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0031-daemon-json-source-gen.md

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0031 Daemon JSON Is Source-Generated

Date: 2026-09-03 Amended: 2026-09-27

Status

Accepted. Daemon-owned wires use source-generated System.Text.Json. 0032 dropped Native AOT; source-gen stays because those wires must not depend on reflection serializers. Envelope tools/call shape still follows 0027.

Do not rename ToolHelpers to McpJson.

Context

Daemon and MCP DTOs used to serialize through reflection. The MCP C# SDK keeps its protocol graph internal and McpJsonUtilities.DefaultOptions is read-only, so app DTOs must Insert(0, context) on a copy. Facade collapse landed; remaining object? on invoke/batch DTOs is Follow-Up, not a live plan. Source-gen was first required so 0032 could aim at Native AOT. AOT is no longer a Daemon target. The wire policy remains.

Decision

  1. Source-gen is required on Daemon-owned wires (control pipe, settings, daemon tool DTOs). Do not switch those wires back to reflection. This is not an AOT milestone.
  2. Two MCP facades, not one options object per assembly:
    • Protocol: ToolHelpers.ProtocolOptions (= SDK DefaultOptions)
    • Daemon DTOs: McpToolJson.Options (chained McpServerJsonContext)
  3. One JsonSerializerContext per named wire (control vs settings vs logs vs pytest vs MTP). A new context means a new wire, not a new project. Do not merge those lifetimes.
  4. tools/call stays spec keys only (content, structuredContent, isError, resultType, _meta). No UseStructuredContent on envelope tools until 0027 is revisited.

Foreign ALC JSON (RuntimeJsonOptions) is not a third wire — it must not produce client tools/call bytes.

Alternatives Considered

  1. Drop source-gen because Daemon AOT was dropped. Rejected — the wires still need a stable serializer that does not walk properties by reflection.
  2. One mega-context for all JSON. Rejected — different wires, TFMs, and compatibility lifetimes.

Follow-Up

  • InvokeCapabilityResponse.Result / ResourceReadResult.Result are still object?. JIT batch reads[] writes through McpToolJson.Options (SDK chain) and works. Bare McpServerJsonContext typeinfo throws — keep that characterization until the union is JsonElement? (or equivalent) without reflection. Capture batch goldens first; counts/shape may change.
  • Landed work: 2026-09-03-stj-facade-0028