fix: MCP tool execution preserve message parts through Zod validation

Root cause: Zod's default .strip() mode on messageSchema silently
removed the 'parts' field from messages during request validation.
The AI SDK stores tool invocations in this field, so
processToolInvocations() never received any tool calls, causing
an infinite loop where the LLM kept proposing the same tool.

Fix: Added .passthrough() to messageSchema to preserve all AI SDK
fields including 'parts' for MCP tool invocation processing.

Also improved mcpService.ts:
- Added parts.length === 0 to early return guard
- Added debug logging for successful tool calls
- Added warning when tool has no execute function

Author: zebbern

app/lib/services/mcpService.ts

@@ -378,7 +378,7 @@ export class MCPService {
     const lastMessage = messages[messages.length - 1];
     const parts = lastMessage.parts;
 
-    if (!parts) {
+    if (!parts || parts.length === 0) {
       return messages;
     }
 
@@ -410,11 +410,13 @@ export class MCPService {
                 messages: convertToCoreMessages(messages),
                 toolCallId,
               });
+              logger.debug(`tool "${toolName}" returned successfully`);
             } catch (error) {
               logger.error(`error while calling tool "${toolName}":`, error);
               result = TOOL_EXECUTION_ERROR;
             }
           } else {
+            logger.warn(`tool "${toolName}" has no execute function`);
             result = TOOL_NO_EXECUTE_FUNCTION;
           }
         } else if (toolInvocation.result === TOOL_EXECUTION_APPROVAL.REJECT) {

app/routes/api.chat.ts

@@ -36,11 +36,13 @@ export const action = withSecurity(chatAction, {
 const logger = createScopedLogger('api.chat');
 
 // Zod schema for chat request validation
-const messageSchema = z.object({
-  id: z.string().optional(),
-  role: z.enum(['user', 'assistant', 'system']),
-  content: z.string(),
-});
+const messageSchema = z
+  .object({
+    id: z.string().optional(),
+    role: z.enum(['user', 'assistant', 'system']),
+    content: z.string(),
+  })
+  .passthrough(); // Preserve 'parts' and other AI SDK fields for MCP tool invocations
 
 const designSchemeSchema = z
   .object({