What causes "Session not found" in zed-industries/zed?

Only enqueue turns after session creation has resolved

Session not found

Error message

Session not found

What it means

run_turn looks session_id up in agent.sessions to obtain the Thread and AcpThread entities; a miss logs 'Session not found in run_turn' and returns a ready Err task — the per-turn closure f that would start generation is never invoked.

Source

Thrown at crates/agent/src/agent.rs:2147

        self.0
            .update(cx, |this, cx| this.load_thread(id, project, cx))
    }

    fn run_turn(
        &self,
        session_id: acp::SessionId,
        cx: &mut App,
        f: impl 'static
        + FnOnce(Entity<Thread>, &mut App) -> Result<mpsc::UnboundedReceiver<Result<ThreadEvent>>>,
    ) -> Task<Result<acp::PromptResponse>> {
        let Some((thread, acp_thread)) = self.0.update(cx, |agent, _cx| {
            agent
                .sessions
                .get_mut(&session_id)
                .map(|s| (s.thread.clone(), s.acp_thread.clone()))
        }) else {
            log::error!("Session not found in run_turn: {}", session_id);
            return Task::ready(Err(anyhow!("Session not found")));
        };
        log::debug!("Found session for: {}", session_id);

        let response_stream = match f(thread, cx) {
            Ok(stream) => stream,
            Err(err) => return Task::ready(Err(err)),
        };
        Self::handle_thread_events(
            response_stream,
            acp_thread.downgrade(),
            Some(self.clone()),
            cx,
        )
    }

    fn handle_thread_events(
        mut events: mpsc::UnboundedReceiver<Result<ThreadEvent>>,
        acp_thread: WeakEntity<AcpThread>,

View on GitHub (pinned to bc538def45)

Solutions

Only enqueue turns after session creation has resolved
On this error, recreate the session and resend the turn
Serialize session teardown with pending turn dispatch so turns cannot race removal

Defensive patterns

Strategy: try-catch

Validate before calling

// Only enqueue turns for sessions the agent actually holds
if !agent.has_session(&session_id) {
    let session = agent.create_session(project, cx).await?;
    session_id = session.id;
}
agent.run_turn(session_id, cx, move |thread, cx| thread.send(prompt, cx))

Try / catch

match run_turn(...).await {
    Err(error) if error.to_string() == "Session not found" => {
        // Recreate the session and replay the turn once.
        let session = agent.create_session(project, cx).await?;
        agent.run_turn(session.id, cx, replay_closure).await
    }
    other => other,
}

Prevention

Enqueue turns only after session creation resolves
Drop queued turns when a session is torn down instead of letting them race
On reconnect, rebuild the session map before resuming the conversation

When it happens

Trigger: Sending a chat turn for a session id not present in the in-memory sessions map: never created in this agent, removed after ending, or stale after an agent restart.

Common situations: Client reconnect after agent restart while reusing pre-restart ids; session ended concurrently with a queued turn; turn dispatched during session teardown.

Related errors

AI-assisted analysis of zed-industries/zed@bc538def45 (2026-08-16). Data as JSON: /api/errors/743f61c5802c77a6. Report an issue: GitHub.