Keep a warm bridge connection + ACP session across turns of one
conversation (gated by FUSION_CLAUDE_ACP_REUSE=1, default OFF), so
multi-turn lanes skip the cold bridge/claude spawn and session/new
round-trip and send only the latest-turn delta (buildResumePrompt).
A stable router indirection serves each turn's handlers.
Addresses the adversarial review of the reuse path:
- P0: a warm-child death routes failure to the CURRENT owner turn via
router.fail, so a reuse turn fails fast instead of hanging until the
30-min inactivity timeout.
- P1: eviction is cache-identity-aware (evictCachedAcpConn only deletes
the map key when it still points at the entry), so a concurrent cold
turn / stale close handler / idle timer can't evict or kill a newer
live entry's child.
- P1: an empty resume delta cold-starts instead of issuing an empty
prompt that could hang.
- P2: a per-turn token drops cross-turn stray updates on the shared
warm connection.
- The idle reaper is unref'd so it never pins the process.
Default OFF → the cold path is functionally unchanged (reviewer-verified).
Adds multi-turn tests: reuse skips spawn+session/new, flag-off spawns
fresh, fail-fast on warm-child death, empty-resume cold fallback.
346/346 pass, tsc clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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