A rule part your parser doesn't implement should error, never disappear

Parsing FREQ, INTERVAL and BYDAY while silently dropping BYSETPOS or WKST produces availability the caller believes is restricted and the system never actually restricted.

This is one of the things Scheduling & Availability Core already handles. Bookings and availability that survive timezones, repeating events and daylight saving — without double-booking.

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Is this you?

A parser that recognizes the common RRULE parts and simply ignores anything it doesn't implement -- BYSETPOS, WKST, BYWEEKNO -- produces a rule that looks accepted and behaves as if the unsupported part was never written. The caller believes their rule is scoped exactly as specified; the system silently computed something broader or different, and nothing in the response signals the gap.

Why this one is easy to get wrong

Parsing the well-known, high-frequency RRULE parts (FREQ, INTERVAL, BYDAY, COUNT) and quietly skipping over an unrecognized token is the natural behavior of a parser built incrementally against real-world examples -- it never crashes, it never errors, and every rule that only uses the common parts works exactly as expected, which is most rules most of the time.

What you get instead

Every unsupported part -- BYWEEKNO, BYYEARDAY, BYHOUR, BYMINUTE, BYSECOND, RSCALE/SKIP, and sub-day FREQ values -- is rejected at parse time with a named error, never silently dropped. WKST specifically is called out as non-cosmetic: RFC 5545's own published example shows the same rule producing different dates under WKST=MO versus WKST=SU, which the harness asserts directly rather than treating WKST as an ignorable detail.

Source: ARCHITECTURE.md 'RRULE subset (decided -- and rejections are loud)' — 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 22 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

3 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 →