<!-- GENERATED FROM prd.json — DO NOT EDIT AS AN INDEPENDENT SOURCE. prd.json IS AUTHORITATIVE. -->
# DGR-021: Define the versioned named-tensor stream envelope

- **Status / triage:** completed; `passes: true`
- **Execution mode:** `AFK`
- **Milestone:** `M1`
- **Dependencies:** `DGR-018`
- **Blocks (derived):** `DGR-022`, `DGR-023`, `DGR-025`, `DGR-031`, `DGR-035`, `DGR-046`
- **Labels:** `area:protocol`, `type:infrastructure`, `priority:p0`, `ready-for-agent`
- **Evidence class:** `model-free`
- **Hardware:** `none`
- **Model:** `generic`
- **Upstream:** `no`

## Objective / description

Fresh Ralph session: read `.scratch/distributed-gguf-runtime/RALPH-CONTEXT.md`, source issue `.scratch/distributed-gguf-runtime/issues/021-define-the-versioned-named-tensor-stream-envelope.md`, and evidence READMEs for dependencies (DGR-018) before changing code. Inspect live source/tests rather than trusting legacy pass states. Objective: Establish the backend-neutral protobuf envelope used by direct and relayed Shard activation traffic.

## Acceptance criteria

- [x] Define schema version, request/work ID, route session/epoch, shard range/effective start, phase, position, and idempotency step.
- [x] Define named tensors with shape, dtype, byte order, bounded fragments, compression identity, and checksum.
- [x] Reserve extensible fields for token-ID sidebands, architecture state, recurrent state, and MTP without claiming implementations.
- [x] Add deterministic serialization, fragmentation, checksum, unknown-field, and size-limit tests.
- [x] 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

Verified evidence: `.scratch/distributed-gguf-runtime/evidence/DGR-021/README.md`. Legacy evidence remains provenance only and grants no implementation completion credit.
