diff --git a/packages/ai/src/providers/openai-codex-responses.ts b/packages/ai/src/providers/openai-codex-responses.ts
index 3a563d44..0b163cb9 100644
--- a/packages/ai/src/providers/openai-codex-responses.ts
+++ b/packages/ai/src/providers/openai-codex-responses.ts
@@ -282,7 +282,7 @@ function buildRequestBody(
 
 function clampReasoningEffort(modelId: string, effort: string): string {
 	const id = modelId.includes("/") ? modelId.split("/").pop()! : modelId;
-	if (id.startsWith("gpt-5.2") && effort === "minimal") return "low";
+	if ((id.startsWith("gpt-5.2") || id.startsWith("gpt-5.3")) && effort === "minimal") return "low";
 	if (id === "gpt-5.1" && effort === "xhigh") return "high";
 	if (id === "gpt-5.1-codex-mini") return effort === "high" || effort === "xhigh" ? "high" : "medium";
 	return effort;
diff --git a/packages/ai/test/openai-codex-stream.test.ts b/packages/ai/test/openai-codex-stream.test.ts
index ad2be006..8f7685cf 100644
--- a/packages/ai/test/openai-codex-stream.test.ts
+++ b/packages/ai/test/openai-codex-stream.test.ts
@@ -231,6 +231,102 @@ describe("openai-codex streaming", () => {
 		await streamResult.result();
 	});
 
+	it("clamps gpt-5.3-codex minimal reasoning effort to low", async () => {
+		const tempDir = mkdtempSync(join(tmpdir(), "pi-codex-stream-"));
+		process.env.PI_CODING_AGENT_DIR = tempDir;
+
+		const payload = Buffer.from(
+			JSON.stringify({ "https://api.openai.com/auth": { chatgpt_account_id: "acc_test" } }),
+			"utf8",
+		).toString("base64");
+		const token = `aaa.${payload}.bbb`;
+
+		const sse = `${[
+			`data: ${JSON.stringify({
+				type: "response.output_item.added",
+				item: { type: "message", id: "msg_1", role: "assistant", status: "in_progress", content: [] },
+			})}`,
+			`data: ${JSON.stringify({ type: "response.content_part.added", part: { type: "output_text", text: "" } })}`,
+			`data: ${JSON.stringify({ type: "response.output_text.delta", delta: "Hello" })}`,
+			`data: ${JSON.stringify({
+				type: "response.output_item.done",
+				item: {
+					type: "message",
+					id: "msg_1",
+					role: "assistant",
+					status: "completed",
+					content: [{ type: "output_text", text: "Hello" }],
+				},
+			})}`,
+			`data: ${JSON.stringify({
+				type: "response.completed",
+				response: {
+					status: "completed",
+					usage: {
+						input_tokens: 5,
+						output_tokens: 3,
+						total_tokens: 8,
+						input_tokens_details: { cached_tokens: 0 },
+					},
+				},
+			})}`,
+		].join("\n\n")}\n\n`;
+
+		const encoder = new TextEncoder();
+		const stream = new ReadableStream<Uint8Array>({
+			start(controller) {
+				controller.enqueue(encoder.encode(sse));
+				controller.close();
+			},
+		});
+
+		const fetchMock = vi.fn(async (input: string | URL, init?: RequestInit) => {
+			const url = typeof input === "string" ? input : input.toString();
+			if (url === "https://api.github.com/repos/openai/codex/releases/latest") {
+				return new Response(JSON.stringify({ tag_name: "rust-v0.0.0" }), { status: 200 });
+			}
+			if (url.startsWith("https://raw.githubusercontent.com/openai/codex/")) {
+				return new Response("PROMPT", { status: 200, headers: { etag: '"etag"' } });
+			}
+			if (url === "https://chatgpt.com/backend-api/codex/responses") {
+				const body = typeof init?.body === "string" ? (JSON.parse(init.body) as Record<string, unknown>) : null;
+				expect(body?.reasoning).toEqual({ effort: "low", summary: "auto" });
+
+				return new Response(stream, {
+					status: 200,
+					headers: { "content-type": "text/event-stream" },
+				});
+			}
+			return new Response("not found", { status: 404 });
+		});
+
+		global.fetch = fetchMock as typeof fetch;
+
+		const model: Model<"openai-codex-responses"> = {
+			id: "gpt-5.3-codex",
+			name: "GPT-5.3 Codex",
+			api: "openai-codex-responses",
+			provider: "openai-codex",
+			baseUrl: "https://chatgpt.com/backend-api",
+			reasoning: true,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 400000,
+			maxTokens: 128000,
+		};
+
+		const context: Context = {
+			systemPrompt: "You are a helpful assistant.",
+			messages: [{ role: "user", content: "Say hello", timestamp: Date.now() }],
+		};
+
+		const streamResult = streamOpenAICodexResponses(model, context, {
+			apiKey: token,
+			reasoningEffort: "minimal",
+		});
+		await streamResult.result();
+	});
+
 	it("does not set conversation_id/session_id headers when sessionId is not provided", async () => {
 		const tempDir = mkdtempSync(join(tmpdir(), "pi-codex-stream-"));
 		process.env.PI_CODING_AGENT_DIR = tempDir;