crux

A CLI that wraps your test runner and emits a compact, failures-only summary. Includes an MCP server (crux-mcp) for agents.

What it solves

Test runner output is loud. A 200-test suite with 3 failures prints hundreds of lines of pass/fail status, ANSI escape codes, and stack traces stuffed with node_modules frames. Agents parsing that output spend tokens on noise. Humans skim past it.

crux strips the output to one summary line plus one block per failure with file:line and the matcher diff. Exit code matches the runner's, unchanged.

Proof

Raw vitest:

$ npx vitest run
 ✓ tests/users.test.ts (15) 124ms
 ✓ tests/orders.test.ts (29) 412ms
[...30 more lines...]
 ✗ tests/auth.test.ts > rejects invalid tokens
   AssertionError: expected 401 to be 500
     at Object.<anonymous> (tests/auth.test.ts:42:17)
     at runMicrotasks (node:internal/process/task_queues:96:5)
     at processTicksAndRejections (node:internal/process/task_queues:81:21)
[...more frames, summary, watch hint...]

crux:

$ crux
3 of 47 tests failed in 1.28s

### FAIL tests/auth.test.ts:42 - auth > rejects invalid tokens
expected 401, received 500

### FAIL tests/billing.test.ts:23 - billing > applies discount
expected 50, received 60

### FAIL tests/users.test.ts:88 - users > deletes own profile
expected true, received false

Same exit code. Same test outcome. Less noise.

Measured against claude-opus-4-7 via Anthropic's count_tokens API (ANSI-stripped) across 7 real OSS projects (22 to 4,456 tests, including honojs/hono, vuejs/core, and nuxt/nuxt) plus 3 controlled failure fixtures: 78% to 99.9% token reduction (median ~89%). Compared to other agent reduction strategies on the same hono run: tail -200 silently drops 2 of 4 failures, vitest --reporter=json is 1.39M tokens (more than 2,100x crux), and a seven-step jq pipeline produces several hundred to a few thousand tokens depending on the truncation cap. crux is 662 tokens for the hono run, smaller than every alternative measured, with zero per-runner setup. Reduction ratios reproduce within 1 percentage point on Sonnet 4.6 (with ~25% fewer absolute tokens). Methodology, full table, and reproduce steps in docs/report-v0.1.md.

Quickstart

npm install -g crux-cli
crux                                    # auto-detect, summarize
crux --json                             # machine-readable
crux --full                             # disable smart truncation
crux -- npx vitest run path/to/test.ts  # explicit command

Or use without a global install:

npx crux-cli           # CLI
npx --package=crux-cli crux-mcp   # MCP server

Use with agents

crux ships two agent-integration templates. Copy the one your agent reads into your own project so the agent prefers crux over invoking the test runner directly:

• Shell-using agents that read AGENTS.md (Aider, Codex, and others following the convention): copy docs/AGENTS.md to your project root.
• Claude Code: copy docs/skills/crux/ to your project's .claude/skills/ directory (or to ~/.claude/skills/ for global use). Step-by-step in docs/using-claude-code.md.
• MCP-using agents (Claude Desktop, Cursor, Cline, others): one-time host registration of the crux-mcp server, no template copy needed. See docs/mcp-hosts.md.

Supported runners (v0.1)

Runner	Status
vitest	shipped
jest	planned (v0.2)
pytest	planned (v0.3)
cargo test	planned (v0.4)
go test	planned (v0.5)

JSON schema

crux --json and the MCP run_tests tool both emit a cruxVersion: 1 envelope. The schema is locked across v0.1 to v1.x; field additions are additive, removals and type changes are major-version-only.

{
  "cruxVersion": 1,
  "runner": "vitest",
  "exitCode": 1,
  "summary": { "passed": 44, "failed": 3, "skipped": 0, "total": 47, "durationMs": 1280 },
  "failures": [
    {
      "testName": "auth > rejects invalid tokens",
      "filePath": "tests/auth.test.ts",
      "line": 42,
      "column": 17,
      "message": "expected 401 but received 500",
      "expected": "401",
      "received": "500",
      "topFrame": { "filePath": "src/auth.ts", "line": 18, "column": null }
      // expectedTruncated and receivedTruncated are optional fields,
      // present and set to true when the sanitize pass modified the
      // value (collapsed a .rejects/.resolves diff body, stripped
      // Symbol-keyed graphs, or hard-capped at 2,048 chars). Omitted
      // entirely when no modification occurred. Bypass via --full or
      // CRUX_FULL=1.
    }
  ],
  "runnerError": null
}

Built on

Subprocess capture and replay for crux's own test suite uses shell-cassette. Recommended for crux-using agents that need replayable test invocations.

License

MIT.