diff --git a/packages/sdk/src/__tests__/lifecycle-hooks.test.ts b/packages/sdk/src/__tests__/lifecycle-hooks.test.ts
index 200fd4e32..73de966e0 100644
--- a/packages/sdk/src/__tests__/lifecycle-hooks.test.ts
+++ b/packages/sdk/src/__tests__/lifecycle-hooks.test.ts
@@ -6,6 +6,7 @@ import type {
   AfterAgentSpawnContext,
   BeforeAgentReleaseContext,
   BeforeAgentSpawnContext,
+  BeforeAgentSpawnHandler,
   SpawnPatch,
 } from '../lifecycle-hooks.js';
 import type { SpawnPtyInput } from '../types.js';
@@ -230,6 +231,34 @@ describe('AgentRelayClient lifecycle hooks', () => {
     expect(fn).toHaveBeenCalledTimes(1);
   });
 
+  it('accepts a BeforeAgentSpawnHandler-typed function without a cast', async () => {
+    // Regression for the addListener overload: a handler that's typed
+    // separately as BeforeAgentSpawnHandler (return: void | SpawnPatch
+    // | Promise<void | SpawnPatch>) must satisfy the addListener signature
+    // without `as` gymnastics. Before the overload landed this assignment
+    // failed with TS2345 because the default `void`-returning shape didn't
+    // cover the SpawnPatch return.
+    const { fetchFn, captures } = makeMockFetch();
+    const client = makeClient(fetchFn);
+
+    const handler: BeforeAgentSpawnHandler = (ctx) => {
+      if (ctx.input.cli !== 'claude') return;
+      return { args: [...(ctx.input.args ?? []), '--from-typed-handler'] };
+    };
+    client.addListener('beforeAgentSpawn', handler);
+
+    await client.spawnPty({ name: 'typed', cli: 'claude', args: ['--orig'] });
+
+    expect(captures[0].body).toMatchObject({
+      args: ['--orig', '--from-typed-handler'],
+    });
+
+    // removeListener must accept the same handler shape without casting.
+    client.removeListener('beforeAgentSpawn', handler);
+    await client.spawnPty({ name: 'typed-2', cli: 'claude', args: ['--orig'] });
+    expect(captures[1].body).toMatchObject({ args: ['--orig'] });
+  });
+
   it('patch shape: extending args requires explicit spread', async () => {
     // Documents the array-replace contract: a patch's `args` overrides
     // the previous `args` outright; handlers must spread to extend.
diff --git a/packages/sdk/src/client.ts b/packages/sdk/src/client.ts
index f2459bfdb..5d5f18b30 100644
--- a/packages/sdk/src/client.ts
+++ b/packages/sdk/src/client.ts
@@ -259,20 +259,38 @@ export class AgentRelayClient {
    * function. Equivalent to `client.eventBus.addListener(...)` but mirrors
    * the `AgentRelay` facade API so direct-client callers don't need to
    * reach through `.eventBus`.
+   *
+   * `beforeAgentSpawn` is the one event whose handler may return a
+   * `SpawnPatch` to mutate the spawn input — the dedicated overload
+   * keeps that contract type-safe without forcing other events to accept
+   * non-void returns.
    */
+  addListener(event: 'beforeAgentSpawn', handler: BeforeAgentSpawnHandler): () => void;
   addListener<K extends keyof AgentRelayEvents>(
     event: K,
     handler: (...args: AgentRelayEvents[K]) => void | Promise<void>
+  ): () => void;
+  addListener<K extends keyof AgentRelayEvents>(
+    event: K,
+    handler: ((...args: AgentRelayEvents[K]) => void | Promise<void>) | BeforeAgentSpawnHandler
   ): () => void {
-    return this.eventBus.addListener(event, handler);
+    return this.eventBus.addListener(
+      event,
+      handler as (...args: AgentRelayEvents[K]) => void | Promise<void>
+    );
   }
 
   /** Remove a previously-registered listener. */
+  removeListener(event: 'beforeAgentSpawn', handler: BeforeAgentSpawnHandler): void;
   removeListener<K extends keyof AgentRelayEvents>(
     event: K,
     handler: (...args: AgentRelayEvents[K]) => void | Promise<void>
+  ): void;
+  removeListener<K extends keyof AgentRelayEvents>(
+    event: K,
+    handler: ((...args: AgentRelayEvents[K]) => void | Promise<void>) | BeforeAgentSpawnHandler
   ): void {
-    this.eventBus.removeListener(event, handler);
+    this.eventBus.removeListener(event, handler as (...args: AgentRelayEvents[K]) => void | Promise<void>);
   }
 
   /**
diff --git a/packages/sdk/src/event-bus.ts b/packages/sdk/src/event-bus.ts
index 1867b1503..90cd3817a 100644
--- a/packages/sdk/src/event-bus.ts
+++ b/packages/sdk/src/event-bus.ts
@@ -18,22 +18,37 @@
  */
 export type EventMap = Record<string, readonly unknown[]>;
 
-export type EventHandler<Args extends readonly unknown[]> = (...args: Args) => void | Promise<void>;
+/**
+ * Handler signature for an event. The optional `R` generic lets a
+ * specific event widen its return type past `void` — used by the
+ * `beforeAgentSpawn` lifecycle hook, whose handlers may return a
+ * `SpawnPatch` that the dispatcher folds into the spawn input. The
+ * default `R = void` keeps the common case strict.
+ */
+export type EventHandler<Args extends readonly unknown[], R = void> = (...args: Args) => R | Promise<R>;
 
 export class EventBus<E extends EventMap> {
-  private handlers: Map<keyof E, Set<EventHandler<readonly unknown[]>>> = new Map();
+  // Stored type uses `unknown` for `R` so a single Set can hold handlers
+  // from any overload — the dispatcher casts back when it cares about a
+  // non-void return value (see `runBeforeSpawn` in `client.ts`).
+  private handlers: Map<keyof E, Set<EventHandler<readonly unknown[], unknown>>> = new Map();
 
   /**
    * Register a handler for `event`. Returns an unsubscribe function that
    * removes the handler when called.
+   *
+   * The `R` generic is inferred from the handler return type; callers
+   * that don't care about the return get the default `void` shape, while
+   * events like `beforeAgentSpawn` can pass handlers that return a
+   * `SpawnPatch`.
    */
-  addListener<K extends keyof E>(event: K, handler: EventHandler<E[K]>): () => void {
+  addListener<K extends keyof E, R = void>(event: K, handler: EventHandler<E[K], R>): () => void {
     let set = this.handlers.get(event);
     if (!set) {
       set = new Set();
       this.handlers.set(event, set);
     }
-    set.add(handler as EventHandler<readonly unknown[]>);
+    set.add(handler as EventHandler<readonly unknown[], unknown>);
     return () => {
       // Re-read the current Set from the map so a stale closure can't blow
       // away the new Set if the caller unsubscribes, re-registers a fresh
@@ -42,7 +57,7 @@ export class EventBus<E extends EventMap> {
       // every listener for the event.
       const current = this.handlers.get(event);
       if (current !== set) return;
-      current.delete(handler as EventHandler<readonly unknown[]>);
+      current.delete(handler as EventHandler<readonly unknown[], unknown>);
       if (current.size === 0) {
         this.handlers.delete(event);
       }
@@ -50,10 +65,10 @@ export class EventBus<E extends EventMap> {
   }
 
   /** Remove a previously-registered handler. Idempotent. */
-  removeListener<K extends keyof E>(event: K, handler: EventHandler<E[K]>): void {
+  removeListener<K extends keyof E, R = void>(event: K, handler: EventHandler<E[K], R>): void {
     const set = this.handlers.get(event);
     if (!set) return;
-    set.delete(handler as EventHandler<readonly unknown[]>);
+    set.delete(handler as EventHandler<readonly unknown[], unknown>);
     if (set.size === 0) {
       this.handlers.delete(event);
     }
@@ -65,9 +80,9 @@ export class EventBus<E extends EventMap> {
   }
 
   /** Snapshot the handlers for `event` so iteration is safe under concurrent mutation. */
-  listeners<K extends keyof E>(event: K): Array<EventHandler<E[K]>> {
+  listeners<K extends keyof E, R = void>(event: K): Array<EventHandler<E[K], R>> {
     const set = this.handlers.get(event);
-    return set ? (Array.from(set) as Array<EventHandler<E[K]>>) : [];
+    return set ? (Array.from(set) as Array<EventHandler<E[K], R>>) : [];
   }
 
   /**
diff --git a/packages/sdk/src/relay.ts b/packages/sdk/src/relay.ts
index b32b64da2..d336d232e 100644
--- a/packages/sdk/src/relay.ts
+++ b/packages/sdk/src/relay.ts
@@ -31,7 +31,7 @@ import { RelayCast } from '@relaycast/sdk';
 
 import { AgentRelayClient, type AgentRelayBrokerInitArgs, type AgentRelaySpawnOptions } from './client.js';
 import { EventBus } from './event-bus.js';
-import type { AgentRelayEvents } from './lifecycle-hooks.js';
+import type { AgentRelayEvents, BeforeAgentSpawnHandler } from './lifecycle-hooks.js';
 import {
   buildPersonaSpawnSpec,
   composePersonaTask,
@@ -466,20 +466,35 @@ export class AgentRelay {
    * can register for the same event; they fire sequentially in
    * registration order. Async handlers are awaited. Handler exceptions
    * are caught and logged; one bad listener never blocks the others.
+   *
+   * `beforeAgentSpawn` is the one event whose handler may return a
+   * `SpawnPatch` to mutate the spawn input before the broker POST — the
+   * dedicated overload below keeps that contract type-safe without
+   * forcing other events to accept non-void returns.
    */
+  addListener(event: 'beforeAgentSpawn', handler: BeforeAgentSpawnHandler): () => void;
   addListener<K extends keyof AgentRelayEvents>(
     event: K,
     handler: (...args: AgentRelayEvents[K]) => void | Promise<void>
+  ): () => void;
+  addListener<K extends keyof AgentRelayEvents>(
+    event: K,
+    handler: ((...args: AgentRelayEvents[K]) => void | Promise<void>) | BeforeAgentSpawnHandler
   ): () => void {
-    return this.bus.addListener(event, handler);
+    return this.bus.addListener(event, handler as (...args: AgentRelayEvents[K]) => void | Promise<void>);
   }
 
   /** Remove a previously-registered listener. Idempotent. */
+  removeListener(event: 'beforeAgentSpawn', handler: BeforeAgentSpawnHandler): void;
   removeListener<K extends keyof AgentRelayEvents>(
     event: K,
     handler: (...args: AgentRelayEvents[K]) => void | Promise<void>
+  ): void;
+  removeListener<K extends keyof AgentRelayEvents>(
+    event: K,
+    handler: ((...args: AgentRelayEvents[K]) => void | Promise<void>) | BeforeAgentSpawnHandler
   ): void {
-    this.bus.removeListener(event, handler);
+    this.bus.removeListener(event, handler as (...args: AgentRelayEvents[K]) => void | Promise<void>);
   }
 
   // ── Public accessors ────────────────────────────────────────────────────