<!-- GENERATED FROM prd.json — DO NOT EDIT AS AN INDEPENDENT SOURCE. prd.json IS AUTHORITATIVE. -->
# DGR-024: Implement in-memory fake gRPC seam transport

- **Status / triage:** specification only; `ready-for-agent`; `passes: false`
- **Execution mode:** `AFK`
- **Milestone:** `M1`
- **Dependencies:** `DGR-022`, `DGR-023`
- **Blocks (derived):** `DGR-033`, `DGR-042`
- **Labels:** `area:protocol`, `area:testing`, `type:vertical-slice`, `priority:p0`, `ready-for-agent`
- **Evidence class:** `fixture`
- **Hardware:** `none`
- **Model:** `fake`
- **Upstream:** `no`

## Objective / description

Fresh Ralph session: read `.scratch/distributed-gguf-runtime/RALPH-CONTEXT.md`, source issue `.scratch/distributed-gguf-runtime/issues/024-implement-in-memory-fake-grpc-seam-transport.md`, and evidence READMEs for dependencies (DGR-022, DGR-023) before changing code. Inspect live source/tests rather than trusting legacy pass states. Objective: Exercise the complete streaming protocol deterministically before a real model or worker exists.

## Acceptance criteria

- [ ] Provide a fake bidirectional stream supporting prefill fragments, decode fast-path frames, release, cancel, and structured errors.
- [ ] Test flow-control blocking, deadlines, malformed fragments, checksum failure, duplicates, and stale epochs.
- [ ] Verify direct and opaque-relay framing preserve identical protobuf bytes.
- [ ] Tests require no sockets outside localhost, model downloads, or native accelerator.
- [ ] Applicable shared quality gates in `prd.json` pass, and the evidence handoff records exact commands/results, changed files, limitations, and dependency handoff.

## Shared quality gates

- Targeted deterministic tests pass; Python changes also pass `python -m compileall packages tests`.
- `git diff --check` passes.
- Default tests are model-download-free, API-credit-free, and GPU-free.
- Evidence README records exact changed files, commands/results, limitations, and dependency handoff; no fabricated evidence or inherited completion credit.
- Native changes pass focused out-of-tree CMake build and CTest; patch changes verify clean apply/check/reverse against the exact llama.cpp pin.
- Runs are opt-in and record exact artifact/split hashes, runtime/upstream pin, backend/driver, hardware, network, commands, and raw metrics. Model artifacts use configured mounted-drive storage and never `/home`.
- Preserve existing Transformers behavior and backend-agnostic Tracker routing/load balancing/billing/relay semantics unless an explicit versioned contract says otherwise. One scoped story commit is expected during execution, but this specification-materialization change is not committed.

## Evidence handoff

Write and verify `.scratch/distributed-gguf-runtime/evidence/DGR-024/README.md`. Until every criterion and applicable gate has real evidence, this story remains `passes: false`. Legacy evidence is provenance only, not completion credit.
