diff --git a/packages/agent-runtime/src/pi/adapter.test.ts b/packages/agent-runtime/src/pi/adapter.test.ts
index 45899b6a7a..4042f2fbe6 100644
--- a/packages/agent-runtime/src/pi/adapter.test.ts
+++ b/packages/agent-runtime/src/pi/adapter.test.ts
@@ -808,6 +808,82 @@ describe("pi provider adapter", () => {
     expect(events.some((event) => event.type === "provider/error")).toBe(false);
   });
 
+  it("completes extension-triggered turns when agent_end includes string custom content", () => {
+    const adapter = createPiProviderAdapter();
+    const context = { threadId: "pi-thread-1" };
+    const agentEndEvent = {
+      type: "agent_end",
+      messages: [
+        {
+          role: "custom",
+          customType: "pi-processes",
+          content: "Process completed successfully",
+          display: true,
+          timestamp: 1777995780000,
+        },
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "The process finished." }],
+          api: "anthropic-messages",
+          provider: "anthropic",
+          model: "claude-haiku-4-5",
+          usage: {
+            input: 10,
+            output: 5,
+            cacheRead: 0,
+            cacheWrite: 0,
+            totalTokens: 15,
+            cost: {
+              input: 0,
+              output: 0,
+              cacheRead: 0,
+              cacheWrite: 0,
+              total: 0,
+            },
+          },
+          stopReason: "stop",
+          timestamp: 1777995781000,
+        },
+      ],
+      willRetry: false,
+    } satisfies AgentSessionEvent;
+
+    adapter.translateEvent(
+      {
+        jsonrpc: "2.0",
+        method: "sdk/message",
+        params: {
+          threadId: context.threadId,
+          message: { type: "agent_start" },
+        },
+      },
+      context,
+    );
+
+    const events = adapter.translateEvent(
+      {
+        jsonrpc: "2.0",
+        method: "sdk/message",
+        params: {
+          threadId: context.threadId,
+          message: agentEndEvent,
+        },
+      },
+      context,
+    );
+
+    expect(events).toContainEqual(
+      expect.objectContaining({
+        type: "turn/completed",
+        scope: turnScope("turn-1"),
+        status: "completed",
+      }),
+    );
+    expect(events.some((event) => event.type === "provider/unhandled")).toBe(
+      false,
+    );
+  });
+
   it("translateEvent agent_end surfaces Pi assistant stop errors as failed turns", () => {
     const adapter = createPiProviderAdapter();
     const context = { threadId: "bb-thread-1" };
diff --git a/packages/agent-runtime/src/pi/adapter.ts b/packages/agent-runtime/src/pi/adapter.ts
index 82ae6f7d3e..ba48a11e2f 100644
--- a/packages/agent-runtime/src/pi/adapter.ts
+++ b/packages/agent-runtime/src/pi/adapter.ts
@@ -260,7 +260,9 @@ const piAssistantMessageSchema = z
 const piConversationMessageSchema = z
   .object({
     role: z.string(),
-    content: z.array(piMessageContentBlockSchema).optional(),
+    content: z
+      .union([z.string(), z.array(piMessageContentBlockSchema)])
+      .optional(),
     stopReason: z.string().optional(),
     errorMessage: z.string().optional(),
     provider: z.string().optional(),