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. Both are first-class: one test suite runs against both, and they are released together under one version. Pick whichever stack you already run; the few declared differences between them, and the known divergences still being fixed, are listed in docs/compatibility.md.

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

Both runtimes expose the same 15 tools — 7 read, 4 monitor, 2 alarm-action and 2 control — defined once in contract/tools.json.

Tool	Access	Hints	What it does
read_opcua_nodes	read	read-only, idempotent	Read the current value of one or more OPC UA nodes in a single request.
browse_opcua_nodes	read	read-only, idempotent	Explore the OPC UA address space: list a node's children, walk a subtree, resolve a human-readable browse path to a node ID, or search for nodes by name.
read_opcua_history †	read	read-only, idempotent	Read what a node's value has been over time.
read_event_history †	read	read-only, idempotent	Read events the OPC UA server stored, for a time range that has already passed.
get_server_status	read	read-only, idempotent	Report whether this MCP server is connected to the OPC UA server and what that server says about itself: its endpoint and connection security, its ServerStatus (state, current time, start time, build info) and its NamespaceArray as index -> URI.
list_subscriptions	read	read-only, idempotent	List the active OPC UA data-change subscriptions, each with the value changes buffered for it since it was created.
list_active_alarms	read	read-only, idempotent	List the alarm and condition instances the server is currently retaining — those that are active, unacknowledged, or both.
subscribe_opcua_nodes	monitor	—	Watch one or more OPC UA nodes for value changes instead of polling them.
unsubscribe_opcua_nodes	monitor	—	Cancel one or more active data-change subscriptions and discard what they had buffered.
subscribe_events	monitor	—	Start buffering OPC UA events (alarms, condition changes, plain events) from a notifier node.
read_events	monitor	—	Read the events buffered by subscribe_events, oldest first.
acknowledge_alarm	alarm-action	—	Acknowledge an alarm or condition, identified by the event_id of the event that reported it (from list_active_alarms or read_events).
act_on_alarm	alarm-action	—	Confirm, annotate or shelve an alarm — the rest of the operator workflow that acknowledge_alarm starts.
write_opcua_nodes	control	destructive, idempotent	Write a value to one or more OPC UA nodes.
call_opcua_method	control	destructive	Call a method on an OPC UA object.

Access decides which OPCUA_PROFILE offers a tool. read and monitor tools are offered under every profile, including the default observe. control tools need operator, which offers them only for allowlisted targets, or full; alarm-action tools need operator with OPCUA_ALLOW_ACKNOWLEDGE_ALARMS, or full. Both also need a verified server — a secured channel and an authenticated server, pinned with OPCUA_SERVER_CERT or validated by OPCUA_SERVER_TRUST_STORE — unless a lab override is set: OPCUA_ALLOW_INSECURE_CONTROL for a channel with no security, OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL for an unauthenticated server. Hints are the MCP tool annotations each tool advertises.

† Needs a server feature: always listed, and refused at call time with capability_not_supported or capability_unknown when the connected server does not advertise what it needs — read_opcua_history on AccessHistoryDataCapability or AggregateFunctions; read_event_history on AccessHistoryEventsCapability.

read_opcua_history needs historical access (AccessHistoryDataCapability) or aggregates (a non-empty AggregateFunctions folder), and its aggregate_function argument needs the latter. read_event_history needs AccessHistoryEventsCapability — keeping values and keeping events are different features and a server commonly does one without the other. Every tool is listed whatever the plant is doing, with the same schema, because MCP clients keep the first list they get and nothing tells them reliably to ask again. A call the connected server cannot serve is refused before anything is sent, with a code to act on — capability_not_supported, capability_unknown, or endpoint_offline when the server cannot be reached — and what to use instead. get_server_status → capabilities reports what the server was found to offer, including its aggregate function names, on which session and when.

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.

Bounded, and honest about it. Every request is bounded before it reaches the OPC UA server — at most 500 nodes per read, 100 per write, 64 method arguments, 1 MiB of arguments, and fewer where the OPC UA server publishes lower OperationLimits; the full list is in the tools reference. A request over a bound is refused whole, never partly sent. And every result that can be partial — history, events, browse, subscriptions — carries a completeness object beside result in structuredContent: test completeness.complete, and never infer it from how many records came back.

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 or Codex

Rather than editing claude_desktop_config.json (or Codex's config.toml) by hand, let the server write it:

opcua-mcp-server --install claude-desktop --dry-run \
  --url opc.tcp://192.168.0.10:4840 \
  --security-policy Basic256Sha256 \
  --client-cert /etc/opcua/client.pem --client-key /etc/opcua/client_key.pem \
  --server-cert /etc/opcua/server.pem

--dry-run validates the files and the combination with the server's own startup parsers and prints a redacted preview and a security summary; drop it to write (--install codex for Codex). 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. The profile defaults to read-only; a control profile must be chosen with --profile, and for a remote endpoint needs a pinned --server-cert. Passwords are never accepted as flags. --force replaces an existing opcua entry. Every flag and rule is in docs/install.md.

There is also a downloadable .mcpb bundle for Claude Desktop — which carries the Node runtime, because Claude Desktop supplies Node rather than Python — and single-file executables of this runtime 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. Every one is declared, with its type, default and secrecy, in the repository's contract/config.json, which both runtimes are tested against and which ships inside this package:

32 settings in six groups. A blank value means the default, whatever the type; a boolean accepts 1, true, yes, on and 0, false, no, off.

Connection — Which OPC UA server to talk to.

Variable	Default	Description
OPCUA_SERVER_URL	opc.tcp://localhost:4840	URL of the OPC UA server to connect to, including any path the server expects. Read once at startup: one process serves one endpoint. Verify who answers with an exact server certificate pin or a configured CA trust store.
OPCUA_PYTHON_BACKEND	asyncua	Select the Python client implementation. The maintained asyncua client is the default; legacy provides the one-release rollback path for uncovered vendor incompatibilities. This setting affects the Python server only. One of asyncua, legacy.

Channel security — How the OPC UA secure channel is signed, encrypted and verified.

Variable	Default	Description
OPCUA_SECURITY_POLICY	None	Encryption suite to negotiate for the secure channel. Anything other than None needs a client certificate and key. The two AES suites are Node-only: the Python runtime refuses them at startup rather than downgrading. One of None, Basic128Rsa15, Basic256, Basic256Sha256. None is unencrypted and unauthenticated, and keeps control tools blocked unless insecure control is explicitly allowed. An unknown policy stops the server at startup.
OPCUA_SECURITY_MODE	SignAndEncrypt once a security policy is set, otherwise None	Message security mode for the secure channel. Sign authenticates messages without encrypting them. One of None, Sign, SignAndEncrypt. A policy with mode None, or a mode without a policy, stops the server at startup; leaving it blank never downgrades a policy to signing only.
OPCUA_CLIENT_CERT	—	Path to the certificate (PEM or DER) this client presents as its application identity. Required by any security policy other than None. A path that does not exist stops the server at startup.
OPCUA_CLIENT_KEY	—	Path to the private key matching the client certificate. A path, never key material. Whoever can read the file can impersonate this client: keep it readable only by the account running the server. A path that does not exist stops the server at startup.
OPCUA_APPLICATION_URI	the subjectAltName URI of the client certificate	Application URI announced to the server. Set it only for a certificate that carries no URI of its own. A configured URI that conflicts with the certificate is refused before connecting.
OPCUA_SERVER_CERT	—	Path to the OPC UA server's own certificate (PEM or DER), pinned: a server presenting any other certificate cannot complete the handshake. Control tools (writes, method calls, alarm actions) require it, or OPCUA_SERVER_TRUST_STORE instead. Requires a security policy other than None. Unset, encryption protects against eavesdropping but not against an impostor endpoint, so control tools are refused unless OPCUA_SERVER_TRUST_STORE validates the server or OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL is set. Set without a security policy, it stops the server at startup rather than pinning nothing; a pinned certificate that has expired or is not yet valid refuses to connect.
OPCUA_SERVER_TRUST_STORE	—	Directory with trusted/certs root CAs, issuers/certs intermediate CAs, and trusted/crl plus issuers/crl signed current revocation lists. Authenticate the OPC UA peer by its validated chain and expected server URI. Requires secured channel and OPCUA_SERVER_APPLICATION_URI; Python requires the maintained asyncua backend. Missing, stale, forged or revoked trust material refuses connection. Cannot be combined with an exact server certificate pin.
OPCUA_SERVER_APPLICATION_URI	—	ApplicationUri of the intended OPC UA server. Must match the peer certificate URI SAN and advertised application identity. Required with OPCUA_SERVER_TRUST_STORE.

User identity — Who the OPC UA session logs in as.

Variable	Default	Description
OPCUA_USERNAME	—	OPC UA username. The session is anonymous when unset. Cannot be combined with a user certificate. Requires a password; either one alone stops the server at startup.
OPCUA_PASSWORD	—	Secret. Password for the username. Without a security policy it crosses the network in clear text. Supply it through the client's secret storage, never on a command line.
OPCUA_USER_CERT	—	Path to the certificate identifying the user, for X.509 user authentication: a different key pair from the client certificate, which secures the channel. Requires the user key and a security policy other than None; cannot be combined with a username. Any of those combinations broken stops the server at startup.
OPCUA_USER_KEY	—	Path to the private key for the user certificate. It signs the server's challenge and is never sent. Whoever can read the file can log in as this user: keep it readable only by the account running the server.

Tool policy — Which MCP tools are offered, and what a control tool may touch.

Variable	Default	Description
OPCUA_PROFILE	observe	Which tools are offered. observe reads, browses, reads history and monitors; operator adds explicitly allowlisted writes, method calls and alarm actions; full exposes every tool and is meant only for a tightly scoped OPC UA account. One of observe, operator, full (read-only means observe; readonly means observe). Unset fails closed to observe; an unknown profile stops the server at startup.
OPCUA_POLICY_FILE	—	Path to a version-1 JSON policy file, the only place per-node value bounds, writable_subtrees, preconditions, the alarm scope and deny_read can be set. Every policy variable that is set overrides what the file says. An unreadable or invalid file stops the server at startup.
OPCUA_ALLOWED_TOOLS	—	Comma-separated tool allowlist. It can only narrow the selected profile, never widen it. An unknown tool name stops the server at startup.
OPCUA_ALLOWED_WRITE_NODES	—	Comma-separated exact node IDs the operator profile may write, as ns=2;i=5 or, stable across a server restart, nsu=<namespace-uri>;i=5 or a browse path from the Root folder such as /Objects/Line1/Setpoint. Setting it replaces the policy file's list, bounds included. Empty fails closed: operator can write nothing. A batch write with any target outside the list is refused before it reaches OPC UA.
OPCUA_ALLOWED_METHODS	—	Comma-separated object_node_id|method_node_id pairs the operator profile may call. Either half may be a browse path from the Root folder. Empty fails closed: operator can call nothing. An entry without | stops the server at startup.
OPCUA_ALLOW_ACKNOWLEDGE_ALARMS	false	Let the operator profile act on alarms: acknowledge_alarm and every act_on_alarm action.
OPCUA_ALLOW_INSECURE_CONTROL	false	Permit control tools on an OPC UA channel with no security policy (None). Does not cover a secured channel to an unverified server; that is OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL. Fail-open override: writes, method calls and alarm actions then travel unauthenticated and unencrypted. Reported as control=INSECURE-OVERRIDE in the startup line, get_server_status and every audit record. Never enable it against production equipment.
OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL	false	Permit control tools on a secured OPC UA channel whose server is not authenticated: neither pinned with OPCUA_SERVER_CERT nor validated by OPCUA_SERVER_TRUST_STORE. Does not cover a channel with no security policy; that is OPCUA_ALLOW_INSECURE_CONTROL. Fail-open override: writes, method calls and alarm actions are then encrypted to whichever server answered the endpoint, which may be an impostor. Reported as control=UNVERIFIED-OVERRIDE in the startup line, get_server_status and every audit record. Never enable it against production equipment.
OPCUA_ALLOW_OUT_OF_RANGE_WRITES	false	Allow a write outside the EURange the OPC UA server itself published for the node. A published InstrumentRange still applies. Fail-open override: it discards the only value bound a deployment with no policy file has.

Audit — Where the control audit trail goes, and what it is labelled with.

Variable	Default	Description
OPCUA_AUDIT_FILE	—	Path to an append-only file for the control audit trail, one JSON object per line, written beside stderr. Created owner-only (0600) if it does not exist; reopened when rotated away. A file that cannot be opened, or that is a symlink, not a regular file, another account's, or group/world-writable, stops the server rather than falling back to stderr alone. While set, a control call whose record cannot be written is refused (fail closed); reads continue.
OPCUA_AUDIT_FSYNC	always	When an audit-file record counts as written. always fsyncs each record before the control call proceeds, so it survives a power loss; none leaves it to the operating system, so it survives only a crash of this process. One of always, none. none can lose the records of control calls that already reached the plant if the machine loses power.
OPCUA_AUDIT_CHAIN	none	Adds seq, prev_hash and hash to every audit record so edits, deletions and reordering can be detected with --verify-audit. sha256 catches accidental or naive edits; hmac-sha256 also resists anyone without the key. Requires the audit file. One of none, sha256, hmac-sha256. Set without an audit file, or hmac-sha256 without a key file, stops the server. No chain detects a truncated tail.
OPCUA_AUDIT_CHAIN_KEY_FILE	—	Path to the HMAC key for the hmac-sha256 audit chain: at least 32 bytes, for example from openssl rand -hex 32. Surrounding whitespace is ignored. Whoever can read the key can forge the chain: a key readable or writable by group or others stops the server. Refused unless the chain is hmac-sha256.
OPCUA_OPERATOR_ID	—	Label stamped on every audit record as operator_label, so a shipped log says which deployment a control call came from. Configured, never verified: it is not an identity, and mcp_principal stays null.

Reconnection — How a refused or dropped connection is retried.

Variable	Default	Description
OPCUA_RECONNECT_INITIAL_DELAY_MS	1000	Wait before the first attempt to repair a dropped or refused connection, in milliseconds. It doubles with each further attempt.
OPCUA_RECONNECT_MAX_DELAY_MS	8000	Ceiling for the doubling retry delay, in milliseconds.
OPCUA_RECONNECT_MAX_RETRY	3	Retries after the first attempt, per connection round, before a tool call gives up: a whole number from -1 to 1000. 0 never retries; -1 never stops trying, in bounded rounds of four retries so no single call waits forever.
OPCUA_SESSION_TIMEOUT_MS	60000	Session lifetime asked of the OPC UA server, in milliseconds. It also sets the keep-alive period, so it decides how quickly an idle connection notices the server has gone.

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 (the Python SDKs' 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 verified server: a security policy and an authenticated server — its certificate pinned with OPCUA_SERVER_CERT, or validated against a CA trust store with OPCUA_SERVER_TRUST_STORE + OPCUA_SERVER_APPLICATION_URI. Encrypted is not enough — without either the channel is encrypted to whoever answered. Two lab-only overrides say otherwise in as many words, one per missing property: OPCUA_ALLOW_INSECURE_CONTROL=true for a channel with no security, and OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL=true for a secured one to an unauthenticated server. A refused control call names the variable to set, and get_server_status → server_identity says which of the three is in force.

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 owner-only, append-only JSON-lines file beside it, fsync'd per record by default. A control call whose record cannot be written is refused rather than sent; reads carry on. OPCUA_AUDIT_CHAIN adds an optional hash chain, checked with opcua-mcp-server --verify-audit FILE… [--key-file KEY]. What the file can and cannot prove is in SECURITY.md.

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. Event subscriptions are re-created too, and the next read_events notes that events raised while the connection was down were not received. 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.

The MCP server starts whether or not the OPC UA server is reachable: it answers initialize at once and makes its first connection in the background, so an endpoint that is down is reported rather than holding the MCP client back for a whole round of backoff.

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 authenticate the endpoint: pin it with OPCUA_SERVER_CERT, or validate it against a CA trust store with OPCUA_SERVER_TRUST_STORE + OPCUA_SERVER_APPLICATION_URI (maintained asyncua backend only). Control tools need that authentication, not only an encrypted channel: encryption says nobody can read the traffic, the pin or trust store says who is on the other end. Each can be waived for a lab by its own explicit override (OPCUA_ALLOW_INSECURE_CONTROL, OPCUA_ALLOW_UNVERIFIED_SERVER_CONTROL), and never by the other's.

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.