diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index cc31aca671..86e39f2554 100644
--- a/open-sse/translator/index.ts
+++ b/open-sse/translator/index.ts
@@ -31,6 +31,7 @@ import {
   recordReplay,
   requiresReasoningReplay,
 } from "../services/reasoningCache.ts";
+import { normalizeResponsesReasoningEffort } from "./request/openai-responses/helpers.ts";
 
 bootstrapTranslatorRegistry();
 export { register } from "./registry.ts";
@@ -61,10 +62,37 @@ function normalizeResponsesInputItem(item) {
   return item;
 }
 
+// Promote a stray top-level Chat-Completions-shaped `reasoning_effort` into the
+// Responses-shaped `reasoning:{effort}` object, in place, removing the top-level key.
+// No-op when `reasoning` is already present (an explicit Responses-shaped value always
+// wins) or when `reasoning_effort` is absent.
+//
+// This exists for the SAME-FORMAT lane (source === target === OPENAI_RESPONSES), where
+// translateRequest's hub-and-spoke translation block is skipped entirely (#7631): a
+// caller that lands a top-level `reasoning_effort` there — e.g. applyNoThinkingAlias
+// on the OpenAI path, which runs upstream of model-format resolution and cannot know
+// yet whether the target lane is Responses-native — would otherwise reach the upstream
+// with BOTH an unrecognized top-level field AND no `reasoning.effort`, so suppression
+// silently does not take effect. The cross-format path (openai -> openai-responses)
+// already performs the equivalent promotion in toResponses.ts; this covers the lane
+// that promotion never runs on.
+function promoteStrayReasoningEffort(body) {
+  if (!body || typeof body !== "object") return body;
+  if (body.reasoning !== undefined) return body;
+  if (body.reasoning_effort === undefined) return body;
+
+  const effort = normalizeResponsesReasoningEffort(body.reasoning_effort);
+  if (effort) {
+    body.reasoning = { effort };
+  }
+  delete body.reasoning_effort;
+  return body;
+}
+
 function normalizeOpenAIResponsesRequest(body) {
   if (!body || typeof body !== "object") return body;
 
-  const normalized = { ...body };
+  const normalized = promoteStrayReasoningEffort({ ...body });
 
   if (typeof normalized.input === "string") {
     normalized.input = [
diff --git a/tests/unit/default-reasoning-effort-6879.test.ts b/tests/unit/default-reasoning-effort-6879.test.ts
index 1cff3f7e54..3ed6a592bc 100644
--- a/tests/unit/default-reasoning-effort-6879.test.ts
+++ b/tests/unit/default-reasoning-effort-6879.test.ts
@@ -137,3 +137,70 @@ test("a lane known to reject reasoning_effort still drops it downstream (delete-
   const stripped = stripUnsupportedParams("github", "claude-3-5-sonnet", body);
   assert.equal("reasoning_effort" in stripped, false);
 });
+
+// ---------------------------------------------------------------------------
+// #7631: the same-format /v1/responses lane (source === target === OPENAI_RESPONSES)
+// skips translateRequest's hub-and-spoke translation block entirely, so a stray
+// top-level `reasoning_effort` (set upstream by applyNoThinkingAlias on the OpenAI
+// path, before model-format resolution knows the target lane is Responses-native)
+// must still be promoted into the Responses-shaped `reasoning.effort`, or thinking
+// suppression silently does not take effect on that lane.
+// ---------------------------------------------------------------------------
+
+test("7631: translateRequest promotes a stray top-level reasoning_effort into reasoning.effort on the same-format OPENAI_RESPONSES lane", async () => {
+  const { translateRequest } = await import("../../open-sse/translator/index.ts");
+  const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+  const body: Record<string, unknown> = {
+    model: "gpt-5.1-codex",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning_effort: "none",
+  };
+
+  const result = translateRequest(
+    FORMATS.OPENAI_RESPONSES,
+    FORMATS.OPENAI_RESPONSES,
+    "gpt-5.1-codex",
+    body
+  );
+
+  assert.equal("reasoning_effort" in result, false);
+  assert.deepEqual(result.reasoning, { effort: "none" });
+});
+
+test("7631: same-format OPENAI_RESPONSES lane leaves an explicit reasoning object untouched (no double-promotion)", async () => {
+  const { translateRequest } = await import("../../open-sse/translator/index.ts");
+  const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+  const body: Record<string, unknown> = {
+    model: "gpt-5.1-codex",
+    input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning: { effort: "high", summary: "auto" },
+  };
+
+  const result = translateRequest(
+    FORMATS.OPENAI_RESPONSES,
+    FORMATS.OPENAI_RESPONSES,
+    "gpt-5.1-codex",
+    body
+  );
+
+  assert.deepEqual(result.reasoning, { effort: "high", summary: "auto" });
+  assert.equal("reasoning_effort" in result, false);
+});
+
+test("7631: cross-format openai -> openai-responses promotion is unchanged (no regression)", async () => {
+  const { translateRequest } = await import("../../open-sse/translator/index.ts");
+  const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+  const body: Record<string, unknown> = {
+    model: "gpt-5.1-codex",
+    messages: [{ role: "user", content: "hi" }],
+    reasoning_effort: "none",
+  };
+
+  const result = translateRequest(FORMATS.OPENAI, FORMATS.OPENAI_RESPONSES, "gpt-5.1-codex", body);
+
+  assert.equal("reasoning_effort" in result, false);
+  assert.deepEqual(result.reasoning, { effort: "none" });
+});