fix: avoid messages fallback for generic file uploads

src/server/proxy-core/capabilities/conversationFileCapabilities.test.ts

@@ -60,6 +60,16 @@ describe('conversationFileCapabilities', () => {
       document: 'inline_only',
       preservesRemoteDocumentUrl: false,
     });
+
+    expect(resolveConversationFileEndpointCapability({
+      sitePlatform: 'new-api',
+      endpoint: 'messages',
+    })).toMatchObject({
+      image: 'native',
+      audio: 'unsupported',
+      document: 'unsupported',
+      preservesRemoteDocumentUrl: false,
+    });
   });
 
   it('ranks document-capable endpoints ahead of lossy fallbacks', () => {
@@ -74,6 +84,6 @@ describe('conversationFileCapabilities', () => {
       },
     });
 
-    expect(ranked).toEqual(['responses', 'messages', 'chat']);
+    expect(ranked).toEqual(['responses', 'chat']);
   });
 });

src/server/proxy-core/capabilities/conversationFileCapabilities.ts

@@ -218,7 +218,7 @@ export function resolveConversationFileEndpointCapability(input: {
     return {
       image: 'native',
       audio: 'unsupported',
-      document: 'inline_only',
+      document: 'unsupported',
       preservesRemoteDocumentUrl: false,
     };
   }
@@ -241,27 +241,34 @@ export function rankConversationFileEndpoints(input: {
     return [...input.requestedOrder];
   }
 
+  const isDocumentCompatible = (endpoint: ConversationFileEndpoint) => {
+    const capability = resolveConversationFileEndpointCapability({
+      sitePlatform: input.sitePlatform,
+      endpoint,
+    });
+    if (capability.document === 'unsupported') return false;
+    if (input.summary.hasRemoteDocumentUrl && !capability.preservesRemoteDocumentUrl) return false;
+    return true;
+  };
+
   const preferredDocumentOrder = input.preferMessagesForClaudeModel === true
     ? ['messages', 'responses', 'chat']
     : ['responses', 'messages', 'chat'];
 
   const supportedDocumentEndpoints = preferredDocumentOrder.filter((endpoint) => {
     if (!input.requestedOrder.includes(endpoint as ConversationFileEndpoint)) return false;
-    const capability = resolveConversationFileEndpointCapability({
+    return isDocumentCompatible(endpoint as ConversationFileEndpoint);
-      sitePlatform: input.sitePlatform,
-      endpoint: endpoint as ConversationFileEndpoint,
-    });
-    if (capability.document === 'unsupported') return false;
-    if (input.summary.hasRemoteDocumentUrl && !capability.preservesRemoteDocumentUrl) return false;
-    return true;
   }) as ConversationFileEndpoint[];
 
   if (supportedDocumentEndpoints.length <= 0) {
-    return [...input.requestedOrder];
+    return [];
   }
 
   return [
     ...supportedDocumentEndpoints,
-    ...input.requestedOrder.filter((endpoint) => !supportedDocumentEndpoints.includes(endpoint)),
+    ...input.requestedOrder.filter((endpoint) => (
+      !supportedDocumentEndpoints.includes(endpoint)
+      && isDocumentCompatible(endpoint)
+    )),
   ];
 }

src/server/routes/proxy/upstreamEndpoint.test.ts

@@ -193,7 +193,7 @@ describe('resolveUpstreamEndpointCandidates', () => {
       },
     );
 
-    expect(order).toEqual(['responses', 'messages', 'chat']);
+    expect(order).toEqual(['responses', 'chat']);
   });
 
   it('remembers the last successful endpoint per site capability profile', async () => {
@@ -252,7 +252,7 @@ describe('resolveUpstreamEndpointCandidates', () => {
       },
     );
 
-    expect(order).toEqual(['responses', 'messages', 'chat']);
+    expect(order).toEqual(['responses']);
   });
 
   it('does not remember messages fallback success for generic /v1/responses requests', async () => {