fix(proxy): harden Anthropic request transformation semantics

- enforce strict tool_result ordering and pairing against assistant tool_use ids
- reject tool_result image payloads that cannot map to OpenAI tool messages
- preserve raw tool schemas on the /v1/messages proxy path instead of silently tightening them
- forward Anthropic tool_choice semantics and cover adaptive routing plus upstream payload checks

tests/unit/proxy/transformers/request-transformer-regressions.test.ts

@@ -74,8 +74,114 @@ describe('ProxyRequestTransformer regressions', () => {
     ).toThrow('tool_result requires user role');
   });
 
-  it('translates url images and error tool results while coalescing repeated turns', () => {
+  it('rejects orphaned, incomplete, or mixed-order tool_result blocks', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'user',
+            content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: 'orphan' }],
+          },
+        ],
+      })
+    ).toThrow('tool_result requires a preceding assistant tool_use');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              { type: 'tool_use', id: 'toolu_1', name: 'search', input: { q: 'docs' } },
+              { type: 'tool_use', id: 'toolu_2', name: 'open', input: { url: 'https://example.com' } },
+            ],
+          },
+          {
+            role: 'user',
+            content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: 'partial' }],
+          },
+        ],
+      })
+    ).toThrow('must provide tool_result blocks for all pending tool_use ids');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: [
+              { type: 'text', text: 'Here you go' },
+              { type: 'tool_result', tool_use_id: 'toolu_1', content: 'result' },
+            ],
+          },
+        ],
+      })
+    ).toThrow('tool_result blocks must come before other user content');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: 'plain follow-up',
+          },
+        ],
+      })
+    ).toThrow('must start with tool_result blocks for pending tool_use ids');
+  });
+
+  it('rejects tool_result content that cannot be represented as OpenAI tool text', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: [
+              {
+                type: 'tool_result',
+                tool_use_id: 'toolu_1',
+                content: [{ type: 'image', source: { type: 'url', url: 'https://example.com/error.png' } }],
+              },
+            ],
+          },
+        ],
+      })
+    ).toThrow('type "image" is not supported in tool_result content');
+  });
+
+  it('rejects unsupported assistant blocks instead of silently dropping them', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'server_tool_use', id: 'srv_1' }],
+          },
+        ],
+      })
+    ).toThrow('type "server_tool_use" is not supported');
+  });
+
+  it('translates url images and tool_choice while coalescing repeated turns', () => {
     const result = new ProxyRequestTransformer().transform({
+      tool_choice: {
+        type: 'tool',
+        name: 'vision',
+        disable_parallel_tool_use: true,
+      },
+      tools: [{ name: 'vision', description: 'Inspect image', input_schema: { type: 'object' } }],
       messages: [
         {
           role: 'user',
@@ -97,16 +203,19 @@ describe('ProxyRequestTransformer regressions', () => {
               type: 'tool_result',
               tool_use_id: 'toolu_1',
               is_error: true,
-              content: [
-                { type: 'text', text: 'fetch failed' },
-                { type: 'image', source: { type: 'url', url: 'https://example.com/error.png' } },
-              ],
+              content: [{ type: 'text', text: 'fetch failed' }],
             },
           ],
         },
       ],
     });
 
+    expect(result.tool_choice).toEqual({
+      type: 'function',
+      function: { name: 'vision' },
+    });
+    expect(result.parallel_tool_calls).toBe(false);
+
     expect(result.messages[0]).toEqual({
       role: 'user',
       content: [
@@ -131,10 +240,16 @@ describe('ProxyRequestTransformer regressions', () => {
     expect(result.messages[2]).toEqual({
       role: 'tool',
       tool_call_id: 'toolu_1',
-      content: [
-        { type: 'text', text: 'Error: fetch failed' },
-        { type: 'image_url', image_url: { url: 'https://example.com/error.png' } },
-      ],
+      content: 'Error: fetch failed',
     });
   });
+
+  it('defaults tools to auto tool_choice when none is specified', () => {
+    const result = new ProxyRequestTransformer().transform({
+      messages: [{ role: 'user', content: 'hello' }],
+      tools: [{ name: 'search', description: 'Search docs', input_schema: { type: 'object' } }],
+    });
+
+    expect(result.tool_choice).toBe('auto');
+  });
 });