From 1f0d8d97500dd42d26fc5b7066877b5b91dec21e Mon Sep 17 00:00:00 2001
From: can1357 <me@can.ac>
Date: Fri, 17 Jul 2026 03:57:41 +0200
Subject: [PATCH] fix(codex): preserve Lite tool-use constraints

---
 .../openai-codex/request-transformer.ts       | 20 ++++++++++---------
 .../test/openai-codex-responses-lite.test.ts  | 19 ++++++++++++++++++
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/packages/ai/src/providers/openai-codex/request-transformer.ts b/packages/ai/src/providers/openai-codex/request-transformer.ts
index 63b178b3c..d7887485f 100644
--- a/packages/ai/src/providers/openai-codex/request-transformer.ts
+++ b/packages/ai/src/providers/openai-codex/request-transformer.ts
@@ -279,14 +279,14 @@ export interface CodexLiteShapedBody {
  * `build_responses_request` with `use_responses_lite`): strips pinned image
  * detail, forces parallel tool calling off, moves tools into a leading
  * `additional_tools` developer item and the base instructions into a
- * developer message, then omits top-level `instructions`/`tools` and forces
- * `tool_choice: "auto"`. Because the rewrite removes top-level `tools`, any
- * forced hosted-tool choice (e.g. `{ type: "web_search" }`) would leave the
- * backend unable to validate the choice against a tools collection and it
- * rejects the request with HTTP 400 (#5771); codex-rs always sends
- * `tool_choice: "auto"` here. Shared by normal turns and both
- * remote-compaction paths — codex-rs routes `/responses/compact` through the
- * same builder.
+ * developer message, then omits top-level `instructions`/`tools`. Because the
+ * rewrite removes top-level `tools`, a forced hosted-tool choice (e.g.
+ * `{ type: "web_search" }`) would leave the backend unable to validate the
+ * choice against a tools collection and it rejects the request with HTTP 400
+ * (#5771). Such choices must fall back to `"auto"`; explicit string constraints
+ * such as `"none"` and `"required"` remain valid. Shared by normal turns and
+ * both remote-compaction paths — codex-rs routes `/responses/compact` through
+ * the same builder.
  */
 export function applyCodexResponsesLiteShape(body: CodexLiteShapedBody): void {
 	const input = Array.isArray(body.input) ? body.input : [];
@@ -303,7 +303,9 @@ export function applyCodexResponsesLiteShape(body: CodexLiteShapedBody): void {
 		});
 	}
 	body.input = [...prefix, ...input];
-	body.tool_choice = "auto";
+	if (body.tool_choice !== "none" && body.tool_choice !== "required") {
+		body.tool_choice = "auto";
+	}
 	delete body.instructions;
 	delete body.tools;
 }
diff --git a/packages/ai/test/openai-codex-responses-lite.test.ts b/packages/ai/test/openai-codex-responses-lite.test.ts
index 69d1d8c2b..dd3face59 100644
--- a/packages/ai/test/openai-codex-responses-lite.test.ts
+++ b/packages/ai/test/openai-codex-responses-lite.test.ts
@@ -344,6 +344,25 @@ describe("openai-codex Responses Lite input shaping", () => {
 		expect(noTools.parallel_tool_calls).toBe(false);
 	});
 
+	it("falls back from forced hosted tool choices without weakening explicit tool-use constraints", async () => {
+		const model = createCodexModel("gpt-5.6-terra");
+		const tools = [{ type: "function", name: "handoff", parameters: { type: "object" } }];
+
+		const forced = await transformRequestBody(
+			{ model: model.id, tools, tool_choice: { type: "web_search" } },
+			model,
+			{ responsesLite: true },
+		);
+		expect(forced.tool_choice).toBe("auto");
+		expect(forced.tools).toBeUndefined();
+
+		const disabled = await transformRequestBody({ model: model.id, tools, tool_choice: "none" }, model, {
+			responsesLite: true,
+		});
+		expect(disabled.tool_choice).toBe("none");
+		expect(disabled.tools).toBeUndefined();
+	});
+
 	it("moves instructions and tools into input items under lite", async () => {
 		const model = createCodexModel("gpt-5.6-terra");
 		const tools = [{ type: "function", name: "shot", parameters: { type: "object" } }];