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## What this does In a channel, people often run several unrelated conversations at once (separate threads). Today the agent treats the whole channel as one conversation, so unrelated threads share the same running session — their context bleeds together and independent tasks can step on each other. This change gives the agent a **separate session per thread** inside a channel. Direct messages stay as one conversation (unchanged). The channel is still the boundary for who is allowed in and what is visible — only the agent's working context is now split by thread. ## How it is turned on Off by default. Operators opt in with one setting: - `BUZZ_ACP_SESSION_POLICY=channel` — default, current behavior - `BUZZ_ACP_SESSION_POLICY=thread` — new per-thread behavior Being behind a flag means we can enable it for a few agents, watch how it behaves, and roll back instantly without a code change. ## Key design decisions - **Decide the thread once, up front.** When a message arrives we work out which thread it belongs to a single time and tag it. Everything after that (which line it waits in, which session runs it, what history it sees) uses that tag instead of re-guessing later, which avoids mismatches. - **Default stays identical to today.** Under the default setting a "thread" is just "the whole channel," so existing behavior and every existing test are unchanged. The new, riskier behavior is strictly opt-in. - **Give the agent only its thread's history.** On a reply the agent sees that thread's messages (including ones that did not mention it), not the whole channel transcript — less noise and smaller prompts. - **Don't let one channel use more memory than before.** More threads means more live sessions, so the existing per-channel limit now caps all of a channel's threads together — splitting into threads can't multiply how much work is held. ## Bugs found and fixed while iterating (from review) - **Same thread, two sessions.** If the worker already holding a thread's session was busy, a new message for that thread could start a *second* session on another worker and split its history. Now it waits for the right worker instead of forking. - **Interrupting the wrong thread.** A follow-up meant for thread A could interrupt thread B in the same channel. Interrupts now target the exact thread. - **Stuck thread after a crash.** If a thread's turn crashed, its slot wasn't cleared and stayed blocked for up to ~2 hours. It now clears right away and retries. - **Lost the original request.** When a thread was interrupted and then had to wait for a busy worker, only the follow-up was kept and the original request was dropped. The full request is now preserved on retry. - **Same thread seen as two.** Two spellings of the same thread id (upper/lower case) could be treated as different threads. Normalized so they count as one. ## Not in this PR - The desktop Settings toggle and rollout wiring for managed agents — https://github.com/block/buzz/pull/6909 - One pre-existing retry edge case (present today without this flag, unrelated to this change) — tracked separately so this PR stays focused. ## Testing The full `buzz-acp` test suite passes (830+ unit and integration tests), plus new focused tests for thread routing, session reuse, interrupt targeting, crash recovery, and request preservation. Behavior with the flag off is unchanged. --------- Signed-off-by: Salman Mohammed <smohammed@squareup.com> Signed-off-by: Leo <5faf251baee50ee6bcde338aef6acdd70bb3e60115664c2cd490d94a55dfc488@buzz.block.builderlab.xyz> Co-authored-by: Leo <5faf251baee50ee6bcde338aef6acdd70bb3e60115664c2cd490d94a55dfc488@buzz.block.builderlab.xyz>
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Session Model
You are one session of your agent identity — not the only copy. In channels, each thread gets its own independent conversation context, including a new thread rooted at a top-level mention. DMs stay one conversation, not separate sessions per thread. Multiple sessions of the same agent may be active in different channels or different threads in the same channel at the same time. Sessions share your core memory, your workspace on disk, relay access, and channel authorization. They do NOT share conversation context, in-progress reasoning, or in-context task state.
When a human references work "you" are doing in another channel or a sibling channel thread, that work belongs to a different session of you. Unless the human asks you to take it over or coordinate it from this session, leave execution with the owning session — answer from what you can verify (core memory, workspace files, relay messages) and assume the owning session has it handled.