fix(translator): inject web_search tool in Responses-API flat shape (#2390)

The omniroute_web_search fallback tool was always built in Chat Completions
nested shape ({type, function:{name}}). On the Responses->Responses passthrough
path nothing flattens it, so Codex/relay upstreams rejected it with
'Missing required parameter: tools[0].name'. buildFallbackTool and the
tool_choice injection now emit the flat Responses-API shape ({type, name})
when the target provider speaks the Responses API.

open-sse/services/webSearchFallback.ts

@@ -109,14 +109,23 @@ function buildFallbackParameters(tool: JsonRecord): JsonRecord {
   };
 }
 
-function buildFallbackTool(tool: JsonRecord): JsonRecord {
+function buildFallbackTool(tool: JsonRecord, targetFormat?: string | null): JsonRecord {
+  const name = OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME;
+  const description = buildFallbackDescription(tool);
+  const parameters = buildFallbackParameters(tool);
+
+  // Responses API expects FLAT function tools ({ type, name, parameters }), whereas
+  // Chat Completions expects NESTED ({ type, function: { name, parameters } }). On the
+  // Responses→Responses passthrough path nothing flattens the injected tool, so a nested
+  // shape reaches the upstream as `tools[0].function.name` and is rejected with
+  // "Missing required parameter: 'tools[0].name'." (issue #2390).
+  if (targetFormat === FORMATS.OPENAI_RESPONSES) {
+    return { type: "function", name, description, parameters };
+  }
+
   return {
     type: "function",
-    function: {
-      name: OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME,
-      description: buildFallbackDescription(tool),
-      parameters: buildFallbackParameters(tool),
-    },
+    function: { name, description, parameters },
   };
 }
 
@@ -182,8 +191,12 @@ export function prepareWebSearchFallbackBody<T extends JsonRecord>(
     return true;
   });
 
+  const isResponsesTarget = options.targetFormat === FORMATS.OPENAI_RESPONSES;
+
   if (!toolNames.has(OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME)) {
-    preservedTools.unshift(buildFallbackTool(toRecord(builtInSearchTools[0])));
+    preservedTools.unshift(
+      buildFallbackTool(toRecord(builtInSearchTools[0]), options.targetFormat)
+    );
   }
 
   const nextBody: T = {
@@ -192,10 +205,12 @@ export function prepareWebSearchFallbackBody<T extends JsonRecord>(
   };
 
   if (isBuiltInWebSearchToolChoice(body.tool_choice)) {
-    nextBody.tool_choice = {
-      type: "function",
-      function: { name: OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME },
-    } as T["tool_choice"];
+    // Match the injected tool shape: flat for Responses API, nested for Chat Completions.
+    nextBody.tool_choice = (
+      isResponsesTarget
+        ? { type: "function", name: OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME }
+        : { type: "function", function: { name: OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME } }
+    ) as T["tool_choice"];
   }
 
   return {

tests/unit/web-search-fallback-format.test.ts

@@ -0,0 +1,74 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { prepareWebSearchFallbackBody, OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME } =
+  await import("../../open-sse/services/webSearchFallback.ts");
+
+// Regression for #2390: when the target is a Responses-API provider, the injected
+// omniroute_web_search tool must use the FLAT function shape ({ type, name }), not the
+// nested Chat Completions shape ({ type, function: { name } }). On the Responses→Responses
+// passthrough path nothing flattens it, so a nested tool reaches the upstream as
+// tools[0].function.name and is rejected with "Missing required parameter: 'tools[0].name'".
+
+function makeBody() {
+  return {
+    model: "gpt-5.5",
+    messages: [{ role: "user", content: "search the web" }],
+    tools: [{ type: "web_search" }],
+  };
+}
+
+test("#2390 web_search fallback is FLAT for Responses API target", () => {
+  const { body, fallback } = prepareWebSearchFallbackBody(makeBody(), {
+    targetFormat: "openai-responses",
+    nativeCodexPassthrough: false,
+  });
+
+  assert.equal(fallback.enabled, true);
+  const injected = body.tools[0] as Record<string, unknown>;
+  assert.equal(injected.type, "function");
+  assert.equal(injected.name, OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME);
+  assert.equal(
+    injected.function,
+    undefined,
+    "Responses API tool must not be nested under .function"
+  );
+  assert.ok(injected.parameters, "flat tool keeps top-level parameters");
+});
+
+test("#2390 web_search fallback stays NESTED for Chat Completions target", () => {
+  const { body, fallback } = prepareWebSearchFallbackBody(makeBody(), {
+    targetFormat: "openai",
+    nativeCodexPassthrough: false,
+  });
+
+  assert.equal(fallback.enabled, true);
+  const injected = body.tools[0] as Record<string, unknown>;
+  assert.equal(injected.type, "function");
+  const fn = injected.function as Record<string, unknown> | undefined;
+  assert.ok(fn, "Chat Completions tool must be nested under .function");
+  assert.equal(fn?.name, OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME);
+  assert.equal(
+    injected.name,
+    undefined,
+    "Chat Completions tool must not expose a flat top-level name"
+  );
+});
+
+test("#2390 tool_choice matches the injected tool shape per target format", () => {
+  const responses = prepareWebSearchFallbackBody(
+    { ...makeBody(), tool_choice: { type: "web_search" } },
+    { targetFormat: "openai-responses", nativeCodexPassthrough: false }
+  );
+  const rChoice = responses.body.tool_choice as Record<string, unknown>;
+  assert.equal(rChoice.name, OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME);
+  assert.equal(rChoice.function, undefined);
+
+  const chat = prepareWebSearchFallbackBody(
+    { ...makeBody(), tool_choice: { type: "web_search" } },
+    { targetFormat: "openai", nativeCodexPassthrough: false }
+  );
+  const cChoice = chat.body.tool_choice as Record<string, unknown>;
+  const cFn = cChoice.function as Record<string, unknown> | undefined;
+  assert.equal(cFn?.name, OMNIROUTE_WEB_SEARCH_FALLBACK_TOOL_NAME);
+});