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Unified Remote API and MCP

AXSG now has one shared remote-operation layer that serves LSP, MCP, preview orchestration, and studio/runtime remote surfaces.

The important design choice is:

  • reuse JSON-RPC transport infrastructure across LSP and MCP
  • do not reuse LSP semantics as if they were MCP

Why MCP is not “just LSP”

LSP and MCP both use JSON-RPC 2.0 and commonly run over framed stdio transports, but they solve different problems:

  • LSP is document and editor oriented
  • MCP is tool and resource oriented

AXSG therefore shares:

  • message framing
  • serialization options
  • request and response helpers
  • transport-neutral command routers

AXSG does not share:

  • lifecycle assumptions
  • method names
  • editor-specific payloads

That keeps the remote stack reusable without forcing editor concepts onto MCP clients.

Current adapter layout

Surface Adapter Shared layer underneath
Standalone language server XamlToCSharpGenerator.LanguageServer.Tool RemoteProtocol.JsonRpc plus shared query services
Workspace MCP host XamlToCSharpGenerator.McpServer.Tool RemoteProtocol.JsonRpc plus McpServerCore plus shared query services
Preview helper command transport PreviewHostCommandRouter shared preview payload contracts
Preview MCP host PreviewHostMcpServer shared preview payload contracts plus shared MCP core
Studio remote design server AxsgStudioRemoteCommandRouter shared runtime query services plus shared studio contracts
Runtime MCP host XamlSourceGenRuntimeMcpServer shared runtime query services plus shared MCP core

Shared packages

XamlToCSharpGenerator.RemoteProtocol

This package is the reusable transport and contract layer. It contains:

  • JSON-RPC framing helpers
  • MCP server core
  • preview host contracts
  • studio remote contracts

It is the package to reuse when you need AXSG remote protocols without copying transport code.

Shared operation services

The transport-neutral query layer now lives in services such as:

  • AxsgPreviewQueryService
  • AxsgRuntimeQueryService

Those are consumed by both LSP and MCP hosts so the same operation shape does not get reimplemented per adapter.

MCP host split

AXSG intentionally has more than one MCP host because workspace state and live runtime state are not the same thing.

Workspace MCP host

This host:

  • runs independently as axsg-mcp
  • sees files, projects, and workspace options
  • is best for preview project-context resolution and general tooling queries

It does not automatically attach to a live Avalonia app.

Runtime MCP host

This host:

  • runs inside the Avalonia process
  • sees hot reload, hot design, and studio state directly
  • supports resources/subscribe and event-style resource updates

This is the host you want for dotnet watch, hot reload, hot design, and live-app inspection.

Preview MCP host

This host:

  • runs inside the preview helper process
  • exposes explicit preview lifecycle and preview hot-reload tools/resources
  • supports dynamic tool and resource catalogs as the active preview session starts or stops

This is the host you want when preview is the system under test.

Why the VS Code extension still has its own runtime behavior

The VS Code extension is a product surface, not just a transport wrapper. It still owns:

  • activation behavior
  • lazy language-server startup
  • VS Code preview webview integration
  • editor middleware and inline-C# projection interop

The extension can reuse the shared remote-operation layer, but it still needs editor-specific orchestration that does not belong in MCP or LSP core contracts.

Subscription model

AXSG now uses a capability-based model:

  • workspace MCP host: query-oriented, poll after changes
  • runtime MCP host: subscribe to status and event resources
  • preview MCP host: subscribe to lifecycle resources and react to list-changed notifications

This split matches the real ownership model of the processes instead of pretending one host can answer every question equally well.

Preview hot reload over MCP

Preview MCP now supports in-process live preview application directly through:

  • axsg.preview.hotReload

That operation is intentionally different from the lower-level:

  • axsg.preview.update

hotReload waits for the preview session to complete the in-process apply and returns the concrete result payload. update is still available for dispatch-only flows and compatibility with clients that already consume the lifecycle resources asynchronously.

This keeps the preview host aligned with the rest of AXSG’s shared remote-operation model:

  • transport-neutral preview session/router underneath
  • MCP adapter on top
  • status and event resources as the subscription surface