OMP-0 — Freeze the terminal/transcript Native Chat parity contract #52

Parent epic: #21
Canonical design: #20

Objective

Create the authoritative before/after parity matrix for migrating OMP Native Chat from the current terminal/transcript path to ACP.

No ACP cutover may be approved until every required existing guarantee has an ACP equivalent that is equal or stronger.

Inventory current behavior

Trace the existing OMP path across:

• transcript discovery/resolution;
• OMP JSONL transcript decoder;
• transcript tail/watch behavior;
• PTY send/injection path;
• OMP status/readiness hooks;
• model discovery/switching;
• thinking/session options;
• slash-command handling;
• skills/autocomplete;
• image/file attachment;
• questions/approvals;
• local commands such as /usage;
• context/usage reporting;
• compaction boundaries;
• session resume/reopen;
• mobile;
• local/remote/runtime eligibility;
• error/failure rendering.

Use current tests and merged behavior as the contract, not historical assumptions.

Matrix

For each behavior record:

Classify each row:

• must preserve — user-visible/reliability contract;
• may improve — ACP can be stronger;
• legacy-only — preserve only for fallback sessions;
• not actually supported today — do not accidentally turn an aspiration into a parity blocker.

Reliability guarantees

Explicitly inventory:

• ordering;
• exactly-once visible messages;
• transcript tail deduplication;
• status/turn completion;
• cancellation/interrupt behavior;
• process death handling;
• session identity;
• restart/reopen;
• stale-event rejection;
• unsupported environment behavior.

ACP mapping

For every row identify whether the replacement is:

• standard ACP;
• shared Orca ACP translator;
• OMP ACP standard config/update;
• OMP extension/unknown ACP message;
• fallback-only behavior.

Do not add implementation in this issue.

Deliverable

Commit/update the parity matrix in #20 or a dedicated design doc and link each future migration issue to the rows it owns.

Acceptance criteria

• Every current OMP Native Chat feature has been audited.
• Every current reliability guarantee has been audited.
• Unsupported/aspirational features are separated from existing guarantees.
• ACP mapping is identified for each row.
• OMP-2 — Migrate OMP history, reasoning, tools, edits, usage, and timeline parity to ACP #54–OMP-5 — Make ACP the default OMP Chat path with safe legacy-session adoption and fallback #57 can reference concrete parity row IDs rather than vague “match old behavior” language.