Extract Tool/Prompt/Resource metadata from XML doc comments

[mikekistler]

Is your feature request related to a problem? Please describe.

Minimal APIs added support for extracting metadata from XML doc comments to include in generated OpenAPI documents. This was a highly requested feature. It would be nice to provide similar support for extracting MCP metadata (tool descriptions, tool parameter descriptions, etc) from XML doc comments.

Describe the solution you'd like

Allow developers to use XML Doc comments to specify tool / tool parameter / prompt / resource descriptions and have these extracted and included in the tools/list, etc.

Describe alternatives you've considered

The current mechanism to provide tool descriptions is with the DescriptionAttribute. This has some drawbacks, particularly for larger descriptions that can't be flowed in a nice way in text strings.

Additional context

Some parts of the XML doc comment support in Minimal APIs can probably be reused. @captainsafia is a good contact for this.

[stephentoub]

The XML comments aren't part of the binary; they're generated by the compiler into a separate .xml file, which isn't necessarily generated or distributed with the app. How do we deal with that for OpenAPI?

[captainsafia]

How do we deal with that for OpenAPI?

For OpenAPI, it's implemented via a source generator that discovers XML comments and source generates a cache that we query into via OpenAPI tranformers at runtime (ref). There are two entrypoints for how the XML comments get discovered:

For the assembly under compilation, XML comments are available in the semantic model and we query from there.

For XML comments from references, which are separate from the assmbly, we use MSBuild to resolve the XML files for applicable references and add those to the <AdditionalFiles> item group for processing by the source generator (ref).

[stephentoub]

it's implemented via a source generator

Ok. Yes, that'd be viable, albeit a non-trivial addition.

[captainsafia]

Ok. Yes, that'd be viable, albeit a non-trivial addition.

Yep, when I first started working on the XML generator, there was some interest in shipping it as a general implementation that could be consumed from OpenAPI/documentation engines/anything else. I didn't end up pursuing them due to time constraints but the current implementation does lend itself to being lifted to a non-OpenAPI specific implementation if this feature ends up being desirable for MCP.