fix(sse): accept Responses API custom tool_choice (#13122) (#13775)

Merged in the 2026-09-16 sweep of the maintainer's own open PRs, at the owner's explicit instruction. No push was made to the PR branch: the merge took the head as the owning session left it (verified OPEN, non-draft and MERGEABLE against the release tip immediately before merging).

changelog.d/fixes/13122-responses-custom-tool-choice.md

@@ -0,0 +1 @@
+- **fix(sse):** stop rejecting a Responses API `tool_choice.type: "custom"` (e.g. Codex CLI forcing `functions__exec`) with a 400 `unsupported_feature` error (#13122) — thanks @phamtienduceng-eng

open-sse/translator/request/openai-responses.ts

@@ -716,6 +716,12 @@ export function openaiResponsesToOpenAIRequest(
       result.tool_choice = { type: "function", function: { name: tc.name } };
     } else if (tcType === "local_shell") {
       result.tool_choice = { type: "function", function: { name: "shell" } };
+    } else if (tcType === "custom" && tc.name !== undefined) {
+      // #13122: forced custom/freeform tool_choice (Codex CLI's wire_api="responses"
+      // sends this to force functions__exec-style tools). Custom tools are already
+      // normalized into a Chat { input: string } function schema above, so forcing that
+      // same declared name via Chat's tool_choice selects it correctly.
+      result.tool_choice = { type: "function", function: { name: tc.name } };
     } else if (tcType === "allowed_tools") {
       const mode = toString(tc.mode);
       if (mode !== "auto" && mode !== "required") {

tests/unit/responses-custom-tool-choice-13122.test.ts

@@ -0,0 +1,136 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { openaiResponsesToOpenAIRequest } =
+  await import("../../open-sse/translator/request/openai-responses.ts");
+const { openaiToOpenAIResponsesResponse } =
+  await import("../../open-sse/translator/response/openai-responses.ts");
+const { initState } = await import("../../open-sse/translator/index.ts");
+const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+// #13122: Codex CLI (wire_api="responses") sends tool_choice.type = "custom" when it wants
+// to force the model to call a declared freeform/custom tool (e.g. functions__exec). OmniRoute
+// rejects this with an unsupported_feature error, even though the same request's `tools[].type
+// = "custom"` declaration is already accepted and normalized into a Chat { input: string }
+// function schema a few lines above (see translator-openai-responses-custom-tool-1007.test.ts).
+test("Responses -> Chat: tool_choice.type = 'custom' forces the named custom tool instead of throwing (#13122)", () => {
+  const body = {
+    model: "kr/gpt-5.6-sol",
+    tools: [
+      {
+        type: "custom",
+        name: "functions__exec",
+        description: "Execute freeform code",
+      },
+    ],
+    tool_choice: {
+      type: "custom",
+      name: "functions__exec",
+    },
+    input: [
+      {
+        type: "message",
+        role: "user",
+        content: [{ type: "input_text", text: 'Call functions__exec with exactly: text("ok")' }],
+      },
+    ],
+    stream: false,
+  };
+
+  // Previously (buggy) this threw:
+  //   "Unsupported Responses API feature: tool_choice type 'custom' is not supported by omniroute"
+  // reproducing the exact error body from issue #13122's Reproduction 1 curl.
+  const result = openaiResponsesToOpenAIRequest("kr/gpt-5.6-sol", body, false, {});
+
+  assert.deepEqual(result.tool_choice, {
+    type: "function",
+    function: { name: "functions__exec" },
+  });
+});
+
+// A {type:"custom"} tool_choice with no `name` is not spec-compliant (Responses API's
+// ToolChoiceCustom always requires `name`) — it must still fall through to the existing
+// unsupported_feature throw rather than silently producing a malformed tool_choice.
+test("Responses -> Chat: tool_choice.type = 'custom' without a name still throws unsupported_feature (#13122)", () => {
+  const body = {
+    model: "kr/gpt-5.6-sol",
+    tools: [{ type: "custom", name: "functions__exec", description: "Execute freeform code" }],
+    tool_choice: { type: "custom" },
+    input: [
+      {
+        type: "message",
+        role: "user",
+        content: [{ type: "input_text", text: "hi" }],
+      },
+    ],
+    stream: false,
+  };
+
+  assert.throws(
+    () => openaiResponsesToOpenAIRequest("kr/gpt-5.6-sol", body, false, {}),
+    /Unsupported Responses API feature: tool_choice type 'custom' is not supported by omniroute/
+  );
+});
+
+// End-to-end: the Chat tool_choice produced above (forcing a call to the declared custom
+// tool) must, once the model actually calls it, round-trip back out through the Responses
+// response translator as a `custom_tool_call` item with a raw (unwrapped) `input` string —
+// not a `function_call` item with JSON arguments. This is the behavior Codex CLI actually
+// depends on to complete the custom_tool_call / custom_tool_call_output lifecycle.
+test("OpenAI -> Responses: forced custom tool call round-trips as custom_tool_call with raw input (#13122)", () => {
+  const state = initState(FORMATS.OPENAI_RESPONSES);
+  state.customToolNames = new Set(["functions__exec"]);
+
+  const events: unknown[] = [];
+  const chunks = [
+    {
+      id: "chatcmpl-1",
+      model: "kr/gpt-5.6-sol",
+      choices: [
+        {
+          index: 0,
+          delta: {
+            tool_calls: [
+              {
+                index: 0,
+                id: "call_1",
+                type: "function",
+                function: { name: "functions__exec", arguments: '{"input":"text(' },
+              },
+            ],
+          },
+          finish_reason: null,
+        },
+      ],
+    },
+    {
+      id: "chatcmpl-1",
+      model: "kr/gpt-5.6-sol",
+      choices: [
+        {
+          index: 0,
+          delta: { tool_calls: [{ index: 0, function: { arguments: '\\"ok\\")"}' } }] },
+          finish_reason: "tool_calls",
+        },
+      ],
+    },
+    null,
+  ];
+  for (const chunk of chunks) {
+    const result = openaiToOpenAIResponsesResponse(chunk, state);
+    if (result) events.push(...result);
+  }
+
+  const added = events.find((e: { event?: string }) => e.event === "response.output_item.added") as
+    { data: { item: { type: string; name: string } } } | undefined;
+  assert.ok(added);
+  assert.equal(added.data.item.type, "custom_tool_call");
+  assert.equal(added.data.item.name, "functions__exec");
+
+  const itemDone = events.find(
+    (e: { event?: string; data?: { item?: { type?: string } } }) =>
+      e.event === "response.output_item.done" && e.data?.item?.type === "custom_tool_call"
+  ) as { data: { item: { input: string } } } | undefined;
+  assert.ok(itemDone);
+  assert.equal(itemDone.data.item.input, 'text("ok")');
+});