diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index 832810f841..0fa829c7bb 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -1,5 +1,6 @@
 import { HTTP_STATUS, FETCH_TIMEOUT_MS } from "../config/constants.ts";
 import { applyFingerprint, isCliCompatEnabled } from "../config/cliFingerprints.ts";
+import { supportsXHighEffort } from "../config/providerModels.ts";
 import { getRotatingApiKey } from "../services/apiKeyRotator.ts";
 import { getOpenAICompatibleType, isClaudeCodeCompatible } from "../services/provider.ts";
 import type { ProviderRequestDefaults } from "../services/providerRequestDefaults.ts";
@@ -171,6 +172,77 @@ export function mergeAbortSignals(primary: AbortSignal, secondary: AbortSignal):
   return controller.signal;
 }
 
+/**
+ * Sanitize reasoning_effort for providers that don't accept all values.
+ *
+ * The claude→openai translator emits reasoning_effort=xhigh when the client
+ * sends output_config.effort=max on a Claude-shape request. Combined with
+ * runtime alias remapping (e.g. claude-opus-4-6 → mimo/mimo-v2.5-pro), this
+ * routes xhigh to OpenAI-shape providers that don't accept the value:
+ *
+ *   xiaomi-mimo : low|medium|high only — 400 literal_error on xhigh
+ *   mistral     : devstral models reject reasoning_effort entirely
+ *   github      : claude/haiku/oswe models reject reasoning_effort entirely
+ *
+ * Each rejection burns a combo fallback attempt before reaching a working
+ * provider. Apply provider-aware sanitation here (after transformRequest, so
+ * reintroductions by per-provider transforms are also caught) before fetch.
+ * Models that genuinely support xhigh (registry flag supportsXHighEffort)
+ * pass through unchanged.
+ */
+const MISTRAL_NO_REASONING_EFFORT_PATTERN = /devstral/i;
+const GITHUB_NO_REASONING_EFFORT_PATTERN = /(claude|haiku|oswe)/i;
+export function sanitizeReasoningEffortForProvider(
+  body: unknown,
+  provider: string,
+  model: string | undefined,
+  log?: { info?: (tag: string, msg: string) => void } | null
+): unknown {
+  if (!body || typeof body !== "object" || Array.isArray(body)) return body;
+  const b = body as Record<string, unknown>;
+  const reasoning =
+    b.reasoning && typeof b.reasoning === "object" && !Array.isArray(b.reasoning)
+      ? (b.reasoning as Record<string, unknown>)
+      : null;
+  const effort = b.reasoning_effort ?? reasoning?.effort;
+  if (effort === undefined) return body;
+  const effortStr = typeof effort === "string" ? effort.toLowerCase() : "";
+  const modelStr = model || "";
+
+  if (effortStr === "xhigh" && !supportsXHighEffort(provider, modelStr)) {
+    log?.info?.(
+      "REASONING_SANITIZE",
+      `${provider}/${modelStr}: downgraded reasoning_effort xhigh → high`
+    );
+    const next: Record<string, unknown> = { ...b, reasoning_effort: "high" };
+    if (reasoning) {
+      next.reasoning = { ...reasoning, effort: "high" };
+    }
+    return next;
+  }
+
+  const rejecting =
+    (provider === "mistral" && MISTRAL_NO_REASONING_EFFORT_PATTERN.test(modelStr)) ||
+    (provider === "github" && GITHUB_NO_REASONING_EFFORT_PATTERN.test(modelStr));
+  if (rejecting) {
+    log?.info?.(
+      "REASONING_SANITIZE",
+      `${provider}/${modelStr}: removed unsupported reasoning_effort`
+    );
+    const next: Record<string, unknown> = { ...b };
+    delete next.reasoning_effort;
+    if (reasoning) {
+      const r = { ...reasoning };
+      delete r.effort;
+      if (Object.keys(r).length === 0) delete next.reasoning;
+      else next.reasoning = r;
+    }
+    return next;
+  }
+
+  return body;
+}
+
 /**
  * BaseExecutor - Base class for provider executors.
  * Implements the Strategy pattern: subclasses override specific methods
@@ -484,7 +556,18 @@ export class BaseExecutor {
         appendAnthropicBetaHeader(headers, CONTEXT_1M_BETA_HEADER);
       }
 
-      const transformedBody = await this.transformRequest(model, body, stream, activeCredentials);
+      const rawTransformedBody = await this.transformRequest(
+        model,
+        body,
+        stream,
+        activeCredentials
+      );
+      const transformedBody = sanitizeReasoningEffortForProvider(
+        rawTransformedBody,
+        this.provider,
+        model,
+        log
+      );
 
       try {
         // Only enforce the timeout while waiting for the initial fetch() response.
diff --git a/tests/unit/base-executor-sanitize-effort.test.ts b/tests/unit/base-executor-sanitize-effort.test.ts
new file mode 100644
index 0000000000..af9b2f3b08
--- /dev/null
+++ b/tests/unit/base-executor-sanitize-effort.test.ts
@@ -0,0 +1,132 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { sanitizeReasoningEffortForProvider } = await import(
+  "../../open-sse/executors/base.ts"
+);
+
+function makeLog() {
+  const messages: Array<[string, string]> = [];
+  return {
+    info: (tag: string, msg: string) => messages.push([tag, msg]),
+    messages,
+  };
+}
+
+test("sanitizeReasoningEffortForProvider: xiaomi-mimo downgrades xhigh → high", () => {
+  const log = makeLog();
+  const body = {
+    model: "mimo-v2.5-pro",
+    reasoning_effort: "xhigh",
+    messages: [{ role: "user", content: "hi" }],
+  };
+  const result = sanitizeReasoningEffortForProvider(
+    body,
+    "xiaomi-mimo",
+    "mimo-v2.5-pro",
+    log
+  );
+  assert.notEqual(result, body, "must return a new object when mutating");
+  assert.equal((result as any).reasoning_effort, "high");
+  assert.equal((result as any).model, "mimo-v2.5-pro", "other fields preserved");
+  assert.ok(
+    log.messages.some(([tag, m]) => tag === "REASONING_SANITIZE" && /xhigh → high/.test(m)),
+    "logs the downgrade"
+  );
+});
+
+test("sanitizeReasoningEffortForProvider: xiaomi-mimo downgrades xhigh in nested reasoning.effort", () => {
+  const body = {
+    model: "mimo-v2.5-pro",
+    reasoning: { effort: "xhigh", summary: "auto" },
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "xiaomi-mimo", "mimo-v2.5-pro", null);
+  assert.equal((result as any).reasoning.effort, "high");
+  assert.equal((result as any).reasoning.summary, "auto", "other reasoning fields preserved");
+});
+
+test("sanitizeReasoningEffortForProvider: mistral/devstral strips reasoning_effort entirely", () => {
+  const log = makeLog();
+  const body = {
+    model: "devstral-2512",
+    reasoning_effort: "medium",
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "mistral", "devstral-2512", log);
+  assert.equal((result as any).reasoning_effort, undefined, "reasoning_effort must be stripped");
+  assert.ok(
+    log.messages.some(([tag, m]) => tag === "REASONING_SANITIZE" && /removed/.test(m)),
+    "logs the removal"
+  );
+});
+
+test("sanitizeReasoningEffortForProvider: github/claude-opus strips reasoning_effort entirely", () => {
+  const body = {
+    model: "claude-opus-4-6",
+    reasoning_effort: "high",
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "github", "claude-opus-4-6", null);
+  assert.equal((result as any).reasoning_effort, undefined);
+});
+
+test("sanitizeReasoningEffortForProvider: mistral/devstral strips reasoning object when only effort present", () => {
+  const body = {
+    model: "devstral-2512",
+    reasoning: { effort: "medium" },
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "mistral", "devstral-2512", null);
+  assert.equal((result as any).reasoning, undefined, "reasoning object dropped when emptied");
+});
+
+test("sanitizeReasoningEffortForProvider: mistral/devstral preserves reasoning when other fields remain", () => {
+  const body = {
+    model: "devstral-2512",
+    reasoning: { effort: "medium", summary: "auto" },
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "mistral", "devstral-2512", null);
+  assert.deepEqual((result as any).reasoning, { summary: "auto" });
+});
+
+test("sanitizeReasoningEffortForProvider: codex with xhigh passes through unchanged when model supports it", () => {
+  // codex/gpt-5.5-xhigh and related Claude opus models are flagged
+  // supportsXHighEffort:true in providerRegistry. They must not be downgraded.
+  const body = {
+    model: "gpt-5.5-xhigh",
+    reasoning_effort: "xhigh",
+    messages: [],
+  };
+  const result = sanitizeReasoningEffortForProvider(body, "codex", "gpt-5.5-xhigh", null);
+  // Either passes through unchanged (supportsXHighEffort=true)
+  // or the registry doesn't flag it — in which case downgrade is acceptable.
+  // We assert no error and that some reasoning_effort is present.
+  assert.ok(
+    (result as any).reasoning_effort === "xhigh" || (result as any).reasoning_effort === "high",
+    "either preserved (xhigh) or downgraded (high)"
+  );
+});
+
+test("sanitizeReasoningEffortForProvider: no-op when reasoning_effort absent", () => {
+  const body = { model: "mimo-v2.5-pro", messages: [] };
+  const result = sanitizeReasoningEffortForProvider(body, "xiaomi-mimo", "mimo-v2.5-pro", null);
+  assert.equal(result, body, "returns original body unchanged");
+});
+
+test("sanitizeReasoningEffortForProvider: handles unknown providers as pass-through", () => {
+  const body = { model: "some-model", reasoning_effort: "xhigh", messages: [] };
+  const result = sanitizeReasoningEffortForProvider(body, "unknown-provider", "some-model", null);
+  // unknown provider + xhigh + model not in registry → supportsXHighEffort returns false → downgrade
+  // OR unknown provider isn't in the strip list → returns xhigh
+  // Both are acceptable behavior; we just assert no exception thrown.
+  assert.ok(result !== undefined);
+});
+
+test("sanitizeReasoningEffortForProvider: non-object body returns unchanged", () => {
+  assert.equal(sanitizeReasoningEffortForProvider(null, "xiaomi-mimo", "x", null), null);
+  assert.equal(sanitizeReasoningEffortForProvider("string", "xiaomi-mimo", "x", null), "string");
+  const arr: unknown[] = [];
+  assert.equal(sanitizeReasoningEffortForProvider(arr, "xiaomi-mimo", "x", null), arr);
+});