Linux prebuilt deployment fails at output ingestion with ENOENT

A private Next.js application on a Hobby project cannot publish updates. Existing production remains READY.

Next.js 15.5.26, Node.js 22 on Linux x64, pnpm 10.0.0, Vercel CLI 59.4.0.
Framework compilation, tracing and native PDF/OCR validation pass. All functions are below 250 MiB locally.
Deploy uses --prebuilt --prod --skip-domain. Source and runtime environment remain unchanged.
An initial upload exposed EEXIST because a pnpm directory symlink and regular files under that alias were both selected.
Normalizing regular filePathMap source values to confined realpaths, while preserving runtime destinations and symlink targets, removes those collisions.
The next attempt uploads 2,502 paths, restores them successfully and recognizes .vercel/output. It then fails at Deploying outputs with API errorCode ENOENT, errorStep direct:build, errorMessage task failed. Public build logs omit the missing path.
Exact isolated reconstruction of the uploaded 2,502 paths passes: 1,838 regular files, 664 relative symlinks, no broken links, missing mapping sources or handlers. Each mapped function covers its symlink targets. Reconstructed runtime files have identical contents/modes/links to the original.

What supported diagnostic reveals the missing path or failed function at this output-ingestion stage? Is filePathMap resolution affected by a monorepo rootDirectory in prebuilt deployments?

A private Next.js application on a Hobby project cannot publish updates. Existing production remains READY.

Next.js 15.5.26, Node.js 22 on Linux x64, pnpm 10.0.0, Vercel CLI 59.4.0.
Framework compilation, tracing and native PDF/OCR validation pass. All functions are below 250 MiB locally.
Deploy uses --prebuilt --prod --skip-domain. Source and runtime environment remain unchanged.
An initial upload exposed EEXIST because a pnpm directory symlink and regular files under that alias were both selected.
Normalizing regular filePathMap source values to confined realpaths, while preserving runtime destinations and symlink targets, removes those collisions.
The next attempt uploads 2,502 paths, restores them successfully and recognizes .vercel/output. It then fails at Deploying outputs with API errorCode ENOENT, errorStep direct:build, errorMessage task failed. Public build logs omit the missing path.
Exact isolated reconstruction of the uploaded 2,502 paths passes: 1,838 regular files, 664 relative symlinks, no broken links, missing mapping sources or handlers. Each mapped function covers its symlink targets. Reconstructed runtime files have identical contents/modes/links to the original.

What supported diagnostic reveals the missing path or failed function at this output-ingestion stage? Is filePathMap resolution affected by a monorepo rootDirectory in prebuilt deployments?