diff --git a/packages/coding-agent/src/modes/acp/acp-agent.ts b/packages/coding-agent/src/modes/acp/acp-agent.ts
index 247147a8d79..f0e9acc3a6e 100644
--- a/packages/coding-agent/src/modes/acp/acp-agent.ts
+++ b/packages/coding-agent/src/modes/acp/acp-agent.ts
@@ -5274,7 +5274,9 @@ export class AcpAgent implements Agent {
 		const { outcome } = waiter.terminal;
 		this.#rememberSettledPromptCorrelation(id, record, waiter.correlation);
 		if (outcome.kind === "stopped") {
-			waiter.resolve({ stopReason: outcome.reason });
+			// Once the SDK has acknowledged a client cancel, the cancellation owns the
+			// turn even if a stale terminal frame races in with another stopped reason.
+			waiter.resolve({ stopReason: waiter.cancelAcknowledged ? "cancelled" : outcome.reason });
 			return;
 		}
 		// `phase` is the host's own claim, and a host that omits it leaves `terminalOutcome`
diff --git a/packages/coding-agent/test/acp/acp-cancel-settlement.test.ts b/packages/coding-agent/test/acp/acp-cancel-settlement.test.ts
index 5fcab0154f0..64e296032f6 100644
--- a/packages/coding-agent/test/acp/acp-cancel-settlement.test.ts
+++ b/packages/coding-agent/test/acp/acp-cancel-settlement.test.ts
@@ -3296,3 +3296,20 @@ test("successful remote session retirement releases an unresolved abort owner",
 		fixture.dispose();
 	}
 });
+
+test("cancel 100ms after background prompt start settles cancelled and follow-up end_turn", async () => {
+	const fixture = await createFixture({ cancelSettlementGraceMs: 25 });
+	try {
+		const background = prompt(fixture, "sleep 5000");
+		await bounded(fixture.promptDelivered, "background prompt delivery");
+		await Bun.sleep(100);
+		await bounded(fixture.agent.cancel({ sessionId: fixture.sessionId }), "background cancel acknowledgement");
+		expect(await bounded(background, "background cancelled settlement")).toEqual({ stopReason: "cancelled" });
+		const followUp = prompt(fixture, "follow-up");
+		await waitFor(() => fixture.promptDeliveryCount() === 2, "follow-up prompt delivery");
+		fixture.sendStopped("end_turn");
+		expect(await bounded(followUp, "follow-up completion")).toEqual({ stopReason: "end_turn" });
+	} finally {
+		fixture.dispose();
+	}
+});