diff --git a/src/agents/pi-embedded-runner/run/attempt.test.ts b/src/agents/pi-embedded-runner/run/attempt.test.ts
index 39b2abe4da7..d4a651719fa 100644
--- a/src/agents/pi-embedded-runner/run/attempt.test.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.test.ts
@@ -114,6 +114,34 @@ describe("resolvePromptBuildHookResult", () => {
     expect(result.prependContext).toBe("from-hook");
   });
 
+  it("includes legacy hook agent context fields only when present", async () => {
+    const hookRunner = createLegacyOnlyHookRunner();
+    const messages = [{ role: "user", content: "ctx" }];
+
+    await resolvePromptBuildHookResult({
+      prompt: "hello",
+      messages,
+      hookCtx: {
+        sessionKey: "agent:assistant-beta:main",
+        agentId: "assistant-beta",
+      },
+      hookRunner,
+    });
+
+    expect(hookRunner.runBeforeAgentStart).toHaveBeenCalledWith(
+      {
+        prompt: "hello",
+        messages,
+        sessionKey: "agent:assistant-beta:main",
+        agentId: "assistant-beta",
+      },
+      {
+        sessionKey: "agent:assistant-beta:main",
+        agentId: "assistant-beta",
+      },
+    );
+  });
+
   it("merges prompt-build and legacy context fields in deterministic order", async () => {
     const hookRunner = {
       hasHooks: vi.fn(() => true),
diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index 080f49efea1..474841c10fb 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -1439,8 +1439,8 @@ export async function resolvePromptBuildHookResult(params: {
             {
               prompt: params.prompt,
               messages: params.messages,
-              sessionKey: params.hookCtx.sessionKey,
-              agentId: params.hookCtx.agentId,
+              ...(params.hookCtx.sessionKey ? { sessionKey: params.hookCtx.sessionKey } : {}),
+              ...(params.hookCtx.agentId ? { agentId: params.hookCtx.agentId } : {}),
             },
             params.hookCtx,
           )