diff --git a/crates/app-tauri/src/commands.rs b/crates/app-tauri/src/commands.rs
index 0a20663b..67203f6e 100644
--- a/crates/app-tauri/src/commands.rs
+++ b/crates/app-tauri/src/commands.rs
@@ -4447,8 +4447,7 @@ pub async fn spawn_background_command(
 
 /// `cancel_background_task` — request cancellation of a running background task.
 ///
-/// The terminal `Cancelled` state is written by the completion sink; this returns
-/// the task as currently persisted (absent if it no longer exists).
+/// Returns the task as currently persisted after the cancel request.
 ///
 /// # Errors
 /// Returns an [`ErrorDto`] (`INVALID` for a malformed id, `PROCESS`/`STORE` on
@@ -4457,13 +4456,13 @@ pub async fn spawn_background_command(
 pub async fn cancel_background_task(
     task_id: String,
     state: State<'_, AppState>,
-) -> Result<Option<BackgroundTaskDto>, ErrorDto> {
+) -> Result<BackgroundTaskDto, ErrorDto> {
     let id = parse_task_id(&task_id)?;
     state
         .cancel_background_task
         .execute(id)
         .await
-        .map(|out| out.task.map(BackgroundTaskDto::from))
+        .map(|out| BackgroundTaskDto::from(out.task))
         .map_err(ErrorDto::from)
 }
 
diff --git a/crates/application/src/background/mod.rs b/crates/application/src/background/mod.rs
index 7d93132b..3e786bea 100644
--- a/crates/application/src/background/mod.rs
+++ b/crates/application/src/background/mod.rs
@@ -209,33 +209,103 @@ impl SpawnBackgroundCommand {
 /// Output of [`CancelBackgroundTask`].
 #[derive(Debug, Clone)]
 pub struct CancelBackgroundTaskOutput {
-    /// The task as currently persisted, if it still exists. The terminal
-    /// `Cancelled` state is written by the completion sink, not here.
-    pub task: Option<BackgroundTask>,
+    /// The task as currently persisted after the cancel request.
+    pub task: BackgroundTask,
 }
 
 /// Requests cancellation of a running background task.
 pub struct CancelBackgroundTask {
     store: Arc<dyn BackgroundTaskStore>,
     runner: Arc<dyn BackgroundTaskRunner>,
+    clock: Arc<dyn Clock>,
+    events: Option<Arc<dyn EventBus>>,
 }
 
 impl CancelBackgroundTask {
     /// Wires the use case to its ports.
     #[must_use]
-    pub fn new(store: Arc<dyn BackgroundTaskStore>, runner: Arc<dyn BackgroundTaskRunner>) -> Self {
-        Self { store, runner }
+    pub fn new(
+        store: Arc<dyn BackgroundTaskStore>,
+        runner: Arc<dyn BackgroundTaskRunner>,
+        clock: Arc<dyn Clock>,
+    ) -> Self {
+        Self {
+            store,
+            runner,
+            clock,
+            events: None,
+        }
     }
 
-    /// Signals the runner to cancel `task_id` (idempotent).
+    /// Attaches the public event stream publisher.
+    #[must_use]
+    pub fn with_events(mut self, events: Arc<dyn EventBus>) -> Self {
+        self.events = Some(events);
+        self
+    }
+
+    /// Projects `Cancelling` immediately, then signals the runner to cancel `task_id`.
     ///
     /// # Errors
-    /// [`AppError`] if the runner or store fails.
+    /// [`AppError`] if the task is missing, terminal, or the runner/store fails.
     pub async fn execute(&self, task_id: TaskId) -> Result<CancelBackgroundTaskOutput, AppError> {
-        self.runner.cancel(task_id).await.map_err(map_port_err)?;
-        let task = self.store.get(task_id).await.map_err(map_port_err)?;
+        let task = self
+            .store
+            .get(task_id)
+            .await
+            .map_err(map_port_err)?
+            .ok_or_else(|| AppError::NotFound(format!("background task {task_id}")))?;
+        if task.is_terminal() {
+            return Err(AppError::Invalid(format!(
+                "background task {task_id} is already terminal ({:?})",
+                task.state
+            )));
+        }
+
+        let cancelling = if task.state == BackgroundTaskState::Cancelling {
+            task
+        } else {
+            let now = u64::try_from(self.clock.now_millis().max(0)).unwrap_or(0);
+            let cancelling = task
+                .transition(BackgroundTaskState::Cancelling, now)
+                .map_err(|e| AppError::Invalid(e.to_string()))?;
+            self.store.save(&cancelling).await.map_err(map_port_err)?;
+            self.publish_state_changed(&cancelling);
+            cancelling
+        };
+
+        if let Err(err) = self.runner.cancel(task_id).await {
+            let now = u64::try_from(self.clock.now_millis().max(0)).unwrap_or(0);
+            let failed = cancelling
+                .complete(BackgroundTaskResult::CancelFailed {
+                    finished_at_ms: now,
+                    error: format!("failed to cancel background task: {err}"),
+                })
+                .map_err(|e| AppError::Invalid(e.to_string()))?;
+            self.store.save(&failed).await.map_err(map_port_err)?;
+            self.publish_state_changed(&failed);
+            return Err(map_port_err(err));
+        }
+
+        let task = self
+            .store
+            .get(task_id)
+            .await
+            .map_err(map_port_err)?
+            .unwrap_or(cancelling);
         Ok(CancelBackgroundTaskOutput { task })
     }
+
+    fn publish_state_changed(&self, task: &BackgroundTask) {
+        if let Some(events) = &self.events {
+            events.publish(DomainEvent::BackgroundTaskStateChanged {
+                project_id: task.project_id,
+                task_id: task.id,
+                owner_agent_id: task.owner_agent_id,
+                state: task.state,
+            });
+        }
+    }
 }
 
 /// Read-only recall of the command invocation a background task was spawned with.
diff --git a/crates/application/src/orchestrator/service.rs b/crates/application/src/orchestrator/service.rs
index 5e79ff53..7b7b2c59 100644
--- a/crates/application/src/orchestrator/service.rs
+++ b/crates/application/src/orchestrator/service.rs
@@ -942,12 +942,14 @@ impl OrchestratorService {
                 owner_agent_id,
                 rendezvous,
             },
-            BackgroundTaskResult::Failure { .. } => DomainEvent::BackgroundTaskFailed {
-                project_id: project.id,
-                task_id,
-                owner_agent_id,
-                rendezvous,
-            },
+            BackgroundTaskResult::Failure { .. } | BackgroundTaskResult::CancelFailed { .. } => {
+                DomainEvent::BackgroundTaskFailed {
+                    project_id: project.id,
+                    task_id,
+                    owner_agent_id,
+                    rendezvous,
+                }
+            }
             BackgroundTaskResult::Cancelled { .. } | BackgroundTaskResult::Expired { .. } => {
                 DomainEvent::BackgroundTaskCancelled {
                     project_id: project.id,
@@ -1333,9 +1335,11 @@ impl OrchestratorService {
             BackgroundTaskState::Queued => "Queued",
             BackgroundTaskState::Running => "Running",
             BackgroundTaskState::Waiting => "Waiting",
+            BackgroundTaskState::Cancelling => "Cancelling",
             BackgroundTaskState::Completed => "Completed",
             BackgroundTaskState::Failed => "Failed",
             BackgroundTaskState::Cancelled => "Cancelled",
+            BackgroundTaskState::CancelFailed => "CancelFailed",
             BackgroundTaskState::Expired => "Expired",
         };
         Ok(OrchestratorOutcome {
diff --git a/crates/application/src/orchestrator/wake.rs b/crates/application/src/orchestrator/wake.rs
index 5aeb0d78..ef2da4eb 100644
--- a/crates/application/src/orchestrator/wake.rs
+++ b/crates/application/src/orchestrator/wake.rs
@@ -317,6 +317,10 @@ fn background_completion_prompt(task: &BackgroundTask) -> String {
             "La tâche de fond {} est terminée : annulée.\nRaison : {}",
             task.id, reason
         ),
+        Some(BackgroundTaskResult::CancelFailed { error, .. }) => format!(
+            "La tâche de fond {} est terminée : échec de l'annulation.\nErreur : {}",
+            task.id, error
+        ),
         Some(BackgroundTaskResult::Expired { reason, .. }) => format!(
             "La tâche de fond {} est terminée : expirée.\nRaison : {}",
             task.id, reason
diff --git a/crates/application/src/plugin/mod.rs b/crates/application/src/plugin/mod.rs
index ece68647..afc932c7 100644
--- a/crates/application/src/plugin/mod.rs
+++ b/crates/application/src/plugin/mod.rs
@@ -1940,9 +1940,11 @@ fn background_task_state_label(state: BackgroundTaskState) -> &'static str {
         BackgroundTaskState::Queued => "queued",
         BackgroundTaskState::Running => "running",
         BackgroundTaskState::Waiting => "waiting",
+        BackgroundTaskState::Cancelling => "cancelling",
         BackgroundTaskState::Completed => "completed",
         BackgroundTaskState::Failed => "failed",
         BackgroundTaskState::Cancelled => "cancelled",
+        BackgroundTaskState::CancelFailed => "cancelFailed",
         BackgroundTaskState::Expired => "expired",
     }
 }
diff --git a/crates/application/src/workstate/mod.rs b/crates/application/src/workstate/mod.rs
index a1ef5b37..cd7fada2 100644
--- a/crates/application/src/workstate/mod.rs
+++ b/crates/application/src/workstate/mod.rs
@@ -689,6 +689,9 @@ fn flatten_background_result(
         | Some(BackgroundTaskResult::Expired { reason, .. }) => {
             (None, Some(reason.clone()), None, None)
         }
+        Some(BackgroundTaskResult::CancelFailed { error, .. }) => {
+            (None, Some(error.clone()), None, None)
+        }
         None => (None, None, None, None),
     }
 }
diff --git a/crates/application/tests/background_task_cancel.rs b/crates/application/tests/background_task_cancel.rs
new file mode 100644
index 00000000..eb1e1cf6
--- /dev/null
+++ b/crates/application/tests/background_task_cancel.rs
@@ -0,0 +1,306 @@
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+
+use async_trait::async_trait;
+
+use application::CancelBackgroundTask;
+use domain::ports::{
+    BackgroundCompletionStream, BackgroundTaskHandle, BackgroundTaskPortError,
+    BackgroundTaskRunner, BackgroundTaskSpec, BackgroundTaskStore, Clock, EventBus, EventStream,
+};
+use domain::{
+    AgentId, BackgroundTask, BackgroundTaskKind, BackgroundTaskResult, BackgroundTaskState,
+    BackgroundTaskWakePolicy, DomainEvent, ProjectId, TaskId,
+};
+use uuid::Uuid;
+
+fn tid(n: u128) -> TaskId {
+    TaskId::from_uuid(Uuid::from_u128(n))
+}
+
+fn pid(n: u128) -> ProjectId {
+    ProjectId::from_uuid(Uuid::from_u128(n))
+}
+
+fn aid(n: u128) -> AgentId {
+    AgentId::from_uuid(Uuid::from_u128(n))
+}
+
+fn running_task(id: TaskId) -> BackgroundTask {
+    BackgroundTask::new(
+        id,
+        pid(1),
+        aid(2),
+        BackgroundTaskKind::Command {
+            label: "build".to_owned(),
+        },
+        BackgroundTaskWakePolicy::WakeOwner,
+        100,
+        None,
+    )
+    .unwrap()
+    .transition(BackgroundTaskState::Running, 110)
+    .unwrap()
+}
+
+#[derive(Default)]
+struct FakeStore {
+    tasks: Mutex<HashMap<TaskId, BackgroundTask>>,
+    saves: Mutex<Vec<BackgroundTaskState>>,
+}
+
+impl FakeStore {
+    fn insert(&self, task: BackgroundTask) {
+        self.tasks.lock().unwrap().insert(task.id, task);
+    }
+
+    fn saved_states(&self) -> Vec<BackgroundTaskState> {
+        self.saves.lock().unwrap().clone()
+    }
+}
+
+#[async_trait]
+impl BackgroundTaskStore for FakeStore {
+    async fn create(&self, task: &BackgroundTask) -> Result<(), BackgroundTaskPortError> {
+        self.insert(task.clone());
+        Ok(())
+    }
+
+    async fn get(&self, id: TaskId) -> Result<Option<BackgroundTask>, BackgroundTaskPortError> {
+        Ok(self.tasks.lock().unwrap().get(&id).cloned())
+    }
+
+    async fn save(&self, task: &BackgroundTask) -> Result<(), BackgroundTaskPortError> {
+        self.saves.lock().unwrap().push(task.state);
+        self.insert(task.clone());
+        Ok(())
+    }
+
+    async fn list_open_for_agent(
+        &self,
+        agent_id: AgentId,
+    ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {
+        Ok(self
+            .tasks
+            .lock()
+            .unwrap()
+            .values()
+            .filter(|task| task.owner_agent_id == agent_id && !task.is_terminal())
+            .cloned()
+            .collect())
+    }
+
+    async fn list_undelivered_completions(
+        &self,
+    ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {
+        Ok(Vec::new())
+    }
+
+    async fn mark_completion_delivered(
+        &self,
+        _task_id: TaskId,
+    ) -> Result<(), BackgroundTaskPortError> {
+        Ok(())
+    }
+}
+
+struct FakeRunner {
+    result: Mutex<Result<(), BackgroundTaskPortError>>,
+    cancels: Mutex<Vec<TaskId>>,
+}
+
+impl FakeRunner {
+    fn ok() -> Self {
+        Self {
+            result: Mutex::new(Ok(())),
+            cancels: Mutex::new(Vec::new()),
+        }
+    }
+
+    fn failing(message: &str) -> Self {
+        Self {
+            result: Mutex::new(Err(BackgroundTaskPortError::Runner(message.to_owned()))),
+            cancels: Mutex::new(Vec::new()),
+        }
+    }
+
+    fn cancels(&self) -> Vec<TaskId> {
+        self.cancels.lock().unwrap().clone()
+    }
+}
+
+#[async_trait]
+impl BackgroundTaskRunner for FakeRunner {
+    async fn spawn(
+        &self,
+        spec: BackgroundTaskSpec,
+    ) -> Result<BackgroundTaskHandle, BackgroundTaskPortError> {
+        Ok(BackgroundTaskHandle {
+            task_id: spec.task_id,
+        })
+    }
+
+    async fn cancel(&self, task_id: TaskId) -> Result<(), BackgroundTaskPortError> {
+        self.cancels.lock().unwrap().push(task_id);
+        self.result.lock().unwrap().clone()
+    }
+
+    fn subscribe_completions(&self) -> BackgroundCompletionStream {
+        Box::new(std::iter::empty())
+    }
+}
+
+#[derive(Default)]
+struct SpyEvents {
+    events: Mutex<Vec<DomainEvent>>,
+}
+
+impl SpyEvents {
+    fn events(&self) -> Vec<DomainEvent> {
+        self.events.lock().unwrap().clone()
+    }
+}
+
+impl EventBus for SpyEvents {
+    fn publish(&self, event: DomainEvent) {
+        self.events.lock().unwrap().push(event);
+    }
+
+    fn subscribe(&self) -> EventStream {
+        Box::new(std::iter::empty())
+    }
+}
+
+struct FixedClock(i64);
+
+impl Clock for FixedClock {
+    fn now_millis(&self) -> i64 {
+        self.0
+    }
+}
+
+fn usecase(
+    store: Arc<FakeStore>,
+    runner: Arc<FakeRunner>,
+    events: Arc<SpyEvents>,
+) -> CancelBackgroundTask {
+    CancelBackgroundTask::new(store, runner, Arc::new(FixedClock(200))).with_events(events)
+}
+
+#[tokio::test]
+async fn cancel_running_projects_cancelling_immediately_then_calls_runner() {
+    let store = Arc::new(FakeStore::default());
+    let runner = Arc::new(FakeRunner::ok());
+    let events = Arc::new(SpyEvents::default());
+    store.insert(running_task(tid(10)));
+
+    let out = usecase(Arc::clone(&store), Arc::clone(&runner), Arc::clone(&events))
+        .execute(tid(10))
+        .await
+        .unwrap();
+
+    assert_eq!(out.task.state, BackgroundTaskState::Cancelling);
+    assert_eq!(store.saved_states(), vec![BackgroundTaskState::Cancelling]);
+    assert_eq!(runner.cancels(), vec![tid(10)]);
+    assert_eq!(
+        events.events(),
+        vec![DomainEvent::BackgroundTaskStateChanged {
+            project_id: pid(1),
+            task_id: tid(10),
+            owner_agent_id: aid(2),
+            state: BackgroundTaskState::Cancelling,
+        }]
+    );
+}
+
+#[tokio::test]
+async fn cancel_terminal_task_is_explicit_error_without_runner_call() {
+    let store = Arc::new(FakeStore::default());
+    let runner = Arc::new(FakeRunner::ok());
+    let events = Arc::new(SpyEvents::default());
+    let done = running_task(tid(10))
+        .complete(BackgroundTaskResult::Success {
+            finished_at_ms: 120,
+            exit_code: Some(0),
+            summary: "ok".to_owned(),
+            stdout_tail: None,
+            stderr_tail: None,
+        })
+        .unwrap();
+    store.insert(done);
+
+    let err = usecase(Arc::clone(&store), Arc::clone(&runner), events)
+        .execute(tid(10))
+        .await
+        .unwrap_err();
+
+    assert!(matches!(err, application::AppError::Invalid(_)));
+    assert!(runner.cancels().is_empty());
+    assert!(store.saved_states().is_empty());
+}
+
+#[tokio::test]
+async fn cancel_already_cancelling_is_idempotent_and_retries_runner_cancel() {
+    let store = Arc::new(FakeStore::default());
+    let runner = Arc::new(FakeRunner::ok());
+    let events = Arc::new(SpyEvents::default());
+    store.insert(
+        running_task(tid(10))
+            .transition(BackgroundTaskState::Cancelling, 120)
+            .unwrap(),
+    );
+
+    let out = usecase(Arc::clone(&store), Arc::clone(&runner), events)
+        .execute(tid(10))
+        .await
+        .unwrap();
+
+    assert_eq!(out.task.state, BackgroundTaskState::Cancelling);
+    assert_eq!(runner.cancels(), vec![tid(10)]);
+    assert!(store.saved_states().is_empty());
+}
+
+#[tokio::test]
+async fn cancel_kill_failure_persists_cancel_failed_and_returns_error() {
+    let store = Arc::new(FakeStore::default());
+    let runner = Arc::new(FakeRunner::failing("kill denied"));
+    let events = Arc::new(SpyEvents::default());
+    store.insert(running_task(tid(10)));
+
+    let err = usecase(Arc::clone(&store), Arc::clone(&runner), Arc::clone(&events))
+        .execute(tid(10))
+        .await
+        .unwrap_err();
+
+    assert!(matches!(err, application::AppError::Process(_)));
+    assert_eq!(
+        store.saved_states(),
+        vec![
+            BackgroundTaskState::Cancelling,
+            BackgroundTaskState::CancelFailed,
+        ]
+    );
+    let persisted = store.get(tid(10)).await.unwrap().unwrap();
+    assert_eq!(persisted.state, BackgroundTaskState::CancelFailed);
+    assert!(matches!(
+        persisted.result,
+        Some(BackgroundTaskResult::CancelFailed { .. })
+    ));
+    assert_eq!(
+        events.events(),
+        vec![
+            DomainEvent::BackgroundTaskStateChanged {
+                project_id: pid(1),
+                task_id: tid(10),
+                owner_agent_id: aid(2),
+                state: BackgroundTaskState::Cancelling,
+            },
+            DomainEvent::BackgroundTaskStateChanged {
+                project_id: pid(1),
+                task_id: tid(10),
+                owner_agent_id: aid(2),
+                state: BackgroundTaskState::CancelFailed,
+            },
+        ]
+    );
+}
diff --git a/crates/application/tests/workstate.rs b/crates/application/tests/workstate.rs
index c6bbc578..4032c7b0 100644
--- a/crates/application/tests/workstate.rs
+++ b/crates/application/tests/workstate.rs
@@ -568,9 +568,15 @@ fn background_task(
             .unwrap()
             .transition(BackgroundTaskState::Waiting, created_at_ms + 20)
             .unwrap(),
+        BackgroundTaskState::Cancelling => base
+            .transition(BackgroundTaskState::Running, created_at_ms + 10)
+            .unwrap()
+            .transition(BackgroundTaskState::Cancelling, created_at_ms + 20)
+            .unwrap(),
         BackgroundTaskState::Completed
         | BackgroundTaskState::Failed
         | BackgroundTaskState::Cancelled
+        | BackgroundTaskState::CancelFailed
         | BackgroundTaskState::Expired => {
             let result = match state {
                 BackgroundTaskState::Completed => BackgroundTaskResult::Success {
@@ -591,13 +597,18 @@ fn background_task(
                     finished_at_ms: created_at_ms + 20,
                     reason: "cancelled".to_owned(),
                 },
+                BackgroundTaskState::CancelFailed => BackgroundTaskResult::CancelFailed {
+                    finished_at_ms: created_at_ms + 20,
+                    error: "cancel failed".to_owned(),
+                },
                 BackgroundTaskState::Expired => BackgroundTaskResult::Expired {
                     finished_at_ms: created_at_ms + 20,
                     reason: "expired".to_owned(),
                 },
                 BackgroundTaskState::Queued
                 | BackgroundTaskState::Running
-                | BackgroundTaskState::Waiting => unreachable!(),
+                | BackgroundTaskState::Waiting
+                | BackgroundTaskState::Cancelling => unreachable!(),
             };
             let running = base
                 .transition(BackgroundTaskState::Running, created_at_ms + 10)
diff --git a/crates/backend/src/dto.rs b/crates/backend/src/dto.rs
index 89fd0473..3c55ba8d 100644
--- a/crates/backend/src/dto.rs
+++ b/crates/backend/src/dto.rs
@@ -5161,9 +5161,11 @@ fn background_state_label(state: BackgroundTaskState) -> &'static str {
         BackgroundTaskState::Queued => "queued",
         BackgroundTaskState::Running => "running",
         BackgroundTaskState::Waiting => "waiting",
+        BackgroundTaskState::Cancelling => "cancelling",
         BackgroundTaskState::Completed => "completed",
         BackgroundTaskState::Failed => "failed",
         BackgroundTaskState::Cancelled => "cancelled",
+        BackgroundTaskState::CancelFailed => "cancelFailed",
         BackgroundTaskState::Expired => "expired",
     }
 }
@@ -5201,6 +5203,9 @@ impl From<BackgroundTask> for BackgroundTaskDto {
             | Some(BackgroundTaskResult::Expired { reason, .. }) => {
                 (None, Some(reason.clone()), None, None)
             }
+            Some(BackgroundTaskResult::CancelFailed { error, .. }) => {
+                (None, Some(error.clone()), None, None)
+            }
             None => (None, None, None, None),
         };
         Self {
diff --git a/crates/backend/src/lib.rs b/crates/backend/src/lib.rs
index 666eca2c..809c9b53 100644
--- a/crates/backend/src/lib.rs
+++ b/crates/backend/src/lib.rs
@@ -1357,6 +1357,7 @@ fn background_task_tail_bytes(task: &BackgroundTask) -> Vec<u8> {
             .into_bytes(),
         Some(BackgroundTaskResult::Cancelled { reason, .. })
         | Some(BackgroundTaskResult::Expired { reason, .. }) => reason.clone().into_bytes(),
+        Some(BackgroundTaskResult::CancelFailed { error, .. }) => error.clone().into_bytes(),
         None => Vec::new(),
     }
 }
@@ -2452,10 +2453,14 @@ impl BackendCore {
             )
             .with_events(Arc::clone(&events_port)),
         );
-        let cancel_background_task = Arc::new(CancelBackgroundTask::new(
-            Arc::clone(&background_tasks_port),
-            Arc::clone(&background_runner_port),
-        ));
+        let cancel_background_task = Arc::new(
+            CancelBackgroundTask::new(
+                Arc::clone(&background_tasks_port),
+                Arc::clone(&background_runner_port),
+                Arc::clone(&clock) as Arc<dyn Clock>,
+            )
+            .with_events(Arc::clone(&events_port)),
+        );
         let retry_background_task = Arc::new(RetryBackgroundTask::new(
             Arc::clone(&background_tasks_port),
             Arc::clone(&background_runner) as Arc<dyn BackgroundCommandArchive>,
diff --git a/crates/domain/src/background_task.rs b/crates/domain/src/background_task.rs
index 9b48fff1..151f7a5d 100644
--- a/crates/domain/src/background_task.rs
+++ b/crates/domain/src/background_task.rs
@@ -28,12 +28,16 @@ pub enum BackgroundTaskState {
     Running,
     /// Suspended while waiting for an external event.
     Waiting,
+    /// Cancellation has been accepted and the runner is stopping the resource.
+    Cancelling,
     /// Finished successfully.
     Completed,
     /// Finished with an error.
     Failed,
     /// Cancelled before natural completion.
     Cancelled,
+    /// Cancellation was requested but the runner could not stop the resource.
+    CancelFailed,
     /// Expired after its deadline.
     Expired,
 }
@@ -44,7 +48,7 @@ impl BackgroundTaskState {
     pub const fn is_terminal(self) -> bool {
         matches!(
             self,
-            Self::Completed | Self::Failed | Self::Cancelled | Self::Expired
+            Self::Completed | Self::Failed | Self::Cancelled | Self::CancelFailed | Self::Expired
         )
     }
 }
@@ -342,6 +346,13 @@ pub enum BackgroundTaskResult {
         /// Cancellation reason.
         reason: String,
     },
+    /// Cancellation failed.
+    CancelFailed {
+        /// Completion timestamp, epoch milliseconds.
+        finished_at_ms: u64,
+        /// Human-readable error.
+        error: String,
+    },
     /// Expired completion.
     Expired {
         /// Completion timestamp, epoch milliseconds.
@@ -375,6 +386,7 @@ impl BackgroundTaskResult {
             Self::Success { finished_at_ms, .. }
             | Self::Failure { finished_at_ms, .. }
             | Self::Cancelled { finished_at_ms, .. }
+            | Self::CancelFailed { finished_at_ms, .. }
             | Self::Expired { finished_at_ms, .. } => *finished_at_ms,
         }
     }
@@ -408,6 +420,9 @@ impl BackgroundTaskResult {
             Self::Cancelled { reason, .. } | Self::Expired { reason, .. } => {
                 validate_text("reason", reason)?;
             }
+            Self::CancelFailed { error, .. } => {
+                validate_text("error", error)?;
+            }
         }
         Ok(())
     }
@@ -417,6 +432,7 @@ impl BackgroundTaskResult {
             Self::Success { .. } => BackgroundTaskState::Completed,
             Self::Failure { .. } => BackgroundTaskState::Failed,
             Self::Cancelled { .. } => BackgroundTaskState::Cancelled,
+            Self::CancelFailed { .. } => BackgroundTaskState::CancelFailed,
             Self::Expired { .. } => BackgroundTaskState::Expired,
         }
     }
@@ -459,13 +475,22 @@ pub enum BackgroundTaskError {
 }
 
 fn is_transition_allowed(from: BackgroundTaskState, to: BackgroundTaskState) -> bool {
-    use BackgroundTaskState::{Cancelled, Completed, Expired, Failed, Queued, Running, Waiting};
+    use BackgroundTaskState::{
+        CancelFailed, Cancelled, Cancelling, Completed, Expired, Failed, Queued, Running, Waiting,
+    };
 
     matches!(
         (from, to),
-        (Queued, Running | Cancelled | Expired)
-            | (Running, Waiting | Completed | Failed | Cancelled | Expired)
-            | (Waiting, Running | Completed | Failed | Cancelled | Expired)
+        (
+            Queued,
+            Running | Cancelling | Cancelled | CancelFailed | Expired
+        ) | (
+            Running,
+            Waiting | Cancelling | Completed | Failed | Cancelled | CancelFailed | Expired
+        ) | (
+            Waiting,
+            Running | Cancelling | Completed | Failed | Cancelled | CancelFailed | Expired
+        ) | (Cancelling, Cancelled | CancelFailed | Expired)
     )
 }
 
diff --git a/crates/domain/tests/background_task.rs b/crates/domain/tests/background_task.rs
index beac463c..94459473 100644
--- a/crates/domain/tests/background_task.rs
+++ b/crates/domain/tests/background_task.rs
@@ -134,6 +134,11 @@ fn allowed_non_terminal_transitions_update_timestamp() {
         .transition(BackgroundTaskState::Running, 140)
         .unwrap();
     assert_eq!(running_again.state, BackgroundTaskState::Running);
+
+    let cancelling = running_again
+        .transition(BackgroundTaskState::Cancelling, 150)
+        .unwrap();
+    assert_eq!(cancelling.state, BackgroundTaskState::Cancelling);
 }
 
 #[test]
@@ -245,6 +250,18 @@ fn result_variants_determine_terminal_state() {
             .state,
         BackgroundTaskState::Expired
     );
+    assert_eq!(
+        running
+            .transition(BackgroundTaskState::Cancelling, 125)
+            .unwrap()
+            .complete(BackgroundTaskResult::CancelFailed {
+                finished_at_ms: 130,
+                error: "kill failed".into(),
+            })
+            .unwrap()
+            .state,
+        BackgroundTaskState::CancelFailed
+    );
 }
 
 #[test]
diff --git a/crates/infrastructure/src/background_task/runner.rs b/crates/infrastructure/src/background_task/runner.rs
index 6d6b368a..269392a4 100644
--- a/crates/infrastructure/src/background_task/runner.rs
+++ b/crates/infrastructure/src/background_task/runner.rs
@@ -139,11 +139,14 @@ impl CommandBackgroundRunner {
             .filter(|s| !s.is_empty());
 
         // Reap the real exit status (works for a natural exit and a forced kill).
-        let exit_code = pty
-            .kill(&control.pty_handle)
-            .await
-            .ok()
-            .and_then(|status| status.code);
+        let exit_code = match outcome {
+            WaitOutcome::Cancelled => None,
+            WaitOutcome::Expired | WaitOutcome::Exited => pty
+                .kill(&control.pty_handle)
+                .await
+                .ok()
+                .and_then(|status| status.code),
+        };
 
         let finished_at_ms = u64::try_from(clock.now_millis().max(0)).unwrap_or(0);
         let result = match outcome {
@@ -281,8 +284,12 @@ impl BackgroundTaskRunner for CommandBackgroundRunner {
             .cloned();
         // Unknown task ⇒ already terminal or never ours: cancellation is idempotent.
         if let Some(control) = control {
+            self.pty
+                .kill(&control.pty_handle)
+                .await
+                .map_err(|e| BackgroundTaskPortError::Runner(format!("pty kill failed: {e}")))?;
             control.cancelled.store(true, Ordering::SeqCst);
-            control.cancel.notify_one();
+            control.cancel.notify_waiters();
         }
         Ok(())
     }
diff --git a/crates/infrastructure/src/store/background_task.rs b/crates/infrastructure/src/store/background_task.rs
index e4706c52..6787ef85 100644
--- a/crates/infrastructure/src/store/background_task.rs
+++ b/crates/infrastructure/src/store/background_task.rs
@@ -108,7 +108,8 @@ impl FsBackgroundTaskStore {
     /// Reconciles persisted tasks against live runtime handles at boot.
     ///
     /// B2 rule: `Running`/`Waiting` tasks missing from `live_task_ids` are marked
-    /// `Failed` with a synthetic restart-loss failure and become delivery-pending.
+    /// `Failed`; `Cancelling` tasks missing from `live_task_ids` are marked
+    /// `CancelFailed`. Both become delivery-pending.
     /// Already-terminal undelivered tasks are reported as delivery-pending. `Queued`
     /// tasks are left open.
     ///
@@ -127,18 +128,13 @@ impl FsBackgroundTaskStore {
         for (project_id, doc) in &mut state.docs {
             for task in &mut doc.tasks {
                 match task.state {
-                    BackgroundTaskState::Running | BackgroundTaskState::Waiting
+                    BackgroundTaskState::Running
+                    | BackgroundTaskState::Waiting
+                    | BackgroundTaskState::Cancelling
                         if !live.contains(&task.id) =>
                     {
                         let finished_at_ms = now_ms.max(task.updated_at_ms);
-                        let result = BackgroundTaskResult::Failure {
-                            finished_at_ms,
-                            exit_code: None,
-                            error: "background task lost its runtime handle during IdeA restart"
-                                .into(),
-                            stdout_tail: None,
-                            stderr_tail: None,
-                        };
+                        let result = lost_runtime_result(task.state, finished_at_ms);
                         *task = task.complete(result).map_err(invalid_task)?;
                         report.failed_task_ids.push(task.id);
                         report.delivery_pending_task_ids.push(task.id);
@@ -158,18 +154,13 @@ impl FsBackgroundTaskStore {
                 for task in &mut doc.tasks {
                     if matches!(
                         task.state,
-                        BackgroundTaskState::Running | BackgroundTaskState::Waiting
+                        BackgroundTaskState::Running
+                            | BackgroundTaskState::Waiting
+                            | BackgroundTaskState::Cancelling
                     ) && !live.contains(&task.id)
                     {
                         let finished_at_ms = now_ms.max(task.updated_at_ms);
-                        let result = BackgroundTaskResult::Failure {
-                            finished_at_ms,
-                            exit_code: None,
-                            error: "background task lost its runtime handle during IdeA restart"
-                                .into(),
-                            stdout_tail: None,
-                            stderr_tail: None,
-                        };
+                        let result = lost_runtime_result(task.state, finished_at_ms);
                         *task = task.complete(result).map_err(invalid_task)?;
                     }
                 }
@@ -444,3 +435,21 @@ fn io_error(error: std::io::Error) -> BackgroundTaskPortError {
 fn invalid_task(error: domain::BackgroundTaskError) -> BackgroundTaskPortError {
     BackgroundTaskPortError::Invalid(error.to_string())
 }
+
+fn lost_runtime_result(state: BackgroundTaskState, finished_at_ms: u64) -> BackgroundTaskResult {
+    if state == BackgroundTaskState::Cancelling {
+        BackgroundTaskResult::CancelFailed {
+            finished_at_ms,
+            error: "background task cancellation lost its runtime handle during IdeA restart"
+                .into(),
+        }
+    } else {
+        BackgroundTaskResult::Failure {
+            finished_at_ms,
+            exit_code: None,
+            error: "background task lost its runtime handle during IdeA restart".into(),
+            stdout_tail: None,
+            stderr_tail: None,
+        }
+    }
+}
diff --git a/crates/infrastructure/tests/background_task_runner.rs b/crates/infrastructure/tests/background_task_runner.rs
index 32bd3871..627e47d3 100644
--- a/crates/infrastructure/tests/background_task_runner.rs
+++ b/crates/infrastructure/tests/background_task_runner.rs
@@ -33,6 +33,7 @@ struct FakePtyState {
     handle: Option<PtyHandle>,
     status: Option<ExitStatus>,
     kills: usize,
+    fail_kill: bool,
     scrollback: Vec<u8>,
 }
 
@@ -57,6 +58,10 @@ impl FakePty {
         self.state.lock().unwrap().kills
     }
 
+    fn fail_kill(&self) {
+        self.state.lock().unwrap().fail_kill = true;
+    }
+
     fn subscribe_count(&self) -> usize {
         self.subscribe_count.load(Ordering::SeqCst)
     }
@@ -105,6 +110,9 @@ impl PtyPort for FakePty {
     async fn kill(&self, _handle: &PtyHandle) -> Result<ExitStatus, PtyError> {
         let mut state = self.state.lock().unwrap();
         state.kills += 1;
+        if state.fail_kill {
+            return Err(PtyError::Io("kill denied".to_owned()));
+        }
         let status = state.status.get_or_insert(ExitStatus { code: Some(130) });
         Ok(*status)
     }
@@ -314,6 +322,31 @@ async fn deadline_expires_and_kills_when_wait_never_resolves() {
     assert_eq!(pty.subscribe_count(), 0);
 }
 
+#[tokio::test]
+async fn cancel_surfaces_pty_kill_failure_without_completion() {
+    let pty = Arc::new(FakePty::default());
+    pty.fail_kill();
+    let clock = Arc::new(FakeClock::default());
+    let (runner, _completions) = runner_with(Arc::clone(&pty), clock);
+    let task_id = TaskId::from_uuid(id(16));
+
+    runner
+        .spawn(spawn_spec(task_id, None))
+        .await
+        .expect("spawn succeeds");
+
+    let err = runner
+        .cancel(task_id)
+        .await
+        .expect_err("kill failure surfaces");
+    assert!(matches!(err, domain::BackgroundTaskPortError::Runner(_)));
+    assert_eq!(pty.kill_count(), 1);
+    assert!(
+        runner.pty_handle_for(task_id).is_some(),
+        "failed cancel keeps the live handle available for diagnostics/retry"
+    );
+}
+
 #[tokio::test]
 async fn cancel_wins_even_if_output_is_still_available() {
     let pty = Arc::new(FakePty::default());
diff --git a/crates/infrastructure/tests/background_task_store.rs b/crates/infrastructure/tests/background_task_store.rs
index 85a2ba14..a09867a2 100644
--- a/crates/infrastructure/tests/background_task_store.rs
+++ b/crates/infrastructure/tests/background_task_store.rs
@@ -86,6 +86,12 @@ fn running_task(id: u128, project: u128, owner: u128, now_ms: u64) -> Background
         .unwrap()
 }
 
+fn cancelling_task(id: u128, project: u128, owner: u128, now_ms: u64) -> BackgroundTask {
+    running_task(id, project, owner, now_ms)
+        .transition(BackgroundTaskState::Cancelling, now_ms + 2)
+        .unwrap()
+}
+
 fn completed_task(id: u128, project: u128, owner: u128, now_ms: u64) -> BackgroundTask {
     running_task(id, project, owner, now_ms)
         .complete(BackgroundTaskResult::Success {
@@ -337,6 +343,31 @@ async fn reconcile_marks_running_without_live_handle_failed_and_pending() {
     );
 }
 
+#[tokio::test]
+async fn reconcile_marks_cancelling_without_live_handle_cancel_failed_and_pending() {
+    let tmp = TempDir::new();
+    let store = FsBackgroundTaskStore::new(&tmp.project_path());
+    let lost = cancelling_task(1, 10, 100, 1_000);
+
+    store.create(&lost).await.unwrap();
+
+    let report = store.reconcile_boot(&[], 2_000).await.unwrap();
+
+    assert_eq!(report.failed_task_ids, vec![lost.id]);
+    assert_eq!(report.delivery_pending_task_ids, vec![lost.id]);
+
+    let lost_after = store.get(lost.id).await.unwrap().unwrap();
+    assert_eq!(lost_after.state, BackgroundTaskState::CancelFailed);
+    assert!(lost_after.has_pending_completion_delivery());
+    assert!(matches!(
+        lost_after.result,
+        Some(BackgroundTaskResult::CancelFailed {
+            finished_at_ms: 2_000,
+            ..
+        })
+    ));
+}
+
 #[tokio::test]
 async fn reconcile_uses_monotonic_timestamp_when_clock_is_behind() {
     let tmp = TempDir::new();
diff --git a/crates/web-server/src/lib.rs b/crates/web-server/src/lib.rs
index aec765ec..ad2a7362 100644
--- a/crates/web-server/src/lib.rs
+++ b/crates/web-server/src/lib.rs
@@ -2955,7 +2955,7 @@ async fn invoke_cancel_background_task(
         .execute(task_id)
         .await
         .map_err(ErrorDto::from)?;
-    serde_json::to_value(output.task.map(BackgroundTaskDto::from)).map_err(serialization_error)
+    serde_json::to_value(BackgroundTaskDto::from(output.task)).map_err(serialization_error)
 }
 
 async fn invoke_retry_background_task(
diff --git a/frontend/src/adapters/workStateNormalization.ts b/frontend/src/adapters/workStateNormalization.ts
index 9ae54422..a47655a9 100644
--- a/frontend/src/adapters/workStateNormalization.ts
+++ b/frontend/src/adapters/workStateNormalization.ts
@@ -86,7 +86,11 @@ function normalizeBackgroundStatus(
   const raw = typeof value === "string" ? value.toLowerCase() : "";
   if (raw === "completed" || raw === "success" || raw === "succeeded") return "completed";
   if (raw === "failed" || raw === "failure") return "failed";
+  if (raw === "cancelfailed" || raw === "cancel_failed" || raw === "cancel-failed") {
+    return "cancelFailed";
+  }
   if (raw === "cancelled" || raw === "canceled" || raw === "expired") return "cancelled";
+  if (raw === "cancelling" || raw === "canceling") return "cancelling";
   if (raw === "running" || raw === "started") return "running";
   if (raw === "delivered") return "delivered";
   // Backend `queued`/`waiting` collapse to the cancellable "pending" bucket.
diff --git a/frontend/src/domain/index.ts b/frontend/src/domain/index.ts
index 06536921..a7d78f04 100644
--- a/frontend/src/domain/index.ts
+++ b/frontend/src/domain/index.ts
@@ -612,7 +612,15 @@ export interface BackgroundCompletion {
   ownerAgentId: string;
   projectId: string;
   kind: string;
-  status: "running" | "completed" | "failed" | "cancelled" | "pending" | "delivered";
+  status:
+    | "running"
+    | "cancelling"
+    | "completed"
+    | "failed"
+    | "cancelFailed"
+    | "cancelled"
+    | "pending"
+    | "delivered";
   exitCode: number | null;
   /** Human-readable summary / error / reason of a terminal result (ticket #5). */
   summary: string | null;
diff --git a/frontend/src/features/projects/projectActivity.ts b/frontend/src/features/projects/projectActivity.ts
index fcf7db3f..3444f1b4 100644
--- a/frontend/src/features/projects/projectActivity.ts
+++ b/frontend/src/features/projects/projectActivity.ts
@@ -3,7 +3,7 @@ import type { BackgroundCompletion, ProjectWorkState } from "@/domain";
 export type ProjectActivityStatus = "none" | "idle" | "busy";
 
 const ACTIVE_BACKGROUND_STATUSES: ReadonlySet<BackgroundCompletion["status"]> =
-  new Set(["pending", "running"]);
+  new Set(["pending", "running", "cancelling"]);
 
 export function projectActivityStatus(
   state: ProjectWorkState | null | undefined,
diff --git a/frontend/src/features/web/WebWorkspace.tsx b/frontend/src/features/web/WebWorkspace.tsx
index e72fb514..ed70ff31 100644
--- a/frontend/src/features/web/WebWorkspace.tsx
+++ b/frontend/src/features/web/WebWorkspace.tsx
@@ -507,17 +507,23 @@ function AgentLiveRow({
 
 const TASK_STATUS_LABEL: Record<BackgroundCompletion["status"], string> = {
   running: "Running",
+  cancelling: "Cancelling",
   completed: "Completed",
   failed: "Failed",
+  cancelFailed: "Cancel failed",
   cancelled: "Cancelled",
   pending: "Pending",
   delivered: "Delivered",
 };
 
 function taskStatusClass(status: BackgroundCompletion["status"]): string {
-  if (status === "running" || status === "pending") return "bg-warning/15 text-warning";
+  if (status === "running" || status === "pending" || status === "cancelling") {
+    return "bg-warning/15 text-warning";
+  }
   if (status === "completed" || status === "delivered") return "bg-success/10 text-success";
-  if (status === "failed" || status === "cancelled") return "bg-danger/10 text-danger";
+  if (status === "failed" || status === "cancelFailed" || status === "cancelled") {
+    return "bg-danger/10 text-danger";
+  }
   return "bg-raised text-muted";
 }
 
diff --git a/frontend/src/features/workstate/ProjectWorkStatePanel.tsx b/frontend/src/features/workstate/ProjectWorkStatePanel.tsx
index 2d8e4ea7..c36e490a 100644
--- a/frontend/src/features/workstate/ProjectWorkStatePanel.tsx
+++ b/frontend/src/features/workstate/ProjectWorkStatePanel.tsx
@@ -92,10 +92,14 @@ function formatTaskStatus(status: BackgroundCompletion["status"]): string {
   switch (status) {
     case "running":
       return "Running";
+    case "cancelling":
+      return "Cancelling";
     case "completed":
       return "Completed";
     case "failed":
       return "Failed";
+    case "cancelFailed":
+      return "Cancel failed";
     case "cancelled":
       return "Cancelled";
     case "delivered":
@@ -107,16 +111,33 @@ function formatTaskStatus(status: BackgroundCompletion["status"]): string {
 
 /** Terminal (finished) task states that carry a result summary (ticket #5). */
 function isTerminalTaskStatus(status: BackgroundCompletion["status"]): boolean {
-  return status === "completed" || status === "failed" || status === "cancelled";
+  return (
+    status === "completed" ||
+    status === "failed" ||
+    status === "cancelFailed" ||
+    status === "cancelled"
+  );
 }
 
 function taskStatusClass(status: BackgroundCompletion["status"]): string {
-  if (status === "running" || status === "pending") return "bg-warning/15 text-warning";
+  if (status === "running" || status === "pending" || status === "cancelling") {
+    return "bg-warning/15 text-warning";
+  }
   if (status === "completed" || status === "delivered") return "bg-success/10 text-success";
-  if (status === "failed" || status === "cancelled") return "bg-danger/10 text-danger";
+  if (status === "failed" || status === "cancelFailed" || status === "cancelled") {
+    return "bg-danger/10 text-danger";
+  }
   return "bg-raised text-muted";
 }
 
+function canCancelTask(status: BackgroundCompletion["status"]): boolean {
+  return status === "running" || status === "pending" || status === "cancelling";
+}
+
+function canRetryTask(status: BackgroundCompletion["status"]): boolean {
+  return status === "failed" || status === "cancelFailed" || status === "cancelled";
+}
+
 function requesterLabel(ticket: AgentTicketState): string {
   if (ticket.source.kind === "human") return "Human";
   const id = shortTicket(ticket.source.agentId);
@@ -346,8 +367,8 @@ function BackgroundTaskRow({
   const attachmentRef = useRef<BackgroundTaskAttachment | null>(null);
   const attachSeqRef = useRef(0);
   const hasOutput = Boolean(task.stdoutTail || task.stderrTail);
-  const canCancel = task.status === "running" || task.status === "pending";
-  const canRetry = task.status === "failed" || task.status === "cancelled";
+  const canCancel = canCancelTask(task.status);
+  const canRetry = canRetryTask(task.status);
   const canAttachLive = canCancel;
 
   function detachLive(): void {
diff --git a/frontend/src/features/workstate/workstate.test.tsx b/frontend/src/features/workstate/workstate.test.tsx
index 8663ebf7..030e095a 100644
--- a/frontend/src/features/workstate/workstate.test.tsx
+++ b/frontend/src/features/workstate/workstate.test.tsx
@@ -224,6 +224,21 @@ describe("ProjectWorkStatePanel", () => {
               createdAtMs: 16,
               updatedAtMs: 17,
             },
+            {
+              taskId: "task-cancelling-1",
+              kind: "command",
+              state: "cancelling",
+              createdAtMs: 18,
+              updatedAtMs: 19,
+            },
+            {
+              taskId: "task-cancel-failed-1",
+              kind: "command",
+              state: "cancelFailed",
+              summary: "kill denied",
+              createdAtMs: 20,
+              updatedAtMs: 21,
+            },
           ],
         },
       ],
@@ -237,6 +252,8 @@ describe("ProjectWorkStatePanel", () => {
       ["task-queued-1", "pending"],
       ["task-failed-1", "failed"],
       ["task-expired-1", "cancelled"],
+      ["task-cancelling-1", "cancelling"],
+      ["task-cancel-failed-1", "cancelFailed"],
     ]);
     // Optional fields omitted by the backend normalize to null (not undefined/NaN).
     expect(tasks[0]?.exitCode).toBeNull();
@@ -249,25 +266,44 @@ describe("ProjectWorkStatePanel", () => {
     expect(tasks[1]?.targetAgentId).toBe("agent-bg");
     expect(tasks[1]?.conversationId).toBe("conversation-rdv");
     // updatedAtMs (backend-emitted) is carried through for chronological ordering.
-    expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17]);
+    expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17, 19, 21]);
 
     renderPanel(workState);
 
     const list = await screen.findByLabelText("Runner background tasks");
     const cancelButtons = within(list).getAllByRole("button", { name: "Cancel" });
     const retryButtons = within(list).getAllByRole("button", { name: "Retry" });
-    // waiting + queued are cancellable; failed + expired are retryable.
-    expect(cancelButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(2);
-    expect(retryButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(2);
+    // waiting + queued + cancelling are cancellable; failed + expired + cancelFailed are retryable.
+    expect(cancelButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(3);
+    expect(retryButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(3);
+    expect(list.textContent).toContain("Cancelling");
+    expect(list.textContent).toContain("Cancel failed");
+    const rows = within(list).getAllByRole("listitem");
+    const rowTexts = rows.map((row) => row.textContent ?? "");
+    expect(rowTexts).toHaveLength(6);
+    const cancelFailedRow = rows[0] as HTMLElement;
+    const cancellingRow = rows[1] as HTMLElement;
+    expect(within(cancellingRow).getByText("Cancelling")).toBeTruthy();
+    expect(
+      (within(cancellingRow).getByRole("button", {
+        name: "Cancel",
+      }) as HTMLButtonElement).disabled,
+    ).toBe(false);
+    expect(within(cancelFailedRow).getByText("Cancel failed")).toBeTruthy();
+    expect(
+      (within(cancelFailedRow).getByRole("button", {
+        name: "Retry",
+      }) as HTMLButtonElement).disabled,
+    ).toBe(false);
 
     // Rows are ordered most-recently-updated first (updatedAtMs desc), taskId as
     // the deterministic tiebreak — locks the fix for the dead finishedAtMs sort.
-    const text = list.textContent ?? "";
-    const order = ["task-exp", "task-fai", "task-que", "task-wai"].map((id) =>
-      text.indexOf(id),
-    );
-    expect(order).toEqual([...order].sort((a, b) => a - b));
-    expect(order.every((i) => i >= 0)).toBe(true);
+    expect(rowTexts[0]).toContain("Cancel failed");
+    expect(rowTexts[1]).toContain("Cancelling");
+    expect(rowTexts[2]).toContain("Cancelled");
+    expect(rowTexts[3]).toContain("Failed");
+    expect(rowTexts[4]).toContain("Pending");
+    expect(rowTexts[5]).toContain("Pending");
   });
 
   it("shows a terminal task summary and hides it when absent or non-terminal (#5)", async () => {
diff --git a/frontend/src/plugins/runtime/services.ts b/frontend/src/plugins/runtime/services.ts
index a0dea166..9ec77841 100644
--- a/frontend/src/plugins/runtime/services.ts
+++ b/frontend/src/plugins/runtime/services.ts
@@ -142,7 +142,15 @@ export interface BackgroundTaskStatus {
   ownerAgentId: string;
   projectId: string;
   kind: string;
-  status: "pending" | "running" | "completed" | "failed" | "cancelled" | "delivered";
+  status:
+    | "pending"
+    | "running"
+    | "cancelling"
+    | "completed"
+    | "failed"
+    | "cancelFailed"
+    | "cancelled"
+    | "delivered";
   exitCode: number | null;
   summary: string | null;
   stdoutTail: string | null;
@@ -178,7 +186,16 @@ export interface CommandTaskStatus {
   ownerAgentId: string;
   projectId: string;
   kind: string;
-  state: "queued" | "running" | "waiting" | "completed" | "failed" | "cancelled" | "expired";
+  state:
+    | "queued"
+    | "running"
+    | "waiting"
+    | "cancelling"
+    | "completed"
+    | "failed"
+    | "cancelFailed"
+    | "cancelled"
+    | "expired";
   exitCode: number | null;
   summary: string | null;
   stdoutTail: string | null;