fix(devtools): use explicit env flag for dev mode detection (#12694)

## Summary
- Replace path-based `/dist/` check with an explicit
`AI_SDK_DEVTOOLS_DEV` env flag for dev mode detection
- Prevents false positives in monorepos (NX, Turborepo) where `/dist/`
paths are common in the workspace
- The `dev:api` script now sets `AI_SDK_DEVTOOLS_DEV=true`
automatically; production mode is the default when the flag is unset

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: nicoalbanese

.changeset/cuddly-turtles-collect.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/devtools': patch
+---
+
+Use explicit AI_SDK_DEVTOOLS_DEV env flag for dev mode detection

packages/devtools/package.json

@@ -26,7 +26,7 @@
   ],
   "scripts": {
     "dev": "concurrently -k \"pnpm dev:api\" \"pnpm dev:client\"",
-    "dev:api": "tsx src/viewer/server.ts",
+    "dev:api": "AI_SDK_DEVTOOLS_DEV=true tsx src/viewer/server.ts",
     "dev:client": "vite",
     "example": "tsx examples/basic/index.ts",
     "start": "node dist/viewer/server.js",

packages/devtools/src/viewer/server.ts

@@ -36,13 +36,19 @@ const broadcastToClients = (event: string, data: Record<string, unknown>) => {
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
-// Determine if we're running from source (tsx) or built (dist)
-// We're in dev mode if NOT running from the dist folder
-const isDevMode =
-  !__dirname.includes('/dist/') || process.env.NODE_ENV === 'development';
-const projectRoot = isDevMode
-  ? path.resolve(__dirname, '../..')
-  : path.resolve(__dirname, '../..');
+// Determine whether to use the dev-mode Vite proxy or serve the built client.
+//
+// We rely exclusively on an explicit env flag to avoid false positives in
+// monorepos where /dist paths are common.
+//
+//   AI_SDK_DEVTOOLS_DEV=true  → use dev mode (Vite proxy)
+//   AI_SDK_DEVTOOLS_DEV=false → use production mode (serve built client)
+//
+// If the flag is unset, default to production mode.
+const devEnv = process.env.AI_SDK_DEVTOOLS_DEV;
+const isDevMode = devEnv !== undefined && devEnv !== 'false' && devEnv !== '0';
+// __dirname points at packages/devtools/src/viewer, so ../.. is the package root.
+const projectRoot = path.resolve(__dirname, '../..');
 
 // Client directory: dist/client in both cases
 const clientDir = path.join(projectRoot, 'dist/client');