idempotent sync push replay duplicate message

Replaying the same push message twice should be a no-op, for free

Retrying a push after a timeout is only safe if replaying the same message twice does nothing extra -- the pack gets this from HLC equality, not a dedupe table.

This is one of the things Offline Sync Engine already handles. Let your app keep working with no internet, then merge the changes correctly when it reconnects.

Buy for $992 of 3 clean-room builds passed · full refund if it fails on your machine

Is this you?

A client that retries a push after a network timeout (not knowing whether the first attempt actually landed) needs replaying the same message to be harmless -- otherwise a retry after an ambiguous failure can double-apply a write, or a naive dedupe mechanism (tracking which message ids were already seen) adds a whole extra table and its own consistency problems to what should be a simple retry.

Why this one is easy to get wrong

Building an explicit dedupe table (message ids, a seen-before check) is the first idea that comes to mind for "don't apply the same thing twice," mirroring how webhook idempotency is usually solved -- it's not obvious without reading the storage model closely that a cell-based, HLC-stamped design can get the same guarantee from comparison logic that already has to exist for conflict resolution, with no extra table required.

What you get instead

Because a cell write only applies when its HLC is strictly greater than the stored cell's HLC, replaying an already-applied message compares equal on HLC and is a no-op by the same rule that resolves ordinary conflicts -- there's no separate mutation-id tracking or dedupe table anywhere in the design. Any push can be retried blindly, which is exactly the property a client recovering from an ambiguous network failure needs.

Source: ARCHITECTURE.md 'Core model: a synced cell store' — checkable in the pack you receive

How you actually use this

You don’t install a library or wire up an SDK. Your own coding agent builds the code in your project, and you keep it — no runtime dependency on us.

Step 1
Download and unzip
You get a folder: the docs that tell an agent what to build, a starting skeleton, and the test suite that decides when it's done.
Step 2
Open it in Claude Code or Cursor
Point your coding agent at the folder. Nothing to install, no account with us, no API key.
Step 3
Paste one prompt
The pack contains the exact prompt. Paste it as your first message and leave it alone — it works through the build itself, choosing a cheaper or stronger model per task.
Step 4
Run ./verify.sh
One command. It prints a pass or fail for every check. Green means the build is done — the same script we ran to produce the receipt on this page.

Typical build: about 12 minutes of your agent working, mostly unattended. Then you integrate the working module into your app the way you would any code you’d written yourself.

Why you can believe this

2 of 3 runs passed

We ran this pack from an empty folder 3 times and published exactly what happened — every check, the model, the token cost, the wall time. Not a testimonial, and not our opinion: the same verify.sh you run yourself. Read the full receipt →

Buy for $99 14-day refund if verify.sh fails →