diff --git a/changelog.d/fixes/11116-reasoning-effort-capability-discovery.md b/changelog.d/fixes/11116-reasoning-effort-capability-discovery.md
new file mode 100644
index 0000000000..fbc4dfe694
--- /dev/null
+++ b/changelog.d/fixes/11116-reasoning-effort-capability-discovery.md
@@ -0,0 +1 @@
+- **fix(providers):** `reasoning_effort` now learns the accepted values from a provider's own 400/422 response and clamps to the highest one instead of forwarding an unsupported `xhigh`/`max` (or a hardcoded `"high"` fallback) — fixes custom OpenAI-compatible connections and registered providers with no reasoning metadata ([#11116](https://github.com/diegosouzapw/OmniRoute/pull/11116)) — thanks @maxmad64bis
diff --git a/open-sse/config/constants.ts b/open-sse/config/constants.ts
index c45cf542a6..f1fa055c93 100644
--- a/open-sse/config/constants.ts
+++ b/open-sse/config/constants.ts
@@ -171,6 +171,7 @@ export const HTTP_STATUS = {
   FORBIDDEN: 403,
   NOT_FOUND: 404,
   NOT_ACCEPTABLE: 406,
+  UNPROCESSABLE_ENTITY: 422,
   REQUEST_TIMEOUT: 408,
   GONE: 410,
   RATE_LIMITED: 429,
@@ -263,11 +264,17 @@ export const PROVIDER_PROFILES = {
     circuitBreakerReset: envInt("OMNIROUTE_CIRCUIT_BREAKER_API_KEY_RESET_MS", 30000),
     // Provider-level circuit breaker (entire provider cooldown after repeated failures)
     providerFailureThreshold: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_FAILURE_THRESHOLD", 15), // Scaled for 500+ connections (was 5)
-    providerFailureWindowMs: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_FAILURE_WINDOW_MS", 1800000), // 30min window (was 20min)
+    providerFailureWindowMs: envInt(
+      "OMNIROUTE_PROVIDER_BREAKER_API_KEY_FAILURE_WINDOW_MS",
+      1800000
+    ), // 30min window (was 20min)
     providerCooldownMs: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_COOLDOWN_MS", 600000), // 10min cooldown when threshold reached
     degradationThreshold: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_DEGRADATION_THRESHOLD", 7),
     maxBackoffMultiplier: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_MAX_BACKOFF_MULTIPLIER", 4),
-    backoffEscalationCount: envInt("OMNIROUTE_PROVIDER_BREAKER_API_KEY_BACKOFF_ESCALATION_COUNT", 3),
+    backoffEscalationCount: envInt(
+      "OMNIROUTE_PROVIDER_BREAKER_API_KEY_BACKOFF_ESCALATION_COUNT",
+      3
+    ),
   },
   // Local providers (localhost inference backends like Ollama, LM Studio, oMLX).
   // Not yet wired into getProviderProfile() — will be used when local provider_nodes
diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index 66e10b67e9..1c13442aff 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -20,6 +20,10 @@ import {
   recordLearnedThinkingCap,
   parseThinkingBudgetMax,
 } from "../services/learnedThinkingCaps.ts";
+import {
+  recordLearnedReasoningEffort,
+  parseReasoningEffortEnum,
+} from "../services/learnedReasoningEffortCaps.ts";
 import {
   getParamFilterConfig,
   addParamToBlocklist,
@@ -826,6 +830,9 @@ export class BaseExecutor {
     // loop. The learned cap is also recorded process-wide via
     // recordLearnedThinkingCap so future requests skip the 400 entirely.
     let thinkingBudgetClampedMax: number | null = null;
+    // Set by the reasoning_effort 4xx clamp-and-retry below — guards the same
+    // "fires at most once per URL" invariant as thinkingBudgetClampedMax above.
+    let reasoningEffortClamped = false;
 
     for (let urlIndex = 0; urlIndex < fallbackCount; urlIndex++) {
       const requestCredentials = withForcedResponsesUpstream(
@@ -1529,6 +1536,49 @@ export class BaseExecutor {
           }
         }
 
+        // Reasoning-effort enum 4xx clamp-and-retry (any provider/model without a
+        // declared reasoning_effort capability — custom OpenAI-compatible
+        // connections, or a registered provider the registry hasn't caught up
+        // with). Mirrors the thinking_budget clamp-and-retry above: parse the
+        // upstream-advertised accepted values, record them process-wide (so
+        // FUTURE requests clamp proactively via sanitizeReasoningEffortForProvider
+        // → getLearnedReasoningEffort), clamp the live transformedBody by
+        // re-running the sanitizer, and retry the same URL once.
+        if (
+          (response.status === HTTP_STATUS.BAD_REQUEST ||
+            response.status === HTTP_STATUS.UNPROCESSABLE_ENTITY) &&
+          !reasoningEffortClamped &&
+          transformedBody &&
+          typeof transformedBody === "object"
+        ) {
+          const errText = await response
+            .clone()
+            .text()
+            .catch(() => "");
+          const acceptedValues = parseReasoningEffortEnum(errText);
+          if (acceptedValues) {
+            reasoningEffortClamped = true;
+            const learned = recordLearnedReasoningEffort(this.provider, model, acceptedValues);
+            if (learned) {
+              transformedBody = sanitizeReasoningEffortForProvider(
+                transformedBody,
+                this.provider,
+                model,
+                log
+              );
+              let retryBody = JSON.stringify(transformedBody);
+              if (usesClaudeCodeProtocol || this.provider === "claude") {
+                retryBody = await signRequestBody(retryBody);
+              }
+              log?.info?.(
+                "REASONING_SANITIZE",
+                `Upstream ${response.status} rejected reasoning_effort on ${url} — clamped to ${learned} and retrying (learned for ${this.provider}/${model})`
+              );
+              response = await fetchWithStartTimeout(url, { ...fetchOptions, body: retryBody });
+            }
+          }
+        }
+
         // Generic reactive 400 field-downgrade; each field is stripped at most once.
         if (
           response.status === HTTP_STATUS.BAD_REQUEST &&
diff --git a/open-sse/executors/base/reasoningEffort.ts b/open-sse/executors/base/reasoningEffort.ts
index fa416dbc1a..8dd99904fd 100644
--- a/open-sse/executors/base/reasoningEffort.ts
+++ b/open-sse/executors/base/reasoningEffort.ts
@@ -8,6 +8,10 @@ import {
   getProviderModel,
   getProviderModels,
 } from "../../config/providerModels.ts";
+import {
+  getLearnedReasoningEffort,
+  REASONING_EFFORT_ORDER,
+} from "../../services/learnedReasoningEffortCaps.ts";
 
 /**
  * Sanitize reasoning_effort for providers that don't accept all values.
@@ -338,10 +342,24 @@ export function sanitizeReasoningEffortForProvider(
 
   const supportsXHigh = supportsXHighEffort(provider, modelStr);
   const supportsMax = supportsMaxEffortForProvider(provider, modelStr);
+  // Highest value we've actually seen this provider+model accept in a real
+  // upstream 4xx (learnedReasoningEffortCaps.ts) — takes priority over the
+  // static registry (which defaults to "supports everything" when there's no
+  // entry, e.g. custom OpenAI-compatible connections) and over the hardcoded
+  // "high" fallback below (which isn't always valid either).
+  const learnedCap = getLearnedReasoningEffort(provider, modelStr);
+  const learnedRank = learnedCap ? REASONING_EFFORT_ORDER.indexOf(learnedCap) : -1;
 
   // ── xhigh handling ──────────────────────────────────────────────────────
   // xhigh is OmniRoute-internal. Map it to the best effort the model accepts.
   if (effortStr === "xhigh") {
+    if (learnedCap && learnedRank < REASONING_EFFORT_ORDER.indexOf("xhigh")) {
+      log?.info?.(
+        "REASONING_SANITIZE",
+        `${provider}/${modelStr}: clamped reasoning_effort xhigh → ${learnedCap} (learned)`
+      );
+      return writeEffortValue(b, learnedCap, c);
+    }
     if (supportsXHigh) return body; // model accepts xhigh natively
     if (supportsMax) {
       log?.info?.(
@@ -366,6 +384,13 @@ export function sanitizeReasoningEffortForProvider(
   // upstream, and if it 400s the user gets a clear signal. This prevents
   // new models from being unusable for weeks until they're whitelisted (#8057).
   if (effortStr === "max") {
+    if (learnedCap && learnedRank < REASONING_EFFORT_ORDER.indexOf("max")) {
+      log?.info?.(
+        "REASONING_SANITIZE",
+        `${provider}/${modelStr}: clamped reasoning_effort max → ${learnedCap} (learned)`
+      );
+      return writeEffortValue(b, learnedCap, c);
+    }
     if (supportsMax) return body; // explicitly known to accept max
 
     // A model that explicitly advertises its accepted tiers is safe to normalize.
diff --git a/open-sse/services/learnedReasoningEffortCaps.ts b/open-sse/services/learnedReasoningEffortCaps.ts
new file mode 100644
index 0000000000..b0125d8683
--- /dev/null
+++ b/open-sse/services/learnedReasoningEffortCaps.ts
@@ -0,0 +1,126 @@
+/**
+ * Learned Reasoning-Effort Caps — reactive capability memory for providers/models
+ * OmniRoute has no static registry entry for (custom OpenAI-compatible connections,
+ * or any registered provider whose registry entry carries no reasoning metadata).
+ *
+ * Same shape as `learnedThinkingCaps.ts` (thinking_budget), generalized from a
+ * numeric budget to an ordinal reasoning_effort scale: on a 4xx whose body
+ * enumerates the accepted values, `base.ts`'s executor calls
+ * `recordLearnedReasoningEffort`, which stores the highest recognized value in a
+ * module-level Map keyed "provider:model" (lowercased). Subsequent requests for
+ * the same provider+model read the cap via `getLearnedReasoningEffort` (consulted
+ * by `sanitizeReasoningEffortForProvider` in `executors/base/reasoningEffort.ts`)
+ * so the 4xx→retry round-trip is paid at most once per process per provider+model.
+ *
+ * In-memory only (same operator-accepted tradeoff as the thinking-budget cache):
+ * restart resets, the first request after a restart may re-learn at the cost of
+ * one upstream 4xx.
+ */
+
+export const REASONING_EFFORT_ORDER: readonly string[] = [
+  "none",
+  "minimal",
+  "low",
+  "medium",
+  "high",
+  "xhigh",
+  "max",
+];
+
+// key: `${provider}:${model}` lowercased → highest value known to be accepted.
+const learnedCaps = new Map<string, string>();
+
+function buildKey(provider: string | null | undefined, model: string | null | undefined): string {
+  const p = typeof provider === "string" ? provider.trim().toLowerCase() : "";
+  const m = typeof model === "string" ? model.trim().toLowerCase() : "";
+  if (!p || !m) return "";
+  return `${p}:${m}`;
+}
+
+function rankOf(value: string): number {
+  return REASONING_EFFORT_ORDER.indexOf(value);
+}
+
+/**
+ * Return the learned cap for provider+model, or null when nothing has been
+ * learned yet (no upstream 4xx recorded). Keyed case-insensitively.
+ */
+export function getLearnedReasoningEffort(
+  provider: string | null | undefined,
+  model: string | null | undefined
+): string | null {
+  const key = buildKey(provider, model);
+  if (!key) return null;
+  return learnedCaps.get(key) ?? null;
+}
+
+/**
+ * Record that `acceptedValues` is the enum the upstream advertised for
+ * provider+model, and store the highest recognized value as the learned cap.
+ * Returns the stored value, or null when `acceptedValues` contained no token
+ * from `REASONING_EFFORT_ORDER` (nothing usable to learn) or the key is unusable.
+ *
+ * Always monotonically decreases: if a cap already stored ranks lower than the
+ * newly computed highest, the stored (lower) value wins and is returned
+ * unchanged. This keeps a later, laxer-looking response (or a race between
+ * concurrent requests) from ratcheting the cap back up.
+ */
+export function recordLearnedReasoningEffort(
+  provider: string | null | undefined,
+  model: string | null | undefined,
+  acceptedValues: string[]
+): string | null {
+  const key = buildKey(provider, model);
+  if (!key) return null;
+
+  let best: string | null = null;
+  let bestRank = -1;
+  for (const raw of acceptedValues) {
+    const rank = rankOf(raw);
+    if (rank > bestRank) {
+      bestRank = rank;
+      best = raw;
+    }
+  }
+  if (best === null) return null;
+
+  const existing = learnedCaps.get(key);
+  if (existing !== undefined && rankOf(existing) <= bestRank) {
+    return existing; // already learned an equal-or-lower cap; keep it
+  }
+  learnedCaps.set(key, best);
+  return best;
+}
+
+// Matches both prose shapes observed: OVH's `@ai-sdk/openai-compatible`
+// deserializer ("expected one of `a`, `b`") and a generic vendor prose form
+// ("Supported types are a, b, and c").
+const LIST_INTRO = /(?:expected one of|supported (?:types|values) are)[:\s]*([^.]+)/i;
+
+/**
+ * Extract the upstream-advertised accepted reasoning_effort values from a 4xx
+ * error body. Returns only tokens present in REASONING_EFFORT_ORDER (unknown
+ * tokens are dropped defensively) in the order they appeared, or null when the
+ * text names no recognized enum member.
+ */
+export function parseReasoningEffortEnum(errText: unknown): string[] | null {
+  if (typeof errText !== "string" || !errText) return null;
+  const match = LIST_INTRO.exec(errText);
+  if (!match) return null;
+  const tokens = match[1]
+    .split(/,|\band\b|&/i)
+    .map((t) =>
+      t
+        .replace(/`/g, "")
+        .replace(/\([^)]*\)/g, "")
+        .trim()
+        .toLowerCase()
+    )
+    .filter((t) => t.length > 0 && REASONING_EFFORT_ORDER.includes(t));
+  return tokens.length > 0 ? tokens : null;
+}
+
+/** Test-only: clear the learned-cap Map between tests. */
+export function __test_resetLearnedReasoningEffortCaps(): void {
+  learnedCaps.clear();
+}
diff --git a/tests/unit/http-status-unprocessable-entity.test.ts b/tests/unit/http-status-unprocessable-entity.test.ts
new file mode 100644
index 0000000000..636e0ebe37
--- /dev/null
+++ b/tests/unit/http-status-unprocessable-entity.test.ts
@@ -0,0 +1,7 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { HTTP_STATUS } from "../../open-sse/config/constants.ts";
+
+test("HTTP_STATUS declares UNPROCESSABLE_ENTITY as 422", () => {
+  assert.equal(HTTP_STATUS.UNPROCESSABLE_ENTITY, 422);
+});
diff --git a/tests/unit/learned-reasoning-effort-caps.test.ts b/tests/unit/learned-reasoning-effort-caps.test.ts
new file mode 100644
index 0000000000..5d69a342c3
--- /dev/null
+++ b/tests/unit/learned-reasoning-effort-caps.test.ts
@@ -0,0 +1,126 @@
+import { test, after, beforeEach } from "node:test";
+import assert from "node:assert/strict";
+import {
+  REASONING_EFFORT_ORDER,
+  parseReasoningEffortEnum,
+  recordLearnedReasoningEffort,
+  getLearnedReasoningEffort,
+  __test_resetLearnedReasoningEffortCaps,
+} from "../../open-sse/services/learnedReasoningEffortCaps.ts";
+
+beforeEach(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+after(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+// ── REASONING_EFFORT_ORDER ──────────────────────────────────────────────────
+
+test("REASONING_EFFORT_ORDER is none < minimal < low < medium < high < xhigh < max", () => {
+  assert.deepEqual(REASONING_EFFORT_ORDER, [
+    "none",
+    "minimal",
+    "low",
+    "medium",
+    "high",
+    "xhigh",
+    "max",
+  ]);
+});
+
+// ── parseReasoningEffortEnum ────────────────────────────────────────────────
+
+test("parseReasoningEffortEnum extracts the real OVH 422 enum (backtick-quoted)", () => {
+  const err =
+    "Failed to deserialize the JSON body into the target type: reasoning_effort: " +
+    "unknown variant `xhigh`, expected one of `none`, `high`, `medium`, `low`, `minimal`";
+  assert.deepEqual(parseReasoningEffortEnum(err), ["none", "high", "medium", "low", "minimal"]);
+});
+
+test("parseReasoningEffortEnum extracts a bare comma/and-joined enum with annotations", () => {
+  const err =
+    "Unexpected reasoning effort high. Supported types are xhigh (default), medium, and low.";
+  assert.deepEqual(parseReasoningEffortEnum(err), ["xhigh", "medium", "low"]);
+});
+
+test("parseReasoningEffortEnum drops unrecognized tokens", () => {
+  const err = "expected one of `none`, `turbo`, `high`";
+  assert.deepEqual(parseReasoningEffortEnum(err), ["none", "high"]);
+});
+
+test("parseReasoningEffortEnum returns null for unrelated error text", () => {
+  assert.equal(parseReasoningEffortEnum("connection refused"), null);
+  assert.equal(parseReasoningEffortEnum(""), null);
+  assert.equal(parseReasoningEffortEnum(null), null);
+  assert.equal(parseReasoningEffortEnum(undefined), null);
+});
+
+test("parseReasoningEffortEnum returns null when the list has no recognized token", () => {
+  assert.equal(parseReasoningEffortEnum("expected one of `foo`, `bar`"), null);
+});
+
+// ── recordLearnedReasoningEffort / getLearnedReasoningEffort ───────────────
+
+test("records the highest recognized value from the accepted list", () => {
+  const learned = recordLearnedReasoningEffort("ovh", "qwen3-coder-30b-a3b-instruct", [
+    "none",
+    "high",
+    "medium",
+    "low",
+    "minimal",
+  ]);
+  assert.equal(learned, "high");
+  assert.equal(getLearnedReasoningEffort("ovh", "qwen3-coder-30b-a3b-instruct"), "high");
+});
+
+test("returns null and stores nothing when acceptedValues has no recognized token", () => {
+  const learned = recordLearnedReasoningEffort("acme", "model-x", ["foo", "bar"]);
+  assert.equal(learned, null);
+  assert.equal(getLearnedReasoningEffort("acme", "model-x"), null);
+});
+
+test("monotonic decrease: a later, higher accepted-list never ratchets the cap back up", () => {
+  recordLearnedReasoningEffort("acme", "model-x", ["none", "low", "medium"]);
+  const learned = recordLearnedReasoningEffort("acme", "model-x", [
+    "none",
+    "low",
+    "medium",
+    "high",
+    "xhigh",
+  ]);
+  assert.equal(learned, "medium");
+  assert.equal(getLearnedReasoningEffort("acme", "model-x"), "medium");
+});
+
+test("a later, lower accepted-list does ratchet the cap down", () => {
+  recordLearnedReasoningEffort("acme", "model-x", ["none", "low", "medium", "high"]);
+  const learned = recordLearnedReasoningEffort("acme", "model-x", ["none", "low"]);
+  assert.equal(learned, "low");
+  assert.equal(getLearnedReasoningEffort("acme", "model-x"), "low");
+});
+
+test("getLearnedReasoningEffort returns null for unknown provider+model", () => {
+  assert.equal(getLearnedReasoningEffort("acme", "unknown-model"), null);
+});
+
+test("getLearnedReasoningEffort is keyed case-insensitively on provider+model", () => {
+  recordLearnedReasoningEffort("OVH", "Qwen3-Coder-30B", ["none", "high"]);
+  assert.equal(getLearnedReasoningEffort("ovh", "qwen3-coder-30b"), "high");
+  assert.equal(getLearnedReasoningEffort("OVH", "QWEN3-CODER-30B"), "high");
+});
+
+test("different providers for the same model id have independent caps", () => {
+  recordLearnedReasoningEffort("ovh", "shared-model", ["none", "high"]);
+  assert.equal(getLearnedReasoningEffort("openrouter", "shared-model"), null);
+});
+
+test("handles empty/null provider or model gracefully", () => {
+  assert.equal(getLearnedReasoningEffort("", "m"), null);
+  assert.equal(getLearnedReasoningEffort("p", ""), null);
+  assert.equal(getLearnedReasoningEffort(null, "m"), null);
+  assert.equal(getLearnedReasoningEffort("p", null), null);
+  assert.equal(recordLearnedReasoningEffort("", "m", ["high"]), null);
+  assert.equal(recordLearnedReasoningEffort("p", "", ["high"]), null);
+});
diff --git a/tests/unit/reasoning-effort-clamp-and-retry.test.ts b/tests/unit/reasoning-effort-clamp-and-retry.test.ts
new file mode 100644
index 0000000000..a97ac16df0
--- /dev/null
+++ b/tests/unit/reasoning-effort-clamp-and-retry.test.ts
@@ -0,0 +1,110 @@
+import { test, after, beforeEach } from "node:test";
+import assert from "node:assert/strict";
+import { BaseExecutor } from "../../open-sse/executors/base.ts";
+import {
+  getLearnedReasoningEffort,
+  recordLearnedReasoningEffort,
+  __test_resetLearnedReasoningEffortCaps,
+} from "../../open-sse/services/learnedReasoningEffortCaps.ts";
+
+const OVH_422_BODY = JSON.stringify({
+  error: {
+    message:
+      "Failed to deserialize the JSON body into the target type: reasoning_effort: " +
+      "unknown variant `xhigh`, expected one of `none`, `high`, `medium`, `low`, `minimal`",
+  },
+});
+
+// Passthrough executor: returns the body unchanged so we assert on exactly what
+// base.ts sends upstream.
+class SimpleExecutor extends BaseExecutor {
+  constructor() {
+    super("openai-compatible-chat-eaff6869", {
+      baseUrls: ["https://oai.endpoints.kepler.ai.cloud.ovh.net/v1/chat/completions"],
+    });
+  }
+  async transformRequest(_model: string, body: Record<string, unknown>) {
+    return { ...body };
+  }
+}
+
+beforeEach(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+after(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+test("422 'unknown variant xhigh, expected one of ...' clamps reasoning_effort and retries once", async () => {
+  const executor = new SimpleExecutor();
+  const originalFetch = globalThis.fetch;
+  const capturedBodies: Record<string, unknown>[] = [];
+
+  globalThis.fetch = async (_url: string | URL | Request, init: RequestInit = {}) => {
+    const body = JSON.parse(String(init.body));
+    capturedBodies.push(body);
+    if (capturedBodies.length === 1) {
+      return new Response(OVH_422_BODY, {
+        status: 422,
+        headers: { "Content-Type": "application/json" },
+      });
+    }
+    return new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { "Content-Type": "application/json" },
+    });
+  };
+
+  try {
+    const result = await executor.execute({
+      model: "qwen3-coder-30b-a3b-instruct",
+      body: { reasoning_effort: "xhigh" },
+      stream: false,
+      credentials: {},
+    });
+    assert.equal(capturedBodies.length, 2);
+    assert.equal(capturedBodies[0].reasoning_effort, "xhigh");
+    assert.equal(capturedBodies[1].reasoning_effort, "high");
+    assert.equal(
+      getLearnedReasoningEffort("openai-compatible-chat-eaff6869", "qwen3-coder-30b-a3b-instruct"),
+      "high"
+    );
+    assert.equal(result.response.status, 200);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("a second request for the same provider+model sends the learned value on the first try", async () => {
+  const executor = new SimpleExecutor();
+  const originalFetch = globalThis.fetch;
+  const capturedBodies: Record<string, unknown>[] = [];
+
+  globalThis.fetch = async (_url: string | URL | Request, init: RequestInit = {}) => {
+    const body = JSON.parse(String(init.body));
+    capturedBodies.push(body);
+    return new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { "Content-Type": "application/json" },
+    });
+  };
+
+  try {
+    recordLearnedReasoningEffort(
+      "openai-compatible-chat-eaff6869",
+      "qwen3-coder-30b-a3b-instruct",
+      ["none", "high", "medium", "low", "minimal"]
+    );
+    await executor.execute({
+      model: "qwen3-coder-30b-a3b-instruct",
+      body: { reasoning_effort: "xhigh" },
+      stream: false,
+      credentials: {},
+    });
+    assert.equal(capturedBodies.length, 1);
+    assert.equal(capturedBodies[0].reasoning_effort, "high");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
diff --git a/tests/unit/reasoning-effort-learned-capability.test.ts b/tests/unit/reasoning-effort-learned-capability.test.ts
new file mode 100644
index 0000000000..210a451341
--- /dev/null
+++ b/tests/unit/reasoning-effort-learned-capability.test.ts
@@ -0,0 +1,91 @@
+import { test, after, beforeEach } from "node:test";
+import assert from "node:assert/strict";
+import { sanitizeReasoningEffortForProvider } from "../../open-sse/executors/base/reasoningEffort.ts";
+import {
+  recordLearnedReasoningEffort,
+  __test_resetLearnedReasoningEffortCaps,
+} from "../../open-sse/services/learnedReasoningEffortCaps.ts";
+
+beforeEach(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+after(() => {
+  __test_resetLearnedReasoningEffortCaps();
+});
+
+test("unregistered/custom provider+model: no learned cap yet sends xhigh unchanged", () => {
+  const body = { reasoning_effort: "xhigh" };
+  const result = sanitizeReasoningEffortForProvider(
+    body,
+    "openai-compatible-chat-eaff6869",
+    "qwen3-coder-30b-a3b-instruct"
+  ) as { reasoning_effort: string };
+  assert.equal(result.reasoning_effort, "xhigh");
+});
+
+test("unregistered/custom provider+model: a learned cap clamps xhigh down to it", () => {
+  recordLearnedReasoningEffort("openai-compatible-chat-eaff6869", "qwen3-coder-30b-a3b-instruct", [
+    "none",
+    "high",
+    "medium",
+    "low",
+    "minimal",
+  ]);
+  const body = { reasoning_effort: "xhigh" };
+  const result = sanitizeReasoningEffortForProvider(
+    body,
+    "openai-compatible-chat-eaff6869",
+    "qwen3-coder-30b-a3b-instruct"
+  ) as { reasoning_effort: string };
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("learned cap only clamps when the requested effort is above it", () => {
+  recordLearnedReasoningEffort("acme", "model-x", ["none", "low", "medium"]);
+  const body = { reasoning_effort: "low" };
+  const result = sanitizeReasoningEffortForProvider(body, "acme", "model-x") as {
+    reasoning_effort: string;
+  };
+  assert.equal(result.reasoning_effort, "low");
+});
+
+test("registry says supportsXHighEffort:false (and no supportsMax path) with a learned cap below 'high': uses the learned cap, not the hardcoded 'high'", () => {
+  // claude-haiku-4-5 is registered with supportsXHighEffort:false
+  // (open-sse/config/providers/registry/claude/index.ts) and its family is
+  // excluded from supportsClaudeMaxEffort (CLAUDE_MAX_EFFORT_UNSUPPORTED_FAMILY_PATTERNS
+  // in providerModels.ts), so it reaches the hardcoded-"high" line today —
+  // a real registry-covered case. Teach a lower cap and confirm it wins.
+  recordLearnedReasoningEffort("claude", "claude-haiku-4-5-20251001", ["none", "low", "medium"]);
+  const body = { reasoning_effort: "xhigh" };
+  const result = sanitizeReasoningEffortForProvider(
+    body,
+    "claude",
+    "claude-haiku-4-5-20251001"
+  ) as {
+    reasoning_effort: string;
+  };
+  assert.equal(result.reasoning_effort, "medium");
+});
+
+test("registry says supportsXHighEffort:false with no learned cap: falls back to hardcoded 'high' (unchanged behavior)", () => {
+  const body = { reasoning_effort: "xhigh" };
+  const result = sanitizeReasoningEffortForProvider(
+    body,
+    "claude",
+    "claude-haiku-4-5-20251001"
+  ) as {
+    reasoning_effort: string;
+  };
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("deepseek's non-ordinal max<->xhigh translation is untouched by the learned-cap catch-all", () => {
+  recordLearnedReasoningEffort("deepseek", "deepseek-v4", ["none", "low"]);
+  const body = { reasoning_effort: "xhigh" };
+  const result = sanitizeReasoningEffortForProvider(body, "deepseek", "deepseek-v4") as {
+    reasoning_effort: string;
+  };
+  // deepseek's special case returns early — xhigh -> max, never reaches the catch-all.
+  assert.equal(result.reasoning_effort, "max");
+});