diff --git a/src/agents/harness/lifecycle-hook-helpers.test.ts b/src/agents/harness/lifecycle-hook-helpers.test.ts
index 8b27996baf9..210130747cd 100644
--- a/src/agents/harness/lifecycle-hook-helpers.test.ts
+++ b/src/agents/harness/lifecycle-hook-helpers.test.ts
@@ -213,4 +213,36 @@ describe("agent harness lifecycle hook helpers", () => {
       reason: "fix generated baseline\n\nrerun the focused tests",
     });
   });
+
+  it("falls back to retry instruction keys when retry idempotency keys are malformed", async () => {
+    const hookRunner = {
+      hasHooks: () => true,
+      runBeforeAgentFinalize: vi.fn().mockResolvedValue({
+        action: "revise",
+        retry: {
+          instruction: "retry with a safe key",
+          idempotencyKey: { invalid: true },
+          maxAttempts: 1,
+        } as never,
+      }),
+    };
+
+    await expect(
+      runAgentHarnessBeforeAgentFinalizeHook({
+        event: EVENT,
+        ctx: { runId: "run-1", sessionKey: "agent:main:session-1" },
+        hookRunner: hookRunner as never,
+      }),
+    ).resolves.toEqual({
+      action: "revise",
+      reason: "retry with a safe key",
+    });
+    await expect(
+      runAgentHarnessBeforeAgentFinalizeHook({
+        event: EVENT,
+        ctx: { runId: "run-1", sessionKey: "agent:main:session-1" },
+        hookRunner: hookRunner as never,
+      }),
+    ).resolves.toEqual({ action: "continue" });
+  });
 });
diff --git a/src/agents/harness/lifecycle-hook-helpers.ts b/src/agents/harness/lifecycle-hook-helpers.ts
index 12e9b0f2845..5f39e70bf26 100644
--- a/src/agents/harness/lifecycle-hook-helpers.ts
+++ b/src/agents/harness/lifecycle-hook-helpers.ts
@@ -137,19 +137,19 @@ function normalizeBeforeAgentFinalizeResult(
   event?: PluginHookBeforeAgentFinalizeEvent,
 ): AgentHarnessBeforeAgentFinalizeOutcome {
   if (result?.action === "finalize") {
-    return result.reason?.trim()
-      ? { action: "finalize", reason: result.reason.trim() }
-      : { action: "finalize" };
+    const reason = normalizeTrimmedString(result.reason);
+    return reason ? { action: "finalize", reason } : { action: "finalize" };
   }
   if (result?.action === "revise") {
-    const retryInstruction = result.retry?.instruction?.trim();
+    const retryInstruction = normalizeTrimmedString(result.retry?.instruction);
     if (retryInstruction) {
       const maxAttempts =
         typeof result.retry?.maxAttempts === "number" && Number.isFinite(result.retry.maxAttempts)
           ? Math.max(1, Math.floor(result.retry.maxAttempts))
           : 1;
       const retryRunId = event?.runId ?? event?.sessionId ?? "unknown-run";
-      const retryKey = result.retry?.idempotencyKey?.trim() || retryInstruction.slice(0, 160);
+      const retryKey =
+        normalizeTrimmedString(result.retry?.idempotencyKey) || retryInstruction.slice(0, 160);
       const budget = getFinalizeRetryBudget();
       const runBudget = budget.get(retryRunId) ?? new Map<string, number>();
       const nextCount = (runBudget.get(retryKey) ?? 0) + 1;
@@ -161,15 +161,23 @@ function normalizeBeforeAgentFinalizeResult(
       if (nextCount > maxAttempts) {
         return { action: "continue" };
       }
-      const reason = result.reason?.trim();
+      const reason = normalizeTrimmedString(result.reason);
       const revisedReason =
         reason && reason.includes(retryInstruction)
           ? reason
           : [reason, retryInstruction].filter(Boolean).join("\n\n");
       return { action: "revise", reason: revisedReason };
     }
-    const reason = result.reason?.trim();
+    const reason = normalizeTrimmedString(result.reason);
     return reason ? { action: "revise", reason } : { action: "continue" };
   }
   return { action: "continue" };
 }
+
+function normalizeTrimmedString(value: unknown): string | undefined {
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  const trimmed = value.trim();
+  return trimmed ? trimmed : undefined;
+}