# Agent Note: Default local instruction overlay

Status: implemented

English | [中文](2026-07-21-local-instruction-overlay.zh.md)

## Problem

Personal, git-ignored guidance (`AGENTS.local.md` / `CLAUDE.local.md`) is a Claude Code convention for per-developer overrides that are deliberately not committed. The [agent-instructions plugin](2026-06-24-workspace-context.md) loaded only one candidate per directory, so a `.local.` name could only be reached by adding it to `instructionFileCandidates`, where — because a directory has one winner — it would *shadow* the committed base file instead of supplementing it. That inverts the additive "base plus personal overlay" model the names evoke, and it was off by default.

## Decision

The plugin loads a second, independent candidate list per project directory. `localInstructionFileCandidates` defaults to `['AGENTS.local.md', 'CLAUDE.local.md']` and is resolved with the same same-directory validation as `instructionFileCandidates`. In every project directory from the root to the session cwd, the plugin loads the base candidates and then, additively, the local candidates; the local files are ordered after the base files so their guidance takes precedence within the byte budget. Both lists load in full under [per-directory content dedup](2026-07-21-instruction-load-all-dedup.md). An empty `localInstructionFileCandidates` disables the overlay.

The default lives in the plugin `Config` schema rather than a product `cordis.yml`, so every embedder (TUI, ACP, headless) reads `.local.` files consistently and a deployment overrides or disables the behavior in one place. This is symmetric with the plugin-owned `instructionFileCandidates` default.

The fixed user-global `$DSH_HOME/AGENTS.md` has no local overlay and stays base-only.

## Independent scopes per candidate

The base and local candidates in one directory must stay independent across baseline freezing, the pending window, the version cache, and reconciliation, so a change to one never suppresses the other. Each `(directory, candidateName)` pair is its own scope key — see [per-candidate scope keys](2026-07-21-instruction-load-all-dedup.md), which replaced the earlier base/local tier sentinel. Discovery iterates the base list and then the local list in each project directory, `reconcileInstructionContext` enumerates every configured candidate per directory, and `probeScopeInstruction` decodes the candidate name to read exactly that file. The model-facing prompt derives its human directory label from the file display path, so the scope key never reaches the model.

## Alternatives considered

**Higher-priority first-wins (`.local.` loaded instead of the base file).** Rejected: a personal overlay that replaces the committed file drops shared project guidance whenever the overlay exists, which is the opposite of the additive Claude Code model.

**Keep it opt-in through `instructionFileCandidates`.** Rejected: one directory has a single winner, so a `.local.` name added to that list shadows the base file rather than supplementing it. The packages guidance to keep opt-ins out of shipped defaults is outweighed here by strong prior art and the user-facing expectation that `.local.` files are always read.

**Default at the product `cordis.yml` level instead of the plugin schema.** Rejected: it would enable `.local.` only for whichever entry point remembered to opt in, splitting behavior across TUI/ACP/headless and duplicating a value that belongs beside the existing candidate default.

**Reuse the bare directory as the scope key for base and local files.** Rejected: base and local files in one directory would collide in every scope-keyed map, so a change to one would suppress or overwrite the other. A distinct scope key per candidate keeps them independent without widening the persisted metadata shape.

**Extend the overlay to the user-global scope.** Deferred: `$DSH_HOME` is a single fixed `AGENTS.md` with no committed base to supplement, so it stays base-only until a concrete need appears.

## Consequences

`.local.` guidance is read by default across all products with no per-deployment configuration, matching neighboring tools. Each project directory can contribute a durable scope per existing candidate rather than one, so dynamic discovery, edits, and removals reconcile the base and local files independently. The scope key is now [per-candidate](2026-07-21-instruction-load-all-dedup.md); `dsh-session` keeps no compatibility promise for older sessions, so this is a free change. The user-global scope remains base-only, recorded as a Known Limitation in the package README.
