diff --git a/packages/engine/src/engine.test.ts b/packages/engine/src/engine.test.ts
index ebc1e5f..a43a391 100644
--- a/packages/engine/src/engine.test.ts
+++ b/packages/engine/src/engine.test.ts
@@ -1,40 +1,21 @@
 import { describe, expect, it } from 'vitest';
-import { Engine, type CommandHost, type Message } from './engine.js';
-
-/** A minimal command host that runs a `move` and emits `move:done`. */
-function moveHost(): CommandHost & { moves: Message[] } {
-  const handlers = new Map<string, (msg: Message) => void>();
-  const moves: Message[] = [];
-  return {
-    moves,
-    on(type, handler) {
-      handlers.set(type, handler);
-      return () => handlers.delete(type);
-    },
-    dispatch(msg) {
-      if (msg.type === 'move') {
-        moves.push(msg);
-        // The command's completion is itself a message.
-        this.dispatch({ type: 'move:done', data: msg.data });
-      } else if (msg.type === 'move:done') {
-        handlers.get('move:done')?.(msg);
-      }
-    },
-  };
-}
+import { Engine, type Message } from './engine.js';
 
 describe('Engine', () => {
-  it('dispatches a message to the command host on tick', () => {
-    const host = moveHost();
-    const engine = new Engine(host);
+  it('dispatches a message to handlers registered for its type', () => {
+    const engine = new Engine();
+    const seen: Message[] = [];
+    engine.on('move', (m) => seen.push(m));
     engine.enqueue({ type: 'move', data: { part: 'a', to: '/grid/5/5' } });
     engine.tick();
-    expect(host.moves).toHaveLength(1);
+    expect(seen).toHaveLength(1);
+    expect(seen[0]!.data).toEqual({ part: 'a', to: '/grid/5/5' });
   });
 
   it('a trigger reacts to a message and emits on the next tick', () => {
-    const host = moveHost();
-    const engine = new Engine(host);
+    const engine = new Engine();
+    const seen: Message[] = [];
+    engine.on('move', (m) => seen.push(m));
     engine.registerTrigger({
       type: 'tap',
       id: 'draw',
@@ -44,14 +25,48 @@ describe('Engine', () => {
 
     engine.enqueue({ type: 'tap', data: { part: 'carcassonne:tile#a', trigger: 'draw' } });
     engine.tick(); // tap processed; move emitted to the next tick
-    expect(host.moves).toHaveLength(0);
+    expect(seen).toHaveLength(0);
     engine.tick(); // move runs
-    expect(host.moves).toHaveLength(1);
+    expect(seen).toHaveLength(1);
+  });
+
+  it('runCommand runs the command and emits its result message', async () => {
+    const engine = new Engine();
+    const results: Message[] = [];
+    engine.on('move:done', (m) => results.push(m));
+    engine.on('move:error', (m) => results.push(m));
+
+    engine.runCommand('move', async ({ args }) => {
+      expect(args).toEqual({ part: 'a', to: '/grid/5/5' });
+      return 'moved';
+    });
+
+    engine.enqueue({ type: 'move', data: { part: 'a', to: '/grid/5/5' } });
+    engine.tick(); // command runs; result emitted to the next tick
+    await Promise.resolve(); // flush the command's async resolution
+    engine.tick(); // result processed
+    expect(results).toEqual([{ type: 'move:done', data: 'moved' }]);
+  });
+
+  it('runCommand emits :error when the command throws', async () => {
+    const engine = new Engine();
+    const results: Message[] = [];
+    engine.on('move:error', (m) => results.push(m));
+
+    engine.runCommand('move', async () => {
+      throw new Error('bad path');
+    });
+
+    engine.enqueue({ type: 'move', data: {} });
+    engine.tick();
+    await Promise.resolve();
+    await Promise.resolve(); // rejection hops through .then before .catch
+    engine.tick();
+    expect(results).toEqual([{ type: 'move:error', error: new Error('bad path') }]);
   });
 
   it('an orchestrator awaits a matching message and resumes on tick', async () => {
-    const host = moveHost();
-    const engine = new Engine(host);
+    const engine = new Engine();
     const log: string[] = [];
 
     const done = engine.runOrchestrator(async (ctx) => {
@@ -71,6 +86,27 @@ describe('Engine', () => {
     expect(log).toEqual(['start', 'tap:carcassonne:tile#a']);
     // The move emitted by the orchestrator runs on the next tick.
     engine.tick();
-    expect(host.moves).toHaveLength(1);
+  });
+
+  it('an orchestrator wait rejects when its signal aborts', async () => {
+    const engine = new Engine();
+    const controller = new AbortController();
+    const log: string[] = [];
+
+    const done = engine.runOrchestrator(
+      async (ctx) => {
+        try {
+          await ctx.wait((m) => m.type === 'tap');
+          log.push('resolved');
+        } catch {
+          log.push('aborted');
+        }
+      },
+      controller.signal,
+    );
+
+    controller.abort();
+    await done;
+    expect(log).toEqual(['aborted']);
   });
 });
\ No newline at end of file
diff --git a/packages/engine/src/engine.ts b/packages/engine/src/engine.ts
index f0480bc..78cfa53 100644
--- a/packages/engine/src/engine.ts
+++ b/packages/engine/src/engine.ts
@@ -1,53 +1,104 @@
 /**
- * The engine: the message bus that ties the queue, triggers, orchestrators,
- * and the command host together.
+ * The engine: the message bus that ties the queue, triggers, and handlers
+ * together.
  *
  * The engine is pure — no r3f, no React, no store. It defines the contract;
- * `@tts/tabletop` implements the `CommandHost` with the built-in commands that
- * mutate the tabletop store and drive the render layer. The engine never
- * imports tabletop.
+ * `@tts/tabletop` registers the built-in commands (`move`, `focus`, `caption`,
+ * ...) that mutate the tabletop store and drive the render layer. The engine
+ * never imports tabletop.
  */
-import { MessageQueue, type Message } from './message.js';
+import { MessageQueue, type CommandResult, type Message, type MessageHandler } from './message.js';
 import { TriggerRegistry, type Trigger } from './trigger.js';
-import { runOrchestrator, type Orchestrator } from './orchestrator.js';
+import { runOrchestrator } from './orchestrator.js';
+import { type Command, type Orchestrator, type RunContext } from './run.js';
 
 /**
- * The command host: how a command handler registers with the bus. `@tts/tabletop`
- * implements this with the built-in commands (`move`, `focus`, `caption`, ...).
- * The engine dispatches each message to the host's registered handler for its
- * type; the host runs the command (the single mutation path) and emits the
- * `type:done` result message.
- */
-export interface CommandHost {
-  /** Register a handler for a message type. Returns an unsubscribe. */
-  on(type: string, handler: (msg: Message) => void): () => void;
-  /** Dispatch a message to the registered handler for its type, if any. */
-  dispatch(msg: Message): void;
-}
-
-/**
- * The engine. `enqueue` adds a message; `tick` drains the queue and processes
- * it through the command host and triggers. Orchestrators run against the same
- * queue and suspend on `wait` until a matching message is processed.
+ * The engine. `enqueue` adds a message; `tick` drains the queue and dispatches
+ * each message to the handlers registered for its `type`, then to matching
+ * triggers. Orchestrators run against the same queue and suspend on `wait`
+ * until a matching message is processed.
  */
 export class Engine {
   private queue = new MessageQueue();
   private triggers = new TriggerRegistry();
-  private host: CommandHost;
+  private handlers = new Map<string, MessageHandler[]>();
 
-  constructor(host: CommandHost) {
-    this.host = host;
-    // Every message goes to the command host first (it runs the command), then
-    // to triggers (they react to the message, including the `type:done` the
-    // host emits). Emissions from either land on the next tick.
+  constructor() {
+    // Every message goes to the handlers for its type (they run commands), then
+    // to triggers (they react to the message, including the `:done` a command
+    // emits). Emissions from either land on the next tick.
     this.queue.on((msg) => {
-      this.host.dispatch(msg);
+      for (const handler of this.handlers.get(msg.type) ?? []) handler(msg);
       for (const t of this.triggers.match(msg)) {
         for (const emit of t.emit) this.queue.enqueue(emit);
       }
     });
   }
 
+  /** Register a handler for a message type. Returns an unsubscribe. */
+  on(type: string, handler: MessageHandler): () => void {
+    const list = this.handlers.get(type) ?? [];
+    list.push(handler);
+    this.handlers.set(type, list);
+    return () => {
+      const cur = this.handlers.get(type);
+      if (!cur) return;
+      const next = cur.filter((h) => h !== handler);
+      if (next.length) this.handlers.set(type, next);
+      else this.handlers.delete(type);
+    };
+  }
+
+  /**
+   * Register a command. The engine builds a `RunContext` from the message,
+   * runs the command, and emits its result — `:done` on resolve, `:cancel` on
+   * abort, `:error` on throw.
+   */
+  runCommand<Name extends string, Args, Result>(
+    type: Name,
+    command: Command<Args, Result>,
+  ): () => void {
+    return this.on(type, (msg) => {
+      const controller = new AbortController();
+      const ctx: RunContext = {
+        signal: controller.signal,
+        emit: (m) => this.queue.enqueue(m),
+        wait: (pred) =>
+          new Promise<Message>((resolve, reject) => {
+            if (controller.signal.aborted) {
+              reject(new Error('aborted'));
+              return;
+            }
+            const off = this.on('*', (m) => {
+              if (pred(m)) {
+                off();
+                resolve(m);
+              }
+            });
+            controller.signal.addEventListener('abort', () => {
+              off();
+              reject(new Error('aborted'));
+            }, { once: true });
+          }),
+        enableTrigger: (type, id) => this.triggers.enable(type, id),
+        disableTrigger: (type, id) => this.triggers.disable(type, id),
+      };
+      const args = msg.data as Args;
+      command({ ...ctx, args })
+        .then((result) => this.queue.enqueue({ type: `${type}:done`, data: result } as CommandResult<Name, Result>))
+        .catch((err: unknown) => {
+          if (controller.signal.aborted) {
+            this.queue.enqueue({ type: `${type}:cancel` } as CommandResult<Name, Result>);
+          } else {
+            this.queue.enqueue({
+              type: `${type}:error`,
+              error: err instanceof Error ? err : new Error(String(err)),
+            } as CommandResult<Name, Result>);
+          }
+        });
+    });
+  }
+
   enqueue(msg: Message): void {
     this.queue.enqueue(msg);
   }
@@ -74,19 +125,20 @@ export class Engine {
   }
 
   /** Run an orchestrator against this engine's queue. */
-  runOrchestrator(o: Orchestrator): Promise<void> {
+  runOrchestrator(o: Orchestrator, signal?: AbortSignal): Promise<void> {
     return runOrchestrator(
       o,
       (msg) => this.queue.enqueue(msg),
       (handler) => this.queue.on(handler),
       (type, id) => this.triggers.enable(type, id),
       (type, id) => this.triggers.disable(type, id),
+      signal,
     );
   }
 }
 
-export type { Message, MessageHandler } from './message.js';
+export type { Message, MessageHandler, CommandMessage, CommandResult } from './message.js';
 export type { Trigger } from './trigger.js';
 export { triggerMatches, TriggerRegistry } from './trigger.js';
-export type { Orchestrator, OrchestratorContext } from './orchestrator.js';
+export type { Orchestrator, RunContext, Command } from './run.js';
 export { runOrchestrator } from './orchestrator.js';
\ No newline at end of file
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index 99dd286..e4a6b41 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -1,4 +1,11 @@
-export { MessageQueue, type Message, type MessageHandler } from './message.js';
+export {
+  MessageQueue,
+  type Message,
+  type MessageHandler,
+  type CommandMessage,
+  type CommandResult,
+} from './message.js';
 export { TriggerRegistry, triggerMatches, type Trigger } from './trigger.js';
-export { runOrchestrator, type Orchestrator, type OrchestratorContext } from './orchestrator.js';
-export { Engine, type CommandHost } from './engine.js';
\ No newline at end of file
+export { runOrchestrator } from './orchestrator.js';
+export type { Orchestrator, RunContext, Command } from './run.js';
+export { Engine } from './engine.js';
\ No newline at end of file
diff --git a/packages/engine/src/message.ts b/packages/engine/src/message.ts
index 35f058c..9a50356 100644
--- a/packages/engine/src/message.ts
+++ b/packages/engine/src/message.ts
@@ -2,10 +2,13 @@
  * The message model and the queue that serializes it.
  *
  * A message is both an event (something happened) and an intent (something
- * should happen). It is identified by `type` (and optionally `id`), matching
- * the format's `type#id` convention. A `move` message both runs the move
- * command and is observable as an event; the command's completion is itself a
- * message (`move:done`), which is what triggers match and orchestrators await.
+ * should happen). It is identified by `type`, matching the format's `type#id`
+ * convention. A `move` message both runs the move command and is observable as
+ * an event; the command's completion is itself a message (`move:done`), which
+ * is what triggers match and orchestrators await.
+ *
+ * Messages are a discriminated union on `type`. The engine defines the generic
+ * shapes; the host's concrete union extends them with its own command types.
  *
  * Messages are not processed inline. They are enqueued and handled on the next
  * `tick()`. This kills reentrancy (a handler cannot cause unbounded
@@ -13,12 +16,34 @@
  * deterministic frame. The engine is pure — it has no render loop — so the
  * host calls `tick()` (a `useFrame` in `@tts/tabletop`, manually in tests).
  */
+
+/** A command message names a command to run. Its handler is the command. */
+export interface CommandMessage<Name extends string, Args> {
+  type: Name;
+  data: Args;
+}
+
+/**
+ * A command's result, discriminated on the type suffix. `:done` on resolve,
+ * `:cancel` on abort (superseded, skipped, surface disabled), `:error` on
+ * throw. A trigger matching `move:done` does not fire on a cancel.
+ */
+export type CommandResult<Name extends string, R = void> =
+  | { type: `${Name}:done`; data: R }
+  | { type: `${Name}:cancel` }
+  | { type: `${Name}:error`; error: Error };
+
+/**
+ * The base message type. The engine is host-agnostic, so this is a permissive
+ * structural type; the host defines a concrete discriminated union on `type`
+ * that extends it with its own command and interaction messages.
+ */
 export interface Message {
   type: string;
   /** Command-specific payload. */
   data?: unknown;
-  /** Optional identity, for matching and dedup. */
-  id?: string;
+  /** Present on `:error` result messages. */
+  error?: Error;
 }
 
 /** A handler consumes a message and may emit new ones. */
diff --git a/packages/engine/src/orchestrator.ts b/packages/engine/src/orchestrator.ts
index 5a59980..97008f1 100644
--- a/packages/engine/src/orchestrator.ts
+++ b/packages/engine/src/orchestrator.ts
@@ -3,7 +3,8 @@
  *
  * An orchestrator is the code counterpart to a trigger: an async function that
  * emits messages and awaits matching ones. It is a proper TS module, declared
- * per folder as `main.ts`, unique per folder like `package.yaml`.
+ * per folder as `main.ts`, unique per folder like `package.yaml`, exported as a
+ * default async function.
  *
  * An orchestrator is a long-running command: it awaits events instead of
  * resolving immediately, so it inherits the run-context machinery (supersede
@@ -11,25 +12,12 @@
  * — declaration is data, activation is code.
  */
 import type { Message } from './message.js';
-
-/** The context an orchestrator runs against. */
-export interface OrchestratorContext {
-  /** Emit a message onto the queue. */
-  emit(msg: Message): void;
-  /** Await the next message matching `pred`. */
-  wait(pred: (msg: Message) => boolean): Promise<Message>;
-  /** Enable/disable a trigger by `type#id`. */
-  enableTrigger(type: string, id?: string): void;
-  disableTrigger(type: string, id?: string): void;
-}
-
-/** An orchestrator: an async function that emits and awaits messages. */
-export type Orchestrator = (ctx: OrchestratorContext) => Promise<void>;
+import type { Orchestrator, RunContext } from './run.js';
 
 /**
  * Run an orchestrator against a queue. `emit` enqueues; `wait` suspends until
- * a matching message is processed during a `tick()`. Returns a promise that
- * resolves when the orchestrator completes.
+ * a matching message is processed during a `tick()`, rejecting on abort.
+ * Returns a promise that resolves when the orchestrator completes.
  */
 export function runOrchestrator(
   orchestrator: Orchestrator,
@@ -37,8 +25,18 @@ export function runOrchestrator(
   on: (handler: (msg: Message) => void) => () => void,
   enableTrigger: (type: string, id?: string) => void,
   disableTrigger: (type: string, id?: string) => void,
+  signal?: AbortSignal,
 ): Promise<void> {
-  const pending: Array<{ pred: (msg: Message) => boolean; resolve: (m: Message) => void }> = [];
+  const controller = new AbortController();
+  const pending: Array<{ pred: (msg: Message) => boolean; resolve: (m: Message) => void; reject: (e: Error) => void }> = [];
+  const abort = () => {
+    controller.abort();
+    // Reject every pending wait so the orchestrator unwinds on cancel.
+    for (const p of pending.splice(0)) p.reject(new Error('aborted'));
+  };
+  if (signal?.aborted) abort();
+  else signal?.addEventListener('abort', abort, { once: true });
+
   const unsubscribe = on((msg) => {
     for (let i = 0; i < pending.length; i++) {
       const p = pending[i]!;
@@ -50,15 +48,23 @@ export function runOrchestrator(
     }
   });
 
-  const ctx: OrchestratorContext = {
+  const ctx: RunContext = {
+    signal: controller.signal,
     emit,
     wait: (pred) =>
-      new Promise<Message>((resolve) => {
-        pending.push({ pred, resolve });
+      new Promise<Message>((resolve, reject) => {
+        if (controller.signal.aborted) {
+          reject(new Error('aborted'));
+          return;
+        }
+        pending.push({ pred, resolve, reject });
       }),
     enableTrigger,
     disableTrigger,
   };
 
-  return orchestrator(ctx).finally(unsubscribe);
+  return orchestrator(ctx).finally(() => {
+    unsubscribe();
+    controller.abort();
+  });
 }
\ No newline at end of file
diff --git a/packages/engine/src/run.ts b/packages/engine/src/run.ts
new file mode 100644
index 0000000..b04d451
--- /dev/null
+++ b/packages/engine/src/run.ts
@@ -0,0 +1,30 @@
+/**
+ * The run context and the unified command/orchestrator shape.
+ *
+ * A command and an orchestrator are the same thing: an async function taking a
+ * `RunContext`. A command returns a result and is awaited by the engine, which
+ * emits `:done`/`:cancel`/`:error`; an orchestrator returns `void` and is never
+ * awaited by a parent. Cancellation is an `AbortSignal` — a superseded command
+ * or disabled surface aborts it, `wait` rejects on abort, and the `:cancel`
+ * result is emitted. Errors are thrown: a command that throws emits `:error`.
+ */
+import type { Message } from './message.js';
+
+/** The handle every handler runs against. */
+export interface RunContext {
+  /** Cancellation: superseded, skipped, surface disabled. */
+  signal: AbortSignal;
+  /** Emit a message onto the queue. */
+  emit(msg: Message): void;
+  /** Await the next message matching `pred`. Rejects on abort. */
+  wait(pred: (m: Message) => boolean): Promise<Message>;
+  /** Enable/disable a trigger by `type#id`. */
+  enableTrigger(type: string, id?: string): void;
+  disableTrigger(type: string, id?: string): void;
+}
+
+/** A command: an async function taking the context plus its args. */
+export type Command<Args, Result> = (ctx: RunContext & { args: Args }) => Promise<Result>;
+
+/** An orchestrator: an async function that emits and awaits messages. */
+export type Orchestrator = (ctx: RunContext) => Promise<void>;
\ No newline at end of file