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MCP Product Contract

External AI clients reach host capabilities through DevTools.Daemon. In-host MCP runtime is shared across registered hosts.

Behavior

  • Daemon owns stdio MCP, gateway, auth, host discovery, and a fixed external tool/prompt surface (ListChanged = false).
  • Infrastructure tools remain on the daemon (list_processes, launch_host, read_file_info). list_machines is answered by the gateway.
  • Host capabilities are not projected into daemon tools/list. Clients use code_mode(code, readOnly?), whose code is a C# async method body. Search and dispatch run inside that program:
    • SearchAsync returns ranked candidates from the in-memory catalog. Treat them as options, not an instruction to invoke the first result. Inspect the description and call DescribeAsync for the schema when intent or arguments are ambiguous. Search is schema-aware lexical retrieval, not a guarantee of semantic equivalence. The detailed policy is in decision 0040.
    • Specialized capabilities are preferred by default. C# and Python execution tools are the fallback when no specialized candidate is sufficiently relevant; explicit code-execution queries may surface them directly.
    • DescribeAsync returns the SDK Tool; InvokeAsync returns the host CallToolResult (including image and audio blocks); ReadAsync returns ReadResourceResult. The model receives only the program's return value. readOnly defaults to false; when true, InvokeAsync throws unless the tool's ReadOnlyHint is true.
    • Text over 40,000 characters keeps its start and end. The full text is saved under %TEMP%, and the return includes that path. Image, audio, and blob payloads over 1 MiB are rejected. Ordinary objects and arrays are returned as JSON text; code_mode does not advertise an outputSchema. A returned CallToolResult passes through when its text is within the limit.
  • execute_csharp_code and execute_python_code descriptions include the required entry pattern. From code_mode, source is passed as one string and quotes inside that string must be doubled. Search and invocation locators remain inside the Daemon; neither search_dynamic nor invoke_dynamic is an external MCP tool.
  • MRTR is not a product workflow (0027). The working loop is one code_mode program: search, call, and return a projection. Execute error tags ([COMPILATION ERROR], [RUNTIME ERROR], [ROLLBACK]) are retried inside that program. Destructive tools use structured warning + dryRun (e.g. revit_delete_elements), not elicitation. The host hop may still serialize InputRequiredResult if a tool throws InputRequiredException (plumbing); do not build agent features on Gateway/Cursor elicitation, ElicitAsync, or Python Resolve(Elicit). Isolated .NET toolsets: low-level throw/retry only; high-level MrtrContext suspend is unsupported on the sync ALC invoker.
  • read_file_info defaults to detail=summary for on-disk file peek; pass detail=full for complete transmission/link metadata. Success responses include SDK StructuredContent plus compact JSON in Content (prefer StructuredContent for machine parsing).
  • list_processes emits StructuredContent on success (manual path — no OutputSchema on tools/list until clients accept inferred schemas).
  • Agent-facing JSON from code_mode results, read_file_info, and tool errors uses compact SDK McpJsonUtilities (not indented pretty-print).
  • Fixed prompts (revit_code, acad_code) are daemon-owned via native prompts/list / prompts/get and never contact a host.
  • Two named-pipe protocols stay separate:
    • Pytest/control: DevTools_{Host}_{Version}_{PID} (BridgeMessage length-prefixed frames)
    • MCP: DevToolsMcp_{Host}_{Version}_{PID} (newline-delimited JSON-RPC)
  • Host SDK server advertises listChanged so HostConnections can refresh only that process catalog; the external daemon collections stay unchanged.
  • Call observability is always-on at protocol boundaries. Host in-process MCP logs via McpLogFilters and ILogger; daemon code_mode and infrastructure tools log the same shape. Arguments and results are protocol JSON via SDK McpJsonUtilities (not hash-only summaries). Binary blocks are described by type / mime / length — never base64 Data on monitor lines.
  • MCP Tasks extension (io.modelcontextprotocol/tasks) is advertised on the daemon SDK server. TaskSelection returns Synchronous or Optional only. launch_host and code_mode are Optional. The other infrastructure tools stay synchronous. Required is unused until clients advertise the tasks extension; it rejects the call before the tool runs. Daemon-to-host tools/call stays synchronous; task polling is client ↔ daemon only.
  • view_screenshot captures at 1280 px width (Revit 150 DPI unchanged; AutoCAD 1280×720). Return the ImageContentBlock from code_mode when the model must see it.

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