promptlint-mcp
Lint AI prompts like code. Save tokens. Catch contradictions.
A static analyzer for AI prompts — system prompts, agent instructions, tool descriptions — that runs as a CLI and as an MCP server so Claude Code (or any MCP-aware agent) can lint prompts before sending them.
Quick demo
Run promptlint on a typical (silently broken) system prompt:
<!-- TODO: record an asciinema cast of the three CLI modes (default / --json / --trim) and link here as `[](...)` once uploaded. The text block below is the verbatim `NO_COLOR=1` output — it ships the idea without needing the cast to land first. -->$ node src/cli.mjs examples/bad-prompt.md
promptlint examples/bad-prompt.md
score 14/100
tokens 367 → 307 (-60, 16% saved)
issues 14
info [ambiguous-pronoun] Pronoun "that" has no clear referent (line 1)
→ Replace with a concrete noun, e.g. "this" → "the config file".
...
error [contradiction] Contradicting directives (length): "concise" vs "detailed"
→ Pick one, or scope each with a clear condition (e.g. by task type).
error [contradiction] Contradicting directives (commenting): "do not add comments" vs "explain each step"
→ Pick one, or scope each with a clear condition (e.g. by task type).
info [trailing-fluff] "please" fluff (match 1 of 1)
→ Delete. AIs do not need politeness tokens.
· Please
...
Two error-level contradictions caught, score clamped to 14/100, 60 tokens (16%) reclaimable via --trim.
And here's --trim in action on a small input:
$ echo "You are a helpful assistant. Please always be concise. Thank you for your help! When the user asks a question, handle it carefully." | node src/cli.mjs - --trim
You are a helpful assistant. Always be concise. When the user asks a question, handle it carefully.
See docs/promo/ for posting drafts and the (TODO) asciinema cast.
Why
- Contradictions:
"be concise"+"be thorough"ships silently and wastes tokens on every request. promptlint flags paired opposites (length / tone / frequency / commenting / asking) aserror. - Redundancy: near-duplicate sentences (Jaccard > 0.6) get one
warnapiece — delete one. - Ambiguous pronouns:
"use it"/"handle that"without a mid-sentence referent is flagged with a line number. Start-of-sentence capitals do NOT count as referents, so normal English doesn't false-positive. - Long code examples: fenced code blocks over ~1200 chars or 30 lines
trigger a
warn— examples are almost always trimmable to shape-only. - Politeness fluff:
Please/Thanks/I hope this helps/Let me know if…/Feel free to…are detected, counted, and (with--trim) stripped while keeping your actual imperatives intact.
Scope: English prompts, prose — not source code. A Japanese prompt or a file of JavaScript will look clean even if it isn't.
Install
Install both the CLI and the MCP server in one go:
npm install -g promptlint-mcp
Two binaries are placed on your $PATH:
| Bin | What it is |
|---|---|
promptlint | the CLI (lint a file or stdin) |
promptlint-mcp | the MCP server (stdio transport) |
Run without installing
# CLI: lint a file
npx -p promptlint-mcp promptlint <file>
# CLI: lint stdin
echo "be concise and thorough" | npx -p promptlint-mcp promptlint -
# MCP server (auto-launched by Claude Code via mcp config; see below)
npx -y promptlint-mcp
From source (hacking on it)
git clone https://github.com/sean-sunagaku/promptlint-mcp.git
cd promptlint-mcp
npm install
node src/cli.mjs examples/bad-prompt.md
Requires Node >= 18. Zero runtime deps other than the MCP SDK.
CLI usage
Default human report
node src/cli.mjs examples/bad-prompt.md
promptlint examples/bad-prompt.md
score 14/100
tokens 367 → 307 (-60, 16% saved)
issues 14
info [ambiguous-pronoun] Pronoun "that" has no clear referent (line 1)
→ Replace with a concrete noun, e.g. "this" → "the config file".
...
error [contradiction] Contradicting directives (length): "concise" vs "detailed"
→ Pick one, or scope each with a clear condition (e.g. by task type).
error [contradiction] Contradicting directives (commenting): "do not add comments" vs "explain each step"
→ Pick one, or scope each with a clear condition (e.g. by task type).
info [trailing-fluff] "thanks" fluff (match 1 of 2)
→ Delete. AIs do not need politeness tokens.
· Thank you
info [trailing-fluff] "please" fluff (match 1 of 1)
→ Delete. AIs do not need politeness tokens.
· Please
info [trailing-fluff] "hope-helps" fluff (match 1 of 1)
→ Delete. AIs do not need politeness tokens.
· I hope this helps
...
A clean prompt (see examples/good-prompt.md) returns score 100/100 with
zero issues — silence is a feature.
--json for machine consumption
node src/cli.mjs examples/bad-prompt.md --json
{
"score": 14,
"issues": [
{
"rule": "ambiguous-pronoun",
"severity": "info",
"message": "Pronoun \"that\" has no clear referent (line 1)",
"line": 1,
"suggestion": "Replace with a concrete noun, e.g. \"this\" → \"the config file\"."
},
{
"rule": "contradiction",
"severity": "error",
"message": "Contradicting directives (length): \"concise\" vs \"detailed\"",
"suggestion": "Pick one, or scope each with a clear condition (e.g. by task type)."
}
],
"trimmed": "…",
"tokensBefore": 367,
"tokensAfter": 307,
"saved": 60,
"savedPercent": 16
}
Empty input adds "note": "empty input". Pure-fluff input (e.g. "Thanks!")
that would otherwise trim to empty instead returns the original with
"trimmerFallback": true — a safe drop-in never hands a blank prompt to your
LLM.
--trim to print trimmed output (for piping)
echo "You are a helpful coding assistant. Please always be concise. Thank you for your help! When the user asks a question, handle it carefully. If they provide code, review it. Let me know if anything is unclear. I hope this helps!" \
| node src/cli.mjs - --trim
You are a helpful coding assistant. Always be concise. When the user asks a question, handle it carefully. If they provide code, review it.
Fenced code blocks are preserved verbatim. Quoted strings ("…") and inline
backticks (`…`) are masked before fluff removal so example text survives
untouched.
Stdin
cat prompt.md | node src/cli.mjs -
The report header shows promptlint <stdin>.
Other flags
node src/cli.mjs --rules # list the 5 rule IDs, one per line
node src/cli.mjs --help # usage
Exit codes
| Code | Meaning |
|---|---|
| 0 | no errors (warn/info ok) |
| 1 | at least one error-severity issue |
| 2 | bad usage (missing file, read failure, etc.) |
MCP server usage
Register with Claude Code in one command (no clone, no global install needed):
claude mcp add promptlint -- npx -y promptlint-mcp
Or add this to ~/.claude.json (user scope) or project .mcp.json
(project scope):
{
"mcpServers": {
"promptlint": {
"command": "npx",
"args": ["-y", "promptlint-mcp"]
}
}
}
If you've globally installed the package, the absolute-path form also works
and skips npx's first-run download:
{
"mcpServers": {
"promptlint": {
"command": "promptlint-mcp"
}
}
}
Tools exposed
| Tool | Input | Returns |
|---|---|---|
lint_prompt | { text: string } | Two text content items: [0] human one-line summary ("score: X/100 · issues: N · tokens: A → B …"), [1] full JSON of the lint() result (parse with JSON.parse(result.content[1].text)). |
trim_prompt | { text: string } | Two text content items: [0] the trimmed prompt, [1] a savings footer ("Saved N tokens (P%). Original: A, trimmed: B."). Does NOT resolve contradictions, redundancy, or ambiguity — run lint_prompt for those. |
Both tools are pure / idempotent. No network, no filesystem, no state.
Rules
| ID | Severity | What it catches | Example trigger |
|---|---|---|---|
contradiction | error | Paired opposing directives on length / tone / frequency / prohibition / commenting / asking | "be concise" + "be thorough" |
redundancy | warn | Two sentences with Jaccard word-set similarity > 0.6 | Two paragraphs that both say "respond in English" with different wording |
long-example | warn | Fenced code block > 1200 chars or > 30 lines | A 40-line helper function pasted inline |
ambiguous-pronoun | info | it / this / that / these / those with no mid-sentence concrete referent | "Use it to figure out what to do." |
trailing-fluff | info | Politeness / filler: please, thanks, I hope this helps, let me know if…, feel free to…, sorry, certainly/absolutely/of course | "Please refactor the code. Thanks!" |
Score formula: 100 - (errors*25 + warns*10 + infos*3), then clamped to <=59
if any error exists and <=79 if 2+ warnings exist. Severity ceilings are a
traffic-light: any error = red zone, regardless of count.
Pricing & Roadmap
Free (OSS, MIT)
- CLI (
node src/cli.mjs) — all three modes (report / JSON / trim), all 5 rules. - MCP server (
node src/mcp.mjs) — both tools, unlimited use. - No account, no telemetry, no network.
Pro (planned, $9/mo)
- Team-shared custom rule dictionary (your project's bannned phrases, required headers, forbidden contradictions).
- Per-project lint history with diff view.
- Cross-project token-savings dashboard ("your team saved 1.2M tokens this month").
Pro is not shipped yet. No signup form today — 👍 the early-access issue (#1) to register interest. We DM invites in 👍 order.
Development
Smoke test
npm run lint:self # lints examples/bad-prompt.md
Expected: exit 1 (2 error contradictions), score in the low teens, all
rules firing. If this drifts, a rule regressed.
Baseline
examples/good-prompt.md must always score 100/100 with 0 issues. If
a change breaks that, the change is wrong.
Adding a rule
Rules live in src/linter.mjs. Each rule is a plain function:
function ruleFooBar(text) {
// return an array of { rule, severity, message, samples?, suggestion, line? }
return [];
}
Add it to the issues array inside lint() and append its ID to the RULES
export. If it targets prose (not code), run it against the maskCodeBlocks()
copy so fenced examples don't trigger it. If it needs sentence splitting,
reuse splitSentences (quote-aware) or splitSentencesPreservingDelims
(quote-aware, keeps delimiters for the trimmer).
Contradictions
Extend CONTRADICTION_PAIRS at the top of src/linter.mjs. Each entry is:
{ a: ["keyword-a", ...], b: ["keyword-b", ...], label: "short-label" }
Existing labels: length, tone, frequency, prohibition, commenting,
asking.
Community
- 💬 Discussions — questions, ideas, prompt war stories
- 🐛 Issues — bugs, feature requests
- ⭐ Star the repo if it saves you tokens — discovery helps other AI users find it.
How this was built
This package was produced end-to-end by two AI agent teams:
- ai-auto-improve-app — built and polished the product. Customer AI uses the tool and writes feedback → Developer AI patches the code → repeat. Each round's commit (
round-001…round-003) is the developer-AI's response to that round's customer-AI feedback. - ai-launch-team — published and distributed it. 5 sub-agents handled npm publish, GitHub Release, awesome-mcp PRs, Discussions threads, social copy drafts, and SVG/PNG asset generation.
Combined, this took the product from "empty repo" to "v0.1.2 on npm + 2 awesome-mcp PRs + 2 Discussions seed threads + 4 Issues + full social drafts" with the only manual step being the platform-mandated SMS / OTP authentications (X account creation, npm 2FA setup) — the rest was AI.
License
MIT — see LICENSE.