Continue a Pi session checkpoint into a new Git commit

Traceability

• Spec ID: session-checkpoint-executor-poc
• Status: Implemented

Intent

Prove the smallest safe execution path that combines an immutable Git source checkpoint with one exact point in a coding-agent session. Given a repository, a base commit, a Pi session JSONL file, a session entry id, and a continuation prompt, Better Harness creates an isolated detached worktree, continues the Pi conversation from that entry, and records the resulting file changes as a new commit whose single parent is the resolved base commit.

The POC treats the Git commit and the Pi entry as a caller-supplied checkpoint pair. It records and revalidates both halves, but it cannot infer that the historical worktree used by the original session exactly matched the supplied commit. The resulting commit is kept reachable through a namespaced ref; no user branch, index, or working tree is switched or updated.

Acceptance Scenarios

• AC-1: harness-session-executor plan resolves the repository, base commit and tree, parses the Pi JSONL session without rewriting it, validates the selected entry's parent chain, and writes a versioned plan containing full immutable ids and SHA-256 digests. Planning does not create a worktree, commit, or ref.
• AC-2: harness-session-executor run --plan <file> --yes revalidates the plan digest, base commit/tree, source session digest, session identity, selected entry, and output-ref absence before invoking a model or mutating Git state.
• AC-3: execution forks the Pi JSONL into execution-owned metadata, selects the exact entry as the active leaf, and continues it inside a detached worktree at the base commit. The live Pi runtime exposes only repository-contained read, list, edit, and write tools; shell commands, extensions, skills, prompt templates, deletion, and paths through .git are unavailable. The runner also aborts after 64 tool calls or 15 minutes.
• AC-4: when the continuation changes files, the executor stages only the isolated worktree and creates a commit with exactly one parent (the base commit), deterministic provenance trailers, and a