Validate and normalize untrusted paths

When accepting file paths or names from untrusted input (e.g., prompts, API payloads) that will be written to disk or used to run external tools, enforce strict allow-list validation before use: reject absolute paths (both POSIX and Windows styles), path traversal segments, control characters, reserved device names (CON, PRN, AUX, NUL, COMn, LPTn), trailing dots/spaces, unpaired surrogates, and references to sensitive directories (e.g., .git, .env). In addition, normalize each path segment consistently (Unicode NFC normalization plus case folding) before using it as a deduplication key or writing it to disk, since filesystems can be case-insensitive and/or Unicode-normalizing. Without this normalization, visually distinct inputs (e.g., Unicode confusables like ‘σ’ vs final-sigma ‘ς’, or differently-cased names) can collide unexpectedly on disk, causing unintended overwrites, cache poisoning, or validation bypass. Example:

// Validate
if (path.posix.isAbsolute(name) || path.win32.isAbsolute(name) || /\.\.|[\u0000-\u001f<>:"|?*]/.test(name)) {
  throw new Error(`Invalid path: ${name}`);
}
// Normalize for collision-safe keys
const target = name.normalize('NFC').toLowerCase();
if (seen.has(target)) {
  throw new Error(`Duplicate path after normalization: ${name}`);
}

Apply this pattern whenever untrusted names are mapped to filesystem resources, ensuring both strict rejection of dangerous inputs and normalization that matches the actual OS/filesystem semantics.