diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8dbf17187..1529f6cca 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,15 @@ matching `## [X.Y.Z]` section (plus install instructions). Do not maintain
 parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 `## [Unreleased]` to `## [X.Y.Z] - YYYY-MM-DD`, then run the release script.
 
+## [Unreleased]
+
+### Fixed
+
+- **Codex Responses no longer sends `reasoning.summary: "auto"`.** ChatGPT
+  Codex rejects that value for gpt-5.6-terra / gpt-5.3-codex family models
+  (HTTP 400 at turn 0). The adapter now sends `{ effort }` only, matching
+  Codex CLI catalog `default_reasoning_summary=none` (CL-6893).
+
 ## [0.2.103] - 2026-08-23
 
 ### TUI
diff --git a/src/provider/codex-responses-adapter.ts b/src/provider/codex-responses-adapter.ts
index fe17667e3..0327f8fe3 100644
--- a/src/provider/codex-responses-adapter.ts
+++ b/src/provider/codex-responses-adapter.ts
@@ -321,9 +321,12 @@ function buildRequest(
   }
   // reasoning_effort rides in providerOptions (same place the OpenAI-compatible
   // path reads it); map it onto the Responses `reasoning.effort` field.
+  // ChatGPT Codex rejects summary:"auto" for gpt-5.6-terra / gpt-5.3-codex
+  // family (HTTP 400; supported: concise | detailed | none). Codex CLI catalog
+  // default_reasoning_summary is none — send effort only (CL-6893).
   const effort = options.providerOptions?.["reasoning_effort"];
   if (typeof effort === "string" && effort !== "none") {
-    body["reasoning"] = { effort, summary: "auto" };
+    body["reasoning"] = { effort };
   }
   if (sessionId !== undefined) body["prompt_cache_key"] = sessionId;
 
diff --git a/tests/unit/codex-responses-adapter.test.ts b/tests/unit/codex-responses-adapter.test.ts
index 4acfce362..2c642619d 100644
--- a/tests/unit/codex-responses-adapter.test.ts
+++ b/tests/unit/codex-responses-adapter.test.ts
@@ -132,9 +132,26 @@ describe("codex-responses buildRequest", () => {
     const body = JSON.parse(
       adapter().buildRequest([userTurn("x")], "gpt-5-codex", options).body,
     ) as Record<string, unknown>;
-    expect(body["reasoning"]).toEqual({ effort: "high", summary: "auto" });
+    expect(body["reasoning"]).toEqual({ effort: "high" });
   });
 
+  // CL-6893: ChatGPT Codex rejects reasoning.summary "auto" for the gpt-5.6-terra /
+  // gpt-5.3-codex family (HTTP 400). Codex CLI catalog default is none — omit summary
+  // (effort only) so Terra/Luna request bodies never send summary:"auto".
+  test.each(["gpt-5.6-terra", "gpt-5.6-luna"] as const)(
+    "omits reasoning.summary auto for %s (CL-6893)",
+    (model) => {
+      const options: InferenceOptions = {
+        providerOptions: { ...baseOptions.providerOptions, reasoning_effort: "high" },
+      };
+      const body = JSON.parse(
+        adapter().buildRequest([userTurn("x")], model, options).body,
+      ) as Record<string, unknown>;
+      expect(body["reasoning"]).toEqual({ effort: "high" });
+      expect(body["reasoning"]).not.toHaveProperty("summary");
+    },
+  );
+
   test("roundtrips encrypted reasoning signature from prior assistant turn into Responses reasoning item", () => {
     // Prior turn's assistant content included a thinking block with signature.
     // buildRequest must emit the "reasoning" item (with encrypted_content) before