OPC UA MCP Server

A Model Context Protocol (MCP) server that provides seamless integration with OPC UA servers. This server enables AI assistants and other MCP clients to interact with industrial automation systems through standardized OPC UA communication protocols.

There is a Node runtime exposing the same tools from the same contract — pick whichever stack you already run.

Overview

This MCP server acts as a bridge between AI assistants and OPC UA servers, allowing for:

• Reading sensor data and system variables, with the engineering unit and range the node publishes, so a 51.75 arrives labelled °C rather than bare
• Writing control values to actuators and systems, bounded by the range the equipment itself declared
• Browsing OPC UA node hierarchies, with each node's type definition
• Calling OPC UA methods for system operations
• Subscribing to data changes, so a node can be watched rather than polled, with an optional deadband so jitter is discarded at the server
• Collecting events, listing the alarms a server retains, and acknowledging, confirming, annotating or shelving them
• Reading history — values, raw or summarised by a server-side aggregate, and events the server stored for a range that has already passed
• Reporting connection and server health, including the OPC UA server's own diagnostics counters, and reconnecting by itself when the server is restarted

One OPC UA endpoint per process: OPCUA_SERVER_URL is read once, every tool targets it, and each MCP client opens its own OPC UA session.

Tools

Fifteen tools, defined once in contract/tools.json and shared with the Node runtime so the two cannot drift apart.

Tool	What it does
read_opcua_nodes	Read one or more nodes — value, data type, status, timestamps, engineering unit and range
browse_opcua_nodes	List children, walk a subtree, resolve a browse path, search by name
write_opcua_nodes	Write to one or more nodes
call_opcua_method	Invoke a method on an object node
get_server_status	Connection state, server health, the namespace array and the server's own diagnostics
subscribe_opcua_nodes	Watch nodes for data changes instead of polling them, with an optional deadband
list_subscriptions	The active subscriptions, each with its buffered changes
unsubscribe_opcua_nodes	Cancel subscriptions
subscribe_events	Start collecting events from a notifier node
read_events	Read the events collected since the last read
list_active_alarms	The alarms the server is currently retaining
acknowledge_alarm	Acknowledge one of them, with a comment
act_on_alarm	Confirm, annotate or shelve an alarm — the rest of the operator workflow
read_opcua_history †	Historical values, raw or summarised by a server-side aggregate
read_event_history †	Events the server stored, for a range that has already passed

† Capability-gated. read_opcua_history appears only when the connected server advertises historical access (AccessHistoryDataCapability) or aggregates (a non-empty AggregateFunctions folder). read_event_history is gated separately on AccessHistoryEventsCapability — keeping values and keeping events are different features and a server commonly does one without the other. What a server cannot do is not on the menu, rather than failing at call time.

One tool per operation, not one per arity. Reading one node and reading fifty is the same request with a longer list, so it is one tool and one code path.

See the central per-tool reference in docs/examples.md for full tool signatures, parameters and return formats.

Resources

One resource, opcua://subscriptions: the active data-change subscriptions and the values each has buffered, as JSON. It is the same set of records list_subscriptions returns, re-readable without spending a tool call. See Subscriptions for the shape and the worked example.

Features

Key Capabilities

• Automatic Connection Management: Handles OPC UA client lifecycle with proper connection setup and teardown, and rebuilds a dead session rather than staying down until restart
• Type-Safe Operations: Automatic type conversion based on existing node data types, with values checked against the node's published range and any policy bounds before the write is sent
• Tool Profiles and Audit: An observe-only default, an allowlist for control, and every control call recorded with its targets, outcome, endpoint and session
• Error Handling: Comprehensive error reporting for debugging and monitoring; a failure never comes back as an empty message
• Async Support: Built on the mcp SDK's MCPServer for efficient asynchronous operations
• Data-Change Subscriptions: Monitored items buffered as they arrive, re-created on a new session after a reconnect
• Configurable: Environment-based endpoint, security policy, credentials and authorization policy

Installation

Prerequisites

• Python 3.10 or higher
• Access to an OPC UA server (local or remote)
• UV package manager (recommended) or pip

From PyPI (recommended)

uvx opcua-mcp-server            # run without installing
uv tool install opcua-mcp-server   # or install permanently
pip install opcua-mcp-server       # or with pip

Point it at your OPC UA endpoint:

export OPCUA_SERVER_URL="opc.tcp://localhost:4840"

Registering with Claude Desktop

Rather than editing claude_desktop_config.json by hand, let the server write it:

opcua-mcp-server --install claude-desktop --url opc.tcp://192.168.0.10:4840

It merges into the existing config, backs the old one up, and records absolute paths — Claude Desktop is launched from the GUI and does not inherit a login shell's PATH, so a bare "command": "uvx" often works in a terminal and fails in the app. Add --dry-run to see the result first, --force to replace an existing opcua entry.

There is also a downloadable .mcpb bundle for Claude Desktop and single-file executables that need no Python at all — see docs/install.md.

In an MCP client:

{
  "mcpServers": {
    "opcua": {
      "command": "uvx",
      "args": ["opcua-mcp-server"],
      "env": { "OPCUA_SERVER_URL": "opc.tcp://localhost:4840" }
    }
  }
}

From source (development)

1. Install dependencies for the whole workspace (run from the repo root):

   uv sync --all-packages
   

2. Configure the OPC UA server URL:

   export OPCUA_SERVER_URL="opc.tcp://localhost:4840"
   

Configuration

The server is configured entirely through environment variables:

Variable	Default	Meaning
OPCUA_SERVER_URL	opc.tcp://localhost:4840	OPC UA endpoint to connect to
OPCUA_SECURITY_POLICY	None	None, Basic128Rsa15, Basic256 or Basic256Sha256 (the AES suites are Node-only)
OPCUA_SECURITY_MODE	SignAndEncrypt once a policy is set, otherwise None	None, Sign or SignAndEncrypt
OPCUA_CLIENT_CERT	—	Client certificate (PEM/DER). Required for any policy other than None
OPCUA_CLIENT_KEY	—	Private key for OPCUA_CLIENT_CERT
OPCUA_APPLICATION_URI	the subjectAltName URI of OPCUA_CLIENT_CERT	Application URI announced to the server. Set it only for a certificate that carries no URI of its own
OPCUA_SERVER_CERT	—	The OPC UA server's certificate, pinned. Without it, encryption protects against eavesdropping but not against an impostor endpoint. Requires a policy other than None
OPCUA_USERNAME	—	Username identity; the session is anonymous when unset
OPCUA_PASSWORD	—	Password for OPCUA_USERNAME
OPCUA_USER_CERT	—	Certificate identifying the user, for X.509 authentication. A different key pair from OPCUA_CLIENT_CERT, which secures the channel. Cannot be combined with OPCUA_USERNAME
OPCUA_USER_KEY	—	Private key for OPCUA_USER_CERT. Signs the server's challenge; never sent
OPCUA_PROFILE	observe	observe, operator, or full tool profile (read-only is an alias for observe)
OPCUA_POLICY_FILE	—	Optional version-1 JSON policy file; environment variables override it
OPCUA_ALLOWED_TOOLS	—	Comma-separated allowlist that can only narrow the selected profile
OPCUA_ALLOWED_WRITE_NODES	—	Comma-separated node IDs writable by the operator profile. ns=2;i=5 or, preferably, nsu=<namespace-uri>;i=5
OPCUA_ALLOWED_METHODS	—	Comma-separated object_node_id|method_node_id pairs callable by operator
OPCUA_ALLOW_ACKNOWLEDGE_ALARMS	false	Allow operator to act on alarms — acknowledge_alarm and every act_on_alarm action
OPCUA_ALLOW_INSECURE_CONTROL	false	Lab-only override permitting control tools without OPC UA channel security
OPCUA_ALLOW_OUT_OF_RANGE_WRITES	false	Allow a write outside the EURange the OPC UA server itself published for that node
OPCUA_AUDIT_FILE	—	Append-only file for the control audit trail, one JSON object per line, written beside stderr. A file that cannot be opened stops the server rather than falling back
OPCUA_OPERATOR_ID	—	Label stamped on every audit record, so a shipped log says which deployment a control call came from
OPCUA_RECONNECT_INITIAL_DELAY_MS	1000	Delay before the first reconnection attempt; doubles each attempt
OPCUA_RECONNECT_MAX_DELAY_MS	8000	Ceiling for that doubling
OPCUA_RECONNECT_MAX_RETRY	3	Retries after the first attempt. 0 disables retrying, -1 retries forever
OPCUA_SESSION_TIMEOUT_MS	60000	Session timeout asked of the OPC UA server; also sets the keep-alive period

Names are case-insensitive, and a policy on its own implies SignAndEncrypt. An unusable combination — a mode without a policy, a policy without a certificate, a username without a password — is refused at startup with a message naming the variable. With no security configured the connection is unencrypted and unauthenticated, and the server says so on stderr; see SECURITY.md. Making a client certificate and getting it trusted: docs/certificates.md.

On the Python runtime, certificate and key files are parsed as PEM only when they are named *.pem and as DER otherwise (a python-opcua rule), so a PEM key called client.key fails to load — name it client_key.pem. The Node runtime sniffs the contents and accepts either name.

export OPCUA_SERVER_URL="opc.tcp://plc.example.internal:4840"
export OPCUA_SECURITY_POLICY="Basic256Sha256"     # implies SignAndEncrypt
export OPCUA_CLIENT_CERT="/etc/opcua/client.pem"
export OPCUA_CLIENT_KEY="/etc/opcua/client_key.pem"
export OPCUA_SERVER_CERT="/etc/opcua/plc_server.pem"
export OPCUA_USERNAME="mcp-operator"
export OPCUA_PASSWORD="…"

Deciding what the agent may do

Three profiles, and the default is the restrictive one:

OPCUA_PROFILE	What it offers
observe (default)	Read, browse, history and monitoring. No writes, no methods
operator	The above, plus only the write targets and methods you allowlist
full	Every tool

operator is the one worth understanding. A write to a node outside OPCUA_ALLOWED_WRITE_NODES is refused before anything reaches OPC UA, and one forbidden target rejects an entire batch rather than letting part of it through. Both operator and full also require a secured OPC UA channel unless OPCUA_ALLOW_INSECURE_CONTROL=true says otherwise in as many words.

The policy is enforced again on every call, not only when tools are listed — an MCP client may hold a stale catalogue, and a hidden tool is a usability feature rather than a security boundary.

A writable_nodes entry in a policy file may also bound the value, not only the node: min, max, enum and max_change narrow what an allowlisted node will accept, so a model that picks the right node and hallucinates 9999 instead of 99.9 is refused. The node's own published EURange applies on top, which is the only value bound that exists with no policy file at all. See Bounding the value, not only the node.

Every control call is recorded — its targets, its outcome, the endpoint it went to, the session it rode on and the attempt number — to stderr and, with OPCUA_AUDIT_FILE set, to an append-only JSON-lines file beside it.

Staying connected

The connection is re-established by itself: a dropped or refused session is retried with exponential backoff, and the read and write paths rebuild a dead session rather than failing until the process is restarted. python-opcua has no reconnection of its own, so this server owns the whole of it — including building a fresh client per attempt, because a restarted server may present a new certificate. Data-change subscriptions are re-created on the new session, so the IDs keep working and the values already buffered are still there to be read. Tune it with OPCUA_RECONNECT_INITIAL_DELAY_MS, OPCUA_RECONNECT_MAX_DELAY_MS, OPCUA_RECONNECT_MAX_RETRY and OPCUA_SESSION_TIMEOUT_MS above.

Concurrent callers during an outage share one rebuild rather than each running their own backoff, and a caller arriving mid-rebuild takes that attempt's answer.

get_server_status reports whether the connection is up and what the OPC UA server says about itself; it is the one tool that answers while the connection is down, and calling it is also what brings a dropped one back. See Staying connected.

A control call whose session died before its response arrived is not silently re-sent. OPC UA lets a Write partially succeed and defines no operation order, so a dead session never proves the write did not land: the server rebuilds the connection and then says plainly that the request may or may not have reached the plant, naming what it was aimed at.

Usage

Running the Server

After uv sync --all-packages from the repo root:

uv run --no-sync opcua-mcp-server

Or run it directly against this package from anywhere in the repo:

uv --directory packages/server-python run opcua-mcp-server

Integration with MCP Clients

Add to your MCP client configuration (e.g., config.json):

{
  "mcpServers": {
    "opcua-mcp": {
      "command": "/path/to/uv",
      "args": [
        "--directory",
        "/path/to/packages/server-python",
        "run",
        "opcua-mcp-server"
      ],
      "env": {
        "OPCUA_SERVER_URL": "opc.tcp://localhost:4840"
      }
    }
  }
}

Example Usage in Conversation

Once configured, you can ask Claude to perform real-world OPC UA operations:

Reading Sensor Values

• "What is the current temperature reading from the reactor vessel?"
• "Check the pressure level in Tank A"
• "Read all temperature sensors in the cooling system"
• "Get the current flow rate on production line B"

Controlling Equipment

• "Turn on the conveyor motor"
• "Set the mixer speed to 80 RPM"
• "Open valve V-101 to 75% position"
• "Start the circulation pump"
• "Set the heating element to 150°C"

Alarms

• "What alarms are active right now?"
• "Acknowledge the high-temperature alarm with a note about the cause"
• "Shelve that chattering level alarm for thirty minutes"
• "What fired overnight on the packaging line?"

System Monitoring

• "Read all sensor values from the water treatment plant"
• "Check the status of all pumps and motors"
• "Is the plant server refusing our connections, or just slow?"
• "Watch the reactor temperature and ignore changes under 0.5°C"
• "What variables are available on this OPC UA server?"

Real Example Scenarios

User: "What's the temperature in reactor R-101?"
Assistant: I'll read the temperature sensor for reactor R-101.
Tool call: read_opcua_nodes with node_ids ["ns=2;i=11"]
Result: "Reactor R-101 temperature: 87.3 °C (range 0–150 °C)"

User: "Turn on the main conveyor motor"
Assistant: I'll start the main conveyor motor for you.
Tool call: write_opcua_nodes with nodes [{node_id: "ns=2;i=12", value: true}]
Result: "Main conveyor motor started successfully"

User: "Set the pump speed to 65%"
Assistant: I'll adjust the pump speed to 65%.
Tool call: write_opcua_nodes with nodes [{node_id: "ns=2;i=13", value: 65}]
Result: "Pump speed set to 65%"

User: "What variables are available on this OPC UA server?"
Assistant: I'll browse the OPC UA server's address space.
Tool call: browse_opcua_nodes
Result: Found 5 variables:
- Temperature (ns=2;i=2): 25.3 °C - AnalogItemType
- Pressure (ns=2;i=3): 5.0 bar - AnalogItemType
- MotorSpeed (ns=2;i=4): 1500 RPM - AnalogItemType
- MotorState (ns=2;i=5): True - Motor ON/OFF state
- ValvePosition (ns=2;i=6): False - Valve OPEN/CLOSED position

Security

The connection defaults to no security (SecurityPolicy.None) and the tool profile defaults to observe-only. For anything beyond local development, configure both channel security and an operator allowlist, and pin the endpoint with OPCUA_SERVER_CERT. Control tools are blocked on an insecure channel unless the explicit lab override is set.

The MCP policy is defense in depth, not a replacement for OPC UA authorization. Scope the OPC UA account to the same nodes and methods; use a separate read-only account for observe deployments.

See SECURITY.md for the full posture, what the servers do and do not verify, and how to report a vulnerability.

API Reference

See the central per-tool reference in docs/examples.md for full tool signatures, parameters, and return formats. The shared tool surface is defined in contract/tools.json.