fix: prevent duplicate worker spawns from parallel tool calls

src/agent/channel_dispatch.rs

@@ -324,6 +324,25 @@ async fn check_worker_limit(state: &ChannelState) -> std::result::Result<(), Age
     reserve_worker_slot_local(active_worker_count, &state.channel_id, max_workers)
 }
 
+/// Reject spawn if an active worker already has the same task.
+///
+/// This prevents duplicate workers when the LLM emits multiple spawn_worker
+/// calls in a single response and one fails then gets retried on the next
+/// depth.
+async fn check_duplicate_task(
+    state: &ChannelState,
+    task: &str,
+) -> std::result::Result<(), AgentError> {
+    let status = state.status_block.read().await;
+    if let Some(existing_id) = status.find_duplicate_worker_task(task) {
+        return Err(AgentError::DuplicateWorkerTask {
+            channel_id: state.channel_id.to_string(),
+            existing_worker_id: existing_id.to_string(),
+        });
+    }
+    Ok(())
+}
+
 /// Spawn a worker from a ChannelState. Used by the SpawnWorkerTool.
 pub async fn spawn_worker_from_state(
     state: &ChannelState,
@@ -332,8 +351,9 @@ pub async fn spawn_worker_from_state(
     suggested_skills: &[&str],
 ) -> std::result::Result<WorkerId, AgentError> {
     check_worker_limit(state).await?;
-    ensure_dispatch_readiness(state, "worker");
     let task = task.into();
+    check_duplicate_task(state, &task).await?;
+    ensure_dispatch_readiness(state, "worker");
 
     let rc = &state.deps.runtime_config;
     let prompt_engine = rc.prompts.load();
@@ -466,8 +486,9 @@ pub async fn spawn_opencode_worker_from_state(
     interactive: bool,
 ) -> std::result::Result<crate::WorkerId, AgentError> {
     check_worker_limit(state).await?;
-    ensure_dispatch_readiness(state, "opencode_worker");
     let task = task.into();
+    check_duplicate_task(state, &task).await?;
+    ensure_dispatch_readiness(state, "opencode_worker");
     let directory = std::path::PathBuf::from(directory);
 
     let rc = &state.deps.runtime_config;

src/agent/status.rs

@@ -263,6 +263,22 @@ impl StatusBlock {
         self.active_workers.iter().any(|w| w.id == worker_id)
     }
 
+    /// Check if an active worker already exists with a matching task.
+    ///
+    /// The status block stores OpenCode tasks with a `[opencode] ` prefix, so
+    /// comparisons strip that prefix before matching. Returns the existing
+    /// worker's ID if found.
+    pub fn find_duplicate_worker_task(&self, task: &str) -> Option<WorkerId> {
+        let normalized = task.strip_prefix("[opencode] ").unwrap_or(task);
+        self.active_workers.iter().find_map(|worker| {
+            let existing = worker
+                .task
+                .strip_prefix("[opencode] ")
+                .unwrap_or(&worker.task);
+            (existing == normalized).then_some(worker.id)
+        })
+    }
+
     /// Get the number of active branches.
     pub fn active_branch_count(&self) -> usize {
         self.active_branches.len()

src/error.rs

@@ -187,6 +187,14 @@ pub enum AgentError {
     #[error("max concurrent workers ({max}) reached for channel {channel_id}")]
     WorkerLimitReached { channel_id: String, max: usize },
 
+    #[error(
+        "duplicate worker task on channel {channel_id}: worker {existing_worker_id} is already running this task"
+    )]
+    DuplicateWorkerTask {
+        channel_id: String,
+        existing_worker_id: String,
+    },
+
     #[error("worker state transition failed: {0}")]
     InvalidStateTransition(String),
 

src/tools/spawn_worker.rs

@@ -147,6 +147,19 @@ impl Tool for SpawnWorkerTool {
         let readiness = self.state.deps.runtime_config.work_readiness();
         let is_opencode = args.worker_type.as_deref() == Some("opencode");
 
+        // Reject if an active worker already has the same task. This prevents
+        // duplicate workers when the LLM emits multiple spawn_worker calls in
+        // a single response and one fails/retries.
+        {
+            let status = self.state.status_block.read().await;
+            if let Some(existing_id) = status.find_duplicate_worker_task(&args.task) {
+                return Err(SpawnWorkerError(format!(
+                    "a worker is already running this task (worker {existing_id}). \
+                     Wait for it to complete or cancel it before spawning another."
+                )));
+            }
+        }
+
         let worker_id = if is_opencode {
             let directory = args.directory.as_deref().ok_or_else(|| {
                 SpawnWorkerError("directory is required for opencode workers".into())