fix: signal compression boundary to context engine#16306
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When _compress_context rotates session_id (compression split), fire on_session_start(new_sid, boundary_reason="compression", old_session_id=<old>) on the active context engine. Plugin engines (e.g. hermes-lcm) use this to preserve DAG lineage across the rollover instead of re-initializing fresh per-session state. Built-in ContextCompressor.on_session_start accepts **kwargs and ignores them — no behavior change for default users. Closes hermes-lcm#68 symptom: after Hermes compressed and minted a new physical session, LCM was treating the split as a fresh /new and losing continuity (compression_count: 1, store_messages: 0, dag_nodes: 0). Credit: @Tosko4 (PR #13370) — minimized scope to the boundary_reason signal only; the broader session-lifecycle refactor will be taken in separate PRs if justified by concrete plugin need.
This was referenced Apr 27, 2026
This was referenced Apr 27, 2026
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External context engine plugins (e.g. hermes-lcm) now receive
boundary_reason="compression"when Hermes rotates session_id during context compression, so they can preserve DAG / store lineage across the split instead of treating it as a fresh/new.Changes
run_agent.py(+16): after the compression-split rotatessession_id, fireself.context_compressor.on_session_start(new_sid, boundary_reason="compression", old_session_id=<old>). Guarded withhasattr+try/exceptso built-inContextCompressor(which accepts**kwargsand ignores them) is unaffected.tests/run_agent/test_compression_boundary_hook.py(new): three tests — boundary hook fires on rollover, no hook whensession_dbis absent (no rotation happens), plugin exception does not break compression.Root cause
Closes hermes-lcm#68 symptom: after Hermes compressed and minted a new physical session, LCM saw the new session_id and reset per-session state, giving
compression_count: 1, store_messages: 0, dag_nodes: 0. The built-in compressor had no way to tell plugins that the rotation was a compression-boundary continuation rather than a fresh user-initiated/new. This adds that signal.Validation
tests/run_agent/test_compression_boundary_hook.py(new)tests/run_agent/test_compression_persistence.py+tests/cli/test_manual_compress.pyCredit
Salvaged from @Tosko4's PR #13370 — scope minimized to the
boundary_reasonsignal only. The broader session-lifecycle refactor (new_transition_context_engine_sessionhelper, off-thread memory flush on/new,HERMES_SESSION_IDcontextvar,conversation_session_idfield onProcessSession,inspect.signatureintrospection forfocus_topic, plugin discovery re-ordering) will be taken in separate PRs if justified by concrete plugin need. Contributor authorship preserved on the fix commit.