diff --git a/crates/maestro-cli/src/sessions.rs b/crates/maestro-cli/src/sessions.rs
index f58fd98..ed51946 100644
--- a/crates/maestro-cli/src/sessions.rs
+++ b/crates/maestro-cli/src/sessions.rs
@@ -108,6 +108,7 @@ fn print_session_table(out: &mut impl Write, sessions: &[Session], info: &Daemon
             ActivityState::Active => "active",
             ActivityState::Idle => "idle",
             ActivityState::AwaitingInput => "awaiting",
+            ActivityState::Error => "error",
         };
         // Show the live foreground command alongside the spawn command when
         // the daemon has observed one that differs (NAP-340).
diff --git a/crates/maestro-protocol/src/lib.rs b/crates/maestro-protocol/src/lib.rs
index 391d0bd..e960175 100644
--- a/crates/maestro-protocol/src/lib.rs
+++ b/crates/maestro-protocol/src/lib.rs
@@ -126,6 +126,9 @@ pub enum ActivityState {
     Idle,
     /// Blocked on a `read()` from the pty — waiting on the user.
     AwaitingInput,
+    /// The agent running in the session reported a failure (#102). Never
+    /// inferred by the daemon; only set when an agent reports it (#105).
+    Error,
 }
 
 /// Optional worktree request attached to a `CreateSessionRequest`.
@@ -617,6 +620,16 @@ mod tests {
         assert_eq!(full, round_trip(&full));
     }
 
+    // #102: nothing infers `Error`; only #105's agent-reported failure sets it.
+    #[test]
+    fn error_activity_state_serializes_pascal_case_and_idle_stays_default() {
+        assert_eq!(
+            serde_json::to_value(ActivityState::Error).unwrap(),
+            serde_json::json!("Error")
+        );
+        assert_eq!(ActivityState::default(), ActivityState::Idle);
+    }
+
     #[test]
     fn activity_state_serializes_pascal_case_and_defaults_idle() {
         assert_eq!(