Refactor reasoning handling and metadata management in responses

- Updated chat stream tests to clarify reasoning behavior and ensure correct handling of include lists.
- Enhanced responses proxy route to normalize include lists and apply default reasoning includes based on request context.
- Improved transformer metadata handling, ensuring proper serialization and deserialization of metadata records.
- Added tests for preserving reasoning signatures and managing citations in OpenAI-compatible responses.
- Refined the logic for injecting default includes in responses and ensured compatibility with codex surface defaults.

src/server/transformers/anthropic/messages/conversion.test.ts

@@ -358,6 +358,41 @@ describe('convertOpenAiBodyToAnthropicMessagesBody', () => {
     ]);
   });
 
+  it('preserves top-level reasoning_signature when rebuilding assistant thinking blocks for messages fallback', () => {
+    const body = convertOpenAiBodyToAnthropicMessagesBody(
+      {
+        model: 'gpt-5',
+        messages: [
+          {
+            role: 'assistant',
+            content: 'final answer',
+            reasoning_content: 'internal only',
+            reasoning_signature: 'metapi:anthropic-signature:sig-top-level-1',
+          },
+        ],
+      },
+      'claude-opus-4-6',
+      false,
+    );
+
+    expect(body.messages).toEqual([
+      {
+        role: 'assistant',
+        content: [
+          {
+            type: 'thinking',
+            thinking: 'internal only',
+            signature: 'sig-top-level-1',
+          },
+          {
+            type: 'text',
+            text: 'final answer',
+          },
+        ],
+      },
+    ]);
+  });
+
   it('strips cache_control from thinking blocks and empty text blocks', () => {
     const body = convertOpenAiBodyToAnthropicMessagesBody(
       {

src/server/transformers/anthropic/messages/conversion.ts

@@ -301,9 +301,10 @@ function sanitizeAnthropicContentBlock(item: Record<string, unknown>): Record<st
 
   if (type === 'thinking' || type === 'redacted_thinking' || type === 'reasoning') {
     const text = asTrimmedString(item.thinking ?? item.text ?? item.content ?? item.data);
-    if (!text) return null;
     const signature = type === 'redacted_thinking' ? undefined : resolveAnthropicThinkingSignature(item);
     if (signature === null) return null;
+    const hasThinkingCarrier = type !== 'redacted_thinking' && signature !== undefined;
+    if (!text && !hasThinkingCarrier) return null;
     const next: Record<string, unknown> = { ...item };
     if (type === 'redacted_thinking') {
       next.type = 'redacted_thinking';
@@ -772,6 +773,15 @@ export function convertOpenAiBodyToAnthropicMessagesBody(
     if (role !== 'assistant') return;
 
     const contentBlocks = convertOpenAiContentToAnthropicBlocks(item.content);
+    const reasoningCarrier = sanitizeAnthropicContentBlock({
+      type: 'thinking',
+      thinking: asTrimmedString(item.reasoning_content ?? item.reasoning),
+      reasoning_signature: item.reasoning_signature,
+      signature: item.signature,
+    });
+    if (reasoningCarrier) {
+      contentBlocks.unshift(reasoningCarrier);
+    }
 
     const rawToolCalls = Array.isArray(item.tool_calls) ? item.tool_calls : [];
     for (let index = 0; index < rawToolCalls.length; index += 1) {