fix: prevent task-augmented tool errors from being masked

Re-throw McpErrors for task-augmented requests to avoid masking them
with 'Invalid task creation result' errors in server.ts validation.

The issue occurs when a task-augmented tools/call request fails validation:
1. Error is caught in mcp.ts and wrapped via createToolError()
2. Result is validated against CreateTaskResultSchema in server.ts
3. Validation fails because the error result lacks a task property
4. User sees 'Invalid task creation result' instead of the actual error

This fix re-throws all McpError types for task-augmented requests,
allowing the actual error to propagate instead of being masked.

Fixes #1385

Author: codewithkenzo

.changeset/task-error-masking.md

@@ -0,0 +1,11 @@
+---
+"@modelcontextprotocol/server": patch
+---
+
+fix: prevent task-augmented tool errors from being masked
+
+Re-throw McpErrors for task-augmented requests instead of wrapping them
+in createToolError(). This prevents protocol errors from being masked
+by "Invalid task creation result" errors during CreateTaskResultSchema validation.
+
+Fixes #1385

packages/server/src/server/mcp.ts

@@ -224,8 +224,13 @@ export class McpServer {
                 await this.validateToolOutput(tool, result, request.params.name);
                 return result;
             } catch (error) {
+                // For task-augmented requests, re-throw McpErrors to avoid masking
+                // them with "Invalid task creation result" errors in server.ts
+                if (request.params.task && error instanceof McpError) {
+                    throw error;
+                }
                 if (error instanceof McpError && error.code === ErrorCode.UrlElicitationRequired) {
-                    throw error; // Return the error to the caller without wrapping in CallToolResult
+                    throw error;
                 }
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }