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TestLens

TestLens

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testlens.bigfloppy.netDeveloper Tools0Updated 2d ago
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About TestLens

TestLens checks Python `unittest` suites with mutation testing by changing code and reporting which changes the tests catch or miss. It is for developers and agents evaluating whether tests cover relevant behavior, and is available as an API and stdio MCP server.

Key features

  1. 01Generate code mutants automatically
  2. 02Run Python `unittest` cases against mutants
  3. 03Report mutation score and survivors
  4. 04Show survivor line numbers and hangs
  5. 05Provide an API health check

Use cases

  1. 01Check whether tests catch boundary-condition changes
  2. 02Find behavior changes missed by a test suite
  3. 03Review tests generated by humans or AI

MCP support

The stdio MCP server exposes `testlens_mutate` for running tests against generated mutants and `testlens_health` for checking API availability. It sends code and tests to the TestLens API and returns the verdict and report.

Configuration

Remote servers need no local install. Pick your client, then copy the config or install in one click.

Endpointhttps://testlens.bigfloppy.net/mcp
Copy install command
Claude CodeTerminal
Cursor.cursor/mcp.json
VS Code.vscode/mcp.json
Claude Desktopclaude_desktop_config.json
Windsurf~/.codeium/windsurf/mcp_config.json

Run commands in a terminal; paste JSON into the client’s MCP config and restart. Servers that need sign-in will prompt on first connect.

Tools

Details

From the project README

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.

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