Nexus Legal MCP Server

@nexus-legal/mcp

MCP (Model Context Protocol) server for Nexus Legal — exposes 27 specialised legal capabilities to Claude Desktop, Claude Code, Cursor and any other MCP-compatible client.

What is this? Connect your Claude to Nexus in 30 seconds. From then on Claude can run ISO 31000 legal analysis, Monte Carlo litigation simulation, case-law search, legal drafting, administrative doctrine, adversarial red teaming, multi-jurisdiction comparison (cross-border), citation verification, procedural time-limit computation, the statute↔case-law citation graph, and analysis of Box files by file_id (box_analyze) — without leaving the conversation.

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Quick install

1. Generate an MCP key

Go to legal.nexusquantum.legal/developers, choose "MCP server", give it a name (e.g. "personal MacBook") and press + Create MCP key. Copy the nlk_... key — it is shown only once.

2. Configure your client

Claude Desktop (macOS)

Edit ~/Library/Application Support/Claude/claude_desktop_config.json:

{
  "mcpServers": {
    "nexus-legal": {
      "command": "npx",
      "args": ["-y", "@nexus-legal/mcp"],
      "env": {
        "NEXUS_API_KEY": "nlk_YOUR_KEY_HERE"
      }
    }
  }
}

Restart Claude Desktop. You will see a 🔌 icon in the chat with the 27 Nexus tools available.

Claude Desktop (Windows)

Edit %APPDATA%\Claude\claude_desktop_config.json with the same contents.

Claude Code (CLI)

claude mcp add nexus-legal -- npx -y @nexus-legal/mcp
# Then set your key:
export NEXUS_API_KEY=nlk_YOUR_KEY_HERE

Cursor

Edit ~/.cursor/mcp.json:

{
  "mcpServers": {
    "nexus-legal": {
      "command": "npx",
      "args": ["-y", "@nexus-legal/mcp"],
      "env": { "NEXUS_API_KEY": "nlk_YOUR_KEY_HERE" }
    }
  }
}

3. Try it

In Claude Desktop, type:

Analyse this contract with Nexus in jurisdiction GB, commercial branch, conservative profile: «[paste the contract text here]»

Claude will call the nexus_analyze tool and return the report with certainty locks [L1]/[L2-J]/[L3-NV]/[L4], blocking signals [L5-C]/[L5-P], and a NEXUS-AUDIT-TRAIL block.

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The 27 tools

Billing: the MCP server is not free. Every call requires an account with a credit balance > 0 (paid from minute one). Premium content is per jurisdiction add-on: case law and extra legislation are add-ons (Layer 3) — the subscription includes the legislation of one jurisdiction. Without the matching add-on, searches and lookups for that jurisdiction return 403 CONTENT_NOT_ENTITLED.

On jurisdictions and coverage. 63 jurisdiction codes are selectable (GB, ES, CO, SG, US with the US-CA/US-NY/US-DE/US-TX state overlays, MULTI for cross-border, and others). Selectable is not the same as grounded: what corpus actually backs a given jurisdiction — how many provisions, when our copy was last written, whether search is enabled for it — varies, and it is not something to infer from this list. Call nexus_normativa_coverage with your key: it answers for the jurisdictions your account has active, with the figures of each. A jurisdiction with no grounding corpus still analyses, but without literal citation against an official source.

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Bring-your-own-engine (BYO) accounts — what you will see before registering one

Some accounts are provisioned so that legal generation runs only on the firm's own LLM engine, with no fallback chain. That is not a limitation: it is the guarantee that documents never reach a Nexus engine.

While no provider is registered and enabled, every generative tool (nexus_analyze, nexus_consulta, nexus_chat, nexus_draft, nexus_audit, nexus_adversarial, nexus_redteam, nexus_doctrina, nexus_consensus, nexus_monte_carlo, nexus_opinion, nexus_playbook, nexus_compare_versions, nexus_cross_border_compare, box_analyze) returns:

Error running nexus_analyze: BYO_PROVIDER_REQUIRED: This account requires its own
BYO LLM engine for AI-generative endpoints … This is a configuration state, not a
transient failure — retrying returns the same 409. Register and enable a provider
via POST /api/v1/llm-providers, then retry.

Retrying does not fix it, and no credits are charged for a run that stops there. Register the engine once (POST /api/v1/llm-providers, or header → AI models in the application) and run it again.

Reasoning engines and long analyses

If your engine is a reasoning model (DeepSeek V-pro, o1-style, Claude 5 with thinking), a heavy analysis can take longer than a synchronous HTTP request is allowed to last. The gateway closes the connection at 300 s — that ceiling is ours to live with, not to raise.

MEASURED on 2026-08-18: nexus_analyze in mode: "auditoria" over a ~5 KB contract, on a BYO DeepSeek engine, went past it and returned 502. The reason is not a fault in your engine: a reasoning model spends a large part of its completion budget thinking before it writes, and when it runs out mid-thought Nexus automatically re-issues the call with a wider budget — which costs more wall clock again.

Two ways round it, in order of preference:

1. Use the asynchronous path for that combination: POST /api/v1/jobs with kind: "analyze" returns 202 and a jobId you poll (or a signed webhook). No wall clock. This is the right door for reasoning engines and for auditoria on large documents.
2. Use mode: "standard" through the MCP tool, which is materially shorter.

Platform engines and mode: "standard" are unaffected: the synchronous tool is the right one there and stays the default.

Read-only tools work without an engine, because they generate nothing: nexus_jurisprudencia_search, verify_cita, nexus_verificacion, nexus_normativa_search / _articulo / _pyramid, nexus_corpus_coverage, nexus_normativa_coverage, cruce_normativa_jurisprudencia, plazos_calcular, nexus_conflict_check (deterministic name matching, no LLM) and nexus_inbox.

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Environment variables

Language. This package sends the same tool descriptions to every client — MCP publishes them once, in tools/list, and no tool here takes a language argument — so descriptions and rendered output are in English. Where a tool produces a legal deliverable, the language of that deliverable follows the jurisdiction and the account, not this README.

You can name the role and the party in your own terms. professionalRole takes lawyer, solicitor, barrister, prosecutor, notary, registrar, judge, in-house and individual. proceduralSide takes the term your forum actually uses: claimant (England and Wales, CPR r 2.3), pursuer (Scotland), plaintiff (Northern Ireland, which did not adopt the CPR), defendant, defender. The original Spanish wire values — abogado, demandante, auditoria — are unchanged and will keep working: this adds spellings, it does not retire any. Everything resolves to the same protocol value before it reaches the analysis, so the two are interchangeable in every call.

API version: this server speaks to the Nexus v1 API, the same one used by the TypeScript and Python SDKs. Read-only and verification tools call /api/v1/* routes; generative ones enter through the backend node routes with the same nlk_ key (mcp scope), the same billing and the same BYO enforcement.

This stdio package exposes 27 of the catalogue's capabilities. The remote connector (https://legal.nexusquantum.legal/api/mcp, nothing to install) also exposes the case-file, firm-memory, firm-governance, legal-analytics, account and billing capabilities. If you need any of those, use the remote connector or the v1 API directly.

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Privacy and GDPR

• Zero Retention on EU infrastructure. Documents you send via MCP are processed in RAM on Railway europe-west4-drams3a (Netherlands) and are NOT persisted to a database by default. The Nexus Zero Retention policy applies to 100% of MCP traffic exactly as it does to web traffic.
• PII Gatekeeper. Before anything reaches the LLM, the backend anonymises national ID numbers, IBANs, phone numbers, emails and account numbers with reversible tokens ([DNI_1], [ACCOUNT_3]…). The MCP client only ever sees output already re-identified by our pipeline.
• Audit. Every tool call is recorded in analysis_costs and api_usage_stats (with via_api_key_id) for billing and traceability.

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Support

• Email: support@nexusquantum.legal
• Documentation: https://legal.nexusquantum.legal/developers
• Issues: https://github.com/djtellado/nexus-legal-prod/issues

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Licence

Apache-2.0 — see LICENSE and NOTICE.

Copyright 2026 Nexus Legal · Quantum Nexus Ventures.