Entry 4

The write path, and an init that nearly ate hellbender's skill

2026-09-20cliskilladoption

Filescrates/cairns/src/main.rscrates/cairns/templates/SKILL.mdcrates/cairns-core/src/date.rs

new, init and the skill template close the write path. new refuses an area cairns.toml does not declare - naming the ones it does, since the useful part of that error is the list - and regenerates the index on the way out, so the index is never stale in the window between creating an entry and remembering to run index.

This entry was created with it.

Dates, in two places for one reason

cairns-core computes a civil date from a Unix timestamp with Howard Hinnant's civil_from_days, about ten lines, which keeps the crate dependency-free and wasm32-clean. The binary does not use it for new: it asks jiff for the local date, because an entry written at eleven at night should not be dated tomorrow, and getting a local date right means timezone data the core has no business carrying.

The arithmetic was tested against the epoch, a leap day, and an arbitrary timestamp. The arbitrary one failed - and the code was right, the expectation was wrong. Worth recording only because the reflex on a red test is to look at the code, and the leap day passing first time was the signal that the conversion was sound.

init froze hellbender's slugs, exactly as entry 2 asked

Adoption on a clean clone, with its fifteen areas declared:

$ cairns check          # before
... 13 problems
$ cairns init
froze 13 slugs - these entries keep the names they were published under
$ cairns check          # after
ok

Each of the thirteen gained a slug: line pinning the name it was published under, so the two entries whose titles had drifted and the eleven cut mid-word by the old 60-character rule all keep their URLs. New entries get the better derivation. Adoption preserves rather than improves, which is the only way it can be safe.

It also destroyed the thing it was adopting

The first working init rewrote .claude/skills/worklog/SKILL.md - 58 lines deleted - and hellbender's skill is not a generated file. It carries the rules that make that log what it is: every claim locatable by a VA against HELLBEND.EXE at 0x00400000, a format entry unfinished until the matching docs/formats/ page exists. All of it gone, in a command whose entire purpose is to help a project adopt the tool.

Worse, it announced the opposite:

wrote .claude/skills/worklog/SKILL.md (project section preserved)

The flag behind that message was "a file was already here", not "its project section survived". Two separate failures - a destructive default, and a message asserting the thing that had just not happened - and the second is the one that would have let it go unnoticed, because the output read like success.

init now leaves an existing skill alone unless it carries the <!-- cairns:project --> marker, and says why:

kept .claude/skills/worklog/SKILL.md - it has no <!-- cairns:project --> marker.
Everything below that marker is what init preserves, so put it above the parts
this project wrote and run init again.

With the marker added, regeneration rewrites the generic half from the template with the project's own areas in it and returns the project section verbatim. Both paths are now checked against a fresh hellbender clone.

The general rule this is an instance of: a command that writes into a repo it did not create must default to refusing, not to overwriting, and must never describe what it wishes it had done.