Declare plan support for ACP agents and translate it via session modes

[bradhallett]

Human comments

I hit this running omp as an ACP provider day-to-day: plan-then-approve works on first-party providers and silently doesn't exist for ACP threads, which made the orchestration workflows I use bb for (scouts → review → implement) awkward to gate. The slop-cop repro matches what I saw before filing.

Two things I'd flag for reviewers: the capability only un-gates what the agent actually advertises (no-modes agents get byte-identical passthrough — that was a hard requirement for me, not a nice-to-have), and rewind is deliberately left undeclarable rather than approximated. Built agent-assisted; I reviewed the wire-shape decisions myself.

What was wrong

The plan action and edit-past-message gates were already capability-driven (composerActions: ["plan"], capabilities.fork), but ACP agents could not declare either: the ACP provider hardcoded composerActions to [] and typed AcpAgentDefinition.fork as none|tip, so every ACP agent was excluded by declaration shape rather than by capability. The bridge also dropped promptMode and discarded current-mode updates as noise. Reproduced independently by the slop-cop triage at base 43e3fc519 (report); fixes #3019.

What changed

• AcpAgentDefinition gains supportsPlan (mirroring supportsManualCompaction), threaded through the customAgents setting and mapped to composerActions: ["plan"] in the declaration; acp-omp declares it.
• The ACP bridge now reads the session modes agents advertise (session/new, session/load, session/fork results and current_mode_update notifications) and, when the agent advertises a plan mode, drives it with session/set_mode on plan turns, strips the /plan command mention from the prompt, and returns the session to its initial mode afterward; agents that report no modes keep byte-identical text passthrough.
• Session rewind stays undeclarable for ACP agents: ACP v1 session/fork is tip-only with no fork-point selector and reports no per-turn checkpoints, so no shipped or custom ACP agent can honestly declare checkpoint rewind today.

How you verified

pnpm exec turbo run typecheck test lint --continue plus pnpm format:check. provider-bridge-acp typecheck clean, all 317 tests pass, including the new ones: declaration mapping (composerActions: ["plan"] only for agents declaring it), wire modes/current_mode_update parsing, and bridge plan-turn translation (set_mode "plan" precedes session/prompt with the mention stripped; a no-modes agent gets no set_mode and untouched text; the mode resets to the initial mode after the turn). Server suites that consume provider declarations pass solo (first-party-provider-plugins, plugin-install, plugin-update). Remaining failures are pre-existing on main f6868ad0c, each verified by stash-rerun on a clean tree: secret-storage concurrent-creators, server install-machine-script (macOS Abort trap: 6 on the spawned host-daemon), host-workspace workspace-diff/provisioning, and 183 files flagged by format:check from the pinned oxfmt 0.64.0 at head (formatter default drift); the new code matches the dominant committed style of the files it touches.

Fixes #3019

AGENT GENERATED

[willadamskeane]

I tested this against the shipped OpenCode ACP provider and found two follow-up gaps: acp-opencode is not opted into supportsPlan, and the host-daemon protocol is not advanced even though an older daemon will silently ignore the now-exposed promptMode. I opened a minimal four-file patch directly against this PR branch: bradhallett#1. Focused provider/contract tests and typechecks pass (133 tests total).