Building an MCP Server That Verifies Its Sources: Inside footnote-mcp

footnote-mcp is a Python MCP server installable via pip, Docker, or pipx. No API keys required — it falls back to scraped Bing + DuckDuckGo search and automatic headless Chromium for JavaScript-heavy pages. The Verification Pipeline The core tool is evidence_entailment . It takes a claim and a source text, and returns whether the claim is supported, unsupported, or contradicted. The heuristic backend extracts numeric and named-entity tokens from both the claim and source, then checks for exact matches and contradictions. On its design domain — numeric and factual data claims — it achieves 100% accuracy on a labeled benchmark set. For semantic cases (negation, paraphrase), the ollama backend uses a local LLM as a judge. Three tools build on this: corroborate_claim triangulates a claim across multiple sources, locate_claim_span finds the exact supporting sentence with character offsets, and build_research_debug_report produces a compact report of queries, URLs, and verification gaps. The Fetch Ladder web_read fetches pages through a 5-tier escalation ladder: HTTP (curl_cffi) to rotating proxy to headless Chromium to Chromium through proxy to hosted scrape API (Firecrawl/ScrapingBee). A block/quality detector decides when to escalate, and per-domain rate limiting, circuit breakers, and negative cache keep it polite. Search Backends web_search supports Tavily, Brave, Google, or scraped Bing + DuckDuckGo as fallback. Pass semantic: true to reorder results by meaning using local Ollama embeddings. Structured Data and Browser Tools Beyond text, the server handles tables, CSV/XLSX/PDF/JSON, date validation, unit resolution, and time series reconciliation. For JavaScript-heavy pages, 10 browser tools let you drive a headless Chromium session. When generic parsers fail, the server can synthesize sandboxed extraction code through a controlled recipe system. Benchmark Results The heuristic backend achieves 100% accuracy on numeric and factual data claims (n=15). Overall accurac