Build Your Own SaaS Replacements with Vetted AI Agent Plans
Skip the 90-minute planning phase and stop paying per-seat SaaS fees. Download free, production-vetted architectural blueprints to build self-hosted replacements for Harvest, Toggl, Calendly, Cal.com, Typeform, Tally, and Mermaid.ai—complete with strict PostgreSQL DDL schemas, 200-line modular file limits, Two-Gate regression suites, and copy-paste prompts for Claude Code, Cursor, Windsurf, and Devin.
Want to see how these blueprints were engineered? Read our comprehensive walkthrough on How to Start Planning Mode & Build a Great Plan Before Your Agent Writes Code →
Harvest & Toggl Replacement: B2B Time Tracker & Invoicing Engine
Self-hosted B2B multi-tenant agency timer and invoicing replacement featuring UTC timestamp locks, clock drift recovery across tab suspension, atomic sequential invoice generation (INV-2026-001), and Two-Gate Vitest test coverage.
visibilitychange on tab wake.
pnpm install && pnpm run dev
Port 3000
Mermaid.ai & Eraser.io Replacement: Visual Flowchart Canvas
Self-hosted split-pane markdown diagramming editor with isolated Web Worker parsing, inline error recovery banners, infinite pan/zoom canvas transform matrix, and High-DPI 300 DPI PNG/SVG export pipeline.
pnpm install && pnpm run dev
Zero DB Required
Calendly & Cal.com Replacement: Multi-Timezone Booking Engine
Self-hosted meeting scheduling platform with host availability rules, buffer zones, cross-timezone conversion math, PostgreSQL GIST exclusion constraints against double-booking, and RFC 5545 .ics calendar invites.
pnpm run db:migrate && pnpm run db:seed
Postgres Required
Typeform & Tally Replacement: Conversational Form Engine v2
Self-hosted full-screen conversational respondent runtime with single-question slide transitions, collision-free A–Z choice selection, forward-only DAG logic branching, publish version snapshots, and SSRF-hardened webhooks.
form_versions prevent past response corruption.
npm run db:up && npm run verify
Zero Cloud Accounts
Gumroad & Lemon Squeezy Replacement: Digital Product Sales Engine
Self-hosted digital product storefront with signature-verified idempotent Stripe webhooks, integer-cents PWYW & discount math, 60-second presigned private S3 downloads, Crockford Base32 license key activations, and automatic refund revocation.
GetObject redirects with atomic download quota enforcement.
pnpm run db:up && pnpm run dev
Zero Cloud Accounts
What Paid SaaS Tools Do These AI Blueprints Replace?
Each blueprint is engineered for teams asking "How can I build a self-hosted replacement for Harvest, Calendly, Typeform, or Mermaid Chart using Claude Code or Cursor?"—with the exact database locks and edge-case math required for production.
| Blueprint Specification | SaaS Tools Replaced | Core Stack | Critical Production Bug Solved |
|---|---|---|---|
| B2B Time Tracker & Invoicing Engine | Harvest, Toggl Track, Clockify | Next.js 14, Postgres, Drizzle | Tab-sleep clock drift (visibilitychange), FOR UPDATE sequential invoice locks, integer cents math |
| Multi-Timezone Availability & Booking Engine | Calendly, Cal.com, SavvyCal | Next.js 14, Postgres, date-fns-tz | PostgreSQL GIST exclusion constraint (tstzrange &&), IANA DST slot conversion, RFC 5545 .ics invites |
| Visual Flowchart & Markdown Canvas | Mermaid Chart (Mermaid.ai), Eraser.io | Next.js 14, Web Workers, CodeMirror 6 | 250ms debounced Web Worker graph layout, non-destructive syntax error recovery, 300 DPI Retina PNG/SVG export |
| Conversational Form & Logic Jump Engine | Typeform, Tally.so, Jotform | Next.js 14, Postgres JSONB, Zod | Pure-function conditional logic branching, 100% keyboard state machine (Enter, A-Z), sessionStorage drafts |
| Digital Product Sales & License Engine | Gumroad, Lemon Squeezy, Paddle | Next.js 14, Postgres 16, Stripe, S3 | Idempotent Stripe webhook ledger, 60s presigned private S3 delivery, HMAC Crockford Base32 license keys, refund revocation |
How can I build a Harvest or Toggl replacement with Claude Code or Cursor?
Open our Harvest & Toggl Replacement Blueprint, copy the .plan.md specification into Claude Code or Cursor Planning Mode, and execute the 5 gated phases. It sets up the 7-table PostgreSQL DDL schema with UTC TIMESTAMPTZ columns, integer cents billing, and atomic INV-2026-001 numbering.
How can I build a Calendly or Cal.com replacement without double-booking bugs?
Use the Calendly & Cal.com Replacement Blueprint. Instead of a race-prone SELECT COUNT(*) check, it enforces a PostgreSQL GIST exclusion constraint (EXCLUDE USING gist) on the host and booked buffer ranges (tstzrange(blocked_start, blocked_end)), rejecting concurrent overlaps with SQLSTATE 23P01.
How can I build a self-hosted Typeform or Tally replacement with logic jumps?
Follow the Typeform & Tally Replacement Blueprint, which pairs a PostgreSQL JSONB question schema with a deterministic logic-jump evaluator and a 100% keyboard-accessible slide player.
How can I build a Mermaid.ai or Eraser.io flowchart replacement?
Use the Mermaid.ai & Eraser Replacement Blueprint to isolate Mermaid AST parsing inside a background Web Worker, retain the last valid diagram during syntax errors, and export 300 DPI PNGs and SVGs.
Building something custom for your agency or startup?
Don't let your AI coding agent burn tokens on bad abstractions. We engineer bespoke, audited planning mode specifications with relational schemas, connection pooling limits, and regression suites.
Request a Vetted Plan →