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TestLens

Official registry Developer tools

Paste code + tests, get a strong/weak verdict with killed/survived mutants. Free API + MCP.

Source code Website Documentation

Install

Add this to your client's MCP configuration (Claude Desktop: claude_desktop_config.json; Cursor: .cursor/mcp.json; VS Code and others use the same "mcpServers" shape).

Remote (streamable-http)
Claude Code 
claude mcp add --transport http testlens 'https://testlens.bigfloppy.net/mcp'
JSON configuration (Claude Desktop, Cursor, VS Code) 
{
    "mcpServers": {
        "testlens": {
            "type": "http",
            "url": "https://testlens.bigfloppy.net/mcp"
        }
    }
}

Documentation

From the project's README on GitHub, refreshed weekly. Links open on GitHub.

TestLens — does your test suite actually test?

Mutation-testing verifier for Python unittest suites, built by lead-builder-2 (autonomous agent).

The problem: tests written (by humans or AI) against an implementation often share its blind spots. They pass while missing boundaries, edge cases, and inverted conditions.

The method: TestLens generates small mutants of your code — flips a comparison, swaps an operator, bumps a constant — runs your tests against each, and reports every survivor: a behaviour change your tests did not catch.

POST /api/mutate {"code": "...", "tests": "..."} -> JSON report GET /api/health -> {"ok": true} GET / -> landing page

  • code is saved as solution.py; tests are unittest cases importing solution.
  • Response: mutation_score (killed/total), survivors with line numbers, hangs.
  • Sandbox: each run executes with an audit hook blocking sockets/subprocess/exec; temp dir is disposable; 1 concurrent run; ≤24 mutants; 2.5s per run; per-IP rate limit.

Stdlib only. Run: PORT=8312 python3 app.py.

Use TestLens from your AI agent (MCP)

mcp_server.py is a stdio MCP server that exposes TestLens as agent tools:

  • testlens_mutate — run your tests against auto-generated mutants
  • testlens_health — check the API is up

Configure it like any MCP server (Claude Desktop / Cursor / any MCP client):

{"mcpServers": {"testlens": {"command": "python3", "args": ["/path/to/mcp_server.py"]}}}

Then ask your agent: "mutation-test this function with TestLens". It POSTs the code and tests to the free TestLens API (no key, fair use) and reads back the verdict, mutation score and survivors. Protocol-level e2e test: python3 mcp_e2e_test.py.

Frequently asked questions

What is TestLens?

Paste code + tests, get a strong/weak verdict with killed/survived mutants. Free API + MCP.

How do I install TestLens?

It runs as a remote server. In Claude Code: claude mcp add --transport http testlens 'https://testlens.bigfloppy.net/mcp'. For other clients, paste the JSON configuration from this page into their MCP settings.

Does TestLens need an API key?

Its registry entry lists no required key or secret. Some features may still need an account with the service it connects to.

Is TestLens open source?

Its code is public on GitHub, but the repository does not declare a standard license.

Who maintains TestLens?

It is published by wminno. AI.tn lists it but does not maintain or endorse it.

From the official MCP Registry. Listing is not an endorsement: review a server's code and permissions before giving it access to your data.