fuel-formats

Transport-independent parsers for tensor wire formats:

• safetensors — re-exports from upstream safetensors crate + MmapedFile convenience
• pickle — minimal PyTorch .pth reader (subset of Python pickle protocol 2)
• gguf — llama.cpp's GGUF v1/v2/v3 container
• imatrix — llama.cpp activation-importance matrix

Every parser operates on impl Read / impl Seek / &[u8] / Cow<'_, [u8]> and returns format-typed structs. No item in this crate references Tensor, Device, Storage, or any other backend-frontend type. The only borrowed types are foundational primitives from fuel-core-types: DType, Shape, GgmlDType.

Why a separate crate

A parser tied to a specific transport (file path, mmap) cannot easily serve other transports. Splitting the parser surface from the transport adapter unlocks several concrete use cases:

• Streaming weight load — read safetensors / GGUF off an impl Read (HTTP body, S3 stream, decompressor) without ever materializing the full file on disk.
• Inter-process tensor exchange — two Fuel processes (or Fuel ↔ Lightbulb ↔ mlmf) can exchange tensors as safetensors-on-the-wire over a Unix socket / shared-mem region. The format is already a public, language-agnostic schema — using it as IPC means there is no Fuel-internal protocol to maintain, and any HF-ecosystem tool can read what Fuel emits.
• KV-cache / activation hand-off — between an inference and a draft model in speculative decoding, or between an embedding model and a retrieval model in a pipeline.
• RemoteHostStorage (Phase 7c) — the transport returns typed buffers; the parser surface sits naturally on top.
• Hot weight reload during serving — write current weights to a wire format, parse back into a new process. No special Fuel-native serialization.

Where Tensor-construction lives

Phase 7.5's work item E has landed: the wrappers that turn parsed metadata into a Tensor no longer live in fuel-core. fuel_core::safetensors/fuel_core::quantized were pure compat shims to fuel_loaders (fuel-core dissolution, GAP-347 PR 5/6) and have since been retired outright — current call sites are fuel_loaders::safetensors::load and fuel_loaders::quantized::gguf_file::Content::open (the ggml_file module they once discussed was deleted entirely; see fuel-loaders/src/quantized/mod.rs). PthTensors::get took a different path: pickle's Tensor-construction layer was absorbed directly into fuel_formats::pickle, with no separate fuel-loaders wrapper needed.

Pattern for new transports

A new transport adapter (HTTP, S3, IPC, etc.) follows the same shape as fuel-loaders:

1. Acquire bytes from your transport — Vec<u8>, Cursor<&[u8]>, an impl Read.
2. Hand them to the appropriate fuel-formats parser:
   • fuel_formats::safetensors::SafeTensors::deserialize(&bytes)
   • GGUF: fuel-formats keeps the wire types and the writer only; reading is fuel_loaders::quantized::gguf_file::Content::open/from_bytes over mlmf-gguf (MLMF slice D)
   • fuel_formats::pickle::read_pth_tensor_info(path, false, key) (currently file-bound; a stream variant is straightforward to add)
   • fuel_formats::imatrix::parse(&mut reader)
3. Hand the parsed metadata + raw bytes to whoever builds Tensors on a Device.

The smoke test in tests/transport_independence.rs shows each parser running against an in-memory Cursor<&[u8]> without touching the filesystem.

Status

This crate landed as part of ROADMAP Phase 7.5 work item A (fuel-formats extraction). Subsequent work items (B/C/D/E) will reduce fuel-core further; A2 will move the file-transport adapters into a dedicated fuel-loaders crate once Tensor lives in fuel-tensor.