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MCP resources

Resources are read-only data that an MCP client can fetch with resources/read. UML-MCP serves seven uml:// URIs. Source of truth: mcp_core/resources/diagram_resources.py.

Calling a resource

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "resources/read",
  "params": { "uri": "uml://types" }
}

The response wraps a JSON string in MCP's standard envelope. Every UML-MCP resource returns valid JSON inside contents[0].text.

Resource catalog

URI Purpose When to use
uml://types Available diagram types with backend, description, and supported formats Discover valid diagram_type values before calling generate_uml
uml://templates Starter templates (PlantUML, Mermaid, D2, …) per type Get minimal valid code to customize
uml://examples Realistic example diagrams per type Reference for syntax and structure
uml://formats Supported output_format values per type Pick a valid format before render
uml://capabilities diagram_type → backend + formats matrix Tooling and validation use case
uml://server-info Server name, version, tools, prompts, Kroki URL Discovery and health checks
uml://workflow Recommended plan-then-generate workflow Add to agent system prompts

uml://types

Returns a JSON object: { <type>: { backend, description, formats }, ... }. Same payload as the list_diagram_types tool, which helps when a client cannot call resources but can call tools.

uml://templates

Minimal starter snippets keyed by diagram_type. Use when the user is new to a notation; modify the template instead of writing from scratch.

uml://examples

More realistic example diagrams keyed by diagram_type. Heavier than templates; useful as few-shot context for an LLM.

uml://formats

Format whitelist per type:

{
  "class": ["png", "svg", "pdf", "txt", "base64"],
  "mermaid": ["svg", "png"],
  "d2": ["svg"]
}

uml://capabilities

Combines backend, formats, and description into a single matrix. Same shape validate_uml uses internally to catch unsupported output_format values.

uml://server-info

Server identity and runtime metadata:

{
  "server_name": "uml-mcp",
  "version": "...",
  "description": "...",
  "tools": ["generate_uml", "validate_uml", "list_diagram_types", "generate_uml_batch"],
  "prompts": ["uml_diagram", "uml_diagram_with_thinking", "..."],
  "kroki_server": "https://kroki.io",
  "plantuml_server": "..."
}

uml://workflow

The plan-then-generate workflow text. Add it to agent prompts when you want the model to plan before calling generate_uml.

Tips

Cache resources client-side

Resource payloads are stable across requests within a server version. Cache them on the client to avoid repeated round-trips inside a multi-step flow.

What if my client cannot read resources?

Use the list_diagram_types tool; it returns the same payload as uml://types. For other resources, the data is also embedded in the documentation site (see linked pages above).