What happens to file names when several ComfyUI runs write with the same filename_prefix at once, and how do I keep outputs unique and traceable?

Problem. Parallel jobs may collide on output names. Conditions. Core SaveImage; label: inference from code, not tested. Method. The counter is computed once per save call from a directory listing, with no lock or exclusive-create flag visible in the code; the file is then written by img.save to the computed name. Two processes (for example two servers sharing one output directory) scanning at the same moment could pick the same number and one overwrite the other (inference). Within one server, collisions depend on whether prompts execute one at a time, which was not verified here. Mitigations: give each job its own subfolder or stem (run id, shot id), use %batch_num% inside batches, and read the returned filename and subfolder from history rather than guessing the counter. PreviewImage shows the project's own approach: it writes to the temp directory with a random 5-letter suffix appended to the prefix. Metadata: unless --disable-metadata is set, the prompt and workflow are embedded in each PNG, which records which job made which file. Failure modes. Gaps in numbering (deleted files) do not reset the counter; it follows the maximum. Limitations. No concurrency test was run. Version/date. fb3befd, fetched 2026-10-24. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The collision risk is inferred from reading code; no concurrent run was executed and the server queue model was not checked in this round. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/folder_paths.py",
      "note": "get_save_image_path; PreviewImage.__init__; SaveImage.save_images; researcher reasoning"
    },
    {
      "url": "https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/nodes.py",
      "note": "get_save_image_path; PreviewImage.__init__; SaveImage.save_images; researcher reasoning"
    }
  ]
}