fix(responses): strip mismatched agent_message item ids before passthrough (#215)

Routed threads can replay agent_message items whose ids lost their
amsg_ prefix (dialect-mixed histories). The ChatGPT Responses backend
validates the id prefix per item type and 400s the whole request --
Invalid 'input[N].id': 'msg_...'. Expected an ID that begins with 'amsg'.
-- which bricks the thread on every retry.

stripInvalidItemIds already covers message/reasoning/function_call/
custom_tool_call/tool_search_call/web_search_call; add the missing
agent_message -> amsg_ mapping so mismatched ids are stripped like
every other item type.

Co-authored-by: Robin Choi <robin@Robinui-MacBookPro.local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

src/adapters/openai-responses.ts

@@ -57,6 +57,7 @@ function stripInvalidItemIds(body: unknown): unknown {
 
   const validPrefixes: Record<string, string> = {
     message: "msg_",
+    agent_message: "amsg_",
     reasoning: "rs_",
     function_call: "fc_",
     custom_tool_call: "ctc_",

tests/openai-responses-passthrough.test.ts

@@ -41,6 +41,8 @@ describe("OpenAI Responses passthrough sanitization", () => {
       { item: { type: "tool_search_call", id: "tsc_1", call_id: "call_5", execution: "client", arguments: {} }, expectedId: "tsc_1" },
       { item: { type: "web_search_call", id: "fc_wrong", status: "completed" }, expectedId: undefined },
       { item: { type: "web_search_call", id: "ws_valid", status: "completed" }, expectedId: "ws_valid" },
+      { item: { type: "agent_message", id: "msg_wrong-dialect", content: [{ type: "output_text", text: "routed reply" }] }, expectedId: undefined },
+      { item: { type: "agent_message", id: "amsg_1", content: [{ type: "output_text", text: "routed reply" }] }, expectedId: "amsg_1" },
     ];
     const input = cases.map(({ item }) => item);
     const request = adapter.buildRequest({