diff --git a/packages/poe-acp-client/README.md b/packages/poe-acp-client/README.md
index d8f9e49f1..26d8c7f02 100644
--- a/packages/poe-acp-client/README.md
+++ b/packages/poe-acp-client/README.md
@@ -27,6 +27,10 @@ const update = parseSessionUpdate({
 - Run-report helpers: `generateRunReportFromSessionUpdateStream`, `formatRunReportSummary`, and `saveRunReport`.
 - ACP protocol types and error-code helpers.
 
+## Validation
+
+ACP helpers validate protocol boundaries before forwarding data to callers. Prompt responses must use one of the supported stop reasons: `completed`, `cancelled`, `max_tokens`, or `end_turn`.
+
 ## Config Options
 
 This package does not load a config file. Runtime behavior is controlled by
diff --git a/packages/poe-acp-client/src/acp-client-unified.test.ts b/packages/poe-acp-client/src/acp-client-unified.test.ts
index 813e7d1a8..b274066e4 100644
--- a/packages/poe-acp-client/src/acp-client-unified.test.ts
+++ b/packages/poe-acp-client/src/acp-client-unified.test.ts
@@ -1981,6 +1981,19 @@ describe("AcpClient", () => {
     await expect(turn.response).rejects.toThrow(/stopReason/i);
   });
 
+  it("accepts end_turn prompt responses from ACP agents", async () => {
+    const { transport, sendRequestMock } = createTransportMock();
+    sendRequestMock
+      .mockResolvedValueOnce({ protocolVersion: 1 } satisfies InitializeResponse)
+      .mockResolvedValueOnce({ stopReason: "end_turn" } satisfies PromptResponse);
+    const client = new AcpClient({ transport, protocolVersion: 1 });
+    await client.initialize();
+
+    const turn = client.prompt("session-1", [{ type: "text", text: "Hello agent" }]);
+
+    await expect(turn.response).resolves.toEqual({ stopReason: "end_turn" });
+  });
+
   it("validates prompt content using promptCapabilities", async () => {
     const { transport, sendRequestMock } = createTransportMock();
     sendRequestMock.mockResolvedValueOnce({ protocolVersion: 1 } satisfies InitializeResponse);
diff --git a/packages/poe-acp-client/src/acp-client.ts b/packages/poe-acp-client/src/acp-client.ts
index a9986e9c3..86e8131b7 100644
--- a/packages/poe-acp-client/src/acp-client.ts
+++ b/packages/poe-acp-client/src/acp-client.ts
@@ -114,6 +114,7 @@ const validStopReasons = new Set<PromptResponse["stopReason"]>([
   "completed",
   "cancelled",
   "max_tokens",
+  "end_turn",
 ]);
 
 export interface AcpClientHandlers {
@@ -241,7 +242,7 @@ function assertPromptResponse(value: PromptResponse): void {
   if (!validStopReasons.has(value.stopReason)) {
     throw invalidResponse(
       "session/prompt",
-      '"stopReason" must be "completed", "cancelled", or "max_tokens".'
+      '"stopReason" must be "completed", "cancelled", "max_tokens", or "end_turn".'
     );
   }
 }
diff --git a/packages/poe-acp-client/src/types.compile-check.ts b/packages/poe-acp-client/src/types.compile-check.ts
index bbb798b60..2d731d66d 100644
--- a/packages/poe-acp-client/src/types.compile-check.ts
+++ b/packages/poe-acp-client/src/types.compile-check.ts
@@ -803,5 +803,5 @@ const ignoredLegacyToolCallContent: ToolCallContent = {
   content: { type: "text", text: "legacy" },
 };
 
-// @ts-expect-error stop reason values are restricted to completed/cancelled/max_tokens
-const ignoredInvalidStopReason: StopReason = "end_turn";
+// @ts-expect-error stop reason values are restricted to supported ACP values
+const ignoredInvalidStopReason: StopReason = "totally-not-valid";
diff --git a/packages/poe-acp-client/src/types.ts b/packages/poe-acp-client/src/types.ts
index 2e6b9c225..33a0b02f1 100644
--- a/packages/poe-acp-client/src/types.ts
+++ b/packages/poe-acp-client/src/types.ts
@@ -302,7 +302,7 @@ export type ProtocolVersion = number;
 
 export type RequestId = null | number | string;
 
-export type StopReason = "completed" | "cancelled" | "max_tokens";
+export type StopReason = "completed" | "cancelled" | "max_tokens" | "end_turn";
 
 export interface Implementation extends AcpExtensible {
   name: string;