diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 6966d94a..d5f1525f 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -9,6 +9,7 @@
 
 ### Fixed
 
+- Fixed OpenAI Codex Responses replay after switching from Anthropic extended-thinking sessions by generating unique fallback message item IDs for converted thinking/text blocks ([#5148](https://github.com/earendil-works/pi/issues/5148)).
 - Fixed Anthropic-compatible replay for providers that return empty thinking signatures by adding an opt-in `allowEmptySignature` compatibility flag ([#4464](https://github.com/earendil-works/pi/issues/4464)).
 - Fixed OpenAI and OpenRouter GPT-5.5 Pro thinking level metadata to expose only supported medium, high, and xhigh efforts.
 - Fixed OpenCode Go Kimi K2.6 thinking-off requests to send `thinking: "none"` ([#5078](https://github.com/earendil-works/pi/issues/5078)).
diff --git a/packages/ai/src/providers/openai-responses-shared.ts b/packages/ai/src/providers/openai-responses-shared.ts
index b7bcd192..d769ce19 100644
--- a/packages/ai/src/providers/openai-responses-shared.ts
+++ b/packages/ai/src/providers/openai-responses-shared.ts
@@ -166,6 +166,7 @@ export function convertResponsesMessages<TApi extends Api>(
 				assistantMsg.model !== model.id &&
 				assistantMsg.provider === model.provider &&
 				assistantMsg.api === model.api;
+			let textBlockIndex = 0;
 
 			for (const block of msg.content) {
 				if (block.type === "thinking") {
@@ -176,10 +177,13 @@ export function convertResponsesMessages<TApi extends Api>(
 				} else if (block.type === "text") {
 					const textBlock = block as TextContent;
 					const parsedSignature = parseTextSignature(textBlock.textSignature);
+					const fallbackMessageId =
+						textBlockIndex === 0 ? `pi_msg_${msgIndex}` : `pi_msg_${msgIndex}_${textBlockIndex}`;
+					textBlockIndex++;
 					// OpenAI requires id to be max 64 characters
 					let msgId = parsedSignature?.id;
 					if (!msgId) {
-						msgId = `msg_${msgIndex}`;
+						msgId = fallbackMessageId;
 					} else if (msgId.length > 64) {
 						msgId = `msg_${shortHash(msgId)}`;
 					}
diff --git a/packages/ai/test/openai-responses-message-id.test.ts b/packages/ai/test/openai-responses-message-id.test.ts
new file mode 100644
index 00000000..07669d8c
--- /dev/null
+++ b/packages/ai/test/openai-responses-message-id.test.ts
@@ -0,0 +1,48 @@
+import type { ResponseOutputMessage } from "openai/resources/responses/responses.js";
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.ts";
+import { convertResponsesMessages } from "../src/providers/openai-responses-shared.ts";
+import type { AssistantMessage, Context, Usage } from "../src/types.ts";
+
+const usage: Usage = {
+	input: 0,
+	output: 0,
+	cacheRead: 0,
+	cacheWrite: 0,
+	totalTokens: 0,
+	cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+};
+
+describe("OpenAI Responses message ID conversion", () => {
+	it("generates unique fallback message IDs for multiple text blocks in one assistant turn", () => {
+		const model = getModel("openai-codex", "gpt-5.5");
+		const assistant: AssistantMessage = {
+			role: "assistant",
+			content: [
+				{ type: "thinking", thinking: "private reasoning" },
+				{ type: "text", text: "visible answer" },
+			],
+			api: "anthropic-messages",
+			provider: "anthropic",
+			model: "claude-opus-4-8",
+			usage,
+			stopReason: "stop",
+			timestamp: Date.now() - 1000,
+		};
+		const context: Context = {
+			systemPrompt: "You are concise.",
+			messages: [{ role: "user", content: "hello", timestamp: Date.now() - 2000 }, assistant],
+		};
+
+		const input = convertResponsesMessages(model, context, new Set(["openai", "openai-codex", "opencode"]));
+		const messageIds = input
+			.filter(
+				(item): item is ResponseOutputMessage =>
+					item.type === "message" && "id" in item && typeof item.id === "string",
+			)
+			.map((item) => item.id);
+
+		expect(messageIds).toEqual(["pi_msg_1", "pi_msg_1_1"]);
+		expect(new Set(messageIds).size).toBe(messageIds.length);
+	});
+});