Tool calls fail with "invalid outputSchema: unsupported dialect" — outputSchema declares draft-07 but client validator expects 2020-12

[timhargreaves]

Note: While the text below is AI generated, it was done so at my direction due to a bug I am personally experiencing; it is not an automated detector or reporter.

Summary

Every tool call against the Todoist MCP server fails before it runs, with a client-side schema validation error. The tool's outputSchema declares "$schema": "http://json-schema.org/draft-07/schema#", but the connecting client's default validator only supports JSON Schema 2020-12 and rejects the draft-07 dialect tag outright. The server is unreachable through this client until the schema is fixed.

Environment

• MCP client: Claude (Cowork mode, desktop app), MCP tool-loading layer
• Server: Todoist MCP (mcp__todoist__* tools, connected via the Claude Connectors Directory / official Todoist connector)
• Date observed: 2026-08-17

Steps to reproduce

1. Connect the Todoist MCP server to a client that validates outputSchema against JSON Schema 2020-12 by default.
2. Call any Todoist tool, e.g. find-tasks, find-sections, get-overview, search, or user-info.
3. The call fails immediately, without reaching the server logic.

Expected behavior

The tool call executes normally and returns task/project/label data.

Actual behavior

Every tool called returns the same class of error before any request is sent, e.g.:

Error: Tool 'find-tasks' has an invalid outputSchema: JSON Schema declares an unsupported dialect ("$schema": "http://json-schema.org/draft-07/schema#"). The default validator supports JSON Schema 2020-12 only; pass a pre-configured Ajv instance to AjvJs

The same error, with the tool name substituted, was reproduced on at least: find-tasks, find-sections, get-overview, search, user-info. This suggests every tool in the server shares the same (or a similarly generated) outputSchema header and is equally affected — the server is unusable through this client, not just one tool.

Suspected cause

The server's tool outputSchema objects include an explicit "$schema": "http://json-schema.org/draft-07/schema#" field. MCP has been moving toward standardizing on JSON Schema 2020-12 as the default/expected dialect for embedded schemas (tool inputSchema/outputSchema, elicitation requestedSchema) — see modelcontextprotocol/modelcontextprotocol SEP-1613. Clients built against that expectation validate the dialect tag strictly and reject draft-07, even though the actual schema shape may be otherwise compatible.

Suggested fix

Either:

• Remove the explicit "$schema": "http://json-schema.org/draft-07/schema#" field from tool outputSchema definitions (letting it default), or
• Update the schemas to declare/target JSON Schema 2020-12 ("$schema": "https://json-schema.org/draft/2020-12/schema"), or
• Regenerate the schemas with a 2020-12-compliant schema generator/version.

Impact

Total: no Todoist tool is callable through an affected client until this is fixed. There is no workaround on the client side — retrying does not help, since the failure happens during schema validation before the request is dispatched.

[gnapse]

Thanks for the report. We reproduced this.

MCP does not require $schema; schemas without it default to JSON Schema 2020-12. Our MCP SDK v1 dependency generated an explicit draft-07 declaration, which most clients accept, but Cowork rejects.

We are migrating to SDK v2 in #579. It generates 2020-12 schemas natively, and we confirmed all tools and tests still pass. This should fix the issue once released.