Adding a harness

The adapter boundary is the extension point. A new harness means one new file, registration in two places, and tests:

1. src/adapters/<name>.ts — export a HarnessAdapter. The interface is in src/adapters/types.ts; opencode.ts is the smallest example, codex.ts the most complete. You implement:
   • name, versionCommand, supportedVersions, pathPrepend — identity, and the probe sonata runs to read the installed version
   • plan(input) → a LaunchPlan: the bash script, whether the harness is interactive, and effortHonoured (required — whether the plan passes a pinned <key>@<effort> level through). Optional on the plan: canWriteReport (false when this configuration cannot write report.md at all) and silentUntilExit (true when nothing reaches the pane until the run exits)
   • canPromptForApproval — whether the harness can stop and ask a human
   • promptPatterns / describePrompt — how a pending approval looks
   • approveKeys — the tmux key sequences that answer one
   • usage(query) — required: the tokens a finished run spent, read from the harness's own store, or unobservable with a reason. Never guessed, never zero
   • optional: health(env) — runtime checks beyond "is it installed"; update — how sonata init offers to bring the harness up to date; fallbackReportFile — a file the harness writes its own final message to, used as a report fallback before the pane
2. src/adapters/index.ts — register it.
3. src/config.ts — add the name to KNOWN_HARNESSES.
4. tests/adapters/<name>.test.ts — follow an existing adapter's tests.

Optionally, src/detect.ts for sonata init discovery. It probes opencode, pi, codex (via codex app-server's model/list) and reasonix; the claude harness is the host and is never offered for discovery. Note the id shape — opencode, pi and reasonix take provider/model ids, while codex takes a bare id.

Probe the real binary before you write the adapter. Every adapter bug found so far was invisible in the documentation and obvious on the first real run: OpenCode silently eating a positional argument, codex rejecting a flag that its own exec accepts, both harnesses printing approval prompts that matched none of the patterns written for them. If you claim a harness prints something, capture it into tests/fixtures/panes/ and test against that.