ContractDiff — Paste Two Contract Versions, Get a Plain-English Redline Report in 30 Seconds

Q: Who can build ContractDiff — Paste Two Contract Versions, Get a Plain-English Redline Report in 30 Seconds?

This is a beginner level project. Freelancers and boutique agency owners who review 2-10 client contracts per month — roughly 5M freelancers in the US billing over $5k/month who face contract revisions regularly.

Q: How does ContractDiff — Paste Two Contract Versions, Get a Plain-English Redline Report in 30 Seconds make money?

Credit-based + subscription. 3 free comparisons then $19/month unlimited or $4 per single comparison credit.

Freelancers and agency owners get new contract versions from clients and spend 45 minutes hunting for what changed — usually missing the one clause that costs them $3,000. ContractDiff does a semantic diff of two contract versions and returns a ranked list of changes with plain-English risk labels, so you know what to fight and what to ignore.

𝕏 Post Reddit HN

Difficulty

beginner

What is it?

Every freelancer has signed a contract revision without realizing the client quietly removed the kill fee or doubled the revision rounds. Legal diff tools either require a lawyer or show raw tracked-changes markup that looks like a physics equation. ContractDiff takes two contract text pastes or PDF uploads, runs a semantic diff using Claude, and returns a prioritized change list: High Risk (payment terms, IP ownership, liability caps), Medium Risk (scope additions, deadline changes), Low Risk (formatting, boilerplate). Each change is explained in one plain sentence. No lawyer required, no legal advice given — just clarity. Buildable in one week because Claude handles semantic comparison natively and PDF-to-text via pdf-parse is a one-line install.

Why now?

Claude's instruction-following and long-context capabilities in mid-2026 make semantic contract comparison accurate enough to surface real clause changes without false positives — making a $19/month price point defensible.

▸Paste or PDF upload for both contract versions with side-by-side change highlighting
▸Risk-ranked change list: High, Medium, Low with one plain-English sentence per change
▸Flagged clause types: payment terms, IP ownership, kill fees, scope, liability — pre-labeled by category
▸Export diff report as PDF to share with a lawyer or client

Target Audience

Freelancers and boutique agency owners who review 2-10 client contracts per month — roughly 5M freelancers in the US billing over $5k/month who face contract revisions regularly.

Example Use Case

A freelance web developer receives a revised SOW from a client, pastes both versions into ContractDiff, sees a High Risk flag on the removed kill fee clause in 20 seconds, and negotiates it back before signing.

User Stories

▸As a freelance developer, I want to see exactly what changed between two contract versions, so that I do not accidentally sign away my kill fee.
▸As an agency owner, I want changes risk-ranked by clause type, so that I know which revisions to push back on before my lawyer reviews.
▸As a consultant, I want a PDF diff report, so that I can share a summary with my client during revision negotiations.

Done When

✓Comparison: done when pasting two contract versions and clicking Compare returns a change list in under 30 seconds without an error.
✓Risk labeling: done when each change card displays a color-coded High, Medium, or Low badge and a one-sentence plain-English explanation.
✓PDF upload: done when uploading a PDF contract file extracts readable text and produces the same diff result as a paste input.
✓Billing: done when a user hitting their 3rd free comparison sees a paywall and can purchase a credit pack via Stripe checkout in under 2 minutes.

Is it worth building?

$19/month x 300 freelancers = $5,700 MRR. Math: 5M freelancers billing $5k+/month, target 0.006% via Reddit and Twitter freelance communities.

Unit Economics

CAC: $5 via Reddit organic post. LTV: $228 (12 months at $19/month). Payback: 0.3 months. Gross margin: 85%.

Business Model

Credit-based + subscription

Monetization Path

3 free comparisons then $19/month unlimited or $4 per single comparison credit.

Revenue Timeline

First dollar: week 2 via credit purchase. $1k MRR: month 3. $5k MRR: month 7.

Estimated Monthly Cost

Claude API: $30, Supabase: $25, Vercel: $20, Stripe fees: $10. Total: ~$85/month.

Profit Potential

Side income at $2k MRR, full-time at $8k MRR.

Scalability

High — add clause library, negotiation email drafts per flagged clause, and team plan for agencies reviewing multiple client contracts.

Success Metrics

Week 1: 150 free comparisons run. Month 1: 40 paid subscriptions at $19/month = $760 MRR. Month 3: 300 subscribers = $5,700 MRR.

Launch & Validation Plan

Post a before/after screenshot of a real contract diff result to r/freelance and r/webdev — count upvotes and DMs before writing billing code.

Customer Acquisition Strategy

First customer: post a real contract horror story to r/freelance with a working demo link — offer first 20 subscribers 6 months free in exchange for a written testimonial. Then: Twitter freelance community, ProductHunt, SEO content targeting freelance contract review searches.

What's the competition?

Competition Level

Low

What's the roadmap?

Feature Roadmap

V1 (launch): paste and PDF input, semantic diff, risk labels, PDF export. V2 (month 2-3): negotiation talking points per High Risk clause, clause type filter. V3 (month 4+): team plan for agencies, Notion and Google Docs import.

Milestone Plan

Phase 1 (Week 1): Claude diff pipeline, DiffCard UI, PDF parse working end-to-end. Phase 2 (Week 2): Stripe billing, Supabase auth, PDF export, deploy to Vercel. Phase 3 (Month 2): 100 paying users, SEO content on freelance contract review.

How do you build it?

Tech Stack

Next.js, Claude API, pdf-parse, Supabase, Stripe — build with Lovable for UI, Cursor for diff logic.

Suggested Frameworks

Anthropic SDK, pdf-parse, Vercel AI SDK

Time to Ship

1 week

Required Skills

Claude API prompting, PDF parsing, Next.js API routes, Stripe.

Resources

Anthropic Claude docs, pdf-parse npm package, Stripe billing docs, Supabase quickstart.

MVP Scope

app/page.tsx (two-panel paste input or PDF upload), app/api/diff/route.ts (Claude semantic diff pipeline), app/api/stripe/route.ts (billing), lib/pdf.ts (pdf-parse wrapper), lib/claude.ts (diff prompt + risk classification), lib/db/schema.ts (comparisons + users), components/DiffCard.tsx (risk-labeled change card), .env.example.

Core User Journey

Paste or upload two contract versions -> click Compare -> see risk-ranked change list in 30 seconds -> export PDF report.

Architecture Pattern

Two contract inputs -> pdf-parse if PDF -> Claude API semantic diff prompt -> risk-classified change array -> Supabase comparison log -> UI render -> PDF export via react-pdf.

Data Model

User has many Comparisons. Comparison has two ContractTexts and one DiffReport. DiffReport has many Changes, each with a risk_level and plain_english_summary.

Integration Points

Claude API for semantic diff and risk classification, pdf-parse for PDF text extraction, Supabase for comparison history and user accounts, Stripe for billing, react-pdf for report export.

V1 Scope Boundaries

V1 excludes: negotiation email drafts, clause library, team plans, version history, third-party e-signature integration, multi-document batch.

Success Definition

A freelancer finds ContractDiff via a Google search, runs a comparison on a real client contract revision, sees a High Risk flag they would have missed, and upgrades to paid without contacting the founder.

Challenges

The hardest non-technical problem is that freelancers undervalue contract review until they get burned — marketing must lead with a horror story, not a feature list.

Avoid These Pitfalls

Do not frame this as legal advice or add any prescriptive negotiation recommendations — the legal liability risk is not worth it in v1. Do not add document storage without explicit user consent and clear retention policy. Acquiring first 10 paying freelancers via Reddit requires a real horror story hook — a bland feature post will get ignored.

Security Requirements

Supabase Auth with Google OAuth, RLS on comparisons table, do not retain contract text beyond 30 days without explicit user opt-in, rate limit /api/diff to 20 req/min per IP, legal disclaimer on every exported report.

Infrastructure Plan

Vercel for Next.js, Supabase for Postgres and auth, no long-term file storage in v1, GitHub Actions for CI, Sentry for errors — total ~$65/month.

Performance Targets

300 DAU at launch, Claude diff response under 15 seconds for a 10-page contract, page load under 2s, PDF export under 3 seconds.

Go-Live Checklist

☐Diff accuracy tested on 10 real contract pairs.
☐Stripe credit and subscription flow tested end-to-end.
☐Sentry live on Vercel.
☐Legal disclaimer added to report footer and terms.
☐Custom domain with SSL live.
☐Privacy policy with 30-day retention policy published.
☐5 freelancer beta users confirmed results useful.
☐Rollback: previous Vercel deployment reachable instantly.
☐Reddit r/freelance launch post with horror story hook drafted.

First Run Experience

On first run: two sample contract versions are pre-loaded in the text panels showing a real redline scenario with a removed kill fee. User can immediately click Compare and see the risk-ranked diff without signing up. No manual config required: guest mode allows 3 free comparisons before account creation prompt.

How to build it, step by step

1. Define schema: comparisons(id, user_id, contract_a text, contract_b text, diff_report jsonb, created_at) in Supabase. 2. Run npx create-next-app contractdiff and install @anthropic-ai/sdk, pdf-parse, @supabase/supabase-js, stripe, react-pdf. 3. Build lib/pdf.ts that converts uploaded PDF Buffer to plain text via pdf-parse. 4. Write Claude prompt in lib/claude.ts that takes two contract texts and returns a JSON array of changes with risk_level (high/medium/low), clause_type, and plain_english_summary fields. 5. Build /api/diff route that accepts two text or file inputs, runs Claude prompt, and writes result to Supabase. 6. Build app/page.tsx with two textarea panels, PDF upload buttons, and a Compare button. 7. Build DiffCard component that renders each change with color-coded risk badge and summary sentence. 8. Add PDF export button using react-pdf that renders the full diff report. 9. Add Stripe credit ($4 per comparison) and subscription ($19/month) gating after 3 free runs. 10. Verify: paste two real contract versions with a removed kill fee clause, confirm High Risk flag appears with correct plain-English summary in under 30 seconds — end to end without touching code.

Generated

May 4, 2026

Model

claude-sonnet-4-6

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