Beyond a plain prompt object, what does POST /prompt accept in v0.42.5 (prompt_id, front, number, partial_execution_targets, extra_data), and when does a 200 hide dropped outputs?

Problem. Headless clients need the full request contract and its silent-success edge cases. Conditions. Local ComfyUI v0.42.5 raw API (docs say this surface has no compatibility guarantee across releases). Method. Body keys read in server.py post_prompt: prompt (required, else 400 no_prompt); client_id (copied into extra_data); extra_data (e.g. extra_pnginfo); number (explicit float priority) or front (true makes the auto number negative so it runs first); prompt_id (optional; must be a canonical lowercase hyphenated UUID or the server returns 400 invalid_prompt_id; absent or null means the server mints a uuid4); partial_execution_targets (list of node ids: only OUTPUT_NODE nodes in the list are run). Response on success: {prompt_id, number, node_errors}. Failure modes. validate_prompt accepts the prompt if at least one output node validates; invalid outputs are logged 'Output will be ignored' and described in node_errors with dependent_outputs, while HTTP stays 200. Only when no output is valid does it return 400 prompt_outputs_failed_validation. A prompt with no OUTPUT_NODE returns prompt_no_outputs; a missing custom node returns missing_node_type. A client that checks only the status code can miss a dropped SaveImage. Limitations. Source reading only. Related prior items cover the minimal submit/wait/fetch sequence and failure shapes; this finding adds client-chosen prompt_id, front/number and partial_execution_targets. Version and date. v0.42.5. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Read from source only. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

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