diff --git a/changelog.d/fixes/codex-reasoning-object-whitelist.md b/changelog.d/fixes/codex-reasoning-object-whitelist.md
new file mode 100644
index 0000000000..139d470bae
--- /dev/null
+++ b/changelog.d/fixes/codex-reasoning-object-whitelist.md
@@ -0,0 +1 @@
+- Fixed Codex executor forwarding client `reasoning` sub-fields (`enabled`, `max_tokens`, `exclude`) that the Codex Responses API rejects with HTTP 400, taking down every combo target with a deterministic client error. The reasoning object is now whitelisted to `effort`/`summary`, and `enabled: false` maps to effort `none` when no more specific effort was requested.
diff --git a/config/quality/file-size-baseline.json b/config/quality/file-size-baseline.json
index 79d974eab8..030599c063 100644
--- a/config/quality/file-size-baseline.json
+++ b/config/quality/file-size-baseline.json
@@ -1,4 +1,5 @@
 {
+  "_rebaseline_2026_09_15_13643_combined_growth": "Combined growth of the 2026-09-15 maxmad64bis uplift batch (each PR rebaselined its own growth; the merged sum is larger): open-sse/executors/codex.ts->1528. Every hunk is flag-gated or a verified fix covered by that PR's tests; see the batch report.",
   "_rebaseline_2026_09_15_13609_combined_growth": "Combined growth of the 2026-09-15 maxmad64bis uplift batch (each PR rebaselined its own growth; the merged sum is larger): open-sse/services/accountFallback.ts->2507. Every hunk is flag-gated or a verified fix covered by that PR's tests; see the batch report.",
   "_rebaseline_2026_09_15_13602_combined_growth": "Combined growth of the 2026-09-15 maxmad64bis uplift batch (each PR rebaselined its own growth; the merged sum is larger): src/sse/handlers/chatHelpers.ts->1214. Every hunk is flag-gated or a verified fix covered by that PR's tests; see the batch report.",
   "_rebaseline_2026_09_15_13580_combined_growth": "Combined growth of the 2026-09-15 maxmad64bis uplift batch (each PR rebaselined its own growth; the merged sum is larger): src/sse/handlers/chatHelpers.ts->1202. Every hunk is flag-gated or a verified fix covered by that PR's tests; see the batch report.",
@@ -450,7 +451,7 @@
     "open-sse/executors/antigravity.ts": 1665,
     "open-sse/executors/base.ts": 1753,
     "open-sse/executors/chatgpt-web.ts": 5056,
-    "open-sse/executors/codex.ts": 1505,
+    "open-sse/executors/codex.ts": 1528,
     "open-sse/executors/cursor.ts": 1759,
     "open-sse/executors/muse-spark-web.ts": 1405,
     "open-sse/handlers/chatCore.ts": 6146,
diff --git a/open-sse/executors/codex.ts b/open-sse/executors/codex.ts
index 25b316df7e..8e478cb833 100644
--- a/open-sse/executors/codex.ts
+++ b/open-sse/executors/codex.ts
@@ -1379,8 +1379,16 @@ export class CodexExecutor extends BaseExecutor {
     // Issue #2331: model suffix aliases (for example gpt-5.5-xhigh) represent an
     // explicit model selection, so they must override client-injected defaults such
     // as OpenCode's automatic reasoning.effort=medium for GPT-5-family requests.
+    // OpenRouter-style `enabled: false` asks for reasoning to be off. It
+    // wins over the connection default but still loses to any per-request
+    // effort selection (model suffix, reasoning.effort, or flat
+    // reasoning_effort).
+    const clientDisabledReasoning = reasoningRecord?.enabled === false;
     const rawEffort =
-      modelEffort || explicitReasoning || requestReasoningEffort || fallbackReasoningEffort;
+      modelEffort ||
+      explicitReasoning ||
+      requestReasoningEffort ||
+      (clientDisabledReasoning ? "none" : fallbackReasoningEffort);
 
     if (rawEffort) {
       const clampedEffort = clampEffort(cleanModel, rawEffort);
@@ -1390,6 +1398,24 @@ export class CodexExecutor extends BaseExecutor {
         effort: clampedEffort === "ultra" ? "max" : clampedEffort,
       };
     }
+
+    // The Codex Responses API accepts only `effort` and `summary` inside
+    // `reasoning`. Client ecosystems send OpenRouter-style keys (`enabled`,
+    // `max_tokens`, `exclude`, ...) that the upstream rejects with HTTP 400
+    // "Unknown parameter: 'reasoning.<key>'", so whitelist the object before
+    // it reaches the wire. This must run even when no effort was resolved,
+    // because the client's original object is forwarded unchanged in that
+    // case.
+    const wireReasoning =
+      body.reasoning && typeof body.reasoning === "object" && !Array.isArray(body.reasoning)
+        ? (body.reasoning as Record<string, unknown>)
+        : null;
+    if (wireReasoning) {
+      for (const key of Object.keys(wireReasoning)) {
+        if (key !== "effort" && key !== "summary") delete wireReasoning[key];
+      }
+      if (Object.keys(wireReasoning).length === 0) delete body.reasoning;
+    }
     ensureCodexReasoningSummary(body);
     if (isCompactRequest) {
       delete body.include;
diff --git a/tests/unit/codex-reasoning-wire-whitelist.test.ts b/tests/unit/codex-reasoning-wire-whitelist.test.ts
new file mode 100644
index 0000000000..6b6cb84eaa
--- /dev/null
+++ b/tests/unit/codex-reasoning-wire-whitelist.test.ts
@@ -0,0 +1,96 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { CodexExecutor } from "../../open-sse/executors/codex.ts";
+import { setThinkingBudgetConfig, ThinkingMode } from "../../open-sse/services/thinkingBudget.ts";
+
+// The Codex Responses API accepts only `effort` and `summary` inside
+// `reasoning`. Client ecosystems send OpenRouter-style keys (`enabled`,
+// `max_tokens`, `exclude`, ...) that the upstream rejects with HTTP 400
+// "Unknown parameter: 'reasoning.<key>'", taking down every combo target
+// with the same deterministic client error. The executor must whitelist the
+// object before it reaches the wire; `enabled: false` maps to effort "none"
+// when no more specific effort was requested.
+
+const CTX = { requestEndpointPath: "/responses" };
+
+function transform(body: Record<string, unknown>, model = "gpt-6-astra") {
+  const executor = new CodexExecutor();
+  return executor.transformRequest(model, body, false, CTX) as Record<string, unknown>;
+}
+
+function reasoningOf(result: Record<string, unknown>): Record<string, unknown> | null {
+  const r = result.reasoning;
+  if (r && typeof r === "object" && !Array.isArray(r)) return r as Record<string, unknown>;
+  return null;
+}
+
+test("reasoning.enabled is stripped; explicit effort survives", () => {
+  const r = reasoningOf(transform({ reasoning: { enabled: true, effort: "high" } }));
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.effort, "high");
+  assert.equal("enabled" in r, false);
+});
+
+test("reasoning.enabled:false maps to effort none when nothing more specific is set", () => {
+  const r = reasoningOf(transform({ reasoning: { enabled: false } }));
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.effort, "none");
+  assert.equal("enabled" in r, false);
+  assert.equal("summary" in r, false, "no summary for disabled reasoning");
+});
+
+test("OpenRouter-style reasoning.max_tokens never reaches the wire", () => {
+  const r = reasoningOf(transform({ reasoning: { max_tokens: 2048 } }));
+  assert.ok(!r || !("max_tokens" in r), "max_tokens must be stripped");
+});
+
+test("reasoning.exclude is stripped; sibling effort survives", () => {
+  const r = reasoningOf(transform({ reasoning: { exclude: true, effort: "low" } }));
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.effort, "low");
+  assert.equal("exclude" in r, false);
+});
+
+test("client-provided summary is preserved", () => {
+  const r = reasoningOf(transform({ reasoning: { summary: "detailed", effort: "medium" } }));
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.summary, "detailed");
+  assert.equal(r.effort, "medium");
+});
+
+test("model suffix effort still wins over enabled:false", () => {
+  const r = reasoningOf(transform({ reasoning: { enabled: false } }, "gpt-6-astra-high"));
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.effort, "high");
+});
+
+test("enabled:false wins over an explicit connection reasoning default", () => {
+  setThinkingBudgetConfig({ mode: ThinkingMode.PASSTHROUGH });
+  try {
+    const executor = new CodexExecutor();
+    const result = executor.transformRequest(
+      "gpt-6-astra",
+      { reasoning: { enabled: false } },
+      false,
+      {
+        requestEndpointPath: "/responses",
+        providerSpecificData: { requestDefaults: { reasoningEffort: "high" } },
+      }
+    ) as Record<string, unknown>;
+    const r = reasoningOf(result);
+    assert.ok(r, "reasoning object should be present");
+    assert.equal(r.effort, "none", "client disable must beat the connection default");
+  } finally {
+    setThinkingBudgetConfig({});
+  }
+});
+
+test("flat reasoning_effort path stays clean of extra keys", () => {
+  const result = transform({ reasoning_effort: "low", reasoning: { enabled: true } });
+  assert.equal("reasoning_effort" in result, false, "flat key must never reach the wire");
+  const r = reasoningOf(result);
+  assert.ok(r, "reasoning object should be present");
+  assert.equal(r.effort, "low");
+  assert.equal("enabled" in r, false);
+});