ctxlint

April 13, 2026 · View on GitHub

The linter for AI agent context files and MCP configs.

Lint AGENTS.md, CLAUDE.md, GEMINI.md, .windsurfrules, .clinerules, .aiderules, CONVENTIONS.md, .cursorrules, and .mcp.json. Catch stale file references, dead build commands, hardcoded secrets, token waste, and more — before they hurt your agent.

npx @ctxlint/ctxlint check

Why your context file is probably hurting your agent

Research from ETH Zurich (Gloaguen et al., 2026) found that bloated context files reduce task success rates and increase inference costs by over 20%. References to files that no longer exist, commands that don't match your package.json, directory trees the agent regenerates anyway — all of it is noise that degrades performance.

ctxlint finds that noise and helps you remove it.

What makes ctxlint different

	ctxlint	others
Runtime dependencies	1 (commander only)	5–9
Commands	check, init, slim, diff, mcp	check only
Auto-fix	✓ `slim` removes flagged content	✗
Generates context files	✓ `init` from project metadata	✗
Drift detection	✓ `diff` via git history	✗
MCP config validation	✓ secrets, localhost, SSE, env syntax	✗
Watch mode	✓ `--watch` re-lints on save	✗
SARIF output	✓ GitHub Code Scanning	✗
Multi-language	✓ JS, Rust, Go, Python	JS only
Config file	✓ `.ctxlintrc` per-project config	✗
Strict mode	✓ `--strict` exits 1 on warnings	✗
Research-backed rules	✓ cited papers	✗
Validated precision	~91% on real repos	unknown

Install

# Run without installing (zero setup)
npx @ctxlint/ctxlint check

# Or install globally
npm install -g @ctxlint/ctxlint
ctxlint check

Commands

`check` — lint your context file

ctxlint check [path] [--format terminal|json|sarif] [--severity info|warn|error] [--watch] [--strict]

Runs all rules against every context file found in the project. Exits 0 if no errors, 1 if errors found — suitable for CI.

# Lint current directory
ctxlint check

# Only show warnings and errors
ctxlint check --severity warn

# Exit 1 on warnings too (stricter CI enforcement)
ctxlint check --strict

# Machine-readable JSON for scripts
ctxlint check --format json

# SARIF for GitHub Code Scanning
ctxlint check --format sarif > results.sarif

# Re-lint on every save (great during active editing)
ctxlint check --watch

`mcp` — validate MCP server configs

ctxlint mcp [path] [--format terminal|json] [--severity info|warn|error]

Validates .mcp.json, .cursor/mcp.json, .vscode/mcp.json, and .amazonq/mcp.json. Catches hardcoded secrets, localhost URLs, deprecated SSE transport, and missing server definitions.

# Check current directory
ctxlint mcp

# Machine-readable output for CI
ctxlint mcp --format json

`init` — generate a minimal context file

ctxlint init [path] [--format agents|claude|gemini|all] [--dry-run] [--force]

Generates a context file from your project metadata. Only includes non-inferable content: build commands, version constraints, and non-standard patterns. Never emits a directory tree. Supports JS, Rust, Go, and Python projects.

# Preview without writing
ctxlint init --dry-run

# Write AGENTS.md (default)
ctxlint init

# Write all three formats at once
ctxlint init --format all

`slim` — auto-fix flagged content

ctxlint slim <file> [--dry-run] [--backup]

Automatically removes all error-severity issues from a context file in place. The only context file linter with auto-fix.

# Preview what would be removed
ctxlint slim --dry-run AGENTS.md

# Remove flagged content (keep a backup)
ctxlint slim --backup AGENTS.md

`diff` — detect drift since last update

ctxlint diff [path] [--since <ref>] [--fail-on-stale]

Compares your context file against git history to find references that have gone stale since the file was last updated.

# Check for drift since AGENTS.md was last committed
ctxlint diff

# Check against a specific git ref
ctxlint diff --since main

# Exit 1 if any drift (for pre-commit hooks)
ctxlint diff --fail-on-stale

What it checks

Context file rules

Rule	Severity	Description
`stale-file-ref`	error / warn	References to files/dirs that don't exist. Downgrades to `warn` for monorepo prefix mismatches.
`stale-command`	error	Build commands that don't match package.json scripts, Makefile targets, Cargo.toml, go.mod, or pyproject.toml
`no-directory-tree`	error	Embedded directory trees — agents regenerate these on their own
`redundant-readme`	warn	Content that duplicates README.md (trigram overlap)
`no-inferable-stack`	warn	Tech stack descriptions discoverable from package.json / tsconfig / Cargo.toml
`max-lines`	warn / error	Files over 200 lines almost always contain bloat
`no-style-guide`	info	Style rules that belong in a linter config, not a context file
`token-budget`	warn	Token cost estimate, signal-to-noise ratio, and monthly savings projection
`ci-coverage`	info	CI workflows in `.github/workflows/` not mentioned in the context file

MCP config rules

Rule	Severity	Description
`mcp-schema`	error	`mcpServers` root key missing or malformed
`mcp-missing-command`	error	Server definition has neither `command` nor `url` — can't start
`mcp-hardcoded-secret`	error	API keys or tokens hardcoded in `env` blocks
`mcp-localhost-url`	warn	`url` points to localhost — only works on one machine
`mcp-deprecated-transport`	warn	SSE transport deprecated in MCP spec 2025-03-26
`mcp-env-syntax`	warn	Env var references use wrong syntax for the client (e.g. `${VAR}` instead of `${env:VAR}` in VS Code)

Example output

`ctxlint check`

CLAUDE.md

  ✗ no-directory-tree  Lines 14-34 contain a directory tree (21 lines, ~72 tokens)
     Agents discover file structure via ls/find on their own.

  ✗ stale-command  `npm run test:integration` — script "test:integration" does not exist in package.json
     Available scripts: dev, build, test, lint, typecheck

  ✗ stale-file-ref  references `src/old-api/` but this file does not exist
     Stale references actively mislead agents into searching for non-existent files.

  ⚠ no-inferable-stack  "This is a TypeScript web application built with React 18..."
     Agents infer the tech stack automatically. Keep only non-obvious constraints.

  ℹ no-style-guide  "- Prefer const over let" — your .prettierrc already enforces this
     Agents follow formatter output, not prose instructions.

  ⚠ token-budget  Context file: 58 lines, ~366 tokens
     Signal-to-noise ratio: 0.41 (poor)
     Estimated monthly cost (5 developers): \$0.32 → \$0.00 after fixes

Summary:
  3 errors, 2 warnings, 1 info

`ctxlint mcp`

.mcp.json

  ✗ mcp-hardcoded-secret  `openai-tools.env.OPENAI_API_KEY` appears to contain a hardcoded secret
     Use an environment variable reference instead: "OPENAI_API_KEY": "${OPENAI_API_KEY}"

  ⚠ mcp-localhost-url  `local-search.url` points to localhost (http://localhost:8080/mcp)
     Localhost URLs only work on your machine.

  ⚠ mcp-deprecated-transport  `local-search.transport` uses deprecated SSE transport
     SSE was deprecated in MCP spec 2025-03-26. Remove the transport field — HTTP is now the default.

Summary:
  1 error, 2 warnings, 0 info

Configuration

Create a .ctxlintrc (or .ctxlintrc.json) file in your project root to configure ctxlint per-project:

{
  "checks": ["stale-file-ref", "stale-command", "no-directory-tree"],
  "ignore": ["no-style-guide"],
  "strict": false,
  "contextFiles": ["CUSTOM_AGENTS.md"],
  "tokenThresholds": {
    "info": 500,
    "warning": 2000,
    "error": 5000
  }
}

Field	Type	Default	Description
`checks`	`string[]` or `null`	`null` (all)	Whitelist of rules to run. `null` runs all rules.
`ignore`	`string[]`	`[]`	Rules to skip entirely.
`strict`	`boolean`	`false`	Exit 1 on warnings, not just errors. Overridden by `--strict` CLI flag.
`contextFiles`	`string[]`	`[]`	Additional context file names to lint beyond the defaults.
`tokenThresholds.info`	`number`	`500`	Line count threshold for `info` severity in `token-budget`.
`tokenThresholds.warning`	`number`	`2000`	Line count threshold for `warn` severity in `token-budget`.
`tokenThresholds.error`	`number`	`5000`	Line count threshold for `error` severity in `token-budget`.

CLI flags always take precedence over .ctxlintrc settings.

CI integration

GitHub Actions — Marketplace action (recommended)

name: Lint context file
on: [push, pull_request]

jobs:
  ctxlint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: ctxlint/ctxlint-action@v1

Fail on warnings too:

      - uses: ctxlint/ctxlint-action@v1
        with:
          strict: true

Lint a subdirectory (e.g. in a monorepo):

      - uses: ctxlint/ctxlint-action@v1
        with:
          path: ./packages/my-agent

Inputs

Input	Default	Description
`path`	`.`	Project directory to scan
`strict`	`false`	Exit 1 on warnings in addition to errors
`fail-on-warnings`	`false`	Alias for `strict`

Outputs

Output	Description
`issues-found`	Total number of issues detected
`exit-code`	Raw exit code returned by ctxlint

GitHub Actions — with Code Scanning (SARIF)

name: Lint context file
on: [push, pull_request]

jobs:
  ctxlint:
    runs-on: ubuntu-latest
    permissions:
      security-events: write
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
      - run: npx @ctxlint/ctxlint check --format sarif > ctxlint.sarif
      - uses: github/codeql-action/upload-sarif@v3
        with:
          sarif_file: ctxlint.sarif

Pre-commit hook

# .git/hooks/pre-commit
npx @ctxlint/ctxlint diff --fail-on-stale
npx @ctxlint/ctxlint mcp

Language support

ctxlint is an npm CLI that works on any project, regardless of language.

Ecosystem	`check`	`init`
Node.js (npm/yarn/pnpm/bun)	Validates scripts, detects PM mismatch	Generates commands from package.json
Rust (Cargo)	Flags `cargo` commands if no `Cargo.toml`	Generates `cargo build/test/clippy`
Go	Flags `go` commands if no `go.mod`	Generates `go build/test/fmt ./...`
Python (uv/poetry/pip)	Flags `uv run`/`pytest` if no Python files	Generates `uv run pytest` / `poetry run pytest`
Makefile (any language)	Validates `make <target>` against Makefile	Lists Makefile targets

Current limitations

Semantic staleness — rules check whether a file exists, not whether its description is still accurate.
Precision — validated at ~91% on 5 real-world repos. Some false positives remain in unusual project layouts.

See analysis.md for the full false-positive breakdown.

Contributing

Contributions are welcome — the goal is to make ctxlint the standard linter for AI agent context files.

Good first issues:

Add a new MCP rule (e.g. cross-client config consistency, duplicate server detection)
Improve semantic staleness detection in diff
Improve token cost configuration (--team-size, --sessions-per-day)
Add .ctxlintrc validation (schema errors for unknown fields)

How to contribute:

Fork and clone the repo
npm install
Write your rule in src/rules/ — each rule exports { name, severity, description, run(parsedFile, projectData) → Diagnostic[] }
Add tests in test/rules/ and a fixture in test/fixtures/ if needed
Run npm test — all tests must pass
Open a PR

See AGENTS.md for architecture notes and project conventions.

Research

Gloaguen et al. (2026): Evaluating AGENTS.md: Are Repository-Level Context Files Helpful for Coding Agents? arXiv:2602.11988 — bloated context files reduce task success rates and increase inference cost by over 20%.
Lulla et al. (2026): On the Impact of AGENTS.md Files on the Efficiency of AI Coding Agents: arXiv:2601.20404 — well-structured context files correlate with ~28% lower runtime and ~16% reduced token consumption.