fix(broker): log the whole error chain when a worker fails to spawn

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - `fleet spawn` and SDK placement no longer report `dispatchState: "dispatched"` for a spawn that was placed but never sent. Only `dispatched_node_id`, or a status the node moved, counts as dispatch evidence, so `fleet spawn` and `fleet spawn-status` agree.
+- When a broker worker fails to spawn, the log now includes the underlying OS error, such as a missing binary, instead of only `failed to spawn worker`.
 
 ## [13.4.1] - 2026-10-12
 

crates/broker/src/runtime/relaycast_events.rs

@@ -1125,18 +1125,37 @@ pub(super) async fn spawn_worker_from_request(
             if msg.contains("already exists") {
                 tracing::debug!(child = %name, "agent already spawned via SDK, skipping duplicate relaycast WS spawn");
             } else {
-                tracing::error!(child = %name, error = %e, "failed to spawn worker via relaycast WS");
-                eprintln!("[agent-relay] failed to spawn '{}': {}", name, e);
+                let detail = spawn_failure_detail(&e);
+                tracing::error!(child = %name, error = %detail, "failed to spawn worker via relaycast WS");
+                eprintln!("[agent-relay] failed to spawn '{}': {}", name, detail);
             }
             Err(e)
         }
     }
 }
 
+/// The whole error chain. Display alone stops at the outermost context
+/// ("failed to spawn worker") and drops the OS error that says why.
+fn spawn_failure_detail(error: &anyhow::Error) -> String {
+    format!("{error:#}")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn spawn_failure_detail_keeps_the_os_error() {
+        let error = anyhow::Error::new(std::io::Error::from(std::io::ErrorKind::NotFound))
+            .context("failed to spawn worker");
+        let detail = spawn_failure_detail(&error);
+        assert!(detail.starts_with("failed to spawn worker: "), "{detail}");
+        assert!(
+            detail.contains(&std::io::Error::from(std::io::ErrorKind::NotFound).to_string()),
+            "{detail}"
+        );
+    }
+
     #[test]
     fn duplicate_registration_keeps_already_exists_for_debug_log_classification() {
         let error = ::relaycast::AgentRegistrationError::AlreadyExists {