api cancellation (#1276)

closing the http request to the api now
- sends a cancellation from the api
- writes that canellation in the master
- worker plans off the cancellation
- runner observes that cancellation after every generation step (+1
communication per token)
- cancellation happens synchronously to prevent gpu locks

closes #61

---------

Co-authored-by: Alex Cheema <alexcheema123@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

MISSED_THINGS.md

@@ -5,21 +5,21 @@
 [X] Fetching download status of all models on start
 [X] Deduplication of tasks in plan_step.
 [X] resolve_allow_patterns should just be wildcard now.
-[] no mx_barrier in genreate.py mlx_generate at the end.
+[X] no mx_barrier in genreate.py mlx_generate at the end.
 [] cache assertion not needed in auto_parallel.py PipelineLastLayer.
-[] GPTOSS support dropped in auto_parallel.py.
+[X] GPTOSS support dropped in auto_parallel.py.
-[] sharding changed "all-to-sharded" became _all_to_sharded in auto_parallel.py.
+[X] sharding changed "all-to-sharded" became _all_to_sharded in auto_parallel.py.
-[] same as above with "sharded-to-all" became _sharded_to_all in auto_parallel.py.
+[X] same as above with "sharded-to-all" became _sharded_to_all in auto_parallel.py.
-[] Dropped support for Ministral3Model, DeepseekV32Model, Glm4MoeModel, Qwen3NextModel, GptOssMode in auto_parallel.py.
+[X] Dropped support for Ministral3Model, DeepseekV32Model, Glm4MoeModel, Qwen3NextModel, GptOssMode in auto_parallel.py.
 [] Dropped prefill/decode code in auto_parallel.py and utils_mlx.py.
 [X] KV_CACHE_BITS should be None to disable quantized KV cache.
-[] Dropped _set_nofile_limit in utils_mlx.py.
+[X] Dropped _set_nofile_limit in utils_mlx.py.
-[] We have group optional in load_mlx_items in utils_mlx.py.
+[X] We have group optional in load_mlx_items in utils_mlx.py.
-[] Dropped add_missing_chat_templates for GptOss in load_mlx_items in utils_mlx.py.
+[X] Dropped add_missing_chat_templates for GptOss in load_mlx_items in utils_mlx.py.
-[] Dropped model.make_cache in make_kv_cache in utils_mlx.py.
+[X] Dropped model.make_cache in make_kv_cache in utils_mlx.py.
 [X] We put cache limit back in utils_mlx.py.
-[] topology.py remove_node removes the connections after checking if node is is in self._node_id_to_rx_id_map. on beta_1 it checks after, so would remove stale connections I guess?
+[X] topology.py remove_node removes the connections after checking if node is is in self._node_id_to_rx_id_map. on beta_1 it checks after, so would remove stale connections I guess?
-[] Missing Glm 4.7 model cards (this isn't ready yet but should be picked up, probably create an issue... the blocker is transforemrs version doesn't support the tokenizer for Glm 4.7. rc-1 does but we can't upgrade as it breaks other things.)
+[X] Missing Glm 4.7 model cards (this isn't ready yet but should be picked up, probably create an issue... the blocker is transforemrs version doesn't support the tokenizer for Glm 4.7. rc-1 does but we can't upgrade as it breaks other things.)
 [] try-except in _command_processor only excepts ValueError. This was silently failing leading to un-debuggable errors (we had a KeyError that was happening ). Changed this to catch Exception instead of ValueError. See exo-v2 89ae38405e0052e3c22405daf094b065878aa873 and fb99fea69b5a39017efc90c5dad0072e677455f0.
 [X] In placement.py, place_instance no longer looks at model_meta.supports_tensor and check if this tensor parallel number of nodes is supported by the model's tensor dimensions.
 [X] In placement.py, place_instanec, we no longer have the special case to exclude DeepSeek v3.1 pipeline parallel (it doesn't work).

src/exo/master/adapters/chat_completions.py

@@ -184,7 +184,9 @@ async def generate_chat_stream(
 async def collect_chat_response(
     command_id: CommandId,
     chunk_stream: AsyncGenerator[ErrorChunk | ToolCallChunk | TokenChunk, None],
-) -> ChatCompletionResponse:
+) -> AsyncGenerator[str]:
+    # This is an AsyncGenerator[str] rather than returning a ChatCompletionReponse because
+    # FastAPI handles the cancellation better but wouldn't auto-serialize for some reason
     """Collect all token chunks and return a single ChatCompletionResponse."""
     text_parts: list[str] = []
     tool_calls: list[ToolCall] = []
@@ -234,7 +236,7 @@ async def collect_chat_response(
     combined_text = "".join(text_parts)
     assert model is not None
 
-    return ChatCompletionResponse(
+    yield ChatCompletionResponse(
         id=command_id,
         created=int(time.time()),
         model=model,
@@ -253,4 +255,5 @@ async def collect_chat_response(
             )
         ],
         usage=last_usage,
-    )
+    ).model_dump_json()
+    return

src/exo/master/adapters/claude.py

@@ -161,7 +161,9 @@ async def collect_claude_response(
     command_id: CommandId,
     model: str,
     chunk_stream: AsyncGenerator[ErrorChunk | ToolCallChunk | TokenChunk, None],
-) -> ClaudeMessagesResponse:
+) -> AsyncGenerator[str]:
+    # This is an AsyncGenerator[str] rather than returning a ChatCompletionReponse because
+    # FastAPI handles the cancellation better but wouldn't auto-serialize for some reason
     """Collect all token chunks and return a single ClaudeMessagesResponse."""
     text_parts: list[str] = []
     tool_use_blocks: list[ClaudeToolUseBlock] = []
@@ -212,7 +214,7 @@ async def collect_claude_response(
     input_tokens = last_usage.prompt_tokens if last_usage else 0
     output_tokens = last_usage.completion_tokens if last_usage else 0
 
-    return ClaudeMessagesResponse(
+    yield ClaudeMessagesResponse(
         id=f"msg_{command_id}",
         model=model,
         content=content,
@@ -221,7 +223,8 @@ async def collect_claude_response(
             input_tokens=input_tokens,
             output_tokens=output_tokens,
         ),
-    )
+    ).model_dump_json()
+    return
 
 
 async def generate_claude_stream(

src/exo/master/adapters/responses.py

@@ -122,7 +122,9 @@ async def collect_responses_response(
     command_id: CommandId,
     model: str,
     chunk_stream: AsyncGenerator[ErrorChunk | ToolCallChunk | TokenChunk, None],
-) -> ResponsesResponse:
+) -> AsyncGenerator[str]:
+    # This is an AsyncGenerator[str] rather than returning a ChatCompletionReponse because
+    # FastAPI handles the cancellation better but wouldn't auto-serialize for some reason
     """Collect all token chunks and return a single ResponsesResponse."""
     response_id = f"resp_{command_id}"
     item_id = f"item_{command_id}"
@@ -173,14 +175,15 @@ async def collect_responses_response(
     ]
     output.extend(function_call_items)
 
-    return ResponsesResponse(
+    yield ResponsesResponse(
         id=response_id,
         model=model,
         status="completed",
         output=output,
         output_text=accumulated_text,
         usage=usage,
-    )
+    ).model_dump_json()
+    return
 
 
 async def generate_responses_stream(

src/exo/master/api.py

@@ -125,6 +125,7 @@ from exo.shared.types.commands import (
     PlaceInstance,
     SendInputChunk,
     StartDownload,
+    TaskCancelled,
     TaskFinished,
     TextGeneration,
 )
@@ -540,16 +541,14 @@ class API:
                         break
 
         except anyio.get_cancelled_exc_class():
-            # TODO: TaskCancelled
+            command = TaskCancelled(cancelled_command_id=command_id)
-            """
+            with anyio.CancelScope(shield=True):
-            self.command_sender.send_nowait(
+                await self.command_sender.send(
-                ForwarderCommand(origin=self.node_id, command=command)
+                    ForwarderCommand(origin=self.node_id, command=command)
-            )
+                )
-            """
             raise
         finally:
-            command = TaskFinished(finished_command_id=command_id)
+            await self._send(TaskFinished(finished_command_id=command_id))
-            await self._send(command)
             if command_id in self._text_generation_queues:
                 del self._text_generation_queues[command_id]
 
@@ -644,11 +643,14 @@ class API:
                     "X-Accel-Buffering": "no",
                 },
             )
-
+        else:
-        return await collect_chat_response(
+            return StreamingResponse(
-            command.command_id,
+                collect_chat_response(
-            self._token_chunk_stream(command.command_id),
+                    command.command_id,
-        )
+                    self._token_chunk_stream(command.command_id),
+                ),
+                media_type="application/json",
+            )
 
     async def bench_chat_completions(
         self, payload: BenchChatCompletionRequest
@@ -664,8 +666,7 @@ class API:
         command = TextGeneration(task_params=task_params)
         await self._send(command)
 
-        response = await self._collect_text_generation_with_stats(command.command_id)
+        return await self._collect_text_generation_with_stats(command.command_id)
-        return response
 
     async def _resolve_and_validate_text_model(self, model_id: ModelId) -> ModelId:
         """Validate a text model exists and return the resolved model ID.
@@ -883,6 +884,11 @@ class API:
                         del image_metadata[key]
 
         except anyio.get_cancelled_exc_class():
+            command = TaskCancelled(cancelled_command_id=command_id)
+            with anyio.CancelScope(shield=True):
+                await self.command_sender.send(
+                    ForwarderCommand(origin=self.node_id, command=command)
+                )
             raise
         finally:
             await self._send(TaskFinished(finished_command_id=command_id))
@@ -964,6 +970,11 @@ class API:
 
             return (images, stats if capture_stats else None)
         except anyio.get_cancelled_exc_class():
+            command = TaskCancelled(cancelled_command_id=command_id)
+            with anyio.CancelScope(shield=True):
+                await self.command_sender.send(
+                    ForwarderCommand(origin=self.node_id, command=command)
+                )
             raise
         finally:
             await self._send(TaskFinished(finished_command_id=command_id))
@@ -1221,12 +1232,15 @@ class API:
                     "X-Accel-Buffering": "no",
                 },
             )
-
+        else:
-        return await collect_claude_response(
+            return StreamingResponse(
-            command.command_id,
+                collect_claude_response(
-            payload.model,
+                    command.command_id,
-            self._token_chunk_stream(command.command_id),
+                    payload.model,
-        )
+                    self._token_chunk_stream(command.command_id),
+                ),
+                media_type="application/json",
+            )
 
     async def openai_responses(
         self, payload: ResponsesRequest
@@ -1254,11 +1268,15 @@ class API:
                 },
             )
 
-        return await collect_responses_response(
+        else:
-            command.command_id,
+            return StreamingResponse(
-            payload.model,
+                collect_responses_response(
-            self._token_chunk_stream(command.command_id),
+                    command.command_id,
-        )
+                    payload.model,
+                    self._token_chunk_stream(command.command_id),
+                ),
+                media_type="application/json",
+            )
 
     def _calculate_total_available_memory(self) -> Memory:
         """Calculate total available memory across all nodes in bytes."""

src/exo/master/main.py

@@ -24,6 +24,7 @@ from exo.shared.types.commands import (
     PlaceInstance,
     RequestEventLog,
     SendInputChunk,
+    TaskCancelled,
     TaskFinished,
     TestCommand,
     TextGeneration,
@@ -39,6 +40,7 @@ from exo.shared.types.events import (
     NodeTimedOut,
     TaskCreated,
     TaskDeleted,
+    TaskStatusUpdated,
     TraceEventData,
     TracesCollected,
     TracesMerged,
@@ -279,7 +281,7 @@ class Master:
                         case DeleteInstance():
                             placement = delete_instance(command, self.state.instances)
                             transition_events = get_transition_events(
-                                self.state.instances, placement
+                                self.state.instances, placement, self.state.tasks
                             )
                             for cmd in cancel_unnecessary_downloads(
                                 placement, self.state.downloads
@@ -299,7 +301,7 @@ class Master:
                                 self.state.node_network,
                             )
                             transition_events = get_transition_events(
-                                self.state.instances, placement
+                                self.state.instances, placement, self.state.tasks
                             )
                             generated_events.extend(transition_events)
                         case CreateInstance():
@@ -309,7 +311,7 @@ class Master:
                                 self.state.instances,
                             )
                             transition_events = get_transition_events(
-                                self.state.instances, placement
+                                self.state.instances, placement, self.state.tasks
                             )
                             generated_events.extend(transition_events)
                         case SendInputChunk(chunk=chunk):
@@ -319,6 +321,18 @@ class Master:
                                     chunk=chunk,
                                 )
                             )
+                        case TaskCancelled():
+                            if (
+                                task_id := self.command_task_mapping.get(
+                                    command.cancelled_command_id
+                                )
+                            ) is not None:
+                                generated_events.append(
+                                    TaskStatusUpdated(
+                                        task_status=TaskStatus.Cancelled,
+                                        task_id=task_id,
+                                    )
+                                )
                         case TaskFinished():
                             generated_events.append(
                                 TaskDeleted(
@@ -327,10 +341,9 @@ class Master:
                                     ]
                                 )
                             )
-                            if command.finished_command_id in self.command_task_mapping:
+                            self.command_task_mapping.pop(
-                                del self.command_task_mapping[
+                                command.finished_command_id, None
-                                    command.finished_command_id
+                            )
-                                ]
                         case RequestEventLog():
                             # We should just be able to send everything, since other buffers will ignore old messages
                             # rate limit to 1000 at a time

src/exo/master/placement.py

@@ -22,9 +22,15 @@ from exo.shared.types.commands import (
     PlaceInstance,
 )
 from exo.shared.types.common import NodeId
-from exo.shared.types.events import Event, InstanceCreated, InstanceDeleted
+from exo.shared.types.events import (
+    Event,
+    InstanceCreated,
+    InstanceDeleted,
+    TaskStatusUpdated,
+)
 from exo.shared.types.memory import Memory
 from exo.shared.types.profiling import MemoryUsage, NodeNetworkInfo
+from exo.shared.types.tasks import Task, TaskId, TaskStatus
 from exo.shared.types.worker.downloads import (
     DownloadOngoing,
     DownloadProgress,
@@ -186,6 +192,7 @@ def delete_instance(
 def get_transition_events(
     current_instances: Mapping[InstanceId, Instance],
     target_instances: Mapping[InstanceId, Instance],
+    tasks: Mapping[TaskId, Task],
 ) -> Sequence[Event]:
     events: list[Event] = []
 
@@ -201,6 +208,18 @@ def get_transition_events(
     # find instances to delete
     for instance_id in current_instances:
         if instance_id not in target_instances:
+            for task in tasks.values():
+                if task.instance_id == instance_id and task.task_status in [
+                    TaskStatus.Pending,
+                    TaskStatus.Running,
+                ]:
+                    events.append(
+                        TaskStatusUpdated(
+                            task_status=TaskStatus.Cancelled,
+                            task_id=task.task_id,
+                        )
+                    )
+
             events.append(
                 InstanceDeleted(
                     instance_id=instance_id,

src/exo/master/tests/test_claude_tool_use.py

@@ -4,7 +4,11 @@ import json
 from collections.abc import AsyncGenerator
 from typing import Any, cast
 
-from exo.master.adapters.claude import collect_claude_response, generate_claude_stream
+from exo.master.adapters.claude import (
+    ClaudeMessagesResponse,
+    collect_claude_response,
+    generate_claude_stream,
+)
 from exo.shared.types.api import ToolCallItem
 from exo.shared.types.chunks import ErrorChunk, TokenChunk, ToolCallChunk
 from exo.shared.types.common import CommandId, ModelId
@@ -17,6 +21,18 @@ async def _chunks_to_stream(
         yield chunk
 
 
+async def _collect_response(
+    command_id: CommandId,
+    model: str,
+    chunk_stream: AsyncGenerator[ErrorChunk | ToolCallChunk | TokenChunk, None],
+) -> ClaudeMessagesResponse:
+    """Helper to consume the async generator and parse the JSON response."""
+    parts: list[str] = []
+    async for part in collect_claude_response(command_id, model, chunk_stream):
+        parts.append(part)
+    return ClaudeMessagesResponse.model_validate_json("".join(parts))
+
+
 MODEL = ModelId("test-model")
 COMMAND_ID = CommandId("cmd_test123")
 
@@ -47,7 +63,7 @@ class TestCollectClaudeResponseToolUse:
                 ],
             ),
         ]
-        response = await collect_claude_response(
+        response = await _collect_response(
             COMMAND_ID, "test-model", _chunks_to_stream(chunks)
         )
 
@@ -77,7 +93,7 @@ class TestCollectClaudeResponseToolUse:
                 ],
             ),
         ]
-        response = await collect_claude_response(
+        response = await _collect_response(
             COMMAND_ID, "test-model", _chunks_to_stream(chunks)
         )
 
@@ -102,7 +118,7 @@ class TestCollectClaudeResponseToolUse:
                 ],
             ),
         ]
-        response = await collect_claude_response(
+        response = await _collect_response(
             COMMAND_ID, "test-model", _chunks_to_stream(chunks)
         )
 
@@ -116,7 +132,7 @@ class TestCollectClaudeResponseToolUse:
 
     async def test_no_content_produces_empty_text_block(self):
         chunks: list[ErrorChunk | ToolCallChunk | TokenChunk] = []
-        response = await collect_claude_response(
+        response = await _collect_response(
             COMMAND_ID, "test-model", _chunks_to_stream(chunks)
         )
         assert len(response.content) == 1

src/exo/master/tests/test_placement.py

@@ -239,7 +239,7 @@ def test_get_transition_events_no_change(instance: Instance):
     target_instances = {instance_id: instance}
 
     # act
-    events = get_transition_events(current_instances, target_instances)
+    events = get_transition_events(current_instances, target_instances, {})
 
     # assert
     assert len(events) == 0
@@ -252,7 +252,7 @@ def test_get_transition_events_create_instance(instance: Instance):
     target_instances: dict[InstanceId, Instance] = {instance_id: instance}
 
     # act
-    events = get_transition_events(current_instances, target_instances)
+    events = get_transition_events(current_instances, target_instances, {})
 
     # assert
     assert len(events) == 1
@@ -266,7 +266,7 @@ def test_get_transition_events_delete_instance(instance: Instance):
     target_instances: dict[InstanceId, Instance] = {}
 
     # act
-    events = get_transition_events(current_instances, target_instances)
+    events = get_transition_events(current_instances, target_instances, {})
 
     # assert
     assert len(events) == 1

src/exo/shared/types/commands.py

@@ -48,6 +48,10 @@ class DeleteInstance(BaseCommand):
     instance_id: InstanceId
 
 
+class TaskCancelled(BaseCommand):
+    cancelled_command_id: CommandId
+
+
 class TaskFinished(BaseCommand):
     finished_command_id: CommandId
 
@@ -89,6 +93,7 @@ Command = (
     | PlaceInstance
     | CreateInstance
     | DeleteInstance
+    | TaskCancelled
     | TaskFinished
     | SendInputChunk
 )

src/exo/shared/types/tasks.py

@@ -24,6 +24,7 @@ class TaskStatus(str, Enum):
     Complete = "Complete"
     TimedOut = "TimedOut"
     Failed = "Failed"
+    Cancelled = "Cancelled"
 
 
 class BaseTask(TaggedModel):
@@ -60,6 +61,11 @@ class TextGeneration(BaseTask):  # emitted by Master
     error_message: str | None = Field(default=None)
 
 
+class CancelTask(BaseTask):
+    cancelled_task_id: TaskId
+    runner_id: RunnerId
+
+
 class ImageGeneration(BaseTask):  # emitted by Master
     command_id: CommandId
     task_params: ImageGenerationTaskParams
@@ -87,6 +93,7 @@ Task = (
     | LoadModel
     | StartWarmup
     | TextGeneration
+    | CancelTask
     | ImageGeneration
     | ImageEdits
     | Shutdown

src/exo/utils/channels.py

@@ -125,7 +125,9 @@ class MpSender[T]:
             self._state.buffer.put(item, block=True)
 
     async def send_async(self, item: T) -> None:
-        await to_thread.run_sync(self.send, item, limiter=CapacityLimiter(1))
+        await to_thread.run_sync(
+            self.send, item, limiter=CapacityLimiter(1), abandon_on_cancel=True
+        )
 
     def close(self) -> None:
         if not self._state.closed.is_set():

src/exo/worker/engines/mlx/utils_mlx.py

@@ -64,8 +64,6 @@ from exo.worker.runner.bootstrap import logger
 Group = mx.distributed.Group
 
 
-# TODO: Test this
-#  ALSO https://github.com/exo-explore/exo/pull/233#discussion_r2549683673
 def get_weights_size(model_shard_meta: ShardMetadata) -> Memory:
     return Memory.from_float_kb(
         (model_shard_meta.end_layer - model_shard_meta.start_layer)
@@ -83,30 +81,6 @@ class ModelLoadingTimeoutError(Exception):
     pass
 
 
-def mx_barrier(group: Group | None = None):
-    mx.eval(
-        mx.distributed.all_sum(
-            mx.array(1.0),
-            stream=mx.default_stream(mx.Device(mx.cpu)),
-            group=group,
-        )
-    )
-
-
-def broadcast_from_zero(value: int, group: Group | None = None):
-    if group is None:
-        return value
-
-    if group.rank() == 0:
-        a = mx.array([value], dtype=mx.int32)
-    else:
-        a = mx.array([0], dtype=mx.int32)
-
-    m = mx.distributed.all_sum(a, stream=mx.Device(mx.DeviceType.cpu), group=group)
-    mx.eval(m)
-    return int(m.item())
-
-
 class HostList(RootModel[list[str]]):
     @classmethod
     def from_hosts(cls, hosts: list[Host]) -> "HostList":
@@ -591,3 +565,23 @@ def mlx_cleanup(
     import gc
 
     gc.collect()
+
+
+def mx_any(bool_: bool, group: Group | None) -> bool:
+    if group is None:
+        return bool_
+    num_true = mx.distributed.all_sum(
+        mx.array(bool_), group=group, stream=mx.default_stream(mx.Device(mx.cpu))
+    )
+    mx.eval(num_true)
+    return num_true.item() > 0
+
+
+def mx_barrier(group: Group | None):
+    if group is None:
+        return
+    mx.eval(
+        mx.distributed.all_sum(
+            mx.array(1.0), group=group, stream=mx.default_stream(mx.Device(mx.cpu))
+        )
+    )

src/exo/worker/main.py

@@ -33,6 +33,7 @@ from exo.shared.types.events import (
 from exo.shared.types.multiaddr import Multiaddr
 from exo.shared.types.state import State
 from exo.shared.types.tasks import (
+    CancelTask,
     CreateRunner,
     DownloadModel,
     ImageEdits,
@@ -224,15 +225,22 @@ class Worker:
                         )
                     )
                 case Shutdown(runner_id=runner_id):
+                    runner = self.runners.pop(runner_id)
                     try:
                         with fail_after(3):
-                            await self.runners.pop(runner_id).start_task(task)
+                            await runner.start_task(task)
                     except TimeoutError:
                         await self.event_sender.send(
                             TaskStatusUpdated(
                                 task_id=task.task_id, task_status=TaskStatus.TimedOut
                             )
                         )
+                    finally:
+                        runner.shutdown()
+                case CancelTask(
+                    cancelled_task_id=cancelled_task_id, runner_id=runner_id
+                ):
+                    await self.runners[runner_id].cancel_task(cancelled_task_id)
                 case ImageEdits() if task.task_params.total_input_chunks > 0:
                     # Assemble image from chunks and inject into task
                     cmd_id = task.command_id
@@ -270,18 +278,18 @@ class Worker:
                         del self.input_chunk_buffer[cmd_id]
                     if cmd_id in self.input_chunk_counts:
                         del self.input_chunk_counts[cmd_id]
-                    await self.runners[self._task_to_runner_id(task)].start_task(
+                    await self._start_runner_task(modified_task)
-                        modified_task
-                    )
                 case task:
-                    await self.runners[self._task_to_runner_id(task)].start_task(task)
+                    await self._start_runner_task(task)
 
     def shutdown(self):
         self._tg.cancel_scope.cancel()
 
-    def _task_to_runner_id(self, task: Task):
+    async def _start_runner_task(self, task: Task):
-        instance = self.state.instances[task.instance_id]
+        if (instance := self.state.instances.get(task.instance_id)) is not None:
-        return instance.shard_assignments.node_to_runner[self.node_id]
+            await self.runners[
+                instance.shard_assignments.node_to_runner[self.node_id]
+            ].start_task(task)
 
     async def _nack_request(self, since_idx: int) -> None:
         # We request all events after (and including) the missing index.
@@ -320,8 +328,6 @@ class Worker:
             for event in self.out_for_delivery.copy().values():
                 await self.local_event_sender.send(event)
 
-    ## Op Executors
-
     def _create_supervisor(self, task: CreateRunner) -> RunnerSupervisor:
         """Creates and stores a new AssignedRunner with initial downloading status."""
         runner = RunnerSupervisor.create(

src/exo/worker/plan.py

@@ -4,6 +4,7 @@ from collections.abc import Mapping, Sequence
 
 from exo.shared.types.common import CommandId, NodeId
 from exo.shared.types.tasks import (
+    CancelTask,
     ConnectToGroup,
     CreateRunner,
     DownloadModel,
@@ -53,13 +54,14 @@ def plan(
 ) -> Task | None:
     # Python short circuiting OR logic should evaluate these sequentially.
     return (
-        _kill_runner(runners, all_runners, instances)
+        _cancel_tasks(runners, tasks)
+        or _kill_runner(runners, all_runners, instances)
         or _create_runner(node_id, runners, instances)
         or _model_needs_download(node_id, runners, global_download_status)
         or _init_distributed_backend(runners, all_runners)
         or _load_model(runners, all_runners, global_download_status)
         or _ready_to_warmup(runners, all_runners)
-        or _pending_tasks(runners, tasks, all_runners, input_chunk_buffer)
+        or _pending_tasks(runners, tasks, all_runners, input_chunk_buffer or {})
     )
 
 
@@ -270,7 +272,7 @@ def _pending_tasks(
     runners: Mapping[RunnerId, RunnerSupervisor],
     tasks: Mapping[TaskId, Task],
     all_runners: Mapping[RunnerId, RunnerStatus],
-    input_chunk_buffer: Mapping[CommandId, dict[int, str]] | None = None,
+    input_chunk_buffer: Mapping[CommandId, dict[int, str]],
 ) -> Task | None:
     for task in tasks.values():
         # for now, just forward chat completions
@@ -284,7 +286,7 @@ def _pending_tasks(
         if isinstance(task, ImageEdits) and task.task_params.total_input_chunks > 0:
             cmd_id = task.command_id
             expected = task.task_params.total_input_chunks
-            received = len((input_chunk_buffer or {}).get(cmd_id, {}))
+            received = len(input_chunk_buffer.get(cmd_id, {}))
             if received < expected:
                 continue  # Wait for all chunks to arrive
 
@@ -292,16 +294,33 @@ def _pending_tasks(
             if task.instance_id != runner.bound_instance.instance.instance_id:
                 continue
 
-            # I have a design point here; this is a state race in disguise as the task status doesn't get updated to completed fast enough
+            # the task status _should_ be set to completed by the LAST runner
-            # however, realistically the task status should be set to completed by the LAST runner, so this is a true race
+            # it is currently set by the first
-            # the actual solution is somewhat deeper than this bypass - TODO!
+            # this is definitely a hack
             if task.task_id in runner.completed:
                 continue
 
-            # TODO: Check ordering aligns with MLX distributeds expectations.
-
             if isinstance(runner.status, RunnerReady) and all(
                 isinstance(all_runners[global_runner_id], (RunnerReady, RunnerRunning))
                 for global_runner_id in runner.bound_instance.instance.shard_assignments.runner_to_shard
             ):
                 return task
+
+
+def _cancel_tasks(
+    runners: Mapping[RunnerId, RunnerSupervisor],
+    tasks: Mapping[TaskId, Task],
+) -> Task | None:
+    for task in tasks.values():
+        if task.task_status != TaskStatus.Cancelled:
+            continue
+        for runner_id, runner in runners.items():
+            if task.instance_id != runner.bound_instance.instance.instance_id:
+                continue
+            if task.task_id in runner.cancelled:
+                continue
+            return CancelTask(
+                instance_id=task.instance_id,
+                cancelled_task_id=task.task_id,
+                runner_id=runner_id,
+            )

src/exo/worker/runner/bootstrap.py

@@ -3,7 +3,7 @@ import os
 import loguru
 
 from exo.shared.types.events import Event, RunnerStatusUpdated
-from exo.shared.types.tasks import Task
+from exo.shared.types.tasks import Task, TaskId
 from exo.shared.types.worker.instances import BoundInstance, MlxJacclInstance
 from exo.shared.types.worker.runners import RunnerFailed
 from exo.utils.channels import ClosedResourceError, MpReceiver, MpSender
@@ -15,6 +15,7 @@ def entrypoint(
     bound_instance: BoundInstance,
     event_sender: MpSender[Event],
     task_receiver: MpReceiver[Task],
+    cancel_receiver: MpReceiver[TaskId],
     _logger: "loguru.Logger",
 ) -> None:
     fast_synch_override = os.environ.get("EXO_FAST_SYNCH")
@@ -38,7 +39,7 @@ def entrypoint(
     try:
         from exo.worker.runner.runner import main
 
-        main(bound_instance, event_sender, task_receiver)
+        main(bound_instance, event_sender, task_receiver, cancel_receiver)
     except ClosedResourceError:
         logger.warning("Runner communication closed unexpectedly")
     except Exception as e:

src/exo/worker/runner/runner.py

@@ -1,5 +1,6 @@
 import base64
 import json
+import math
 import resource
 import time
 from collections.abc import Generator
@@ -88,6 +89,7 @@ from exo.worker.engines.mlx.utils_mlx import (
     initialize_mlx,
     load_mlx_items,
     mlx_force_oom,
+    mx_any,
 )
 from exo.worker.runner.bootstrap import logger
 
@@ -112,6 +114,7 @@ def main(
     bound_instance: BoundInstance,
     event_sender: MpSender[Event],
     task_receiver: MpReceiver[Task],
+    cancel_receiver: MpReceiver[TaskId],
 ):
     soft, hard = resource.getrlimit(resource.RLIMIT_NOFILE)
     resource.setrlimit(resource.RLIMIT_NOFILE, (min(max(soft, 2048), hard), hard))
@@ -129,11 +132,15 @@ def main(
         time.sleep(timeout)
 
     setup_start_time = time.time()
+    cancelled_tasks = set[TaskId]()
 
-    model: Model | DistributedImageModel | None = None
+    # type checker was unhappy with me - splitting these fixed it
+    inference_model: Model | None = None
+    image_model: DistributedImageModel | None = None
     tokenizer = None
     group = None
     kv_prefix_cache: KVPrefixCache | None = None
+    check_for_cancel_every: int | None = None
 
     current_status: RunnerStatus = RunnerIdle()
     logger.info("runner created")
@@ -146,6 +153,7 @@ def main(
             if task.task_id in seen:
                 logger.warning("repeat task - potential error")
             seen.add(task.task_id)
+            cancelled_tasks.discard(TaskId("CANCEL_CURRENT_TASK"))
             event_sender.send(
                 TaskStatusUpdated(task_id=task.task_id, task_status=TaskStatus.Running)
             )
@@ -191,7 +199,7 @@ def main(
                         time.sleep(0.5)
 
                     if ModelTask.TextGeneration in shard_metadata.model_card.tasks:
-                        model, tokenizer = load_mlx_items(
+                        inference_model, tokenizer = load_mlx_items(
                             bound_instance, group, on_timeout=on_model_load_timeout
                         )
                         logger.info(
@@ -203,7 +211,7 @@ def main(
                         ModelTask.TextToImage in shard_metadata.model_card.tasks
                         or ModelTask.ImageToImage in shard_metadata.model_card.tasks
                     ):
-                        model = initialize_image_model(bound_instance)
+                        image_model = initialize_image_model(bound_instance)
                     else:
                         raise ValueError(
                             f"Unknown model task(s): {shard_metadata.model_card.tasks}"
@@ -211,8 +219,6 @@ def main(
                     current_status = RunnerLoaded()
                     logger.info("runner loaded")
                 case StartWarmup() if isinstance(current_status, RunnerLoaded):
-                    assert model
-
                     current_status = RunnerWarmingUp()
                     logger.info("runner warming up")
                     event_sender.send(
@@ -224,16 +230,31 @@ def main(
 
                     logger.info(f"warming up inference for instance: {instance}")
                     if ModelTask.TextGeneration in shard_metadata.model_card.tasks:
-                        assert not isinstance(model, DistributedImageModel)
+                        assert inference_model
                         assert tokenizer
 
+                        t = time.monotonic()
                         toks = warmup_inference(
-                            model=model,
+                            model=inference_model,
                             tokenizer=tokenizer,
                             group=group,
-                            # kv_prefix_cache=kv_prefix_cache,  # supply for warmup-time prefix caching
                         )
                         logger.info(f"warmed up by generating {toks} tokens")
+                        check_for_cancel_every = min(
+                            math.ceil(toks / min(time.monotonic() - t, 0.001)), 100
+                        )
+                        if group is not None:
+                            check_for_cancel_every = int(
+                                mx.max(
+                                    mx.distributed.all_gather(
+                                        mx.array([check_for_cancel_every]), group=group
+                                    )
+                                ).item()
+                            )
+
+                        logger.info(
+                            f"runner checking for cancellation every {check_for_cancel_every} tokens"
+                        )
                         logger.info(
                             f"runner initialized in {time.time() - setup_start_time} seconds"
                         )
@@ -241,8 +262,8 @@ def main(
                         ModelTask.TextToImage in shard_metadata.model_card.tasks
                         or ModelTask.ImageToImage in shard_metadata.model_card.tasks
                     ):
-                        assert isinstance(model, DistributedImageModel)
+                        assert image_model
-                        image = warmup_image_generator(model=model)
+                        image = warmup_image_generator(model=image_model)
                         if image is not None:
                             logger.info(f"warmed up by generating {image.size} image")
                         else:
@@ -262,9 +283,9 @@ def main(
                         )
                     )
                     event_sender.send(TaskAcknowledged(task_id=task.task_id))
-
+                    assert inference_model
-                    assert model and not isinstance(model, DistributedImageModel)
                     assert tokenizer
+                    assert check_for_cancel_every
 
                     try:
                         _check_for_debug_prompts(task_params)
@@ -274,7 +295,7 @@ def main(
 
                         # Generate responses using the actual MLX generation
                         mlx_generator = mlx_generate(
-                            model=model,
+                            model=inference_model,
                             tokenizer=tokenizer,
                             task=task_params,
                             prompt=prompt,
@@ -299,11 +320,11 @@ def main(
                             patch_glm_tokenizer(tokenizer)
 
                         # GPT-OSS specific parsing to match other model formats.
-                        elif isinstance(model, GptOssModel):
+                        elif isinstance(inference_model, GptOssModel):
                             mlx_generator = parse_gpt_oss(mlx_generator)
 
                         if tokenizer.has_tool_calling and not isinstance(
-                            model, GptOssModel
+                            inference_model, GptOssModel
                         ):
                             assert tokenizer.tool_call_start
                             assert tokenizer.tool_call_end
@@ -316,7 +337,18 @@ def main(
                             )
 
                         completion_tokens = 0
+                        tokens_since_last_cancel_check = 0
                         for response in mlx_generator:
+                            tokens_since_last_cancel_check += 1
+                            if tokens_since_last_cancel_check >= check_for_cancel_every:
+                                tokens_since_last_cancel_check = 0
+                                cancelled_tasks.update(cancel_receiver.collect())
+                                want_to_cancel = (task.task_id in cancelled_tasks) or (
+                                    TaskId("CANCEL_CURRENT_TASK") in cancelled_tasks
+                                )
+                                if mx_any(want_to_cancel, group):
+                                    break
+
                             match response:
                                 case GenerationResponse():
                                     completion_tokens += 1
@@ -389,7 +421,7 @@ def main(
                 case ImageGeneration(
                     task_params=task_params, command_id=command_id
                 ) if isinstance(current_status, RunnerReady):
-                    assert isinstance(model, DistributedImageModel)
+                    assert image_model
                     logger.info(f"received image generation request: {str(task)[:500]}")
                     current_status = RunnerRunning()
                     logger.info("runner running")
@@ -402,7 +434,9 @@ def main(
 
                     try:
                         image_index = 0
-                        for response in generate_image(model=model, task=task_params):
+                        for response in generate_image(
+                            model=image_model, task=task_params
+                        ):
                             is_primary_output = _is_primary_output_node(shard_metadata)
 
                             if is_primary_output:
@@ -452,7 +486,7 @@ def main(
                 case ImageEdits(task_params=task_params, command_id=command_id) if (
                     isinstance(current_status, RunnerReady)
                 ):
-                    assert isinstance(model, DistributedImageModel)
+                    assert image_model
                     logger.info(f"received image edits request: {str(task)[:500]}")
                     current_status = RunnerRunning()
                     logger.info("runner running")
@@ -465,7 +499,9 @@ def main(
 
                     try:
                         image_index = 0
-                        for response in generate_image(model=model, task=task_params):
+                        for response in generate_image(
+                            model=image_model, task=task_params
+                        ):
                             if _is_primary_output_node(shard_metadata):
                                 match response:
                                     case PartialImageResponse():
@@ -524,14 +560,20 @@ def main(
                     raise ValueError(
                         f"Received {task.__class__.__name__} outside of state machine in {current_status=}"
                     )
-            event_sender.send(
+            was_cancelled = (task.task_id in cancelled_tasks) or (
-                TaskStatusUpdated(task_id=task.task_id, task_status=TaskStatus.Complete)
+                TaskId("CANCEL_CURRENT_TASK") in cancelled_tasks
             )
+            if not was_cancelled:
+                event_sender.send(
+                    TaskStatusUpdated(
+                        task_id=task.task_id, task_status=TaskStatus.Complete
+                    )
+                )
             event_sender.send(
                 RunnerStatusUpdated(runner_id=runner_id, runner_status=current_status)
             )
             if isinstance(current_status, RunnerShutdown):
-                del model, tokenizer, group
+                del inference_model, image_model, tokenizer, group
                 mx.clear_cache()
                 import gc
 

src/exo/worker/runner/runner_supervisor.py

@@ -47,9 +47,11 @@ class RunnerSupervisor:
     _ev_recv: MpReceiver[Event]
     _task_sender: MpSender[Task]
     _event_sender: Sender[Event]
+    _cancel_sender: MpSender[TaskId]
     status: RunnerStatus = field(default_factory=RunnerIdle, init=False)
     pending: dict[TaskId, anyio.Event] = field(default_factory=dict, init=False)
     completed: set[TaskId] = field(default_factory=set, init=False)
+    cancelled: set[TaskId] = field(default_factory=set, init=False)
 
     @classmethod
     def create(
@@ -60,8 +62,8 @@ class RunnerSupervisor:
         initialize_timeout: float = 400,
     ) -> Self:
         ev_send, ev_recv = mp_channel[Event]()
-        # A task is kind of a runner command
         task_sender, task_recv = mp_channel[Task]()
+        cancel_sender, cancel_recv = mp_channel[TaskId]()
 
         runner_process = Process(
             target=entrypoint,
@@ -69,6 +71,7 @@ class RunnerSupervisor:
                 bound_instance,
                 ev_send,
                 task_recv,
+                cancel_recv,
                 logger,
             ),
             daemon=True,
@@ -83,6 +86,7 @@ class RunnerSupervisor:
             initialize_timeout=initialize_timeout,
             _ev_recv=ev_recv,
             _task_sender=task_sender,
+            _cancel_sender=cancel_sender,
             _event_sender=event_sender,
         )
 
@@ -97,6 +101,8 @@ class RunnerSupervisor:
         self._ev_recv.close()
         self._task_sender.close()
         self._event_sender.close()
+        self._cancel_sender.send(TaskId("CANCEL_CURRENT_TASK"))
+        self._cancel_sender.close()
         self.runner_process.join(1)
         if not self.runner_process.is_alive():
             logger.info("Runner process succesfully terminated")
@@ -112,14 +118,6 @@ class RunnerSupervisor:
         logger.critical("Runner process didn't respond to SIGTERM, killing")
         self.runner_process.kill()
 
-        self.runner_process.join(1)
-        if not self.runner_process.is_alive():
-            return
-
-        logger.critical(
-            "Runner process didn't respond to SIGKILL. System resources may have leaked"
-        )
-
     async def start_task(self, task: Task):
         if task.task_id in self.pending:
             logger.warning(
@@ -141,6 +139,17 @@ class RunnerSupervisor:
             return
         await event.wait()
 
+    async def cancel_task(self, task_id: TaskId):
+        if task_id in self.completed:
+            logger.info(f"Unable to cancel {task_id} as it has been completed")
+            return
+        self.cancelled.add(task_id)
+        with anyio.move_on_after(0.5) as scope:
+            await self._cancel_sender.send_async(task_id)
+        if scope.cancel_called:
+            logger.error("RunnerSupervisor cancel pipe blocked")
+            await self._check_runner(TimeoutError("cancel pipe blocked"))
+
     async def _forward_events(self):
         with self._ev_recv as events:
             try:

src/exo/worker/tests/unittests/test_runner/test_event_ordering.py

@@ -1,7 +1,9 @@
 # Check tasks are complete before runner is ever ready.
+import unittest.mock
 from collections.abc import Iterable
 from typing import Callable
 
+import mlx.core as mx
 import pytest
 
 import exo.worker.runner.runner as mlx_runner
@@ -19,6 +21,7 @@ from exo.shared.types.tasks import (
     Shutdown,
     StartWarmup,
     Task,
+    TaskId,
     TaskStatus,
     TextGeneration,
 )
@@ -113,6 +116,7 @@ def patch_out_mlx(monkeypatch: pytest.MonkeyPatch):
     monkeypatch.setattr(mlx_runner, "load_mlx_items", make_nothin((1, MockTokenizer)))
     monkeypatch.setattr(mlx_runner, "warmup_inference", make_nothin(1))
     monkeypatch.setattr(mlx_runner, "_check_for_debug_prompts", nothin)
+    monkeypatch.setattr(mlx_runner, "mx_any", make_nothin(False))
     # Mock apply_chat_template since we're using a fake tokenizer (integer 1).
     # Returns a prompt without thinking tag so detect_thinking_prompt_suffix returns None.
     monkeypatch.setattr(mlx_runner, "apply_chat_template", make_nothin("test prompt"))
@@ -163,6 +167,7 @@ def _run(tasks: Iterable[Task]):
     )
 
     task_sender, task_receiver = mp_channel[Task]()
+    _cancel_sender, cancel_receiver = mp_channel[TaskId]()
     event_sender = EventCollector()
 
     with task_sender:
@@ -173,8 +178,16 @@ def _run(tasks: Iterable[Task]):
         # this is some c++ nonsense
         task_receiver.close = nothin
         task_receiver.join = nothin
-
+        with unittest.mock.patch(
-        mlx_runner.main(bound_instance, event_sender, task_receiver)  # type: ignore[arg-type]
+            "exo.worker.runner.runner.mx.distributed.all_gather",
+            make_nothin(mx.array([1])),
+        ):
+            mlx_runner.main(
+                bound_instance,
+                event_sender,  # pyright: ignore[reportArgumentType]
+                task_receiver,
+                cancel_receiver,
+            )
 
         return event_sender.events