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obsidian-markdown-lint-mcp-server

An MCP server that lints, validates, and renders your Obsidian vault markdown — all inside Docker.

Tools • Quick Start • Configuration • Schemas • Development

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Run markdown linting, front matter validation, and Mermaid-to-SVG rendering as MCP tools callable from Claude Code. Docker isolates Chromium and Node.js from your host machine. The server is stateless — it processes content you send and returns results; Claude Code handles all disk I/O.

This is a stdio MCP server: Claude Code launches it as a subprocess (docker run -i --rm) and talks to it over stdin/stdout. There is no port and no long-running container — the container starts when a session opens and is removed when it ends.

Why Docker

Mermaid rendering requires a Chromium browser (via Puppeteer). Running that, plus dozens of Node.js dependencies, directly on your laptop is a valid security concern. This server packages everything inside a container. Your vault never mounts into it; content travels as strings.

Tools

Tool	What it does
lint_markdown	Runs markdownlint on content, returns errors and a corrected version
validate_front_matter	Validates YAML front matter against a JSON Schema you pass in
render_mermaid_diagrams	Renders all ```mermaid ``` blocks to SVG, replaces them with GitHub image links, embeds the original Mermaid source as base64 in each SVG's <metadata>
extract_mermaid_from_svg	Reads a rendered SVG and returns the original Mermaid source as a code block, ready to edit

The render/extract pair supports a full edit cycle: render → view SVG in Obsidian → extract → edit source → re-render.

Quick start

Prerequisites: Docker Desktop, Node.js ≥ 20, Claude Code.

git clone <repo>
cd obsidian-markdown-lint-mcp-server
npm install
npm run build              # compile TypeScript → dist/
docker build -t obsidian-markdown-lint-mcp .   # or: docker compose build

Register the server with Claude Code. Claude Code runs docker run -i --rm itself, per session — you do not start a container yourself.

Per-project (recommended for a vault) — run this from your vault root; it writes a committable .mcp.json:

claude mcp add obsidian-markdown-lint -s project -- docker run -i --rm obsidian-markdown-lint-mcp

The resulting .mcp.json:

{
  "mcpServers": {
    "obsidian-markdown-lint": {
      "command": "docker",
      "args": ["run", "-i", "--rm", "obsidian-markdown-lint-mcp"]
    }
  }
}

Global — make it available in every project (-s user writes to ~/.claude.json):

claude mcp add obsidian-markdown-lint -s user -- docker run -i --rm obsidian-markdown-lint-mcp

Verify with claude mcp list (should show obsidian-markdown-lint → connected), then start a new Claude Code session — tools are discovered at session start. The four tools are now available.

The -i flag is required: it keeps the container's stdin open for the JSON-RPC stream. Without it the container gets EOF and exits immediately. Do not use docker compose up for this server — it is a stdio subprocess, not a long-running HTTP service.

Hardening (optional)

The server needs no network, no writable root filesystem, and little memory, so you can lock the container down. --network none and --security-opt no-new-privileges are always safe (the image skips the Puppeteer download and renders offline):

claude mcp add obsidian-markdown-lint -s project -- docker run -i --rm --network none --security-opt no-new-privileges --memory 2g obsidian-markdown-lint-mcp

For stricter isolation, add a read-only root with a writable temp dir and drop all Linux capabilities. Chromium renders into /tmp because the server launches it with --disable-dev-shm-usage, so the --tmpfs /tmp is required. Verify a Mermaid render still succeeds under these flags before relying on them, since Chromium's writable paths vary by version:

docker run -i --rm --network none --security-opt no-new-privileges --read-only --tmpfs /tmp --cap-drop ALL --memory 2g obsidian-markdown-lint-mcp

Configuration

Vault layout

The server reads nothing from disk directly. Claude Code reads your vault files and passes content as strings. Place these config files at your vault root:

vault/
  .markdownlint.json     ← linting rules (optional; defaults to markdownlint defaults)
  .schemas/
    _shared.json         ← shared field definitions (reference; not loaded at runtime)
    article.json
    how-to.json
    technical.json
    deep-research.json
    strategy.json
    meeting.json
    brainstorming.json

Pre-built schemas for all seven note types are included in this repo's .schemas/ directory. Copy them to your vault root. The seven type schemas are self-contained, so validate_front_matter only needs the one matching the document's type; _shared.json is the source the type schemas are built from and is kept for maintenance.

CLAUDE.md for your vault

Add a CLAUDE.md to your vault project that tells Claude how to use the server. Minimum viable example:

## MCP: obsidian-markdown-lint-mcp-server

Vault attachments directory: attachments
Schemas directory: .schemas/
Linting config: .markdownlint.json

When asked to "process a vault file":
1. Read the file
2. Read .markdownlint.json and call lint_markdown
3. Read the type field from front matter, read .schemas/{type}.json, call validate_front_matter
4. Call render_mermaid_diagrams with attachments_dir="attachments" and the document title
5. Write the modified_content back to the file
6. Write each SVG from the svgs array to its path field (decoded from base64)

SVG files contain the original Mermaid source base64-encoded in <metadata>.
To edit a diagram: call extract_mermaid_from_svg with the SVG content,
edit the returned mermaid_block, then re-run render_mermaid_diagrams.

Mermaid front matter options

Control rendering per-document:

mermaid_theme: dark          # default | dark | neutral | forest  (default: default)
mermaid_background: white    # any CSS color or "transparent"     (default: white)

After rendering, the server adds mermaid_svg_source: base64-embedded to the front matter so Claude knows SVGs in this document contain extractable source.

Front matter schemas

Seven note types are supported. Each schema lives at .schemas/{type}.json.

Type	Extra required fields
article	core only
how-to	core only
technical	system, component
deep-research	sources (array, min 1)
strategy	related, prepared_for, quarter
meeting	meeting_date, attendees (array, min 1)
brainstorming	core only

Core required fields on every type: type, title, author, category, tags, description, summary, status, version, date_created, date_updated.

The type field is the discriminator. Claude reads it from the front matter to select the right schema file before calling validate_front_matter.

How SVG embedding works

Each rendered SVG contains the original Mermaid source, base64-encoded, inside the SVG's <metadata> element:

<metadata>
  <mermaid:source xmlns:mermaid="http://mermaid.js.org/" encoding="base64">
    Zmxvd2NoYXJ0IExSCiAgICBBIC0tPiBCCg==
  </mermaid:source>
</metadata>

Base64 avoids all whitespace normalization issues — Mermaid is whitespace-sensitive and attribute-value normalization in XML would corrupt indentation. The extract_mermaid_from_svg tool decodes this and returns a ready-to-use ```mermaid ``` block.

SVG output paths

Diagrams render to {attachments_dir}/{document-title-slug}/{diagram-type}-{n}.svg.

For a document titled "System Architecture" with attachments_dir = "attachments" and two flowchart blocks, you get:

attachments/system-architecture/flowchart-1.svg
attachments/system-architecture/flowchart-2.svg

The markdown is updated to:

![flowchart diagram 1](attachments/system-architecture/flowchart-1.svg)

Standard GitHub-flavored markdown — renders in Obsidian, GitHub, and any standard markdown viewer.

Testing

Three layers, all runnable from the repo without building the Docker image:

npm test          # 52 Jest unit tests (ESM), enforced coverage thresholds
npm run eval      # tool-correctness evals against the compiled functions
npm run snapshot  # end-to-end fixture snapshots (requires a real Chromium)

Unit tests (tests/unit/) call the tool functions directly. Coverage is gated (90% lines/functions/statements, 85% branches); src/server.ts and src/create-server.ts are excluded as pure wiring and are covered separately through an in-memory MCP transport. Mermaid rendering is tested with an injected browser/renderer, so no Chromium is needed here. Use npm run test:coverage for the lcov/HTML report.

Evals (tests/evals/) check tool correctness against the compiled functions (no Docker). Output is JSON with summary.passed, summary.failed, and a results array; exit code is 1 if any eval fails. The suite covers all four tools: lint detection, schema validation (pass and fail cases), Mermaid rendering with a mock browser, and SVG round-trip extraction. In Claude Code, say "run evals" or "evaluate the tools" and it runs npm run eval and reports the results.

Snapshots (tests/snapshot/) prove that each input under test-obsidian-vault/original/ reproduces its committed test-obsidian-vault/snap-shot/ output through the real tool pipeline. Unlike the evals, test-1 launches Puppeteer for an actual render, so this needs a real Chromium and is kept out of npm test. SVG geometry is not byte-compared (it varies by Mermaid, Chromium, and font version); the deterministic projection is, namely the modified markdown, the file layout, and the embedded Mermaid source. Output is JSON; exits non-zero on any failure.

JSON Schema format

Each file in .schemas/ is a self-contained JSON Schema draft-07 object. validate_front_matter parses the document's YAML front matter and validates the resulting object against the schema you pass in. A trimmed view of .schemas/article.json:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "Article",
  "type": "object",
  "required": ["type","title","author","category","tags","description","summary","status","version","date_created","date_updated"],
  "properties": {
    "type":         { "const": "article" },
    "title":        { "type": "string", "minLength": 1 },
    "category":     { "type": "string", "enum": ["Software Development", "Security", "DevOps", "..."] },
    "tags":         { "type": "array", "items": { "type": "string", "pattern": "^[a-z0-9][a-z0-9_\\-/]*$" }, "minItems": 4, "maxItems": 12 },
    "status":       { "type": "string", "enum": ["published", "draft", "in-progress"] },
    "version":      { "type": "string", "pattern": "^\\d+\\.\\d+\\.\\d+$" },
    "date_created": { "type": "string", "format": "date" }
  },
  "additionalProperties": false
}

See .schemas/article.json for the complete schema, including the optional fields it also permits (aliases, subtitle, source, sources, reviewers, signoff, mermaid_theme, mermaid_background, mermaid_svg_source). Every schema sets additionalProperties: false, so any front matter key not declared in the schema is a validation error; add a key to the schema before using it in notes.

Important: Date fields must be quoted strings in YAML (date_created: "2026-01-01") because YAML auto-converts unquoted YYYY-MM-DD values to JavaScript Date objects, which fail the type: string check.

The per-type required fields are listed in Front matter schemas above.

Development

npm install
npm run build       # compile TypeScript → dist/

TypeScript source is in src/. After changing code, rebuild the image so the next Claude Code session picks it up:

npm run build && docker build -t obsidian-markdown-lint-mcp .   # or: docker compose build

Then start a new Claude Code session (stdio tools are loaded at session start).

To smoke-test the container by hand without Claude Code, drive it over stdio:

docker compose run --rm obsidian-markdown-lint-mcp
# then paste a JSON-RPC line, e.g. an initialize request, and read the reply

Project structure

src/
  server.ts             stdio entry point (StdioServerTransport bootstrap)
  create-server.ts      builds the McpServer and registers the 4 tools
  tools/
    lint.ts             lint_markdown implementation
    validate.ts         validate_front_matter implementation
    mermaid.ts          render_mermaid_diagrams + extract_mermaid_from_svg
  lib/
    frontmatter.ts      gray-matter parse/update helpers
    svg-metadata.ts     base64 embed/extract helpers
.schemas/               JSON Schema files for all 7 note types
Dockerfile              stdio server image (Chromium for Mermaid)
docker-compose.yml      build/tag helper (not `up` — see comments)

License

MIT © 2026 Philip Senger

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Lint, validate, and render your Obsidian vault markdown without touching your host system.

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