promptdiff

Compare LLM prompt versions. See what broke your AI. Free forever. A gift to the terminal from vøiddo.

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Why promptdiff

You changed a system prompt last Tuesday. Since then, something is subtly wrong — the tone shifted, the outputs grew longer, the cost went up, or the model started hallucinating examples you thought you had removed. You open git diff and get a wall of color-agnostic text because prompts live in markdown and diff(1) has no opinion about them.

promptdiff does the LLM-specific job the generic diff cannot:

• Word- and character-level diff (not just line-level).
• Word-frequency delta — so you see that the word concise quietly got deleted while three please were added.
• Token-impact — "this change adds 247 tokens on claude-opus-4-7 — costs you +$0.006 per call, +$0.60/mo at 100 calls."
• Multi-version timeline — pass three, four, eight prompt versions and see the evolution pair-by-pair.
• Markdown-flavored output for pasting directly into PR review comments.
• JSON output for CI pipelines that want to flag regressions.

Runs locally, no API keys, no telemetry, no upload. Free forever.

Install

# npm
npm install -g @v0idd0/promptdiff

# or pnpm / yarn / bun
pnpm add -g @v0idd0/promptdiff
yarn global add @v0idd0/promptdiff
bun add -g @v0idd0/promptdiff

# one-shot via npx (no install)
npx @v0idd0/promptdiff v1.md v2.md --tokens --model claude

Requires Node.js ≥ 14.

Usage

# classic unified diff
promptdiff prompt-v1.md prompt-v2.md

# side-by-side
promptdiff v1.md v2.md --format side

# inline (great for short diffs)
promptdiff v1.md v2.md --format inline

# markdown — paste straight into a PR comment
promptdiff v1.md v2.md --format markdown -o pr.md

# stats only (word/char/token delta + similarity %)
promptdiff v1.md v2.md --stats

# token impact + cost on a specific model
promptdiff v1.md v2.md --tokens --model claude-opus-4-7 --calls 1000

# word-frequency delta (top 10 terms that gained/lost uses)
promptdiff v1.md v2.md --freq --freq-top 10

# multi-version evolution (3+ files → timeline of pair diffs)
promptdiff v1.md v2.md v3.md v4.md --stats

# compare text directly, no file needed
promptdiff -t "You are a helpful assistant" "You are a rogue assistant"

# character-level diff — useful for short prompts / single-token changes
promptdiff -t "cat" "bat" --char

# read second input from stdin
cat v2.md | promptdiff v1.md -

# JSON output for CI / jq
promptdiff v1.md v2.md --json --tokens --model gpt-5.4 | jq '.pairs[0].tokenImpact.monthly.deltaCost'

# replay a git history — diff against HEAD~1
promptdiff <(git show HEAD~1:prompt.md) prompt.md

Token-impact math

Pricing is a 2026-04-22 snapshot verified against each provider's public pricing page. Cost is computed from an approximate tokenizer (blends char/word signals — close enough within ~5-10% of the vendor's own tokenizer for cost-planning purposes). Supported models include:

gpt-5.4 · gpt-5.4-mini · gpt-5.4-nano · gpt-4.1 · gpt-4o · o3 · o3-mini · o4-mini · claude-opus-4-7 (1M ctx) · claude-opus-4-6 · claude-sonnet-4-6 · claude-haiku-4-5 · gemini-3.1-pro (2M ctx) · gemini-3-flash · gemini-3.1-flash-lite · llama-4-scout (10M ctx) · llama-4-maverick · mistral-large-3 · mistral-small-4 · magistral-medium · grok-4 · grok-4.1-fast (2M ctx) · deepseek-v3.2 · deepseek-r2 · qwen3-max · command-a · command-r7b — full table in src/pricing.js.

Short aliases (pass to --model): gpt, claude, opus, sonnet, haiku, gemini, gemini-pro, llama, mistral, grok, deepseek, qwen, command, reasoning (=o3).

The same pricing table powers @v0idd0/tokcount and @v0idd0/ctxstuff — bumping any of the three gets you fresh numbers across all.

Output formats

All formats support --no-color for pipelines + log files.

Library use

const { diff, lineDiff, stats, wordFrequencyDelta, tokenImpact, formatMarkdown } =
  require('@v0idd0/promptdiff');

const a = fs.readFileSync('v1.md', 'utf8');
const b = fs.readFileSync('v2.md', 'utf8');

const s = stats(a, b, diff(a, b));
console.log(s.delta.tokens, 'token change', s.similarity, '% similar');

const impact = tokenImpact(a, b, 'claude-opus-4-7', /* calls/month */ 500);
console.log('+$' + impact.monthly.deltaCost.toFixed(2), 'per month');

const freq = wordFrequencyDelta(a, b, { minLen: 4, top: 10 });
console.log(freq);

const md = formatMarkdown(lineDiff(a, b), { file1: 'v1.md', file2: 'v2.md' });
console.log(md);  // paste into PR comment

Why free forever

We are vøiddo — a studio building small, sharp tools and a few serious products (scrb, rankd, gridlock, and more). The serious products pay for themselves. The tools are gifts.

We write promptdiff because we iterate on prompts daily, and we needed a fast, local, well-behaved tool for answering "what actually changed, and is it going to cost me more?"

From the same studio

• @v0idd0/tokcount — count LLM tokens + cost across 60+ models
• @v0idd0/ctxstuff — pack codebases into LLM-ready context
• @v0idd0/jsonyo — JSON swiss army knife, 18 commands
• View all tools →

Contributing

Bug, feature idea, stale pricing, new provider? Open an issue at github.com/voidd0/promptdiff/issues or drop a line to support@voiddo.com.

License

MIT — see LICENSE.

Built by vøiddo — a small studio shipping AI-flavoured products, free dev tools, Chrome extensions and weird browser games.