arelle-mcp

The definitive MCP server for XBRL processing, validation, and financial analysis.

Built by [King Hippopotamus](https://github.com/TheKingHippopotamus). Uses Arelle — the world's only free, open-source XBRL-certified processor — as its core engine. No other XBRL MCP server exists. This is the first.

What it does

arelle-mcp gives LLMs (Claude, GPT, etc.) full access to XBRL financial data through 46 tools. That number is the tool registry's, not a figure typed into this file — count it yourself against any checkout:

python -c "import asyncio; from arelle_mcp.server import mcp; \
  print(len(asyncio.run(mcp.list_tools())))"

tests/test_tool_count.py runs that count in CI and fails if this README drifts from it.

Plus 5 resources (reference data) and 5 prompt templates (guided analysis workflows).

Quick Start

Install

pip install arelle-mcp

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "arelle-mcp": {
      "command": "arelle-mcp",
      "env": {
        "ARELLE_MCP_MAX_FILINGS": "5"
      }
    }
  }
}

Claude Code

claude mcp add arelle-mcp -- arelle-mcp

Cursor / Other MCP Clients

{
  "arelle-mcp": {
    "command": "python",
    "args": ["-m", "arelle_mcp"]
  }
}

HTTP Mode (Remote)

ARELLE_MCP_TRANSPORT=streamable-http ARELLE_MCP_PORT=8000 arelle-mcp

Usage Examples

Analyze Apple's Latest 10-K

"Fetch Apple's latest 10-K and give me a financial summary"

The LLM will:

1. Call xbrl_fetch_sec_filing(ticker="AAPL", filing_type="10-K")
2. Extract key metrics with xbrl_extract_facts
3. Render financial statements with xbrl_render_statement

Validate a Filing

"Validate this SEC filing: https://www.sec.gov/Archives/edgar/data/..."

Compare Two Quarters

"Compare Apple's Q2 and Q3 2024 10-Q filings"

Historical Revenue Trend

"Show me Microsoft's revenue history from SEC EDGAR"

The LLM calls xbrl_search_sec_concept(cik="789019", concept="Revenues") — no filing load needed.

Architecture

Core Design Decisions

1. Single-Session Lock — Arelle uses global state that isn't thread-safe. All operations are serialized via asyncio.Lock and offloaded to a ThreadPoolExecutor to avoid blocking the MCP event loop.

2. LRU Filing Cache — Each loaded filing consumes 30-60MB. An OrderedDict-based LRU cache (default: 5 filings) automatically evicts the oldest filing when capacity is reached, calling model.close() to free memory.

3. Lazy Imports — Arelle is heavy (~200MB with taxonomies). All Arelle imports happen lazily inside tool functions, keeping server startup fast.

Project Structure

src/arelle_mcp/
├── server.py            # FastMCP instance, lifespan, registration
├── arelle_wrapper.py    # ArelleManager — session lifecycle, concurrency, caching
├── serializers.py       # Arelle objects → JSON/markdown
├── constants.py         # Arcroles, disclosure systems, SEC config
├── tools/
│   ├── filing.py        # Load, summary, compare, close, list
│   ├── validation.py    # Validate against disclosure systems
│   ├── facts.py         # Extract and filter facts
│   ├── taxonomy.py      # Browse concepts, get details
│   ├── relationships.py # Presentation, calculation, dimension trees
│   ├── edgar.py         # SEC EDGAR API integration
│   └── rendering.py     # Financial statement rendering
├── resources/           # Reference data (disclosure systems, common concepts)
└── prompts/             # Guided analysis workflow templates

stdout is the protocol

Under the stdio transport, stdout is the JSON-RPC wire. A single stray byte on it is a framing error, and a compliant client will tear the transport down and lose every in-flight call.

Arelle is chatty: left to its defaults it prints iXBRL namespace and validation messages (ix11, xmlSchema, ...) to stdout while loading a filing. In v2.1.0 that output landed on the wire and killed long extraction sessions. Since v2.1.1 the server reserves stdout for the protocol at startup (arelle_mcp.stdio_guard), on three independent levels:

• the real stdout is duplicated to a private descriptor and fd 1 is re-pointed

at stderr, so even C-level writes are safe;

• sys.stdout is replaced by a proxy whose .buffer is that private

descriptor — what the MCP transport wraps — while every text write goes to stderr;

• the arelle logger is pinned to stderr, Arelle runs with

logFile="logToStdErr", and every synchronous Arelle call is wrapped in redirect_arelle_output().

Nothing is silenced — all diagnostics still appear on stderr, where your MCP client shows them. tests/test_stdout_is_clean.py proves it end to end by running a real MCP handshake and a real iXBRL load in a child process and asserting the parent sees only parseable JSON-RPC.

Configuration

Development

git clone https://github.com/TheKingHippopotamus/Arelle-MCP.git
cd Arelle-MCP
pip install -e ".[dev]"

# Run tests
pytest

# Type check
mypy src/arelle_mcp

# Lint
ruff check src/

# Test with MCP Inspector
npx @modelcontextprotocol/inspector arelle-mcp

Supported File Formats

• XBRL Instance Documents (.xbrl, .xml)
• Inline XBRL (.htm, .html) — SEC mandated since June 2021
• iXBRL Document Sets
• ZIP archives containing XBRL
• SEC EDGAR URLs (auto-fetched)
• Taxonomy Packages (.zip)

Supported Disclosure Systems

• SEC EFM — US Securities and Exchange Commission
• ESEF — European Single Electronic Format (EU/ESMA)
• HMRC — UK HM Revenue & Customs
• GFM — Global Filing Manual

License

Apache 2.0 — same as Arelle itself.

Built by King Hippopotamus — with zero compromises.