BoundaryML/baml · error · FsPathError

unsupported native path prefix

Error message

unsupported native path prefix: {0}

What it means

FsPathError::UnsupportedPrefix is returned when a native path uses a prefix form baml_path does not support, e.g. Windows verbatim (\\?\C:\...) or device namespace (\\.\) prefixes. Such paths cannot be mapped into the VFS path scheme.

Solutions

1. Strip the \\?\ verbatim prefix before conversion (or use the dunce crate's canonicalize which avoids verbatim form).
2. Use a normal drive-letter path (C:\project\file.baml) instead of a device/namespace path.
3. Don't point the project at UNC/network shares; copy it to a local drive path.
4. Normalize the path with a helper that removes unsupported prefixes at the boundary.

Example fix

// before
let p = std::fs::canonicalize(r"C:\proj\main.baml")?; // \\?\C:\proj\main.baml

// after
let p = dunce::canonicalize(r"C:\proj\main.baml")?; // C:\proj\main.baml

fn has_unsupported_prefix(p: &std::path::Path) -> bool {
    let s = p.as_os_str().to_string_lossy();
    s.starts_with("\\\\?\\") || s.starts_with("\\\\.\\")
}
// strip or reject before conversion

fn is_plain_windows_path(p: &std::path::Path) -> bool {
    let s = p.as_os_str().to_string_lossy();
    !s.starts_with("\\\\?\\") && !s.starts_with("\\\\.\\")
}

When it happens

Trigger: On Windows, passing paths produced by dunce-less canonicalization that yield \\?\ verbatim form, or UNC/device paths (\\.\pipe\..., \\?\UNC\server\share) into native->VFS conversion.

Common situations: Using std::fs::canonicalize on Windows (returns \\?\ prefixed paths) before handing the result to baml_path; working with network shares or named pipes; junctions resolved to verbatim form.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Expected --from ' ' to be a baml_src/ directory, but it…
• Expected --from ' ' to be a baml_src/ directory, but it is…
• native path is not absolute
• a source root already exists at this path
• blob path has no parent

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/70b8f61467783a78. Report an issue: GitHub.

Appendix: source

Thrown at baml_language/crates/baml_path/src/lib.rs:39

    pub fn as_str(&self) -> &str {
        &self.0
    }

    pub fn into_string(self) -> String {
        self.0
    }
}

#[derive(Debug, thiserror::Error)]
pub enum FsPathError {
    #[error("native path is not absolute: {0}")]
    NotAbsolute(PathBuf),

    #[error("native path is not valid Unicode: {0:?}")]
    NonUnicode(PathBuf),

    #[error("unsupported native path prefix: {0}")]
    UnsupportedPrefix(PathBuf),

    #[error("invalid VFS path: {0}")]
    InvalidVfsPath(String),
}

impl From<FsPathError> for vfs::VfsError {
    fn from(error: FsPathError) -> Self {
        vfs::VfsError::from(vfs::error::VfsErrorKind::Other(error.to_string()))
    }
}

fn validate_vfs_path(path: &str) -> Result<(), FsPathError> {
    if path == "/" {
        return Ok(());
    }

    let invalid = !path.starts_with('/')

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