Condenses the substance of PRs #16453, #17453, #16451, #17600, and #13373
into a minimal generic host contract that external context engine plugins
(e.g. hermes-lcm) need to integrate cleanly. Drops scaffolding that
duplicated existing infrastructure or had marginal value.
Five concrete changes:
1. `_transition_context_engine_session()` on AIAgent — generic lifecycle
helper that fires on_session_end → on_session_reset → on_session_start
→ optional carry_over_new_session_context. Engines implement only the
hooks they need; missing hooks are skipped. Built-in compressor keeps
its existing reset-only behavior because callers default to no
metadata. `reset_session_state()` now optionally accepts
previous_messages / old_session_id / carry_over_context and delegates
to the transition helper when provided. (#16453)
2. `conversation_id` passed to `on_session_start()` — both the
agent-init call site and the compression-boundary call site now
forward `self._gateway_session_key` so plugin engines have a stable
conversation identity that survives session_id rotation (compression
splits, /new, resume). The key already existed on AIAgent; it just
wasn't reaching engines. (#16453)
3. Canonical cache buckets forwarded to engines — the usage dict passed
to `update_from_response()` now includes input_tokens, output_tokens,
cache_read_tokens, cache_write_tokens, and reasoning_tokens on top of
the legacy prompt/completion/total keys. Engines can make decisions on
cache-hit ratios and reasoning costs instead of only aggregates. ABC
docstring updated. (#17453)
4. Plugin-registered context engines visible in the picker —
`_discover_context_engines()` in plugins_cmd.py now also includes
engines registered via `ctx.register_context_engine()` from plugin
manifests, deduplicating by name so repo-shipped descriptions win on
collision. (#16451)
5. `_EngineCollector.register_command()` — context engines using the
standard `register(ctx)` pattern can now expose slash commands (e.g.
`/lcm`). Routes to the global plugin command registry with the same
conflict-rejection policy regular plugins use (no shadowing built-ins,
no clobbering other plugins). Previously these calls hit a no-op and
the slash commands silently never appeared. (#17600)
Dropped from the original 5 PRs:
- Compression boundary signal (`boundary_reason="compression"`) from
#16453 — already on main at `agent/conversation_compression.py:412-424`,
landed via the bg-review extraction.
- `discover_plugins()` before fallback in run_agent.py from #16451 —
redundant: `get_plugin_context_engine()` already routes through
`_ensure_plugins_discovered()` which is idempotent.
- Runtime identity diagnostics method + helpers from #13373 (+251 LOC) —
operators can already read engine state via `engine.get_status()`;
the diagnostics view added marginal value relative to its surface area.
- The 553-LOC slash-command machinery from #17600 — replaced with a
20-LOC `register_command` method on the collector that reuses the
existing plugin command registry instead of building a parallel one.
Net: ~215 LOC of host-contract changes + 282 LOC of focused tests, vs
~1,176 LOC across the original 5 PRs.
Co-authored-by: Tosko4 <1294707+Tosko4@users.noreply.github.com>
Closes#16453.
Closes#17453.
Closes#16451.
Closes#17600.
Closes#13373.
Related: stephenschoettler/hermes-lcm#68.
9b5dae17a5