principle-separate-before-serializing-shared-state from edbertchan/catstack is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
Apply when concurrent actors might write to the same file, branch, key, or state object. Eliminate the sharing first; serialize structurally only when one shared writer is a real invariant.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Separate Before Serializing Shared State
When concurrent actors might share mutable state, first ask whether they truly need the same mutable object. If not, eliminate the sharing. When sharing is real, enforce serialization structurally: lockfiles, sequential phases, exclusive ownership. Instructions and conventions are not concurrency control.
**Why:** Concurrent writes to shared state create race conditions that are intermittent, hard to reproduce, and expensive to debug. Telling agents or goroutines to "take turns" does not work.
**Pattern:**
1. **Identify shared mutable state** (files both read and write, branches both push to, APIs both define and consume).
2. **Default: eliminate the shared write target.** Ask: do these actors need one canonical object, or are they publishing independent facts? Give each actor its own owned file, key, branch, or state directory, and merge only at the read/reporting boundary. Two workers writing their own `lastX` field into one `state.json` is still shared mutation; `indexer-state.json` + `metrics-state.json` is not.
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The line at the top of this page pins the label git:20260825.847374d, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
GET https://markdownregistry.com/api/v1/artifacts/art_oltygzlx2ux5wtis
GET https://markdownregistry.com/api/v1/resolve?ref=edbertchan/catstack/principle-separate-before-serializing-shared-state
GET https://markdownregistry.com/api/v1/blob/c99232f6f8521cede46b4dbf91be38c825a094b59195aa12e67d01b33cb910fb
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
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