SMP · Sidecar Saver (legacy)

Every ComfyUI PNG already carries its workflow as embedded metadata, so why a separate sidecar JSON file? Because for a structured-prompt pipeline, the flat prompt string isn't the interesting record - the structured dict is. Sidecar Saver writes both: the PNG, exactly like stock SaveImage, plus a <prefix>_NNNNN_.prompt.json sitting right beside it holding the full PROMPT_DICT, and optionally the structured bundle and your own extra metadata.

The reproducibility angle is the selling point. Re-run with the same seeds and the sidecar JSON comes out byte-identical for the structural fields. Months later you can look at a folder of outputs and know exactly which subject, outfit, and location generated each image - without digging through PNG metadata or hoping the workflow was saved.

How it works

It mirrors ComfyUI's own SaveImage path logic via folder_paths.get_save_image_path, so the naming convention matches what you're used to: ComfyUI/output/SMP/img_00001_.png plus img_00001_.prompt.json next to it. It's an OUTPUT_NODE, so it triggers on Queue even with nothing connected downstream. The JSON embeds the prompt_dict, the optional structured bundle, and whatever you stuff into extra_metadata_json.

The inputs that matter

• images - the decoded batch.
• filename_prefix - defaults to SMP/img.
• prompt_dict (optional) - without it you still get PNGs, just a thin sidecar.
• structured (optional), extra_metadata_json (optional) - extra structure and your own metadata string.

Outputs

None. This is a terminal node, not a passthrough - put it last, after VAEDecode.

Install

No extra dependencies. Base pack install:

cd ComfyUI/custom_nodes
git clone https://github.com/ping1979ping/comfyui-FVMtools

Restart and you're set.

What's inside the JSON

The sidecar is the SMP workflow's audit trail. When you wire the full prompt_dict, it captures subjects[0].age_desc, the resolved outfits.subject_1.garments (with actual color names, no #token# left), the location.elements, and - if the Assembler is connected - the structured prompts and region map. The SMP reference workflow's verification checklist leans on exactly these fields: run, check the sidecar, know the pipeline did what you thought it did.

Common issues

The sidecar comes out missing keys when prompt_dict isn't connected - the node happily saves images alone, it just writes a nearly-empty JSON. And the file lands in ComfyUI/output alongside the PNGs, not in the pack's directory; people grep the wrong folder once. If you're chaining a ShowText to inspect the JSON, remember the sidecar is written at save time, so the file is the truth, not the preview.