fix(server): validate low-level tool inputs

[uuzzrm]

Fixes #2628

Summary

• Remember each tool inputSchema advertised by a low-level tools/list response for the active connection.
• Validate tools/call arguments before dispatch and return an isError tool result when the arguments do not match the advertised schema.
• Keep the high-level McpServer Standard Schema parsing path unchanged, including schemas that intentionally coerce or transform values.
• Add regression coverage for invalid and valid calls, connection-scoped schema state, custom JSON Schema validators, and high-level coercion.
• Update the low-level server and schema validator documentation and example.

Verification

Passed:

• pnpm lint:all
• pnpm typecheck:all
• pnpm build:all
• pnpm --filter @modelcontextprotocol/server test (42 files, 473 tests)
• pnpm --filter @modelcontextprotocol/examples typecheck
• pnpm --filter @modelcontextprotocol/examples exec tsx guides/advanced/low-level-server.examples.ts

The repository-wide pnpm test:all run was blocked by two existing examples/shared tests because the local environment could not load the better-sqlite3 native binding. The server, client, core, and middleware test suites completed successfully.

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commit: e5571a6

[pacocartones]

Verify pass on the current head — no blocking findings.

I reproduced the issue independently against the published @modelcontextprotocol/sdk@1.30.0: the low-level Server runs a tools/call handler with arguments that violate the tool's declared inputSchema (e.g. code: 12345 against {code: string, required}) with no protocol error, while McpServer rejects the identical call with -32602: Input validation error: Invalid arguments for tool …. This PR closes exactly that gap.

Checked against the code rather than the description:

• Schemas are captured from the validated tools/list result (validateResult before _rememberToolInputSchemas), so an invalid tools/list still fails wire validation — the seam does not trust an unvalidated handler response.
• The rejection surface mirrors the high-level path (isError: true + Input validation error: Invalid arguments for tool ${toolName}: …), so clients see one consistent error shape.
• Servers without a declared schema are untouched (_toolInputValidators only populated from declared schemas; the catch around a malformed schema preserves prior behavior).
• The highLevelServers guard against double validation is the right call: McpServer validates via Standard Schema, which may intentionally differ from the advertised JSON Schema (coercion).

Non-blocking notes:

1. The validator runs against the schema recorded at the last tools/list; if a server changes schemas between requests the map is refreshed on each list, so this is fine — worth a line in the docs for readers, though.
2. The ServerOptions.jsonSchemaValidator docstring was updated to mention tool inputs, but docs/advanced/ may still describe it as elicitation-only; a one-line docs sync would avoid confusion.

The CI surface (build, client/server conformance, tests) is green on the current head.

[uuzzrm]

Thanks for the careful verification. I checked the current head dc24b03e against the docs note: docs/advanced/schema-libraries.md already describes low-level Server tool-call validation, elicitation validation, the separate fromJsonSchema validator, and the fact that high-level McpServer uses its registered Standard Schemas. No additional docs change is needed on this head.

The schema map is refreshed from each validated tools/list response as noted. The current CI remains green; this is ready for maintainer review.

[changeset-bot]

🦋 Changeset detected

Latest commit: e5571a6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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