JustFill PDF Forms

Let AI agents (Claude, ChatGPT, n8n — any MCP client) detect, review and fill PDF form fields through justfill.app.

Why agents can trust it

Source	Confidence	What it means
Saved template	1.0	This exact PDF was filled before; geometry is human/agent-verified. No ML runs at all.
AcroForm	1.0	The PDF has embedded form fields — read from the file, filled natively.
ML detection	0.0–0.95	An honest draft. Review it visually (render_preview), fix it, then save_template to lock it in.

ML confidence is calibrated: the detector's raw scores are not probabilities (its server-side filter accepts boxes from raw ~0.02 and auto-accepts at raw 0.15), so they are mapped onto 0–1 to mean what you'd expect — ≥0.75 "detector is sure", 0.4–0.75 "probably right, glance at the preview", <0.4 "borderline accept, verify". The raw detector score is kept on each field as raw_score.

The correction loop (render_preview → add/update/remove_field) exists precisely because ML detection has false positives and negatives. A false positive costs nothing (leave it unfilled or remove it); a false negative is visible on the preview and fixable with one add_field call. Once reviewed, save_template makes every future fill of that form deterministic.

Setup

uv tool install ./mcp-server        # or: pip install ./mcp-server

Authorize once (opens the browser, one click while logged in to justfill.app):

justfill-mcp login

Then the config needs no credentials at all:

{
  "mcpServers": {
    "justfill": { "command": "justfill-mcp" }
  }
}

Alternatives, in the order the server checks them:

1. JUSTFILL_API_KEY env — create a key at justfill.app → Account → API Keys and put "env": {"JUSTFILL_API_KEY": "jf_live_…"} in the config.
2. The key saved by justfill-mcp login (~/.config/justfill/credentials.json).
3. JUSTFILL_EMAIL + JUSTFILL_PASSWORD — legacy fallback; an API key is better (no password in config files, revocable per client, never expires mid-session).

Tools

• open_pdf(path, min_confidence=0.0, max_pages=10, force_detect=False) — template → AcroForm → ML resolution order. Accepts scanned images too (jpg/png/tiff → converted to PDF, deterministically, so templates still match). force_detect=True ignores a saved template and re-runs ML.
• render_preview(page_index) — page image with labeled field boxes (blue = deterministic, green/orange/red = ML confidence)
• render_filled_preview(values, page_index) — the same page with your values drawn in place (checkboxes get an X). Costs no fills — check before you fill.
• list_fields(page_index?)
• add_field(x, y, w, h, name, page_index, field_type, align?, vertical_align?) — coords in % of page, top-left origin
• update_field(field_id, …) / remove_field(field_id)
• update_fields([{field_id, …}, …]) / remove_fields([ids]) — batch versions
• prune_fields(field_type?, confidence_below?, width_below?, height_below?, page_index?, exclude_ids?) — bulk-delete detection noise in one call (criteria AND-ed, removed ids returned)
• fill_pdf(values, output_path, flatten=True) — values = {field_id: text}; responds with warnings for values that will be shrunk/truncated to fit
• save_template(name) — persist the reviewed layout for deterministic repeat fills
• list_templates()

Text alignment: align = left|center|right, vertical_align = top|middle|bottom — set per field (e.g. right for RTL forms, center for boxed digits). Persisted in templates.

Example agent flow

open_pdf("~/forms/w-9.pdf")            → acroform, 27 fields, confidence 1.0
fill_pdf({"f1": "Jane Doe", …}, "~/out/w-9-filled.pdf")

open_pdf("~/forms/scan.jpg")           → converted to PDF; ml, 34 fields
render_preview(0)                      → agent sees noise + one missed line
prune_fields(field_type="cell", width_below=3)   → 16 removed in one call
add_field(x=18, y=62.5, w=40, h=3, name="Phone")
render_filled_preview({…})             → values sit right, no overflow
fill_pdf({…}, "~/out/filled.pdf")
save_template("Client intake form")    → next time: deterministic

Notes

• Auth is a regular justfill.app account; tokens auto-refresh on expiry.
• Pricing follows the same rules as the web app: template and AcroForm resolution never cost anything; ML detection is free on the free plan (daily caps apply) and uses per-page credits on paid plans; downloads consume the account's fill allowance/credits.
• One PDF open at a time per server session (by design — keeps ids stable).
• This repository mirrors released versions of the MCP client (development happens in a private monorepo alongside the justfill.app backend). Bug reports and feature requests are very welcome in the issue tracker here.