MCP Server

MCP Server add-on. Nexus exposes a Model Context Protocol (MCP) server so AI agents (Claude Desktop, Claude Code, or any MCP client) can query Cardano, Bitcoin, Midnight, and market data, and build transactions, as native tools.

The MCP endpoint requires the MCP Server add-on. Tool calls without it fail with errorCode mcp_addon_required (see Errors). The single MCP add-on unlocks all tools below; you do not need the Market Data or Transaction Builder add-ons separately to use their MCP tools.

Connecting

Endpoint: https://nexus.gerowallet.io/mcp (no trailing slash)
Transport: Streamable HTTP, stateless. Every JSON-RPC message is a single POST /mcp, so there is no session to keep open or reconnect.
Auth: sign in with your Nexus account (recommended), or send an API key.

Add the server without any key. The first time you connect, your MCP client opens the Nexus login page in your browser: sign in (email and password, Google or GitHub), then approve the client.

claude mcp add --transport http nexus https://nexus.gerowallet.io/mcp

In Claude Code, run /mcp, pick nexus and choose Authenticate. Any MCP client that supports OAuth works the same way: it discovers the Nexus sign-in on its own, registers itself, and keeps you signed in (access tokens last an hour and renew automatically; after 30 days without use you sign in again). Your account needs the MCP add-on; without it the approval page says so. Signed in this way, the account tools (me, my_api_keys, my_subscription) act as you.

Or use an API key

Send your Nexus API key in the X-Api-Key header (keys start with nxs_). A key with an endpoint allowlist can use MCP only if the list includes /mcp, and public partner (widget) keys are always refused, since they are meant to be embedded in web pages. The account tools refuse API keys, so a leaked key cannot list its sibling keys. Over MCP, the network a key was created for does not limit it: your plan and the MCP add-on decide access, so any eligible key can call every chain’s tools.

claude mcp add --transport http nexus https://nexus.gerowallet.io/mcp --header "X-Api-Key: nxs_your_api_key_here"

Or in a JSON MCP config (Claude Desktop, Claude Code, or any client that supports Streamable HTTP):

{
  "mcpServers": {
    "nexus": {
      "type": "http",
      "url": "https://nexus.gerowallet.io/mcp",
      "headers": { "X-Api-Key": "nxs_your_api_key_here" }
    }
  }
}

The older SSE endpoints (/mcp/sse and /mcp/messages) were retired on 2026-09-26. A client still configured with "type": "sse" gets a 404; switch it to "type": "http" with the URL above.

Health check: GET /mcp/health (no add-on required).

Tools

The server registers 25 tools across six domains.

Cardano (8)

cardano_account_info · cardano_account_history · cardano_address_info · cardano_asset_info · cardano_block_info · cardano_epoch_info · cardano_pool_info · cardano_tx_info

Bitcoin (4)

bitcoin_balance · bitcoin_tx_info · bitcoin_fee_estimate · bitcoin_utxos

Market Data (4)

market_price · market_ohlcv · market_pair_list · market_top

Midnight (4)

midnight_info · midnight_dust_status · midnight_build_unshielded · midnight_submit

Transaction Builder (2)

cardano_build_unsigned · cardano_preview_tx

Account (3)

me · my_api_keys · my_subscription

Tool calls count against your plan’s request quota and rate limit like any other API usage, and every call is audit-logged.

Errors

A refused or failed tool call is a normal MCP tool result with isError: true, not a JSON-RPC protocol error, so the agent sees the reason and can act on it. The result has one text item, and that text is always a JSON object:

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"errorCode\":\"mcp_addon_required\",\"code\":-32001,\"message\":\"This account does not have the MCP Server add-on. Add it at https://nexus.gerowallet.io/dashboard/billing/\",\"upgradeUrl\":\"https://nexus.gerowallet.io/dashboard/billing/\"}"
      }
    ],
    "isError": true
  }
}

Parse content[0].text as JSON. Every error object has errorCode (stable, branch on this), code (numeric) and message (human-readable, may change). Some add fields:

`errorCode`	`code`	Extra fields	Meaning
`unauthenticated`	-32000		An account tool (`me`, `my_api_keys`, `my_subscription`) was called with an API key or without signing in.
`mcp_addon_required`	-32001	`upgradeUrl`	The account has no MCP Server add-on.
`quota_exceeded`	-32002	`resetAt`, `limit`, `tier`	The plan’s request quota is used up until `resetAt` (ISO-8601).
`api_key_scope_denied`	-32003		The API key may not use MCP: a public partner key, or an allowlist without `/mcp`.
`network_disabled`	-32004	`requested`, `enabled`	The `network` argument is not served over MCP; `enabled` lists the ones that are.
`not_found`	-32005		The address, transaction, block or other object does not exist.
`subscription_required`	-32006	`upgradeUrl`	No active subscription.
`network_unsupported`	-32007	`requested`, `supported`	The tool does not serve that network, e.g. a `bitcoin_*` tool called with `cardano-mainnet`; `supported` lists the ones it does.
`invalid_params`	-32602		An argument is invalid; `message` says which.
`internal_error`	-32603		Something failed on the Nexus side. Details are logged, never returned; retry later.

Requests rejected by the MCP layer before a tool runs, such as an unknown tool name or arguments that cannot be read, do not carry this object.

Getting Started

Migrations

Core Concepts

Add-ons & Streaming

API Reference

Connecting

Or use an API key

Tools

Cardano (8)

Bitcoin (4)

Market Data (4)

Midnight (4)

Transaction Builder (2)

Account (3)

Errors

​Connecting

​Sign in with your Nexus account

​Or use an API key

​Tools

​Cardano (8)

​Bitcoin (4)

​Market Data (4)

​Midnight (4)

​Transaction Builder (2)

​Account (3)

​Errors

Connecting

Sign in with your Nexus account

Or use an API key

Tools

Cardano (8)

Bitcoin (4)

Market Data (4)

Midnight (4)

Transaction Builder (2)

Account (3)

Errors