diff --git a/lib/index.js b/lib/index.js
index 1455779..364f4ae 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -96,6 +96,7 @@ function apply(ctx, config) {
     params = params || {};
     if (method === 'harness.status') return computeStatus();
     if (method === 'harness.prompt') return harnessPrompt(params);
+    if (method === 'harness.cancel') return harnessCancel();
     const dot = method.indexOf('.');
     if (dot <= 0) throw new Error('unknown method ' + method);
     const domain = method.slice(0, dot);
@@ -129,6 +130,13 @@ function apply(ctx, config) {
     });
   }
 
+  async function harnessCancel() {
+    const status = await computeStatus();
+    const sessionId = status.activeSessionId;
+    if (!sessionId) throw new Error('no active session to cancel');
+    return api.sessions.cancel({ sessionId });
+  }
+
   function handleRequest(msg, respond) {
     invoke(msg.method || '', msg.params || {}).then((result) => {
       respond({ type: 'response', requestId: msg.requestId, ok: true, result });