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In-Host MCP Runtime

The in-host runtime runs inside Revit/AutoCAD and handles tool/resource execution behind an SDK McpServer on DevToolsMcp_* (protocol 2026-07-28).

Runtime Shape

flowchart TB
    Daemon["DevTools.Daemon"]
    Sessions["HostConnections<br/>scan DevToolsMcp_*"]
    Catalog["ProcessCatalogs"]

    subgraph hosts["Host processes"]
        McpPipe["McpPipeServer<br/>DevToolsMcp_Revit_2025_pid"]
        PytestPipe["DevToolsPipeServer<br/>DevTools_Revit_2025_pid<br/>(test pipe)"]
    end

    Registry["McpCatalogStore"]
    Providers["DotnetMcpRegistry<br/>PythonMcpRegistryProvider"]
    Sources["IMcpSource<br/>BuiltIn / Dotnet / Python"]
    Host["IHostContextExecutor + Python executor"]

    Daemon --> Sessions
    Sessions --> Catalog
    Sessions -->|"SDK McpClient"| McpPipe
    McpPipe --> Registry
    Registry --> Providers
    McpPipe --> Sources
    Sources --> Host
    Pytest["pytest client"] --> PytestPipe
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The Daemon owns external MCP routing and process selection. HostConnections scans for DevToolsMcp_{Host}_{Version}_{PID} whose PID is still live, connects with StreamClientTransport, and hydrates ProcessCatalogs. The host process owns execution, registry loading, and host-thread invocation. Pytest/control remains on the DevTools_* pipe.

Registry Flow

sequenceDiagram
    participant UI as Registry UI
    participant Store as McpCatalogStore
    participant Loader as McpCatalogLoader
    participant Dotnet as DotnetMcpRegistry
    participant Python as PythonMcpRegistryProvider
    participant Settings as ISettingsService
    participant HostServer as McpPipeServer

    UI->>Store: EnsureLoaded (init) / AddPathAsync / ReloadAsync
    Store->>Settings: Read configured paths
    Store->>Loader: LoadCatalog(dotnetPaths, pythonPaths)
    Loader->>Dotnet: Parse assemblies
    Loader->>Python: Parse toolset directories
    Loader-->>Store: RegistryCatalog
    Store->>Settings: Persist accepted paths
    Note over Store,HostServer: CatalogChanged only when tool/resource IDs change
    Store-->>HostServer: CatalogChanged
    HostServer->>HostServer: Rebuild SDK tool/resource collections
    HostServer-->>HostServer: tools/list_changed notifications
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Dispatch Flow

Each connected pipe gets McpServer.Create with tools/resources from McpServerCollections.Populate. Invoke runs on the host thread via IHostContextExecutor inside IMcpSource implementations (BuiltInSource, DotnetSource, PythonSource in DevTools.Execution/External/Mcp/Backends/). McpCallFilter maps InputRequiredException to wire input_required. Cancellation is the request token; tool-internal timeouts stay on the tool.

Host prompts are not registered; guidance lives in daemon fixed prompts.

Primitive .NET path Python path
Tool call (built-in) IBuiltInMcpTool via BuiltInSource PythonSource + ToolInvoke.py
Tool call (.NET toolset, ALC) DotnetSource + ResultBridge.ToHostCallToolResult —
Resource read SDK resource handlers on collections Python resource binding

See Platform boundaries for ALC and MRTR detail.

Parser Library

Shared MCP models and McpSpecKeys live under DevTools.Mcp.Catalog/Core/ (namespaces DevTools.Mcp.Core.*). Daemon catalog indexing is DevTools.Daemon/Mcp/Processes/. Host pipe server is DevTools.Mcp.Catalog/Transport/McpPipeServer.cs. source/DevTools.Ipc/ owns test-pipe framing and pipe name helpers.