From 8f909f4f337aff6594200f13dda17dc42b1cd590 Mon Sep 17 00:00:00 2001
From: Epinephrine <luvs01@hanmail.net>
Date: Sun, 1 Nov 2026 06:44:59 +0000
Subject: [PATCH] fix(responses): reject lossy direct Chat tool choices

---
 scripts/test-layout/layout.json               |  1 +
 src/server/responses/request-prepare.ts       |  6 +++
 tests/fixtures/test-layout-expected.json      |  3 +-
 .../responses-direct-chat-tool-choice.test.ts | 38 +++++++++++++++++++
 4 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 tests/responses/responses-direct-chat-tool-choice.test.ts

diff --git a/scripts/test-layout/layout.json b/scripts/test-layout/layout.json
index 79ff88d6093..6c54fd1cf42 100644
--- a/scripts/test-layout/layout.json
+++ b/scripts/test-layout/layout.json
@@ -131,6 +131,7 @@
     "worker-embed-coverage.test.ts": "lib",
     "jev-stats.test.ts": "usage",
     "responses-devin-401-replay.test.ts": "responses",
+    "responses-direct-chat-tool-choice.test.ts": "responses",
     "responses-oauth-refresh-capacity.test.ts": "responses",
     "responses-grok-devin-preflight.test.ts": "responses",
     "deepseek-quota-currency.test.ts": "providers",
diff --git a/src/server/responses/request-prepare.ts b/src/server/responses/request-prepare.ts
index 8a7c561933a..51c1232900a 100644
--- a/src/server/responses/request-prepare.ts
+++ b/src/server/responses/request-prepare.ts
@@ -68,6 +68,7 @@ import {
   previousResponseProviderState,
 } from "../../responses/state";
 import { formatErrorResponse } from "../../bridge";
+import { chatCanPreserveMandatoryToolChoice } from "./combo-required-tool-contract";
 import { mapCodexAuthContextErrorToResponse } from "./codex-auth-error";
 import type { OcxParsedRequest } from "../../types";
 import { buildToolBridgeMaps } from "./collaboration";
@@ -1438,6 +1439,11 @@ export async function prepareResponsesRequest(
     claudeGoAffinity: options.claudeGoAffinity,
     droidDefaultEffort: options.droidDefaultEffort,
   });
+  if (route.provider.adapter === "openai-chat"
+    && !chatCanPreserveMandatoryToolChoice(parsed._rawBody, route.provider, route.modelId)) {
+    return formatErrorResponse(400, "invalid_request_error",
+      "This mandatory tool_choice cannot be preserved for a translated Chat destination.");
+  }
   try {
     assertPolicyNormalizedRoute(policyScope, admittedPolicyRoute, route);
   } catch (error) {
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index 74bcb8a8604..0235c4dcd09 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -300,7 +300,8 @@
   "openai-chat-tool-call-id-remint.test.ts": "adapters/openai", "coding-agent-json-lines-scaling.test.ts": "providers",
   "usage-snapshot-digest-reuse.test.ts": "usage", "worker-embed.test.ts": "lib",
   "worker-embed-coverage.test.ts": "lib", "jev-stats.test.ts": "usage",
-  "responses-devin-401-replay.test.ts": "responses", "responses-oauth-refresh-capacity.test.ts": "responses", "responses-grok-devin-preflight.test.ts": "responses",
+  "responses-devin-401-replay.test.ts": "responses",
+  "responses-direct-chat-tool-choice.test.ts": "responses", "responses-oauth-refresh-capacity.test.ts": "responses", "responses-grok-devin-preflight.test.ts": "responses",
   "abort-idle-deadline.test.ts": "lib", "tool-envelope-echo-whole-line.test.ts": "adapters",
   "abort-race.test.ts": "adapters", "account-import.test.ts": "server",
   "account-import-project-fallback.test.ts": "server",
diff --git a/tests/responses/responses-direct-chat-tool-choice.test.ts b/tests/responses/responses-direct-chat-tool-choice.test.ts
new file mode 100644
index 00000000000..bed60460fa1
--- /dev/null
+++ b/tests/responses/responses-direct-chat-tool-choice.test.ts
@@ -0,0 +1,38 @@
+import { expect, test } from "bun:test";
+import { handleResponses } from "../../src/server/responses";
+import type { OcxConfig, OcxProviderConfig } from "../../src/types";
+
+test("direct translated Chat rejects an unrepresentable computer_use choice before send", async () => {
+  let sends = 0;
+  const provider: OcxProviderConfig = {
+    adapter: "openai-chat",
+    baseUrl: "https://chat.fixture.invalid/v1",
+    apiKey: "fixture-key",
+    fetch: (async () => {
+      sends += 1;
+      throw new Error("unexpected upstream send");
+    }) as typeof fetch,
+  };
+  const config = {
+    port: 0,
+    defaultProvider: "fixture",
+    providers: { fixture: provider },
+  } as OcxConfig;
+  const response = await handleResponses(new Request("http://localhost/v1/responses", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({
+      model: "fixture/model",
+      input: "Use the computer only.",
+      tools: [
+        { type: "computer_use" },
+        { type: "function", name: "shell", parameters: { type: "object", properties: {} } },
+      ],
+      tool_choice: { type: "computer_use" },
+    }),
+  }), config, { model: "", provider: "" });
+
+  expect(response.status).toBe(400);
+  expect(await response.text()).toContain("mandatory tool_choice cannot be preserved");
+  expect(sends).toBe(0);
+});