diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index a32cb7ef..5647eb74 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed OpenAI Codex user-agent construction to synchronously load Node OS metadata, avoiding a startup race that could report `pi (browser)` in Node/Bun.
+
 ## [0.80.3] - 2026-06-30
 
 ### Added
diff --git a/packages/ai/src/api/openai-codex-responses.ts b/packages/ai/src/api/openai-codex-responses.ts
index 62886e0c..67db7a93 100644
--- a/packages/ai/src/api/openai-codex-responses.ts
+++ b/packages/ai/src/api/openai-codex-responses.ts
@@ -6,20 +6,20 @@ import type {
 	ResponseStreamEvent,
 } from "openai/resources/responses/responses.js";
 
-// NEVER convert to top-level runtime imports - breaks browser/Vite builds
-let _os: typeof NodeOs | null = null;
+type ProcessWithOsBuiltinModule = typeof process & {
+	getBuiltinModule?: (id: "node:os") => typeof NodeOs;
+};
 
-type DynamicImport = (specifier: string) => Promise<unknown>;
-
-const dynamicImport: DynamicImport = (specifier) => import(specifier);
-const NODE_OS_SPECIFIER = "node:" + "os";
-
-if (typeof process !== "undefined" && (process.versions?.node || process.versions?.bun)) {
-	dynamicImport(NODE_OS_SPECIFIER).then((m) => {
-		_os = m as typeof NodeOs;
-	});
+function loadNodeOs(): typeof NodeOs | null {
+	if (typeof process === "undefined" || !(process.versions?.node || process.versions?.bun)) {
+		return null;
+	}
+	return (process as ProcessWithOsBuiltinModule).getBuiltinModule?.("node:os") ?? null;
 }
 
+// NEVER convert to top-level runtime imports - breaks browser/Vite builds
+const _os: typeof NodeOs | null = loadNodeOs();
+
 import { clampThinkingLevel } from "../models.ts";
 import { registerSessionResourceCleanup } from "../session-resources.ts";
 import type {