bun build --app panics with "Runtime file not found" for any custom Bake.Framework #32142

What version of Bun is running?

1.3.14

What platform is your computer?

Darwin arm64 (macOS)

What steps can reproduce the bug?

mkdir bake-repro && cd bake-repro
mkdir pages

cat > pages/index.tsx <<'EOF'
export default function Home() { return "homepage"; }
EOF

cat > server-entry.ts <<'EOF'
export function render(req: Request, meta: any) {
  return new Response(String(meta.pageModule.default()));
}
export async function prerender(meta: any) {
  return { files: { "index.html": String(meta.pageModule.default()) } };
}
EOF

cat > client-entry.ts <<'EOF'
console.log("client loaded");
EOF

cat > bun.app.ts <<'EOF'
export default {
  app: {
    framework: {
      fileSystemRouterTypes: [{
        root: "pages",
        style: "nextjs-pages",
        serverEntryPoint: "./server-entry.ts",
        clientEntryPoint: "./client-entry.ts",
      }],
    },
  },
};
EOF

BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE=1 bun build --app

What is the expected behavior?

dist/index.html is written from prerender.

What do you see instead?

Loading configuration
Bundling routes
Rendering routes
panic(main thread): Runtime file not found. This is an unexpected bug in Bun. Please file a bug report on GitHub.

The client chunk (dist/_bun/<hash>.js) is written, then bun crashes.

Additional information

The bundler only populates OutputFile.bake_extra when is_built_in_react is true:

src/bundler/linker_context/generateChunksInParallel.zig:739 —

if (!c.framework.?.is_built_in_react) break :brk .{};

For any custom Bake.Framework, every chunk's bake_extra.bake_is_runtime therefore stays false, so maybe_runtime_file_index is never assigned, and src/runtime/bake/production.zig:448 panics.

framework: "react" does not hit this panic (it fails earlier on missing react-server-dom-bun deps, which is expected). The dev server (Bun.serve with app: via the default-export-as-server convention) works fine for custom frameworks — only bun build --app is affected.

The fix is presumably to drop the is_built_in_react gate (or at minimum still set bake_is_runtime for custom frameworks).