diff --git a/src/agent-task-executor.ts b/src/agent-task-executor.ts
index 602e340..b29e5f0 100644
--- a/src/agent-task-executor.ts
+++ b/src/agent-task-executor.ts
@@ -98,6 +98,13 @@ export class AgentTaskExecutor implements AgentExecutor {
         resolve();
       });
 
+      // Finalizer: fires exactly once on every terminal path (incl. cancel, which emits no
+      // done/error). Idempotent cleanup — the seam where worktree.dispose()/slot-release will
+      // attach later. Publishes no A2A event (preserves the event stream).
+      runner!.on('settled', () => {
+        this._activeRunners.delete(taskId);
+      });
+
       runner!.start();
     });
   };
diff --git a/src/process-runner.ts b/src/process-runner.ts
index aaaf7af..cad2405 100644
--- a/src/process-runner.ts
+++ b/src/process-runner.ts
@@ -4,7 +4,7 @@ import type { ChildProcess } from 'node:child_process';
 import type { Span } from '@opentelemetry/api';
 import { SpanStatusCode } from '@opentelemetry/api';
 import type { ProcessAdapter, AgentEvent, AgentStats } from './adapters/base.js';
-import type { Runner } from './runner.js';
+import type { Runner, TerminalReason } from './runner.js';
 import type { Config } from './types.js';
 import { tracer, inputTokenCounter, outputTokenCounter, taskDurationHist, taskErrorCounter, context } from './telemetry.js';
 
@@ -30,9 +30,11 @@ export declare interface ProcessRunner {
   on(event: 'agent-event', listener: (e: AgentEvent) => void): this;
   on(event: 'done', listener: (exitCode: number, stderr: string) => void): this;
   on(event: 'error', listener: (err: Error) => void): this;
+  on(event: 'settled', listener: (reason: TerminalReason) => void): this;
   emit(event: 'agent-event', e: AgentEvent): boolean;
   emit(event: 'done', exitCode: number, stderr: string): boolean;
   emit(event: 'error', err: Error): boolean;
+  emit(event: 'settled', reason: TerminalReason): boolean;
 }
 
 /**
@@ -48,6 +50,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
   private _child: ChildProcess | null = null;
   private _cancelled = false;
   private _exited = false;
+  private _settled = false;
   private _timeoutHandle: ReturnType<typeof setTimeout> | null = null;
   private _idleHandle: ReturnType<typeof setTimeout> | null = null;
   private _stderrBuffer = '';
@@ -92,6 +95,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
         'error',
         new Error(`Failed to spawn "${binary}": ${String(err)}`),
       );
+      this._settle('failed');
       return;
     }
 
@@ -100,6 +104,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
     if (child.stdout === null || child.stderr === null) {
       this._finalizeOtel(-1);
       this.emit('error', new Error(`"${binary}" process has no stdout/stderr`));
+      this._settle('failed');
       return;
     }
 
@@ -141,6 +146,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
         }
         taskErrorCounter.add(1, { adapter: adapter.name, error_kind: 'spawn_error' });
         this.emit('error', new Error(`Failed to spawn "${binary}": ${err.message}`));
+        this._settle('failed');
       }
     });
 
@@ -150,9 +156,13 @@ export class ProcessRunner extends EventEmitter implements Runner {
       // Always finalize telemetry regardless of whether this is a normal exit,
       // a cancellation, or a timeout — _finalizeOtel is idempotent.
       this._finalizeOtel(exitCode);
-      if (this._cancelled || this._exited) return;
-      this._exited = true;
-      this.emit('done', exitCode, this._stderrBuffer);
+      if (!this._cancelled && !this._exited) {
+        this._exited = true;
+        this.emit('done', exitCode, this._stderrBuffer);
+      }
+      // `settled` fires on every close, including cancellation (which emits no done/error).
+      // Timeout/idle already settled ('timed-out') so _settle here is a guarded no-op for them.
+      this._settle(this._cancelled ? 'cancelled' : exitCode === 0 ? 'succeeded' : 'failed');
     });
 
     if (config.agentTimeoutMs > 0) {
@@ -163,6 +173,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
             this._exited = true;
             this.emit('done', -1, this._stderrBuffer);
           }
+          this._settle('timed-out');
         }
       }, config.agentTimeoutMs);
     }
@@ -176,7 +187,11 @@ export class ProcessRunner extends EventEmitter implements Runner {
     this._cancelled = true;
     this._clearTimers();
     const child = this._child;
-    if (child === null) return;
+    if (child === null) {
+      // Cancelled before the process started: no close event will fire, so settle here.
+      this._settle('cancelled');
+      return;
+    }
     child.kill('SIGTERM');
     const kill = setTimeout(() => {
       if (!this._exited) child.kill('SIGKILL');
@@ -189,6 +204,13 @@ export class ProcessRunner extends EventEmitter implements Runner {
     this._child?.stdin?.write(answer + '\n');
   }
 
+  /** Emits `settled` exactly once. Idempotent — subsequent terminal paths are no-ops. */
+  private _settle(reason: TerminalReason): void {
+    if (this._settled) return;
+    this._settled = true;
+    this.emit('settled', reason);
+  }
+
   private _processLine(line: string): void {
     const { adapter, config } = this._options;
     if (adapter.isApprovalPrompt(line)) {
@@ -247,6 +269,7 @@ export class ProcessRunner extends EventEmitter implements Runner {
           this._exited = true;
           this.emit('done', 0, this._stderrBuffer);
         }
+        this._settle('timed-out');
       }
     }, idleMs);
   }
diff --git a/src/runner.ts b/src/runner.ts
index 38bb3a5..88b3e82 100644
--- a/src/runner.ts
+++ b/src/runner.ts
@@ -1,5 +1,11 @@
 import type { AgentEvent } from './adapters/base.js';
 
+/**
+ * Why a run reached a terminal state. Emitted with the `settled` event so a finalizer can
+ * run once per run regardless of which path ended it.
+ */
+export type TerminalReason = 'succeeded' | 'failed' | 'cancelled' | 'timed-out';
+
 /**
  * Transport-agnostic execution port.
  *
@@ -22,8 +28,14 @@ export interface Runner {
 
   /** Emitted for each parsed {@link AgentEvent}. */
   on(event: 'agent-event', listener: (e: AgentEvent) => void): this;
-  /** Emitted once when execution finishes (exit code and accumulated stderr). */
+  /** Emitted once when execution finishes normally (exit code and accumulated stderr). */
   on(event: 'done', listener: (exitCode: number, stderr: string) => void): this;
   /** Emitted if execution cannot start or fails unexpectedly. */
   on(event: 'error', listener: (err: Error) => void): this;
+  /**
+   * Emitted **exactly once per run, on every terminal path** — including cancellation, which
+   * emits neither `done` nor `error`. The single hook a finalizer subscribes to for
+   * guaranteed cleanup. `done`/`error` (when they occur) fire before `settled`.
+   */
+  on(event: 'settled', listener: (reason: TerminalReason) => void): this;
 }
diff --git a/tests/unit/agent-task-executor.test.ts b/tests/unit/agent-task-executor.test.ts
index 6af63a3..023fdec 100644
--- a/tests/unit/agent-task-executor.test.ts
+++ b/tests/unit/agent-task-executor.test.ts
@@ -380,3 +380,21 @@ describe('AgentTaskExecutor routing', () => {
     expect(seen).toEqual([undefined, undefined]);
   });
 });
+
+describe('AgentTaskExecutor settled finalizer', () => {
+  it('removes the runner on settled so the next execute starts fresh', () => {
+    const executor = new AgentTaskExecutor(baseConfig, mockAdapter);
+    const ctx = makeContext({ taskId: 'task-s' });
+
+    void executor.execute(ctx, makeBus() as never);
+    expect(vi.mocked(ProcessRunner)).toHaveBeenCalledTimes(1);
+
+    // Finalizer fires — should evict the runner from the active map.
+    getMockRunner().emit('settled', 'succeeded');
+
+    // Same taskId again must build a NEW runner (not hit the resume path).
+    void executor.execute(ctx, makeBus() as never);
+    expect(vi.mocked(ProcessRunner)).toHaveBeenCalledTimes(2);
+    expect(getMockRunner().resume).not.toHaveBeenCalled();
+  });
+});
diff --git a/tests/unit/process-runner.test.ts b/tests/unit/process-runner.test.ts
index 1051c3f..ac3fb30 100644
--- a/tests/unit/process-runner.test.ts
+++ b/tests/unit/process-runner.test.ts
@@ -597,3 +597,51 @@ describe('ProcessRunner', () => {
     });
   });
 });
+
+  describe('settled event', () => {
+    it('emits settled("succeeded") after done on a clean exit', async () => {
+      const runner = new ProcessRunner({ task: 'p', adapter: makeMockAdapter(), config: baseConfig });
+      const order: string[] = [];
+      const settled = new Promise<string>((resolve) => {
+        runner.on('done', () => order.push('done'));
+        runner.on('settled', (reason) => { order.push('settled'); resolve(reason); });
+      });
+      runner.start();
+      exitChild(0);
+      expect(await settled).toBe('succeeded');
+      expect(order).toEqual(['done', 'settled']); // done precedes settled
+    });
+
+    it('emits settled("failed") on a non-zero exit', async () => {
+      const runner = new ProcessRunner({ task: 'p', adapter: makeMockAdapter(), config: baseConfig });
+      const settled = new Promise<string>((resolve) => runner.on('settled', resolve));
+      runner.start();
+      exitChild(1);
+      expect(await settled).toBe('failed');
+    });
+
+    it('emits settled("cancelled") on cancel — the path that emits no done/error', async () => {
+      const runner = new ProcessRunner({ task: 'p', adapter: makeMockAdapter(), config: baseConfig });
+      let doneOrError = false;
+      const settled = new Promise<string>((resolve) => {
+        runner.on('done', () => { doneOrError = true; });
+        runner.on('error', () => { doneOrError = true; });
+        runner.on('settled', resolve);
+      });
+      runner.start();
+      runner.cancel();
+      exitChild(143); // SIGTERM close after cancel
+      expect(await settled).toBe('cancelled');
+      expect(doneOrError).toBe(false);
+    });
+
+    it('emits settled only once even if close arrives twice', async () => {
+      const runner = new ProcessRunner({ task: 'p', adapter: makeMockAdapter(), config: baseConfig });
+      const reasons: string[] = [];
+      runner.on('settled', (r) => reasons.push(r));
+      runner.start();
+      exitChild(0);
+      exitChild(0);
+      expect(reasons).toEqual(['succeeded']);
+    });
+  });