fix(protocol): accept a null trpcCode on rpc_response errors

MsgPack encodes an absent field as null, so a plain (non-tRPC) error
reached the client with trpcCode: null while the schema required a
string. Every provider failure logged a protocol violation next to the
real error. trpcCode is now nullable, matching apiCode.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXRPKSQMTHjNBjrkdNMzUZ

Author: georgi

packages/protocol/src/messages.ts

@@ -907,7 +907,7 @@ export interface RpcErrorPayload {
   message: string;
   retryable: boolean;
   apiCode?: string | null;
-  trpcCode?: string;
+  trpcCode?: string | null;
 }
 
 /**

packages/protocol/src/ws-commands.ts

@@ -472,7 +472,8 @@ export const rpcErrorPayloadOutSchema = z
     message: z.string(),
     retryable: z.boolean(),
     apiCode: z.string().nullable().optional(),
-    trpcCode: z.string().optional()
+    // MsgPack encodes an absent trpcCode as null, so accept both.
+    trpcCode: z.string().nullable().optional()
   })
   .passthrough();
 

packages/protocol/tests/ws-rpc-response.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect } from "vitest";
+import { outboundControlMessageSchemas } from "../src/ws-commands.js";
+
+const schema = outboundControlMessageSchemas.rpc_response;
+
+describe("rpc_response outbound schema", () => {
+  // MsgPack encodes an absent field as null, so a plain (non-tRPC) error
+  // reaches the client with trpcCode: null. Rejecting that logged a protocol
+  // violation on every provider failure.
+  it("accepts a null trpcCode", () => {
+    const parsed = schema.safeParse({
+      type: "rpc_response",
+      request_id: "r1",
+      command: "generate_media",
+      error: {
+        code: "INTERNAL_ERROR",
+        message: "Unprocessable Entity",
+        retryable: true,
+        apiCode: null,
+        trpcCode: null
+      }
+    });
+    expect(parsed.success).toBe(true);
+  });
+
+  it("accepts a string trpcCode", () => {
+    const parsed = schema.safeParse({
+      type: "rpc_response",
+      request_id: "r1",
+      command: "get_asset",
+      error: {
+        code: "NOT_FOUND",
+        message: "missing",
+        retryable: false,
+        apiCode: "not_found",
+        trpcCode: "NOT_FOUND"
+      }
+    });
+    expect(parsed.success).toBe(true);
+  });
+
+  it("rejects a non-string trpcCode", () => {
+    const parsed = schema.safeParse({
+      type: "rpc_response",
+      request_id: "r1",
+      command: "get_asset",
+      error: { code: "X", message: "m", retryable: false, trpcCode: 7 }
+    });
+    expect(parsed.success).toBe(false);
+  });
+});