`--attention-backend` is one field of three: a launch that sets only
`--prefill-attention-backend` or `--decode-attention-backend` leaves the base
field at `None`. Seven decisions read that base field alone and therefore
answered from a field the operator never set. `attention_backends()` is the
pair with the base-field fallback already applied, so each site now asks it for
the half it actually needs:
- `inkling_common/attn` assembles backend-specific kwargs (rel_bias / score
mods) and gates its fused prologue; the backend those describe is the one
`self.attn` dispatches to, so `serving_attention_backend()` selects the pair
member by `forward_batch.forward_mode`, mirroring
`HybridAttnBackend._select_backend` exactly -- draft-extend routes through
the prefill branch like the dispatcher does -- and preferring the
runner-stamped pair, so a draft runner answers with its own backend. That
preference only works if every backend that can enter a ForwardContext
carries the stamp, so `DraftBackendFactory._create_backend` now stamps its
products with the backend it resolved (draft override first), and the
draft-extend conv-sidecar wrapper copies the wrapped backend's stamp -- the
replacement backends the spec workers install had no stamp at all and fell
back to the target's configured pair.
- The chunked-prefix-cache gate is a *prefill* feature -> prefill half. Reading
the base field switched the feature off for every prefill-only configuration.
- `init_deterministic_inference_config` maps *prefill* knobs
(SPLIT_TILE / PREFILL_TRUNCATION_ALIGN) -> prefill half; the map missed and
left truncation unset.
- `two_batch_overlap` computes extend positions -> prefill half.
- mrope's interleaved-rope kernel runs in both phases -> both halves must
support triton. This one is not conservative when it misreads:
`support_triton(None)` answers **True**, so a `--prefill-attention-backend
torch_native` launch took the triton path.
- The req-to-token writer has one caller, `alloc_for_extend` -> prefill half;
its fallback pays several `.item()` syncs per request, so gating it on the
decode half too would send every extend of a mixed launch through the slow
path. `get_last_loc` (the spec-decode allocator's helper) keeps the
both-halves reading: verify tokens are served by either half depending on
`speculative_attention_mode`.
- The flashinfer version floor is a guard; it never fired for a launch that
pinned flashinfer through a split field.
One more site the census found is not converted here: `gpt_oss` derives its
`sinks` parameter dtype from the backend, and a single parameter dtype cannot
serve a split pair (FA4 asserts bfloat16, trtllm_mha consumes float32), so
that one is a behaviour question rather than a config-source one and is fixed
in its own PR.
`test_split_attention_backend_decisions.py` pins the callable decisions by
calling them under a split-only publish, and pins the remaining ones
statically -- the file/why map fails if any of them goes back to the base field
(reverse-verified). It also asserts the `support_triton(None) is True` trap the
sweep exists for.
The stamp comes from the constructor, not the request: every factory leaf
answers ("effective_name", backend), because several map entries do not build
what their key says -- cutedsl_mla draft-extend builds the trtllm-mla backend,
"nsa" is a deprecated alias building dsa, and the hybrid-linear entries pick
fa3/intel_amx/triton by host, which no static rename table can express (a
review catch: on Blackwell the alias stamp reached Inkling's per-forward
kwargs assembly, which asserts a concrete kernel name, and crashed the first
draft-extend forward). The stamping is pinned by unit tests, not only by a
spec e2e: removing the child-stamping loop, stamping an alias from a leaf, or
dropping the wrapper copy goes red (reverse-verified), and a static guard
walks the factory source asserting no leaf answers an alias name. The child loop states its contract explicitly --
`create_decode_backend` passes `stamps_children=True` because its products
are per-step containers by construction, so a container without
`attn_backends` raises instead of being silently skipped by a defensive
probe. The `_version` invalidation names its contract (autograd's in-place
counter: private, chosen because it is the only per-tensor signal that ticks
on copy_-style updates; removal fails loudly). The version-floor guard's file
joins the pair-reader ratchet, and the one runner-seed chain read sharing the
backend's __init__ (`speculative_eagle_topk`) reads the spec bag.
Cheng Wan
d13d5c03ab