fix: surface input validation errors for task-augmented tool calls

[Vadaski]

Problem

When a task-augmented tools/call request fails input schema validation, the error is caught in McpServer._setupToolHandlers and converted to a CallToolResult with isError: true. This result then fails CreateTaskResultSchema validation in server.ts, producing a misleading "Invalid task creation result" error that completely masks the actual validation failure.

Example

{
  "method": "tools/call",
  "params": {
    "name": "batch_process",
    "arguments": { "itemCount": 5, "processingTimeMs": 100 },
    "task": { "ttl": 60000 }
  }
}

Before — client receives:

{ "code": -32602, "message": "Invalid task creation result: ..." }

After — client receives the real error:

{ "code": -32602, "message": "Input validation error: Invalid arguments for tool batch_process: Too small: expected number to be >=500" }

Fix

In mcp.ts, the catch block now re-throws InvalidParams ProtocolErrors that originated from input validation (identified by the "Input validation error:" message prefix set in validateToolInput) when the request is task-augmented. This is targeted to avoid changing semantics for non-validation errors.

Testing

Added a regression test in test/integration/test/server/mcp.test.ts that:

• Registers a task-based tool with z.number().min(500) validation
• Calls it with an invalid argument (duration: 100) and task augmentation
• Asserts the client receives code: -32602 with the real validation message
• Asserts the error does NOT say "Invalid task creation result"

Fixes #1385

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1792369

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1664

@modelcontextprotocol/server

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1664

@modelcontextprotocol/express

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1664

@modelcontextprotocol/hono

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1664

@modelcontextprotocol/node

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1664

commit: 0e534b3