JustFill MCP Server

Let AI agents (Claude, ChatGPT, n8n — any MCP client) detect, review and fill

PDF form fields through justfill.app.

Excel or CSV batch workflow

If the source data is already in a spreadsheet and you need one filled copy of

the same existing PDF per row, an MCP client is optional. The guided browser

workflow imports XLSX or CSV, maps columns to reviewed PDF fields, previews each

record, and exports the approved PDFs in a ZIP.

Try the five-row PDF mail merge sample

— no card or sales call.

Import-ready n8n workflows

Start with the

free reviewed template in the official n8n workflow catalog.

It collects a PDF and JSON payload, reuses the reviewed field names saved for

that exact form, fills the original layout and returns a temporary download

link.

The repository also includes the exact deterministic workflow JSON, synthetic

test PDF, production evidence and a separate two-pass vision workflow for an

unfamiliar form. Both call the hosted MCP endpoint with standard HTTP Request

nodes and can be inspected before adding credentials.

Inspect the source workflows and evidence, or follow the

step-by-step n8n setup.

Business example: recurring supplier intake

An operations team can keep the supplier's required intake PDF unchanged,

save its reviewed field layout once, and let n8n map approved vendor data from

a webhook or CRM record into that exact form. The workflow returns a temporary

filled-PDF link that can be reviewed before it is uploaded to Drive, attached

to a draft email, or written back to the vendor record. The repository's

synthetic supplier-intake PDF exercises this exact path without customer data.

Gemini CLI extension

Install the same reviewed MCP tools plus the included PDF workflow guidance:

gemini extensions install https://github.com/mrmaciej1/justfill-mcp

The extension manifest lives at the repository root and uses the published

`justfill-mcp` package. Gemini CLI asks for normal third-party extension consent

before enabling it.

Why agents can trust it

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 justfill-mcp

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" }
  }
}

For a zero-install configuration, use `uvx` directly:

{
  "mcpServers": {
    "justfill": {
      "command": "uvx",
      "args": ["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()`

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.
• Usage and document-output rules are enforced by the same account service as

the web app. `fill_pdf` reports whether the output is clean or watermarked.

• 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.