Vellum

An MCP server that inspects generated documents (.docx, .pdf) before they reach the user — extracting hard, measurable facts about structure, typography, spacing, and language correctness, so an AI assistant can catch its own formatting mistakes instead of shipping them.

Built against the 2026-07-28 MCP specification. See docs/architecture.md for the full design rationale.

Why

Document-generation skills produce .docx/.pptx/.pdf output, but nothing automatically verifies the result is well-formed: heading levels can be skipped, fonts can be inconsistent, spacing can drift, grammar errors pass through silently. Vellum is a verification layer, not a generator.

Related MCP server: mcp-server-docx

Core principle

The server extracts facts. The model makes judgments.

Vellum never says "this is wrong." It says "here is what is actually true, and here is what is statistically anomalous relative to the rest of the document." Deciding whether a given value is appropriate for a line's role in the document (title vs. caption vs. body) is semantic judgment that stays with the calling model, which has the document's full context — the server does not, and should not try to.

Tools

Every tool returns structured facts plus anomaly flags — never a verdict.

Every tool accepts the document either as a filesystem path or as inline content_base64 (with filename required alongside it) — useful when a document only exists in the conversation (e.g. uploaded directly in a chat interface) rather than saved to disk first. Exactly one of the two input forms must be given.

Status

All six tools (check_docx_structure, check_docx_typography, check_pdf_layout, check_language, check_filename_consistency, full_report) are fully implemented, unit-tested, and validated against a golden set of fixtures with known planted defects. The full MCP surface — resources, prompts, elicitation, cache hints, OAuth 2.1 resource-server auth for the HTTP transport, and correct error semantics — is built and tested (163+ tests passing). See docs/architecture.md and PROGRESS.md for the detailed build history and current open items. Packaging/publication (Phase 7) is in progress — not yet released to PyPI.

Install

git clone https://github.com/Nasser9892/vellum-mcp.git
cd vellum-mcp
pip install -e ".[dev]"

Requires a local LanguageTool instance for check_language — see docs/architecture.md for setup.

Usage (Claude Desktop / any MCP stdio client)

Add to your MCP client config (see examples/claude_desktop_config.json):

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

Development

pip install -e ".[dev]"
pytest                          # unit + integration tests
mcp dev src/vellum_mcp/server.py  # run under MCP Inspector

Golden-set fixtures (sample .docx/.pdf files with deliberately planted defects) live in golden_set/. All checks are validated against these before being trusted — see docs/architecture.md for the discipline.

Validated against MCP Inspector (mcp dev src/vellum_mcp/server.py) and tested against two real MCP clients: Claude Desktop (examples/claude_desktop_config.json) and Cursor (~/.cursor/mcp.json, same {"mcpServers": {"vellum": {"command": "vellum-mcp"}}} shape).

Spec conformance

• All primitives: tools, resources, prompts (not tools-only)

• Stateless core — no reliance on Mcp-Session-Id

• Elicitation via MRTR (InputRequiredResult / inputRequests / inputResponses)

• OAuth 2.1 resource-server posture for the HTTP transport (RFC 9728, RFC 8707, RFC 9207; CIMD over deprecated DCR)

• Tasks extension (io.modelcontextprotocol/tasks) for long-running analysis

• Correct error codes (-32602 for not-found, -32020 for header-bound violations)

• No use of deprecated roots/sampling/logging primitives

License

MIT — see LICENSE.