# MirBuilderBox — Program(JSON v0) → MIR(JSON v0)

Responsibility
- Convert Stage‑B Program(JSON v0) into MIR(JSON v0) for VM/LLVM lines.
- Phase 20.34 starts with a delegate implementation to Runner and progressively internalizes ops.

Interface (stable)
- `emit_from_program_json_v0(program_json: String, opts: Map|Null) -> String|Null`
  - Returns canonical MIR(JSON v0) on success; returns null and prints a tagged diagnostic on failure.

Tags (Fail‑Fast, stable)
- `[mirbuilder/input/null]` — input is null
- `[mirbuilder/input/invalid]` — header missing (version/kind)
- `[mirbuilder/delegate]` — delegate path selected (Runner `--program-json-to-mir`)
- `[mirbuilder/delegate/missing]` — delegate/provider not wired yet

Toggles (default OFF)
- `HAKO_MIR_BUILDER_DELEGATE=1`: Use Runner `--program-json-to-mir` as a temporary provider

Notes
- Box‑First policy: boundary/contract first, then implementation. Keep tags stable; no silent fallback.
- Large payloads: implementation may stream/json-scan later; initial version is string‑based.