diff --git a/src/agents/pi-embedded-runner/run.ts b/src/agents/pi-embedded-runner/run.ts
index 208c76fee48..4a3032ba5eb 100644
--- a/src/agents/pi-embedded-runner/run.ts
+++ b/src/agents/pi-embedded-runner/run.ts
@@ -1133,12 +1133,15 @@ export async function runEmbeddedPiAgent(
             timedOut,
             idleTimedOut,
             timedOutDuringCompaction,
-            timedOutDuringToolExecution,
             sessionIdUsed,
             sessionFileUsed,
             lastAssistant: sessionLastAssistant,
             currentAttemptAssistant,
           } = attempt;
+          // Field is optional in the public harness SDK contract; default to
+          // false here so internal code can rely on a strict boolean. Internal
+          // embedded-runner attempt sets this explicitly. See #52147.
+          const timedOutDuringToolExecution = attempt.timedOutDuringToolExecution ?? false;
           if (sessionIdUsed && sessionIdUsed !== activeSessionId) {
             activeSessionId = sessionIdUsed;
           }
diff --git a/src/agents/pi-embedded-runner/run/types.ts b/src/agents/pi-embedded-runner/run/types.ts
index 81c850eecdf..20cd456ac1c 100644
--- a/src/agents/pi-embedded-runner/run/types.ts
+++ b/src/agents/pi-embedded-runner/run/types.ts
@@ -62,8 +62,13 @@ export type EmbeddedRunAttemptResult = {
    * True if the run-level timer fired while at least one tool execution was
    * still in flight. The LLM had already responded; the timeout is unrelated
    * to the primary model and must not trigger model fallback. Closes #52147.
+   *
+   * Optional for plugin-SDK back-compat: this type is re-exported as
+   * `AgentHarnessAttemptResult` and third-party harnesses cannot necessarily
+   * observe in-flight tool state. Treat absent as `false` at the runner
+   * boundary; internal embedded-runner code always sets it explicitly.
    */
-  timedOutDuringToolExecution: boolean;
+  timedOutDuringToolExecution?: boolean;
   promptError: unknown;
   /**
    * Identifies which phase produced the promptError.