What causes "No subagent session found with id {session_id}" in zed-industries/zed?

Use create_subagent_thread to start a fresh subagent instead of resuming when the id is not from the current process.

No subagent session found with id {session_id}

Error message

No subagent session found with id {session_id}

What it means

resume_subagent_thread looks the id up in agent.sessions and fails when it is absent: the subagent session was either never created in this process or has already been closed.

Source

Thrown at crates/agent/src/agent.rs:3101

            session = parent_thread_entity.read(cx).id().to_string(),
            subagent_session = session_id.to_string(),
            depth,
            is_resumed = false,
        );

        self.prompt_subagent(session_id, subagent_thread, acp_thread)
    }

    pub(crate) fn resume_subagent_thread(
        &self,
        session_id: acp::SessionId,
        cx: &mut App,
    ) -> Result<Rc<dyn SubagentHandle>> {
        let (subagent_thread, acp_thread) = self.agent.update(cx, |agent, _cx| {
            let session = agent
                .sessions
                .get(&session_id)
                .ok_or_else(|| anyhow!("No subagent session found with id {session_id}"))?;
            anyhow::Ok((session.thread.clone(), session.acp_thread.clone()))
        })??;

        let depth = subagent_thread.read(cx).depth();

        if let Some(parent_thread_entity) = self.thread.upgrade() {
            telemetry::event!(
                "Subagent Started",
                session = parent_thread_entity.read(cx).id().to_string(),
                subagent_session = session_id.to_string(),
                depth,
                is_resumed = true,
            );
        }

        self.prompt_subagent(session_id, subagent_thread, acp_thread)
    }

View on GitHub (pinned to bc538def45)

Solutions

Use create_subagent_thread to start a fresh subagent instead of resuming when the id is not from the current process.
Verify the id exists in the session list before attempting resume.
Persist enough context to recreate the subagent rather than relying on in-memory session continuity.

Defensive patterns

Strategy: validation

Validate before calling

// only resume ids that still exist in this process
let resumable = session_ids_from_current_process.contains(&session_id);
if !resumable {
    // create a new subagent and re-feed context instead
}

Try / catch

match env.resume_subagent_thread(session_id.clone(), cx) {
    Ok(handle) => handle,
    Err(e) if e.to_string().contains("No subagent session found") => {
        env.create_subagent_thread(label, cx)? // fall back to fresh subagent
    }
    Err(e) => return Err(e),
}

Prevention

Do not persist subagent session ids across restarts and assume they resume.
Track subagent ids from the same agent instance you resume them against.

When it happens

Trigger: Calling resume_subagent with an id from a previous app run, from a different agent instance, or after the subagent finished and its session entry was removed.

Common situations: Persisted plans referencing subagent session ids after a restart; cross-workspace resume attempts; resuming already-completed subagents.

Related errors

AI-assisted analysis of zed-industries/zed@bc538def45 (2026-08-16). Data as JSON: /api/errors/b79c46d4316565f6. Report an issue: GitHub.