fix: stop policing per-message schemas the upstream owns

1.8.3's request validation whitelisted messages[].role to user/assistant,
but Claude Code v2.1+ sends harness context as role:"system" entries on
/v1/messages?beta=true - every real session died locally with
"400 messages[1].role must be user or assistant" before reaching any
provider. Root cause verified by capturing live Claude Code traffic
through a logging proxy.

The gateway now validates only what routing depends on (body shape,
model, messages array, stream flag) and leaves message schemas to the
upstream protocol, which evolves with the client.

Author: 鲁工

dist/server.d.ts.map

@@ -95,14 +95,10 @@ function requestValidationError(body: unknown): string | null {
   if (!Array.isArray(request.messages) || request.messages.length === 0) {
     return 'messages must be a non-empty array';
   }
-  for (const [index, message] of request.messages.entries()) {
-    if (!message || (message.role !== 'user' && message.role !== 'assistant')) {
-      return `messages[${index}].role must be user or assistant`;
-    }
-    if (typeof message.content !== 'string' && !Array.isArray(message.content)) {
-      return `messages[${index}].content must be a string or content-block array`;
-    }
-  }
+  // Do NOT validate individual message shapes (role, content, ...): the
+  // message schema belongs to the upstream protocol and evolves with the
+  // client - Claude Code v2.1+ already sends role:"system" entries. The
+  // gateway only checks what routing itself depends on.
   if (
     request.max_tokens !== undefined &&
     (!Number.isInteger(request.max_tokens) || request.max_tokens <= 0)

dist/server.js

@@ -247,6 +247,36 @@ describe('POST /v1/messages forwarding', () => {
     expect(text).toContain('message_delta');
   });
 
+  // Claude Code v2.1+ puts harness context into the messages array as
+  // role:"system" entries (POST /v1/messages?beta=true). The gateway must
+  // not police message schemas the upstream owns - 1.8.3's role whitelist
+  // rejected every such request with 400 before it ever left the machine.
+  it('forwards messages with roles beyond user/assistant untouched', async () => {
+    const configFile = writeTempConfig(testConfig(`http://127.0.0.1:${upstreamPort}`));
+    const gateway = await startGateway(configFile);
+
+    const res = await fetch(`${gateway.url}/v1/messages`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        model: 'testmodel-v1',
+        max_tokens: 100,
+        messages: [
+          { role: 'user', content: 'hi' },
+          { role: 'system', content: 'Available agent types for the Agent tool: ...' },
+        ],
+      }),
+    });
+
+    expect(res.status).toBe(200);
+    const forwarded = captured.body?.messages as Array<{ role: string; content: string }>;
+    expect(forwarded).toHaveLength(2);
+    expect(forwarded[1]).toEqual({
+      role: 'system',
+      content: 'Available agent types for the Agent tool: ...',
+    });
+  });
+
   it('returns 400 invalid_model for unknown models', async () => {
     const configFile = writeTempConfig(testConfig(`http://127.0.0.1:${upstreamPort}`));
     const gateway = await startGateway(configFile);