The i3X API Gateway for OPC UA — REST, MCP & Real-Time Streaming

FastAPI gateway implementing the i3X API over OPC UA — exposes OPC UA address spaces as standardized REST and MCP endpoints with JSON responses, OpenAPI docs, and Server-Sent Event subscriptions. Built for teams that do not have deep OPC UA expertise.

Industrial Information Interoperability eXchange (i3X)

The Industrial Information Interoperability Exchange (i3X™) is an open, common API initiative proposed to address a growing interoperability challenge in modern manufacturing architectures: manufacturing data silo proliferation and API chaos. As manufacturers adopt heterogeneous software stacks from multiple vendors, the industry risks repeating past fragmentation seen with protocols, platforms, and namespaces - this time at the API layer.

Architecture Overview

flowchart LR
	subgraph Consumer[consumer]
		Client[i3X Client]
		Agent[AI Agent]
	end

	subgraph Deployment[API Gateway]
		Nginx[nginx]
	end

	subgraph Security[Optional Deployment Security and TLS]
		Auth[Reverse-proxy authentication<br/>configured by deployment]
		Certbot[Optional Certbot / ACME Client]
		CertStore[(Certificate Store)]
		AcmeCA[(ACME CA)]
	end

	API[FastAPI App]

	Client <--> Nginx
	Agent <--> Nginx
	Nginx --> API
	Nginx -. authentication policy .-> Auth
	Certbot -. ACME .-> AcmeCA
	Certbot --> CertStore
	CertStore -. TLS certificate configured at proxy .-> Nginx

	subgraph Core[i3x2ua-instance]
		Routers["REST routers: /v1 and /ua<br/>always mounted"]
		McpRuntime["MCP router and runtime<br/>mounted only when I3X_ENABLE_MCP=1"]
		Deps[Request dependencies]
		Model[Model cache and builder]
		Services[Application services]
		SubSvc["Subscription runtime<br/>native monitored items or shared polling"]
		OpcClient[OPC UA Client]

		API --> Routers
		API -. if enabled .-> McpRuntime
		Routers --> Deps
		Deps --> Model
		Routers --> Services
		Routers --> OpcClient
		McpRuntime -. internally dispatches REST tools .-> Routers
		Services --> SubSvc
		Model --> OpcClient
		SubSvc --> OpcClient
	end

	OpcClient <-->|OPC UA Binary| UaServer[(External OPC UA Server)]

Loading

The proxy authentication and certificate-provisioning components are deployment concerns, not application services. The app does not enforce HTTP authentication; the Docker Compose example configures Basic Auth at nginx, while Certbot/ACME provisioning and renewal are handled separately.

sequenceDiagram
	participant C as Client
	participant R as /v1 Router
	participant D as Dependencies
	participant M as Model Cache/Builder
	participant O as OPC UA Client
	participant U as OPC UA Server
	participant A as Subscription App Service
	participant S as Subscription Runtime

	Note over C,R: Read current values
	C->>R: POST /v1/objects/value
	R->>D: get_or_build_model()
	D->>M: Check model cache
	opt Cache miss
		M->>O: Browse/build address-space model
		O->>U: Browse requests
		U-->>O: Address-space metadata
		O-->>M: Browse results
	end
	M-->>D: BuildResult
	D-->>R: BuildResult
	R->>O: read_data_values(node_ids)
	O->>U: Read Value attributes
	U-->>O: DataValues (value, quality, timestamp)
	O-->>R: Per-node read results
	R-->>C: bulk success/error envelope

	Note over C,R: Streaming subscription
	C->>R: POST /v1/subscriptions
	R->>A: create_subscription()
	A->>S: Create subscription state
	S-->>A: subscriptionId
	A-->>R: subscriptionId
	R-->>C: success

	C->>R: POST /v1/subscriptions/register
	R->>D: get_or_build_model()
	D-->>R: BuildResult
	R->>A: register_monitored_items(elementIds,maxDepth)
	A->>S: Resolve model elements and register nodes
	opt Initial values enabled
		S->>O: Read initial values
		O->>U: Read Value attributes
		U-->>O: Initial DataValues
		O-->>S: Seed update queue
	end
	alt Native monitored items available
		S->>O: Create/reuse native OPC UA subscription
		O->>U: Create MonitoredItems
		U-->>O: Data-change notifications
		O-->>S: Normalized updates
	else Native monitoring unavailable or admission fails
		loop Shared polling interval
			S->>O: read_values(monitored_node_ids)
			O->>U: Batched Read
			U-->>O: Current values
			O-->>S: Normalized updates
		end
	end
	A-->>R: Per-item registration results
	R-->>C: Bulk success/error envelope

	alt SSE stream (push)
		C->>R: POST /v1/subscriptions/stream
		R->>A: begin_stream_session()
		A->>S: Activate stream generation
		S-->>A: Acknowledge sequence and queued updates
		A-->>R: Stream session
		R-->>C: SSE connected comment
		loop Until stream closes
			R->>A: wait for updates
			A->>S: wait_for_updates(afterSequence)
			S-->>A: Updates or timeout
			A-->>R: Updates or keepalive
			R-->>C: SSE data event or keepalive comment
		end
	else Sync endpoint (client pull)
		C->>R: POST /v1/subscriptions/sync
		R->>A: get_pending_updates(acknowledgeSequence)
		A->>S: sync and acknowledge queue
		S-->>A: Pending updates
		A-->>R: Pending updates
		R-->>C: Update batch (206 if queue overflow)
	end

	alt Optional current-value write enabled (I3X_ENABLE_WRITES=1)
		C->>R: PUT /v1/objects/value
		R->>D: get_or_build_model()
		D-->>R: BuildResult
		R->>R: Check feature flag and validate each item
		R->>O: Check write access and resolve OPC UA value type
		O->>U: Read access and value-type attributes
		U-->>O: Access and value-type metadata
		O-->>R: Validation inputs
		R->>O: write_value()
		O->>U: Write value, quality, and source timestamp
		U-->>O: Write status
		O->>U: Read back value, quality, and source timestamp
		U-->>O: Read-back DataValue
		O-->>R: Per-item write and verification outcome
		R-->>C: bulk success/error envelope
	else Optional current-value write disabled
		C->>R: PUT /v1/objects/value
		R-->>C: 501 Not Implemented
	end

	alt Optional historical write enabled (I3X_ENABLE_WRITES=1)
		C->>R: PUT /v1/objects/history
		R->>D: get_or_build_model()
		D-->>R: BuildResult
		R->>R: Validate each VQT against ObjectType schema
		R->>O: Check HistoryWrite access
		O->>U: Read AccessLevel and UserAccessLevel
		U-->>O: HistoryWrite permissions
		O-->>R: Access result
		alt HistoryUpdate supported and permitted
			R->>O: write_history_value(node,value,quality,timestamp)
			O->>U: HistoryUpdate (insert or replace by source timestamp)
			U-->>O: Per-operation status
			O-->>R: Per-item historical write outcome
			R-->>C: Bulk success/error envelope
		else Unsupported service or denied/invalid item
			R-->>C: Error result (501 unsupported, or per-item failure)
		end
	else Optional historical write disabled
		C->>R: PUT /v1/objects/history
		R-->>C: 501 Not Implemented
	end

Loading

Quick Start

Requirements:

• Python 3.14 to 3.15
• uv
• Optional: running OPC UA server

Install dependencies:

uv sync
uv sync --group dev

Use uv sync for runtime dependencies and uv sync --group dev when you need linting, typing, and test tooling.

Start API:

uv run uvicorn i3x_server.main:app --reload --host 127.0.0.1 --port 8000

Start without OPC UA server (PowerShell):

$env:I3X_SKIP_OPCUA_CONNECT="1"
uv run uvicorn i3x_server.main:app --reload --host 127.0.0.1 --port 8000 --loop none

Enable MCP support explicitly when you want the /mcp endpoints and MCP tool catalog to be available:

$env:I3X_ENABLE_MCP="1"
uv run uvicorn i3x_server.main:app --reload --host 127.0.0.1 --port 8000 --loop none

MCP with enhanced Schemas e.g. EnumValues, References, ModelingRules, ...

$env:I3X_ENABLE_MCP="1"
$env:I3X_MCP_INCLUDE_OPCUA_METADATA="1"
uv run uvicorn i3x_server.main:app --reload --host 127.0.0.1 --port 8000 --loop none

If you do not set I3X_ENABLE_MCP, the app starts without MCP support and /mcp returns 404.

Use encrypted OPC UA with the included sample client certificate (development/testing):

uv run python scripts/generate_opcua_client_cert.py

$env:I3X_OPCUA_SECURITY_MODE="SignAndEncrypt"
$env:I3X_OPCUA_SECURITY_POLICY="Basic256Sha256"
$env:I3X_OPCUA_CLIENT_CERT_PATH="./certs/opcua-client-sample/client-cert.pem"
$env:I3X_OPCUA_CLIENT_KEY_PATH="./certs/opcua-client-sample/client-key.pem"
uv run uvicorn i3x_server.main:app --reload --host 127.0.0.1 --port 8000 --loop none

The OPC UA server may require manually trusting the sample client certificate before the secure session can be established.

If you create your own OPC UA client certificates, ensure they include OPC UA-compatible SAN and key usage fields (application URI, DNS names, clientAuth EKU, and signing/encipherment key usages) and match the server's exposed security mode/policy.

OpenAPI/Swagger:

• http://127.0.0.1:8000/openapi.json
• http://127.0.0.1:8000/docs

Operations are grouped in workflow order: Exploratory (address-space discovery), Query (current and historical reads), Update (writes), and Subscribe (data-change subscriptions), followed by Diagnostics and MCP.

The generated document includes request examples, i3X HTTP error envelopes, subscription SSE wire format, sync overflow (206), optional-write failures (501), and conditional-GET headers and bodyless 304 responses. Validation errors return 400, not FastAPI's default 422. Bulk responses can use HTTP 200 while individual items fail: always inspect each item's success, error, and responseDetail. Example element and subscription IDs are placeholders; replace them with IDs returned by discovery or creation.

For production, configure HTTPS and authentication/authorization at the reverse proxy; keep /v1/info public as required by i3X. The application does not enforce an HTTP authentication scheme, and the OpenAPI document deliberately does not claim otherwise. Subscription clientId is a scope identifier, not a credential. Diagnostics and MCP are gateway extensions; restrict their exposure according to your deployment policy. HTTP caching headers are not generated as MCP tool arguments.

When running the nginx reverse proxy from docker compose up -d, use http://localhost:8080/ for the HTTP entrypoint or https://localhost:8443/ for the HTTPS entrypoint. 8443 is mapped to nginx's TLS port, so http://localhost:8443/ will return a 400 error by design.

API Surface

Active endpoints are exposed under /v1 for:

• info and metadata (/info, /namespaces, /objecttypes, /relationshiptypes)
• filtered type queries (/objecttypes/query, /relationshiptypes/query)
• object queries and values (/objects, /objects/list, /objects/related, /objects/value, /objects/history)
• subscriptions (/subscriptions, /subscriptions/register, /subscriptions/unregister, /subscriptions/sync, /subscriptions/list, /subscriptions/delete, /subscriptions/stream)

Current scope emphasis: this implementation prioritizes read/query/subscribe operations and includes optional current-value and historical write support.

OPC UA browse entrypoints

By default, instance discovery starts at the OPC UA Objects folder (ns=0;i=85). Set I3X_OPCUA_BROWSE_ROOTS to a single URI-based ExpandedNodeId or a JSON array to discover only the selected objects and their forward hierarchical descendants:

I3X_OPCUA_BROWSE_ROOTS='nsu=urn:example:plant;s=Machine1'
# Or multiple entrypoints:
I3X_OPCUA_BROWSE_ROOTS='["nsu=urn:example:plant;s=Machine1","nsu=urn:example:plant;s=Machine2"]'

Configured entrypoints must be readable OPC UA Objects. Numeric (i=), string (s=), GUID (g=), and base64 byte-string (b=) identifiers are supported. Namespace URIs are resolved against the connected server on every model build; namespace-index IDs (ns=2;...) and remote-server addressing are not accepted. Configuration is read at startup; restart the gateway after changing it.

Disjoint entrypoints become separate i3X root objects, accessible through GET /v1/objects?root=true. Overlapping entrypoints and shared descendants are deduplicated, preserving modeled hierarchy/composition relationships. No synthetic container or Objects folder is added. These are browse entrypoints, not forced API roots.

Empty or malformed configuration is rejected. An unknown namespace, missing node, unreadable node, or non-Object entrypoint fails the entire model build, without falling back to Objects or publishing a partial model. A failed periodic refresh retains the previous model. To fail startup on model-build errors, set both I3X_MODEL_PRELOAD_BLOCKING=1 and I3X_FAIL_STARTUP_ON_MODEL_PRELOAD_ERROR=1 with I3X_MODEL_PRELOAD_ON_STARTUP=1.

Object-type discovery remains independent and is not restricted to these entrypoints. This setting scopes discovery, not authorization: forward hierarchical cross-links can lead into other branches or namespaces.

Model revisions and conditional GET

Every response built from the address-space model carries I3X-Model-Revision, a content hash of the model. It changes only when the model content changes, not on every periodic rebuild. Model-derived GET endpoints (/objects, /objecttypes, /namespaces, /relationshiptypes, /model/namespace-gaps) also return a weak ETag and Cache-Control: no-cache. Send it back in If-None-Match to get 304 Not Modified with an empty body while the representation is unchanged:

curl -i https://localhost:8443/v1/objects -H 'If-None-Match: W/"<etag>"'

A response served from a stale cache keeps the older ETag, so clients pick up the refreshed data on their next revalidation.

Optional OPC UA diagnostic endpoints are exposed under /ua:

• client state and connectivity (/ua/state, /ua/connection)
• session limits and metrics (/ua/limits, /ua/metrics)

Optional MCP endpoints are exposed only when I3X_ENABLE_MCP=1:

• Streamable HTTP JSON-RPC entry point (POST /mcp; GET /mcp returns 405)
• tool catalog and REST tool call (/mcp/tools, /mcp/call)
• prompt listing, definition, and execution (/mcp/prompts, /mcp/prompts/{name}, /mcp/prompts/execute)
• resource listing and reading (/mcp/resources, /mcp/resources/read)
• plant root listing (custom REST helper /mcp/roots, not MCP filesystem roots)

MCP targets protocol revision 2025-06-18. It supports initialize, ping, tools/list, tools/call, prompts/list, prompts/get, resources/list, resources/read, and resources/templates/list (currently empty). Unsupported versions negotiate back to 2025-06-18; subsequent requests may specify MCP-Protocol-Version: 2025-06-18. Explicit unsupported HTTP version headers are rejected with 400. JSON-RPC batch arrays are not supported by this revision.

Tool catalogs include annotations and an outputSchema. Calls return legacy JSON text alongside structuredContent: {"statusCode": ..., "body": ...} and isError, including per-item failures. Invalid tool arguments use JSON-RPC -32602; execution failures use tool results. Notifications return 202 with no body. Standard prompts/get accepts string arguments and returns rendered messages; custom REST prompt helpers and prompts/execute retain their existing formats.

MCP requests with an Origin must match a loopback request origin or an explicit origin in I3X_CORS_ALLOWED_ORIGINS (wildcards do not authorize MCP origins). Configure the external origin when deploying behind a reverse proxy. Authentication remains the deployment's responsibility; use HTTPS and an authenticated proxy remotely.

MCP write policy: PUT routes are intentionally excluded from MCP tool generation. Object writes are available via REST only.

Current Limitations

i3X contract

• Update methods are optional in i3X. This gateway exposes the standard bulk endpoints PUT /v1/objects/value and PUT /v1/objects/history; historical reads use POST /v1/objects/history.
• Current-value updates use a VQT: value is required and must conform to the ObjectType schema; optional quality defaults to Good and optional RFC 3339 timestamp defaults to server time.
• Historical updates insert or replace records by source timestamp and require a complete VQT. Bad and GoodNoData require a null value, and null requires one of those qualities plus schema-declared nullability.
• The implementation validates writes against the target's published ObjectType JSON Schema. Invalid values fail with 400; missing or invalid schemas fail with 502.

OPC UA gateway integration constraints

• current-value and historical write support is optional and controlled by I3X_ENABLE_WRITES:
  • I3X_ENABLE_WRITES=1: capabilities.update.current=true, capabilities.update.history=true, both bulk write endpoints enabled
  • default (I3X_ENABLE_WRITES unset/0): write endpoints return 501 Not Implemented
• Current-value writes send quality and source timestamp as OPC UA DataValue fields and verify them with a read-back. If the upstream server does not preserve both, that item returns 501 Not Implemented; if verification fails, the outcome is reported as uncertain rather than successful.
• Historical writes require upstream HistoryUpdate support and per-node OPC UA HistoryWrite access. When enabled, capabilities.update.history is true until the gateway receives service-wide BadServiceUnsupported; it then becomes false. Per-node permission differences remain request-time errors. The standard asyncua 2.1.0 server rejects HistoryUpdate.
• This adapter requires historical timestamps in UTC with a Z suffix and accepts fractional precision beyond six digits only when the extra digits are zero. The six-digit precision limit is an OPC UA adapter constraint; i3X's RFC 3339 timestamp requirement does not impose that limit.
• The OPC UA mapping supports historical writes for Variable-backed property elements with built-in Boolean, numeric, String, DateTime, Guid, and base64 ByteString values. Arrays must be rectangular and conform to OPC UA ValueRank and ArrayDimensions. Composition fan-out and custom structured values are not supported.
• Writes are validated against the published ObjectType schema, including formats and nullability. External schema references are not downloaded or accepted. Write element IDs must be printable with no surrounding whitespace.
• Historical mutations are not automatically retried after a transport failure. Explicit service-wide BadServiceUnsupported returns HTTP 501 unless earlier items succeeded, in which case mixed results retain HTTP 200; after that service-wide result, subsequent history writes return 501 without an upstream attempt. Node-level unsupported operations and permission denials remain item-level failures and do not by themselves disable history updates globally.

Docker

Quickstart (Docker image):

docker run -d --name i3x2ua-master -p 8080:8000 -e I3X_ENABLE_MCP=1 -e I3X_OPCUA_ENDPOINT=opc.tcp://opcua.umati.app:4843 ghcr.io/andreasheine/i3x2ua:master

Multiline (bash):

docker run -d \
	--name i3x2ua-master \
	-p 8080:8000 \
	-e I3X_ENABLE_MCP=1 \
	-e I3X_OPCUA_ENDPOINT=opc.tcp://opcua.umati.app:4843 \
	ghcr.io/andreasheine/i3x2ua:master

Run with compose:

docker compose up -d

Build your own image with an explicit server version for /v1/info:

docker build --build-arg BUILD_VERSION=1.1.0 -t i3x2ua:1.1.0 .

If BUILD_VERSION is not set, the API falls back to master.

The stack now starts the API behind an nginx reverse proxy. The app container stays internal, while nginx exposes HTTP and optional HTTPS.

The default compose setup also enables container hardening (read_only, tmpfs, dropped Linux capabilities, no-new-privileges).

Optional environment variables:

• I3X_ENABLE_MCP=1 to enable MCP support; it is disabled by default
• I3X_ENABLE_WRITES=1 to enable current-value writes (PUT /v1/objects/value) and historical writes (PUT /v1/objects/history)
• I3X_OPCUA_CERTS_DIR=./certs to mount OPC UA client/server certificate files into the app container (/app/certs)
• NGINX_HTTPS_ENABLED=1 to enable TLS termination
• NGINX_SSL_CERTS_DIR=./certs with fullchain.pem and privkey.pem
• NGINX_BASIC_AUTH_ENABLED=1 with NGINX_BASIC_AUTH_USER and NGINX_BASIC_AUTH_PASSWORD
• NGINX_SERVER_NAME for the public host name

If you enable HTTPS, mount or place the certificate files in the configured cert directory before starting Compose.

If your OPC UA server runs on the Docker host machine, set I3X_OPCUA_ENDPOINT to opc.tcp://host.docker.internal:<port> instead of 127.0.0.1.

For local HTTPS testing with the nginx reverse proxy, generate sample certificates:

uv run python scripts/generate_https_dev_cert.py

Then use these environment values:

• NGINX_HTTPS_ENABLED=1
• NGINX_SSL_CERTS_DIR=./certs
• NGINX_SSL_CERTIFICATE=/etc/nginx/certs/https-sample/fullchain.pem
• NGINX_SSL_CERTIFICATE_KEY=/etc/nginx/certs/https-sample/privkey.pem

Development

Before running the commands below, make sure dev tools are installed:

uv sync --group dev

uv run ruff check .
uv run ruff format .
uv run mypy .
uv run pytest -q
uv run pytest -q --cov=i3x_server --cov-report=term-missing

Mypy uses explicit package bases from the repository root so namespace packages such as samples have consistent module names during discovery and imports.

Production Deployment and i3X Strict Compliance

This application implements the i3X API specification and is designed to run behind a reverse proxy / api gateway that is responsible for:

• TLS termination — the app itself serves plain HTTP; all HTTPS is handled by nginx.
• Authentication and authorization — the app has no built-in auth layer; token validation, basic auth, or mTLS are enforced at the proxy level.

Required reverse proxy responsibilities for strict i3X compliance

Requirement	Implementation
TLS (HTTPS) for all client-facing traffic	nginx ssl_certificate + ssl_certificate_key
Client authentication (API key / OAuth / mTLS)	nginx auth_request or satisfy any directives
Rate limiting	nginx limit_req_zone
Access logging	nginx access_log

See the NGINX configuration reference and HTTPS guide for details.

License

Open Source License (AGPL-3.0-or-later)

This project is licensed under the GNU Affero General Public License v3.0 or later. See the LICENSE file for the full legal text.

Commercial Licensing

Sponsors at USD 1/month or higher are granted a commercial license while sponsorship remains active, as defined in the sponsor terms. This enables commercial use without AGPL copyleft obligations during active sponsorship.

Sponsor @AndreasHeine via GitHub Sponsors

See COMMERCIAL_LICENSE.md for full grant scope, limits, grace period, and verification details.

Third-Party and Upstream Components

This repository includes third-party and upstream materials with their own licenses, including the bundled i3X/ subtree. See THIRD_PARTY_NOTICES.md for details.

Contributions

By contributing to this repository, you agree that your contributions are provided under AGPL-3.0-or-later for the open-source distribution and may be included in commercially licensed distributions of this project. See CONTRIBUTING.md for sign-off requirements and contribution workflow.