ADR-0008 — Task identity is the hash of its inputs¶

• Status: Accepted · Date: 2026-09-06 · Promotes: notes/transcript.md §9.5

Context¶

The transcript describes a cross-repository task DAG parsed from package.json, Cargo.toml, Makefile, and Taskfile.yaml, using "CAS timestamps" to skip unchanged targets. Timestamps are the wrong key: mtime changes on checkout, restore, and touch without the content changing, and fails to change on same-second writes.

Decision¶

A target's identity is the hash of its inputs — source content, dependency output hashes, the command line, and the declared environment — not a timestamp. A target is skipped when the CAS already holds an output for that input hash. Skips are explainable: the system can say which input changed.

Alternatives rejected¶

• Timestamp comparison. Both false-negative and false-positive, and unusable across machines, containers, or a sync mesh where mtimes are meaningless.
• Always rebuild. Correct and unusable at repository scale.
• Delegate caching to each tool. No cross-repository graph, no shared cache, and every tool's cache is invalidated differently.

Consequences¶

Input declaration must be complete, and an undeclared input produces a wrong cache hit — the classic build-system failure. Undeclared-input detection (sandboxed execution, or filesystem tracing) is therefore part of the epic, not an extra. Cache entries are CAS objects and share its retention policy. Cycles must be reported with the cycle, not merely detected.