Test Writer Loop
HarnessFreeRed/green loop that picks an untested module, writes a failing test, implements it, and iterates with coverage deltas
npx openagents-cli add openagents/test-writer-loop --runtime claude-codeTest Writer Loop
A red/green harness that improves test coverage the disciplined way: pick an untested behavior, write a test that fails first (proving it tests something real), implement just enough to pass, run the full suite, record the coverage delta, and repeat — with explicit stop conditions instead of running until told to stop.
When to use
- A module or file has known-low coverage and you want it improved safely, without the agent quietly weakening tests to hit a number.
- Before a refactor, to build a safety net around code that currently has thin tests.
- As a bounded, reportable task ("spend up to 10 iterations improving coverage on
src/billing/") rather than an open-ended request.
Not a fit for writing the first test infrastructure in a project with none at all (set up a test runner first) or for end-to-end/integration test design, which needs more upfront scoping than a per-behavior loop.
Install
npx openagents-cli add openagents/test-writer-loop| Runtime | Installed to |
|---|---|
claude-code | .claude/skills/test-writer-loop/ |
codex | .codex/skills/test-writer-loop/ |
openai-agents | .openai-agents/test-writer-loop/ |
generic | .openagents/test-writer-loop/ |
Inputs
| name | type | required | default | description |
|---|---|---|---|---|
target_path | path | no | auto | File/directory to focus on; auto-selects the lowest-coverage non-trivial module if omitted |
max_iterations | number | no | 10 | Max red/green cycles per session |
coverage_command | string | no | auto | Coverage command; auto-detected from the package manifest if omitted |
Example run
> Run the test writer loop on src/billing/invoice.ts, max 8 iterations.
The harness baselines current coverage, picks the first untested behavior (e.g. "throws
on negative quantity"), writes a failing test, implements the fix if needed, runs the
full suite, records the delta, and continues until a stop condition in
stop-conditions.md is hit or 8 iterations pass — then reports before/after coverage
and any deferred gaps.
Files
HARNESS.md— overview, setup, guardrails (entry point).loop.md— the per-iteration red/green procedure.stop-conditions.md— hard stops, soft stops, and what's explicitly not a stop condition (coverage % alone).
Limitations
- Needs a working test runner already configured in the project; it does not set one up from scratch.
- Behaviors requiring live infrastructure (a real database, an external API with no test double) are recorded as deferred gaps, not faked.
- Targets coverage as a signal of untested behavior, not a percentage to chase — it will stop before reaching an arbitrary number if remaining gaps are trivial.