diff --git a/open-sse/services/combo.ts b/open-sse/services/combo.ts
index 58371dc93e..df86a79375 100644
--- a/open-sse/services/combo.ts
+++ b/open-sse/services/combo.ts
@@ -1919,11 +1919,18 @@ export async function handleComboChat({
           // skip the same-model retry when `nextTarget` (computed above)
           // actually gives us somewhere to fail over to — with no sibling
           // left, skipping just burns the last attempt for nothing.
+          //
+          // #10217 round-4 fix: this guard reads `failoverBeforeRetryExplicit`
+          // (opt-in only), NOT `config.failoverBeforeRetry` — that field
+          // defaults to true for the separate skipUpstreamRetry mechanism
+          // (see DEFAULT_COMBO_CONFIG comment in comboConfig.ts) and reading
+          // it here would silently skip the same-model retry for every combo,
+          // not just ones that explicitly opted in.
           if (
             retry < maxRetries &&
             isTransient &&
             !providerExhausted &&
-            (!config.failoverBeforeRetry || !nextTarget)
+            (!config.failoverBeforeRetryExplicit || !nextTarget)
           ) {
             if (
               !protectedPriorityTarget &&
@@ -2438,7 +2445,15 @@ async function handleRoundRobinCombo({
 }: HandleRoundRobinOptions): Promise<Response> {
   const config = settings
     ? resolveComboConfig(combo, settings)
-    : { ...getDefaultComboConfig(), ...(combo.config || {}) };
+    : {
+        ...getDefaultComboConfig(),
+        ...(combo.config || {}),
+        // See resolveComboConfig's failoverBeforeRetryExplicit comment in
+        // comboConfig.ts (no `settings` here, so only the combo's own config
+        // can opt in).
+        failoverBeforeRetryExplicit:
+          (combo.config as Record<string, unknown> | undefined)?.failoverBeforeRetry === true,
+      };
   // #9158: clamp combo-level concurrency to a sane bound — a config carrying a
   // huge or negative value would otherwise open an unbounded semaphore and
   // flood targets (or deadlock at 0).
@@ -3163,12 +3178,14 @@ async function handleRoundRobinCombo({
         // just the lower-level skipUpstreamRetry mechanism. Only skip when
         // `offset + 1 < modelCount` means a sibling target is actually left
         // in this rotation; with none left, skipping just wastes the attempt.
+        // #10217 round-4 fix: opt-in only — read failoverBeforeRetryExplicit,
+        // not config.failoverBeforeRetry (see comboConfig.ts comment).
         const hasNextRrTarget = offset + 1 < modelCount;
         if (
           retry < maxRetries &&
           isTransient &&
           !providerExhausted &&
-          (!config.failoverBeforeRetry || !hasNextRrTarget)
+          (!config.failoverBeforeRetryExplicit || !hasNextRrTarget)
         ) {
           continue;
         }
diff --git a/open-sse/services/comboConfig.ts b/open-sse/services/comboConfig.ts
index dd7f220160..97e753ad92 100644
--- a/open-sse/services/comboConfig.ts
+++ b/open-sse/services/comboConfig.ts
@@ -126,12 +126,23 @@ const DEFAULT_COMBO_CONFIG = {
   resetAwareWeeklyWeight: 0.65,
   resetAwareTieBandPercent: 5,
   resetAwareExhaustionGuardPercent: 10,
-  // Opt-in (#2417/#10217): when unset, transient errors retry the same model
-  // up to maxRetries before falling over to the next target — the historical
-  // default. Defaulting this to true silently flipped that behavior for every
-  // combo that never touched the setting, breaking same-model retry semantics
-  // (round 4 base-red bisect: 06f41cda63 vs d2fd88dfbc).
-  failoverBeforeRetry: false,
+  // Historical default (predates #2417/#10217) — true. This value feeds TWO
+  // independent mechanisms and must stay true-by-default for one of them:
+  //   1. skipUpstreamRetry (src/sse/handlers/chat.ts:859,1126) — the
+  //      lower-level executor retry skip. Always default-on; changing this
+  //      default flips that mechanism's behavior for every combo, not just
+  //      opted-in ones.
+  //   2. The #10217 same-model retry guard in this file's combo.ts callers
+  //      (priority/auto + round-robin loops) — meant to be OPT-IN only. That
+  //      guard must NOT read this field directly; it consults the sibling
+  //      `failoverBeforeRetryExplicit` flag computed below in
+  //      resolveComboConfig/resolveComboSetupConfig, which is true only when
+  //      an actual cascade layer (combo/provider/global) set the flag to
+  //      true, not merely inherited from this default. See round-4 base-red
+  //      bisect (06f41cda63 vs d2fd88dfbc) — flipping THIS default to false
+  //      "fixed" mechanism 2 but silently broke mechanism 1's default-on
+  //      behavior for every combo without an explicit opt-in.
+  failoverBeforeRetry: true,
   // Feature 4985: configurable response-body validation predicate (per-combo). When set,
   // a 200 OK whose body fails the predicate fails over to the next target.
   responseValidation: undefined as ResponseValidationConfig | undefined,
@@ -289,15 +300,32 @@ export function resolveComboConfig(
       )
     );
 
+  const cleanGlobal = clean(global);
+  const cleanProviderOverride = clean(providerOverride);
+  const cleanComboConfig = clean(comboConfig);
+
   const merged = {
     ...DEFAULT_COMBO_CONFIG,
-    ...clean(global),
-    ...clean(providerOverride),
-    ...clean(comboConfig),
+    ...cleanGlobal,
+    ...cleanProviderOverride,
+    ...cleanComboConfig,
   };
 
+  // #10217 round-4 fix: `failoverBeforeRetry` defaults to true (see comment on
+  // DEFAULT_COMBO_CONFIG above) and feeds two independent mechanisms. Callers
+  // that gate the OPT-IN same-model retry guard (combo.ts) must NOT read
+  // `merged.failoverBeforeRetry` directly — that stays true unless a layer
+  // explicitly disables it, which can't distinguish "inherited default" from
+  // "operator opted in". This flag is true only when some cascade layer
+  // literally set the value to true, i.e. a genuine opt-in.
+  const failoverBeforeRetryExplicit =
+    cleanComboConfig.failoverBeforeRetry === true ||
+    cleanProviderOverride.failoverBeforeRetry === true ||
+    cleanGlobal.failoverBeforeRetry === true;
+
   return {
     ...merged,
+    failoverBeforeRetryExplicit,
     shadowRouting: {
       ...DEFAULT_COMBO_CONFIG.shadowRouting,
       ...(isRecord(global.shadowRouting) ? clean(global.shadowRouting) : {}),
@@ -327,7 +355,14 @@ export function getDefaultComboConfig() {
  * return type is the single source of truth for ComboContext.config (combo/context.ts).
  */
 export function resolveComboSetupConfig(combo: ComboConfigLike, settings: ComboSettingsLike) {
-  return settings
-    ? resolveComboConfig(combo, settings)
-    : { ...getDefaultComboConfig(), ...((combo?.config as Record<string, unknown>) || {}) };
+  if (settings) return resolveComboConfig(combo, settings);
+  const comboConfig = (combo?.config as Record<string, unknown>) || {};
+  return {
+    ...getDefaultComboConfig(),
+    ...comboConfig,
+    // See resolveComboConfig's failoverBeforeRetryExplicit comment — same
+    // distinction applies here (no `settings`, so only the combo's own config
+    // can opt in).
+    failoverBeforeRetryExplicit: comboConfig.failoverBeforeRetry === true,
+  };
 }