"A tool or add-on for sorting emails on Thunderbird in Linux" matching MCP connectors:

Free platform to test MCP clients without installing anything. Create mock tools with dynamic templates, configurable delays, conditions (if/then), and response sequences. Supports JSON-RPC 2.0 over Streamable HTTP. Built-in text_echo and json_echo tools. Rate-limited tiers: anonymous (5 calls/min, 1 mock tool), registered (10 calls/min, 4 mock tools), premium (60 calls/min, unlimited). Zero setup — no install, no registration required. More info: https://www.testmcp.dev

MCP tool observatory: do registry servers answer, and are their answers true? No key.

A webhook inbox for agents: one call returns a live URL. Mock, verify, inspect and replay.

ship-on-friday MCP — wraps StupidAPIs (requires X-API-Key)

Deploy production REST APIs from JSON schemas in seconds. Manage projects, schemas, and deployments.

Check an MCP endpoint resolves: liveness, live tool list, schema drift. Free.

Discover, pay for, execute, and publish APIs through Apiosk.

Anonymous webhook capture, inspection, waiting, and response configuration for AI agents.

Instant no-signup webhook & HTTP-request inspector for testing webhooks and agent tool-callbacks.

Verify x402 endpoints before payment or publish service profiles for agent discovery.

Deterministic validation for AI-generated artifacts: JSON Schema, OpenAPI response, SQL syntax.

Debug webhooks from your AI agent: inspect and replay captured webhooks on localhost.

API governance for AI agents. Detects breaking changes, scores blast radius, blocks unsafe calls.

End-to-end API testing — generate and run tests from OpenAPI, curl, Postman, or real user traffic.

The official MCP Server for the Mux API

Swagger Petstore API (v1.0.27) as MCP for testing and prototyping powered by the HAPI MCP server

40+ developer tools from Context7, Code Runner, Python Execute, NPM Sentinel, PlantUML, and Microsoft Learn. Docs lookup, code execution, security scanning. Starts at $0.01/call. Get your API key at app.xpay.sh or xpay.tools

Public benchmark where agents submit Q&A answers and get scored on a leaderboard.

Conformance checker for MCP servers. Free, no key, verdicts recomputable and re-measured daily.

MCP server and REST API for querying OpenAPI specs. Endpoint-level extraction, schema resolution, 100+ APIs indexed.