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type: adr id: 0046-isolation-axis-model status: accepted created: 2026-06-05 updated: 2026-06-06 supersedes: superseded_by:

ADR-0046: A task implementing a spec gets a worktree+branch; ad-hoc work stays in-place

Context

corpus specified worktree/branch/merge only inside the parallel-decomposition coordination record. The single-task load path (AGENTS.md → task.md → implement.md) carried no isolation signal, so "implement this spec" gave an agent no reason to leave the base branch — and the per-kind templates gave the inverse cue (the spec-bearing kinds had no Branch field while a few others did). The fix is one small rule on the load path, not a new subsystem.

Decision

Isolation is a binary, decided by reading the task frame; a frame isolation: field overrides it, and it is orthogonal to parallel_group (a single non-parallel task can still need a worktree):

• A code task implementing a spec or audit-remediation (it has a source: *.md / audit-derived spec) → a worktree + branch off the base. The branch is named for what it implements: corpus/<spec-slug> for a whole spec, corpus/<spec-slug>/<task-slug> for one obligation or a fan-out worker (one grammar — single-task and parallel reconcile). base: records the merge target (default main; the dev's HEAD when handed off mid-branch). A spec is implemented off the base, never on it.
• Anything else — a quick ad-hoc edit with no spec, a doc/source-only authoring task, a read-only review → in-place. No ceremony; the absence of a source artifact is the signal.

This lives where the agent reads it: the ## Isolation section of implement.md (both twins) and the isolation:/base: fields on the task.md frame, with a one-line trigger in the starter-kit/AGENTS.md startup. Merge + cleanup are the orchestration lifecycle at worker-count 1 (the cross-worker disjointness condition is vacuous for one writer); an in-flight worktree is recorded under .corpus/status/worktrees/.

Refines 0039 (which scoped isolation to parallel decomposition only) and operationalizes the single-fork clause of 0010.

Alternatives considered

Alternative	Why rejected
Leave isolation specified only for parallel decomposition	The status quo — the single-task path has no signal, so a spec lands on the base by default.
A multi-rung axis / step-ladder / per-task_kind matrix	Over-built for a binary; the extra rungs had no real occupant and didn't cover every kind. The binary + an explicit override is sufficient.
Key isolation off parallel_group	Conflates two orthogonal axes; a single non-parallel spec task still needs a worktree.

Consequences

• An agent can decide worktree-or-not deterministically with no runtime, and name the branch for the spec it implements; a quick ad-hoc edit stays zero-ceremony.
• NO RUNTIME means nothing enforces it — an agent can ignore the rule and land a spec on the base. This is specification completeness, the same soft-control limit as every corpus gate; a future launcher reads the rule.
• No canonical closed set changes; the merge gate and write-surface model are untouched.

Status

Accepted (v0.1).

Affected obligations / constraints

• Adds: the isolation binary + the isolation:/base: frame fields + the branch-naming convention; isolation is orthogonal to parallel_group.
• Modifies: implement.md (a ## Isolation section), the task.md frame, the starter-kit/AGENTS.md startup.
• Refines: 0039; operationalizes 0010.
• Does NOT change: the merge gate, the write-surface conflict model, any canonical closed set.

Ledger note (2026-06-11): refined by ADR-0068 (a change-plan wave decomposes into worktree-isolated tasks; isolation rule unchanged).