Idemcheck
The webhook you processed twice
Problem
A provider retries a webhook because your response was slow. You process it twice. A customer is charged twice, or an email goes twice, or an order ships twice. It happens rarely enough that nobody builds for it and often enough to hurt.
Why now
Nothing new. This is a well-known distributed systems problem that small teams keep rediscovering the hard way, and the number of webhook sources per product keeps growing.
Solution
A receiver that dedupes before your handler sees anything, verifies the signature, and replays what you missed while you were down. Your handler stops needing to be idempotent because the edge already was.
Who it is for
A small team taking webhooks from payment, mail and platform providers. A provider retries because a response was slow, the event is processed twice, and a customer is charged twice or an order ships twice. It happens rarely enough that nobody builds for it and often enough to hurt, which is the exact frequency that keeps a problem unsolved.
What you could earn from Idemcheck
Say you build Idemcheck and charge your own customers $25 a month, the price the score suggests. The numbers below are the revenue that would come from them. This is money you earn, never a fee you pay UpgradIQ, and we never take a cut. It is arithmetic off the price, not a promise the customers arrive on their own.
See what Idemcheck produces
A receiver that dedupes before your handler sees anything, verifies the signature, and replays what you missed while you were down. Your handler stops needing to be idempotent because the edge already was.
Your build kit for Idemcheck
Start the build and the kit opens two steps in, then drip-feeds the rest one phase at a time. Every phase is a copy-paste prompt for your agent with its own verification before the next unlocks, so Idemcheck moves from an empty folder to a running MVP without you ever holding the whole plan in your head.
Progress saves across every device, and the workspace always reopens on the exact step you left, so a build that runs over several weeks survives them instead of dying in a browser tab you eventually close.
How you actually build it
Connect your coding agent once. It pulls the current phase straight from your kit over MCP: the exact prompt, the outcome to aim for, and the check to run before it moves on. You review and ship, it does the typing.
No copy-paste juggling, no thread to lose. The kit stays the single source of truth, and your agent works through it one phase at a time, verifying its own work at each step before the next phase is unlocked.
Take the context file, free
This is the AGENTS.md that keeps your coding agent on scope for the entire build: what Idemcheck is, who it is for, the stack, the non-goals, and the rules it must follow. Copy it into your project now, no account needed, and your agent is briefed before you write a line.
# Idemcheck The webhook you processed twice ## What this is A receiver that dedupes before your handler sees anything, verifies the signature, and replays what you missed while you were down. Your handler stops needing to be idempotent because the edge already was. ## The problem it solves A provider retries a webhook because your response was slow. You process it twice. A customer is charged twice, or an email goes twice, or an order ships twice. It happens rarely enough that nobody builds for it and often enough to hurt. ## Who it is for A small team taking webhooks from payment, mail and platform providers. A provider retries because a response was slow, the event is processed twice, and a customer is charged twice or an order ships twice. It happens rarely enough that nobody builds for it and often enough to hurt, which is the exact frequency that keeps a problem unsolved. ## Stack Next.js + Postgres ## Non-goals Do not build features outside the core job above. This product exists to do one thing well. Every extra feature is a reason it does not ship. ## Rules for the agent - Build one phase at a time. Do not run ahead into later phases. - Validate input at every boundary where data enters. - Never put a permission or plan check in client code. - Ask before adding a dependency that a few lines would cover. - If something is ambiguous, say so rather than inventing a requirement. --- Context file from UpgradIQ · idea idemcheck · prompts kit-v4
Then start free to get the ten-phase kit and run the first prompt.
The path to a running MVP
Foundation
Scaffold the stack, then wire the data model and sign-in. It is the skeleton Idemcheck stands on, and it goes up in an evening rather than a lost week of config.
The product
Build the core feature end to end, then wire how it charges. This is where Idemcheck finally turns into something that a stranger will actually pay you for.
Ship it
Test and harden, deploy it live, then open the launch-day toolkit that lands the first ten users who prove it.
Is Idemcheck for you?
- You want revenue reasonably soon, not a science project
- You can give it evenings and a couple of weekends
- You can reach Teams consuming webhooks
- You are not ready to talk to a first real customer
- You want it fully passive, with zero maintenance
- You expect it to find its own customers without you
The 3 sections that decide whether you should actually build it, plus the full kit, are one click away.
3 more sections decide whether you build Idemcheck
You have the case and the plan. Pro opens how it makes money, the one risk that kills it, and the build notes, plus the 10-phase kit that takes Idemcheck to a running MVP, on this idea and all 200.
How it makes money
Main risk
Build notes
What is free here, and what Pro unlocks
Frequently asked
Is Idemcheck actually validated?
It scored UIQ 78 with a GO verdict, weighed across eight sub-scores on the same rubric as every idea. GO means 75 and above, so demand and buildability were judged, not guessed.
What stack does it use?
Idemcheck is built on Next.js + Postgres, chosen because a coding agent can carry most of it. You are not fighting infrastructure, you are shipping the feature that matters.
How long to a running MVP?
The kit is ten phases, drip-fed one at a time so you always know the single next step. An evening and a weekend for the lighter builds, a few focused weeks for the deeper ones.
What does the $25 a month mean?
It is the price the model suggests Idemcheck could charge its own customers, not a fee to us. It is here so the money question is answered before you build, not after.
Which agent do I need?
Any capable coding agent: Claude Code, Cursor, Lovable, or Bolt. Every phase is a copy-paste prompt with its own check, so the agent does the typing and you keep the judgement.
What is free versus Pro?
The whole case above is free: the problem, timing, solution, buyer, and the full score. Pro adds how it makes money, the one risk that could kill it, and the full kit that carries Idemcheck from an empty folder to a running MVP.
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All 25 Dev tools ideasBuild Idemcheck, not just read about it
You have the problem, the timing, the solution and the buyer. A plan opens how it makes money, the risk that kills it, and the kit that takes it to a running MVP.
Start building Idemcheck