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writing-tla-plus-specs

Use when a protocol, concurrent algorithm, or design needs a model-checked TLA+ or Alloy spec, or a TLC or Apalache trace needs reading. Not for choosing when to model: use validation-first-driven.

.devin/skills/writing-tla-plus-specs/SKILL.md

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# Writing TLA+ specs

## Contract

| Field | Bound contract |
|---|---|
| Trigger | A distributed protocol, concurrent algorithm, or system design needs an explicit-state or symbolic model check of its safety and liveness properties, or an existing TLC, Apalache, Quint, or Alloy run needs its counterexample read and acted on. |
| Authority | Reversible local: writes `.tla`, `.cfg`, `.qnt`, and `.als` files and the tool output directories they produce; rollback is deleting those files. No remote mutation. |
| Side effect | Spec and configuration files on disk, plus TLC's `states` directory and Apalache's `_apalache-out` directory. |
| Done | Every named invariant and temporal property either passes under a recorded bound and configuration, or has a counterexample trace mapped to a named design defect. |

## Inputs

A description of the system: its state variables, the actions that change them, and the properties that must hold. The tool pins from the grounded set: `tla2tools.jar` v1.7.4 (stable; v1.8.0 is a pre-release and the old TLA+ Toolbox GUI is declared unmaintained, so use the `tlaplus.vscode-ide` extension or the command line), Apalache v0.62.2 (download `apalache.zip` or `apalache.tgz` from GitHub Releases), Alloy v6.2.0 (standalone `.jar` from alloytools.org), Quint 0.32.0 (install per https://quint-lang.org/docs/getting-started). A JVM is required for TLC, Apalache, and Alloy. Optional: a state-space bound, a `ConstInit` operator that bounds constants for Apalache, and a Quint spec when the author prefers its surface syntax.