diff --git a/src/agents/pi-embedded-runner/replay-history.ts b/src/agents/pi-embedded-runner/replay-history.ts
index 20f943231c3..25b156998dd 100644
--- a/src/agents/pi-embedded-runner/replay-history.ts
+++ b/src/agents/pi-embedded-runner/replay-history.ts
@@ -227,6 +227,26 @@ function stripStaleAssistantUsageBeforeLatestCompaction(messages: AgentMessage[]
   return touched ? out : messages;
 }
 
+export function normalizeAssistantReplayContent(messages: AgentMessage[]): AgentMessage[] {
+  let touched = false;
+  const out = [...messages];
+  for (let i = 0; i < out.length; i += 1) {
+    const message = out[i] as (AgentMessage & { role?: unknown; content?: unknown }) | undefined;
+    if (!message || message.role !== "assistant" || Array.isArray(message.content)) {
+      continue;
+    }
+    out[i] = {
+      ...(message as unknown as Record<string, unknown>),
+      content:
+        typeof message.content === "string"
+          ? [{ type: "text", text: message.content }]
+          : [{ type: "text", text: "" }],
+    } as AgentMessage;
+    touched = true;
+  }
+  return touched ? out : messages;
+}
+
 function normalizeAssistantUsageSnapshot(usage: unknown) {
   const normalized = normalizeUsage((usage ?? undefined) as UsageLike | undefined);
   if (!normalized) {
@@ -443,8 +463,9 @@ export async function sanitizeSessionHistory(params: {
     params.modelApi === "openai-responses" ||
     params.modelApi === "openai-codex-responses" ||
     params.modelApi === "azure-openai-responses";
+  const normalizedAssistantReplay = normalizeAssistantReplayContent(withInterSessionMarkers);
   const sanitizedImages = await sanitizeSessionMessagesImages(
-    withInterSessionMarkers,
+    normalizedAssistantReplay,
     "session:history",
     {
       sanitizeMode: policy.sanitizeMode,
diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index 0e4bc64c375..c462873b94c 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -138,7 +138,11 @@ import {
   type PromptCacheChange,
 } from "../prompt-cache-observability.js";
 import { resolveCacheRetention } from "../prompt-cache-retention.js";
-import { sanitizeSessionHistory, validateReplayTurns } from "../replay-history.js";
+import {
+  normalizeAssistantReplayContent,
+  sanitizeSessionHistory,
+  validateReplayTurns,
+} from "../replay-history.js";
 import { observeReplayMetadata, replayMetadataFromState } from "../replay-state.js";
 import {
   clearActiveEmbeddedRun,
@@ -1140,6 +1144,9 @@ export async function runEmbeddedAttempt(
         throw new Error("Embedded agent session missing");
       }
       const activeSession = session;
+      const baseConvertToLlm = activeSession.agent.convertToLlm.bind(activeSession.agent);
+      activeSession.agent.convertToLlm = async (messages) =>
+        await baseConvertToLlm(normalizeAssistantReplayContent(messages));
       let prePromptMessageCount = activeSession.messages.length;
       abortSessionForYield = () => {
         yieldAbortSettled = Promise.resolve(activeSession.abort());
@@ -2179,6 +2186,12 @@ export async function runEmbeddedAttempt(
           }
 
           if (!skipPromptSubmission) {
+            const normalizedReplayMessages = normalizeAssistantReplayContent(
+              activeSession.messages,
+            );
+            if (normalizedReplayMessages !== activeSession.messages) {
+              activeSession.agent.state.messages = normalizedReplayMessages;
+            }
             finalPromptText = effectivePrompt;
             const btwSnapshotMessages = activeSession.messages.slice(-MAX_BTW_SNAPSHOT_MESSAGES);
             updateActiveEmbeddedRunSnapshot(params.sessionId, {
diff --git a/src/agents/pi-embedded-runner/sanitize-session-history.tool-result-details.test.ts b/src/agents/pi-embedded-runner/sanitize-session-history.tool-result-details.test.ts
index 54d1afb027a..a951176e2a0 100644
--- a/src/agents/pi-embedded-runner/sanitize-session-history.tool-result-details.test.ts
+++ b/src/agents/pi-embedded-runner/sanitize-session-history.tool-result-details.test.ts
@@ -60,4 +60,25 @@ describe("sanitizeSessionHistory toolResult details stripping", () => {
     const serialized = JSON.stringify(sanitized);
     expect(serialized).not.toContain("Ignore previous instructions");
   });
+
+  it("normalizes malformed assistant string content before replay sanitization", async () => {
+    const sm = SessionManager.inMemory();
+
+    const sanitized = await sanitizeSessionHistory({
+      messages: [
+        { role: "assistant", content: "plain reply", timestamp: 1 } as unknown as AgentMessage,
+        { role: "user", content: "continue", timestamp: 2 } satisfies UserMessage,
+      ],
+      modelApi: "openai-responses",
+      provider: "github-copilot",
+      modelId: "gpt-5-mini",
+      sessionManager: sm,
+      sessionId: "test",
+    });
+
+    expect(sanitized[0]).toMatchObject({
+      role: "assistant",
+      content: [{ type: "text", text: "plain reply" }],
+    });
+  });
 });