2),
-    true,
-    "unit should be at cap when two slots are in use"
-  );
+  try {
+    assert.equal(
+      await isRuntimeUnitAtConcurrencyCap(unit, [], async () => 2),
+      true,
+      "unit should be at cap when two slots are in use"
+    );
 
-  release1();
-  assert.equal(
-    await isRuntimeUnitAtConcurrencyCap(unit, [], async () => 2),
-    false,
-    "unit should have headroom after one slot frees"
+    release1();
+    release1 = () => {};
+    assert.equal(
+      await isRuntimeUnitAtConcurrencyCap(unit, [], async () => 2),
+      false,
+      "unit should have headroom after one slot frees"
+    );
+  } finally {
+    release1();
+    release2();
+  }
+});
+
+test("protected priority model stops at local capacity", async () => {
+  const primaryConnectionId = "conn-protected-capacity";
+  const provider = "featherless-ai";
+  seedConnection(primaryConnectionId, provider, 1);
+  const release = await acquire(
+    buildAccountSemaphoreKey({ provider, accountKey: primaryConnectionId }),
+    {
+      maxConcurrency: 1,
+    }
   );
-  release2();
+  try {
+    const calls: string[] = [];
+    const combo = {
+      name: "protected-model-capacity",
+      strategy: "priority",
+      models: [
+        {
+          kind: "model",
+          model: "featherless-ai/deepseek-ai/DeepSeek-V4-Pro",
+          providerId: provider,
+          connectionId: primaryConnectionId,
+          fallbackOnlyOnQuotaExhaustion: true,
+        },
+        { kind: "model", model: "alibaba/backup", providerId: "alibaba" },
+      ],
+      config: { nestedComboMode: "execute", maxRetries: 0 },
+    };
+    const result = await handleComboChat({
+      body: {},
+      combo,
+      allCombos: [combo],
+      log: createLog(),
+      settings: null,
+      isModelAvailable: async () => true,
+      handleSingleModel: async (_body, modelStr) => {
+        calls.push(modelStr);
+        return okResponse();
+      },
+    });
+    assert.equal(result.status, 503);
+    assert.deepEqual(calls, []);
+  } finally {
+    release();
+  }
+});
+
+test("protected priority combo-ref stops when its child is at local capacity", async () => {
+  const connectionId = "conn-protected-child-capacity";
+  const provider = "featherless-ai";
+  seedConnection(connectionId, provider, 1);
+  const release = await acquire(buildAccountSemaphoreKey({ provider, accountKey: connectionId }), {
+    maxConcurrency: 1,
+  });
+  try {
+    const child = {
+      name: "protected-capacity-child",
+      strategy: "priority",
+      models: [
+        { kind: "model", model: "featherless-ai/child", providerId: provider, connectionId },
+      ],
+      config: { maxRetries: 0 },
+    };
+    const outer = {
+      name: "protected-capacity-outer",
+      strategy: "priority",
+      models: [
+        { kind: "combo-ref", comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+        { kind: "model", model: "alibaba/backup", providerId: "alibaba" },
+      ],
+      config: { nestedComboMode: "execute", maxRetries: 0 },
+    };
+    const calls: string[] = [];
+    const result = await handleComboChat({
+      body: {},
+      combo: outer,
+      allCombos: [outer, child],
+      log: createLog(),
+      settings: null,
+      isModelAvailable: async () => true,
+      handleSingleModel: async (_body, modelStr) => {
+        calls.push(modelStr);
+        return okResponse();
+      },
+    });
+    assert.equal(result.status, 503);
+    assert.deepEqual(calls, []);
+  } finally {
+    release();
+  }
+});
+
+test("protected priority unit keeps non-quota failure trust across retries", async () => {
+  const primary = "runtime-retry-provider/primary";
+  const paidBackup = "anthropic/paid-backup";
+  const paidCombo = {
+    name: "runtime-retry-paid-combo",
+    strategy: "priority",
+    models: [{ kind: "model", model: paidBackup, providerId: "anthropic" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: "runtime-retry-protected-outer",
+    strategy: "priority",
+    models: [
+      {
+        kind: "model",
+        model: primary,
+        providerId: "runtime-retry-provider",
+        fallbackOnlyOnQuotaExhaustion: true,
+      },
+      { kind: "combo-ref", comboName: paidCombo.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 1, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, paidCombo],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr === paidBackup) return okResponse();
+      if (calls.length === 1)
+        return Response.json({ error: "Service unavailable" }, { status: 503 });
+      return Response.json(
+        { error: { message: "Payment required", code: "insufficient_quota" } },
+        { status: 429 }
+      );
+    },
+  });
+
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, [primary, primary]);
+});
+
+test("protected priority unit returns a successful later retry", async () => {
+  const primary = "runtime-success-retry-provider/primary";
+  const paidCombo = {
+    name: "runtime-success-retry-paid-combo",
+    strategy: "priority",
+    models: [{ kind: "model", model: "anthropic/unused", providerId: "anthropic" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: "runtime-success-retry-protected-outer",
+    strategy: "priority",
+    models: [
+      {
+        kind: "model",
+        model: primary,
+        providerId: "runtime-success-retry-provider",
+        fallbackOnlyOnQuotaExhaustion: true,
+      },
+      { kind: "combo-ref", comboName: paidCombo.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 1, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, paidCombo],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      return calls.length === 1
+        ? Response.json({ error: "Service unavailable" }, { status: 503 })
+        : okResponse();
+    },
+  });
+
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, [primary, primary]);
+});
+
+test("protected priority unit advances when every retry is explicit quota exhaustion", async () => {
+  const primary = "runtime-quota-retry-provider/primary";
+  const paidBackup = "anthropic/quota-paid-backup";
+  const paidCombo = {
+    name: "runtime-quota-retry-paid-combo",
+    strategy: "priority",
+    models: [{ kind: "model", model: paidBackup, providerId: "anthropic" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const outer = {
+    name: "runtime-quota-retry-protected-outer",
+    strategy: "priority",
+    models: [
+      {
+        kind: "model",
+        model: primary,
+        providerId: "runtime-quota-retry-provider",
+        fallbackOnlyOnQuotaExhaustion: true,
+      },
+      { kind: "combo-ref", comboName: paidCombo.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 1, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: outer,
+    allCombos: [outer, paidCombo],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr === paidBackup) return okResponse();
+      return Response.json(
+        { error: { message: "Payment required", code: "insufficient_quota" } },
+        { status: 429 }
+      );
+    },
+  });
+
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, [primary, primary, paidBackup]);
+});
+
+test("nested child aggregate prevents protected parent fallback after any non-quota failure", async () => {
+  const transientModel = "runtime-nested-transient/primary";
+  const quotaModel = "runtime-nested-quota/primary";
+  const paidBackup = "anthropic/nested-paid-backup";
+  const quotaLeaf = {
+    name: "runtime-nested-quota-leaf",
+    strategy: "priority",
+    models: [{ kind: "model", model: quotaModel, providerId: "runtime-nested-quota" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const child = {
+    name: "runtime-nested-mixed-child",
+    strategy: "priority",
+    models: [
+      { kind: "model", model: transientModel, providerId: "runtime-nested-transient" },
+      { kind: "combo-ref", comboName: quotaLeaf.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const parent = {
+    name: "runtime-nested-mixed-parent",
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref", comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      { kind: "model", model: paidBackup, providerId: "anthropic" },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: parent,
+    allCombos: [parent, child, quotaLeaf],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr === transientModel) {
+        return Response.json({ error: "Service unavailable" }, { status: 503 });
+      }
+      if (modelStr === quotaModel) {
+        return Response.json(
+          { error: { message: "Payment required", code: "insufficient_quota" } },
+          { status: 429 }
+        );
+      }
+      return okResponse();
+    },
+  });
+
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, [transientModel, quotaModel]);
+});
+
+test("nested child aggregate treats local quality rejection as non-quota evidence", async () => {
+  const invalidModel = "runtime-nested-quality/primary";
+  const quotaModel = "runtime-nested-quality-quota/primary";
+  const paidBackup = "anthropic/nested-quality-paid-backup";
+  const quotaLeaf = {
+    name: "runtime-nested-quality-quota-leaf",
+    strategy: "priority",
+    models: [{ kind: "model", model: quotaModel, providerId: "runtime-nested-quality-quota" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const child = {
+    name: "runtime-nested-quality-child",
+    strategy: "priority",
+    models: [
+      { kind: "model", model: invalidModel, providerId: "runtime-nested-quality" },
+      { kind: "combo-ref", comboName: quotaLeaf.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const parent = {
+    name: "runtime-nested-quality-parent",
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref", comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      { kind: "model", model: paidBackup, providerId: "anthropic" },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: parent,
+    allCombos: [parent, child, quotaLeaf],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr === invalidModel) {
+        return Response.json({ choices: [{ message: {} }] }, { status: 200 });
+      }
+      if (modelStr === quotaModel) {
+        return Response.json(
+          { error: { message: "Payment required", code: "insufficient_quota" } },
+          { status: 429 }
+        );
+      }
+      return okResponse();
+    },
+  });
+
+  assert.equal(result.status, 429);
+  assert.deepEqual(calls, [invalidModel, quotaModel]);
+});
+
+test("nested child aggregate allows protected parent fallback when every failure is quota", async () => {
+  const firstQuotaModel = "runtime-nested-quota/first";
+  const secondQuotaModel = "runtime-nested-quota/second";
+  const paidBackup = "anthropic/nested-all-quota-backup";
+  const quotaLeaf = {
+    name: "runtime-nested-all-quota-leaf",
+    strategy: "priority",
+    models: [{ kind: "model", model: secondQuotaModel, providerId: "runtime-nested-quota" }],
+    config: { maxRetries: 0, retryDelayMs: 0 },
+  };
+  const child = {
+    name: "runtime-nested-all-quota-child",
+    strategy: "priority",
+    models: [
+      { kind: "model", model: firstQuotaModel, providerId: "runtime-nested-quota" },
+      { kind: "combo-ref", comboName: quotaLeaf.name },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const parent = {
+    name: "runtime-nested-all-quota-parent",
+    strategy: "priority",
+    models: [
+      { kind: "combo-ref", comboName: child.name, fallbackOnlyOnQuotaExhaustion: true },
+      { kind: "model", model: paidBackup, providerId: "anthropic" },
+    ],
+    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+  };
+  const calls: string[] = [];
+  const result = await handleComboChat({
+    body: {},
+    combo: parent,
+    allCombos: [parent, child, quotaLeaf],
+    log: createLog(),
+    settings: null,
+    isModelAvailable: async () => true,
+    handleSingleModel: async (_body, modelStr) => {
+      calls.push(modelStr);
+      if (modelStr === paidBackup) return okResponse();
+      return Response.json(
+        { error: { message: "Payment required", code: "insufficient_quota" } },
+        { status: 429 }
+      );
+    },
+  });
+
+  assert.equal(result.ok, true);
+  assert.deepEqual(calls, [firstQuotaModel, secondQuotaModel, paidBackup]);
 });
 
 test("execute fill-first overflows to the next unit when the first connection is at cap", async () => {
@@ -103,43 +501,45 @@ test("execute fill-first overflows to the next unit when the first connection is
   const release1 = await acquire(key, { maxConcurrency: 2 });
   const release2 = await acquire(key, { maxConcurrency: 2 });
 
-  const calls: string[] = [];
-  const combo = {
-    name: "overflow-execute",
-    strategy: "fill-first",
-    models: [
-      {
-        kind: "model",
-        model: "featherless-ai/deepseek-ai/DeepSeek-V4-Pro",
-        providerId: provider,
-        connectionId: primaryConnectionId,
+  try {
+    const calls: string[] = [];
+    const combo = {
+      name: "overflow-execute",
+      strategy: "fill-first",
+      models: [
+        {
+          kind: "model",
+          model: "featherless-ai/deepseek-ai/DeepSeek-V4-Pro",
+          providerId: provider,
+          connectionId: primaryConnectionId,
+        },
+        {
+          kind: "model",
+          model: "alibaba/qwen3.7-max-preview",
+          providerId: "alibaba",
+          connectionId: backupConnectionId,
+        },
+      ],
+      config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
+    };
+
+    const result = await handleComboChat({
+      body: {},
+      combo,
+      handleSingleModel: async (_body, modelStr) => {
+        calls.push(modelStr);
+        return okResponse();
       },
-      {
-        kind: "model",
-        model: "alibaba/qwen3.7-max-preview",
-        providerId: "alibaba",
-        connectionId: backupConnectionId,
-      },
-    ],
-    config: { nestedComboMode: "execute", maxRetries: 0, retryDelayMs: 0 },
-  };
+      isModelAvailable: async () => true,
+      log: createLog(),
+      settings: null,
+      allCombos: [combo],
+    });
 
-  const result = await handleComboChat({
-    body: {},
-    combo,
-    handleSingleModel: async (_body, modelStr) => {
-      calls.push(modelStr);
-      return okResponse();
-    },
-    isModelAvailable: async () => true,
-    log: createLog(),
-    settings: null,
-    allCombos: [combo],
-  });
-
-  assert.equal(result.ok, true);
-  assert.deepEqual(calls, ["alibaba/qwen3.7-max-preview"]);
-
-  release1();
-  release2();
+    assert.equal(result.ok, true);
+    assert.deepEqual(calls, ["alibaba/qwen3.7-max-preview"]);
+  } finally {
+    release1();
+    release2();
+  }
 });
diff --git a/tests/unit/combos-quota-protected.test.ts b/tests/unit/combos-quota-protected.test.ts
index 6c1776a86d..cf2c4ad6ce 100644
--- a/tests/unit/combos-quota-protected.test.ts
+++ b/tests/unit/combos-quota-protected.test.ts
@@ -90,7 +90,11 @@ test("PUT /api/combos/[id] returns 409 for a qtSd/* combo and does NOT mutate it
 
   // Verify the combo was NOT mutated
   const unchanged = await combosDb.getComboById(combo.id);
-  assert.equal(unchanged?.strategy, "priority", "Strategy must remain unchanged after rejected PUT");
+  assert.equal(
+    unchanged?.strategy,
+    "priority",
+    "Strategy must remain unchanged after rejected PUT"
+  );
 });
 
 // ---- non-quota combos still work ----
@@ -116,6 +120,43 @@ test("DELETE /api/combos/[id] succeeds for a regular (non-quota) combo", async (
   assert.equal(gone, null, "Regular combo must be gone after DELETE");
 });
 
+test("PUT merged state rejects partial updates that leave protected priority refs in flatten mode", async () => {
+  const combo = await combosDb.createCombo({
+    name: "protected-ref-update",
+    strategy: "priority",
+    models: [{ kind: "combo-ref", comboName: "child", fallbackOnlyOnQuotaExhaustion: true }],
+    config: { nestedComboMode: "execute" },
+  });
+
+  for (const update of [
+    { config: { nestedComboMode: "flatten" } },
+    {
+      models: [{ kind: "combo-ref", comboName: "child", fallbackOnlyOnQuotaExhaustion: true }],
+      config: {},
+    },
+    { strategy: "priority", config: {} },
+  ]) {
+    const response = await comboRoute.PUT(makePutRequest(combo.id, update), {
+      params: Promise.resolve({ id: combo.id }),
+    });
+    assert.equal(response.status, 400);
+  }
+});
+
+test("PUT merged state accepts dormant weighted protected refs", async () => {
+  const combo = await combosDb.createCombo({
+    name: "dormant-protected-ref-update",
+    strategy: "priority",
+    models: [{ kind: "combo-ref", comboName: "child", fallbackOnlyOnQuotaExhaustion: true }],
+    config: { nestedComboMode: "execute" },
+  });
+  const response = await comboRoute.PUT(
+    makePutRequest(combo.id, { strategy: "weighted", config: { nestedComboMode: "flatten" } }),
+    { params: Promise.resolve({ id: combo.id }) }
+  );
+  assert.equal(response.status, 200);
+});
+
 test("PUT /api/combos/[id] succeeds for a regular (non-quota) combo", async () => {
   const combo = await combosDb.createCombo({
     name: "regular-editable-combo",
@@ -152,10 +193,7 @@ test("DELETE /api/combos/[id] returns 404 when combo does not exist", async () =
 
 test("combos page source filters isHidden from rendered list", async () => {
   const pageSource = fs.readFileSync(
-    new URL(
-      "../../src/app/(dashboard)/dashboard/combos/page.tsx",
-      import.meta.url
-    ).pathname,
+    new URL("../../src/app/(dashboard)/dashboard/combos/page.tsx", import.meta.url).pathname,
     "utf8"
   );
   assert.ok(
diff --git a/tests/unit/ui/combo-quota-exhaustion-option.test.tsx b/tests/unit/ui/combo-quota-exhaustion-option.test.tsx
new file mode 100644
index 0000000000..3103bfa78b
--- /dev/null
+++ b/tests/unit/ui/combo-quota-exhaustion-option.test.tsx
@@ -0,0 +1,91 @@
+// @vitest-environment jsdom
+import React, { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { ComboTargetOptions } from "@/app/(dashboard)/dashboard/combos/ComboQuotaOnlyFallbackToggle";
+import {
+  applyQuotaOnlyFallbackConfig,
+  setQuotaOnlyFallback,
+} from "@/app/(dashboard)/dashboard/combos/comboQuotaOnlyFallback";
+import type { ComboStep } from "@/lib/combos/steps";
+
+const model = (name: string): ComboStep => ({ kind: "model", model: name });
+const comboRef = (): ComboStep => ({ kind: "combo-ref", comboName: "child" });
+const containers: Array<{ root: ReturnType<typeof createRoot>; element: HTMLDivElement }> = [];
+const label = "Only advance on quota exhaustion";
+const shortLabel = "quota-only fallback";
+
+function renderOptions(strategy: string, entry: ComboStep) {
+  const element = document.createElement("div");
+  document.body.appendChild(element);
+  const root = createRoot(element);
+  act(() => {
+    root.render(
+      <ComboTargetOptions
+        strategy={strategy}
+        entry={entry}
+        index={0}
+        hasPricing={false}
+        translate={(key, fallback) =>
+          key === "fallbackOnlyOnQuotaExhaustion"
+            ? label
+            : key === "fallbackOnlyOnQuotaExhaustionShort"
+              ? shortLabel
+              : fallback
+        }
+        onCheckedChange={vi.fn()}
+      />
+    );
+  });
+  containers.push({ root, element });
+  return element;
+}
+
+afterEach(() => {
+  for (const { root, element } of containers.splice(0)) {
+    act(() => root.unmount());
+    element.remove();
+  }
+});
+
+describe("combo quota-only fallback state", () => {
+  it("updates the selected model and combo-ref without mutating other steps", () => {
+    const entries = [model("openai/first"), comboRef(), model("openai/last")];
+    const modelEnabled = setQuotaOnlyFallback(entries, 0, true);
+    const refEnabled = setQuotaOnlyFallback(modelEnabled, 1, true);
+
+    expect(entries[0].fallbackOnlyOnQuotaExhaustion).toBeUndefined();
+    expect(refEnabled[0].fallbackOnlyOnQuotaExhaustion).toBe(true);
+    expect(refEnabled[1].fallbackOnlyOnQuotaExhaustion).toBe(true);
+    expect(refEnabled[2]).toBe(entries[2]);
+    expect(setQuotaOnlyFallback(refEnabled, 1, false)[1]).not.toHaveProperty(
+      "fallbackOnlyOnQuotaExhaustion"
+    );
+  });
+
+  it("ignores an invalid index instead of manufacturing a step", () => {
+    const entries = [model("openai/first")];
+    expect(setQuotaOnlyFallback(entries, 5, true)).toEqual(entries);
+  });
+
+  it("renders accessible human labels for priority models and combo refs only", () => {
+    for (const entry of [model("openai/first"), comboRef()]) {
+      const element = renderOptions("priority", entry);
+      const checkbox = element.querySelector<HTMLInputElement>(`input[aria-label="${label}"]`);
+      expect(checkbox).not.toBeNull();
+      expect(checkbox?.checked).toBe(false);
+      expect(element.textContent).toContain(shortLabel);
+    }
+    expect(renderOptions("weighted", model("openai/first")).querySelector("input")).toBeNull();
+  });
+
+  it("forces execute only for active priority combo refs and preserves dormant config", () => {
+    const protectedRef = setQuotaOnlyFallback([comboRef()], 0, true);
+    expect(applyQuotaOnlyFallbackConfig("priority", protectedRef, {})).toEqual({
+      nestedComboMode: "execute",
+    });
+    expect(applyQuotaOnlyFallbackConfig("weighted", protectedRef, {})).toEqual({});
+    expect(applyQuotaOnlyFallbackConfig("priority", [model("openai/first")], {})).toEqual({});
+  });
+});