feature: .NET source generator for .prompty files (codegen + tracing)

[sethjuarez]

Summary

Build a C# source generator that compiles .prompty files into typed C# classes at build time. This replaces the need for runtime YAML parsing and file loading entirely.

Motivation

Currently, using a .prompty file in C# requires:

var agent = PromptyLoader.Load("path/to/chat.prompty");
var result = await Pipeline.InvokeAgentAsync(agent, inputs: new Dictionary<string, object?> {
    ["firstName"] = "Jane",
    ["question"] = "What is the meaning of life?",
});

With a source generator, the same .prompty file would produce a typed class at build time:

// Auto-generated from chat.prompty
var result = await ChatPrompty.RunAsync(firstName: "Jane", question: "What is the meaning of life?");

This gives you IntelliSense, compile-time validation, and no runtime YAML parsing — the same pattern as Razor pages, gRPC stubs, and protobuf.

Scope

Phase 1: Basic source generator

• Read .prompty files from the project (MSBuild AdditionalFiles or a custom <Prompty> item group)
• Parse frontmatter at build time
• Emit a typed class per .prompty file with:
  • Strongly-typed input parameters from inputSchema.properties
  • Typed return value from outputSchema (or string default)
  • Static RunAsync() / PrepareAsync() methods that delegate to the runtime pipeline
  • The raw prompty content embedded as a string constant (no file I/O at runtime)

Phase 2: Tracing attributes

• Auto-generate OpenTelemetry Activity spans from prompty metadata (name, description, model info)
• Emit [ActivitySource] registration and span creation in the generated code
• Map prompty execution stages (render → parse → execute → process) to child spans
• Include prompty metadata as span attributes (model ID, provider, template kind)

Phase 3: Validation and diagnostics

• Emit compiler warnings/errors for:
  • Invalid YAML frontmatter
  • Missing required inputSchema properties without defaults
  • Unknown model.provider values
  • ${env:VAR} references (informational: "resolved at runtime")
• Roslyn analyzer companion for IDE squiggles

Design Questions

• Should the generated class name come from the name field or the filename?
• Should we generate interfaces for testability (e.g., IChatPrompty)?
• How should ${env:VAR} and ${file:path} references work in codegen context? (They're inherently runtime — the generator can embed them as deferred resolution)
• NuGet packaging: Prompty.SourceGenerator as a separate analyzer package?

Supersedes

• feature: .NET - support Load(Stream, ...) #206 (Load(Stream)) — codegen eliminates the need for stream-based loading
• feature:.NET - support passing file and params as Attributes on a class def #205 (attribute-based loading) — codegen provides a better developer experience

References

• C# Source Generators
• Razor Source Generator (prior art)
• protobuf-net Source Generator (prior art)