Resources#

The OpenSWMM MCP Server exposes model data through nine MCP resources using the swmm:// URI scheme. Resources provide structured, read-only access to model metadata and simulation results without invoking tools.

URI Scheme#

All resource URIs follow the pattern:

swmm://sessions
swmm://session/{session_id}/...

Where {session_id} is the identifier of an active simulation session (e.g. "default").

Available Resources#

swmm://sessions#

List all active simulation sessions.

Returns: JSON array of session metadata objects, each containing:

  • session_id – session identifier

  • state – current lifecycle state

  • working_dir – session working directory

Example:

[
  {
    "session_id": "default",
    "state": "ended",
    "working_dir": "/tmp/openswmm/default"
  }
]

swmm://session/{session_id}/summary#

Model summary for the specified session.

Returns: JSON object with:

  • session_id, state

  • node_count, link_count, subcatchment_count

  • flow_units, route_model

  • start_time, end_time, routing_step

swmm://session/{session_id}/nodes#

List all nodes in the model.

Returns: JSON array of objects, each with node_id, type, and index.

swmm://session/{session_id}/nodes/{node_id}#

Detailed properties and runtime state for a single node.

Returns: JSON object with:

  • node_id, index, node_type

  • invert_elev, max_depth

  • Runtime state (when simulation has run): depth, head, volume, lateral_inflow, overflow

swmm://session/{session_id}/subcatchments#

List all subcatchments in the model.

Returns: JSON array of objects, each with subcatch_id and index.

swmm://session/{session_id}/mass_balance#

Continuity errors and volumetric totals for the simulation.

Returns: JSON object with:

  • runoff_continuity_error, routing_continuity_error

  • quality_continuity_error (may be null)

  • runoff_total – breakdown of runoff volume components

  • routing_total – breakdown of routing volume components

swmm://session/{session_id}/options#

The model’s simulation options.

Returns: JSON object with:

  • flow_units, route_model

  • start_time, end_time, routing_step

Using Resources#

Resources are accessed through the MCP read_resource method. In Claude Code, the LLM reads resources automatically when guided by prompts or when it needs structured model data.

Resources complement tools: where tools perform actions and return results, resources provide passive data access. For example, a prompt might instruct the LLM to “read swmm://session/default/summary” to understand the model before deciding which analysis tools to invoke.