fix(translator): drop Codex image_generation tool in Responses→Chat (#2950)

Codex Desktop injects an image_generation hosted tool into every Responses API
request (even text-only ones). The tool-type validator threw
unsupportedFeature() (400) for it, breaking every Codex Desktop request.

Mirror the tool_search handling: add IMAGE_GENERATION_TOOL_TYPES, allow it past
the validator guard, and drop it from the tools array before forwarding to Chat
Completions.

Closes #2950

CHANGELOG.md

@@ -78,6 +78,11 @@
   `claude-opus-4.6`) instead of the Responses API, which Copilot does not serve for
   non-OpenAI models (returned `[400]`). Native OpenAI `gpt-*` models keep the
   Responses API. (#2911)
+- **translator/responses:** Codex Desktop injects an `image_generation` hosted
+  tool into every Responses API request (even text-only ones), which OmniRoute
+  rejected with `[400] image_generation tool type is not supported`. It is now
+  treated like `tool_search`: allowed past the tool-type validator and dropped
+  silently from the tools array before forwarding to Chat Completions. (#2950)
 
 ### ✨ New Features
 

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

@@ -18,6 +18,9 @@ const WEB_SEARCH_TOOL_TYPES = /^web_search/;
 // tool_search is a Responses API built-in sent by newer Codex clients; it has no Chat Completions
 // equivalent and must be silently dropped (not rejected with 400).
 const TOOL_SEARCH_TOOL_TYPES = /^tool_search/;
+// image_generation is a Responses API hosted tool that Codex Desktop injects into every request
+// (even text-only ones); it has no Chat Completions equivalent and must be silently dropped (#2950).
+const IMAGE_GENERATION_TOOL_TYPES = /^image_generation/;
 
 function toRecord(value: unknown): JsonRecord {
   return value && typeof value === "object" && !Array.isArray(value) ? (value as JsonRecord) : {};
@@ -85,6 +88,7 @@ export function openaiResponsesToOpenAIRequest(
         toolType !== "namespace" &&
         !WEB_SEARCH_TOOL_TYPES.test(toolType) &&
         !TOOL_SEARCH_TOOL_TYPES.test(toolType) &&
+        !IMAGE_GENERATION_TOOL_TYPES.test(toolType) &&
         !tool.function
       ) {
         throw unsupportedFeature(
@@ -267,8 +271,11 @@ export function openaiResponsesToOpenAIRequest(
       .filter((toolValue) => {
         const tool = toRecord(toolValue);
         const toolType = toString(tool.type);
-        // tool_search has no Chat Completions equivalent; drop it silently (issue #2766).
-        return !TOOL_SEARCH_TOOL_TYPES.test(toolType);
+        // tool_search (#2766) and image_generation (#2950) are Responses API built-ins
+        // with no Chat Completions equivalent; drop them silently.
+        return (
+          !TOOL_SEARCH_TOOL_TYPES.test(toolType) && !IMAGE_GENERATION_TOOL_TYPES.test(toolType)
+        );
       })
       .map((toolValue) => {
         const tool = toRecord(toolValue);

tests/unit/translator-openai-responses-req.test.ts

@@ -747,3 +747,53 @@ test("Responses -> Chat: tool_search is stripped from output tools array (issue
   assert.equal(tools[0].type, "function");
   assert.equal(tools[0].function.name, "foo");
 });
+
+// --- Issue #2950: image_generation built-in should be silently dropped ---
+
+test("Responses -> Chat: image_generation does not throw (issue #2950)", () => {
+  // Codex Desktop injects an image_generation hosted tool into every Responses
+  // request, even text-only ones. It has no Chat Completions equivalent and must
+  // be dropped silently, not rejected with 400.
+  assert.doesNotThrow(() =>
+    openaiResponsesToOpenAIRequest(
+      "gpt-4o",
+      {
+        input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+        tools: [{ type: "image_generation", output_format: "png" }],
+      },
+      false,
+      null
+    )
+  );
+});
+
+test("Responses -> Chat: image_generation is stripped from output tools array (issue #2950)", () => {
+  const result = openaiResponsesToOpenAIRequest(
+    "gpt-4o",
+    {
+      input: [{ role: "user", content: [{ type: "input_text", text: "hello" }] }],
+      tools: [
+        { type: "image_generation", output_format: "png" },
+        {
+          type: "function",
+          name: "foo",
+          description: "A function",
+          parameters: { type: "object" },
+        },
+      ],
+    },
+    false,
+    null
+  ) as Record<string, unknown>;
+
+  const tools = result.tools as any[];
+  assert.ok(Array.isArray(tools), "tools array must be present");
+  assert.equal(
+    tools.some((t) => t.type === "image_generation"),
+    false,
+    "image_generation must be stripped from output"
+  );
+  assert.equal(tools.length, 1, "only the function tool must remain");
+  assert.equal(tools[0].type, "function");
+  assert.equal(tools[0].function.name, "foo");
+});