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fix(pd): serve multimodal requests on language-model-only vLLM decode workers - #2640
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… workers A multimodal request on a vLLM P/D deployment whose decode workers run --language-model-only (no vision encoder, encoder-cache budget 0) fails with HTTP 400 decode_worker_failed_to_start: the router forwards the pixel-less mm identity (placeholders, hashes, grid tensors) to the decode leg, the servicer rebuilds a multimodal prompt from it, and vLLM's input preprocessor raises "exceeds the pre-allocated encoder cache size 0". The router cannot detect the pairing because the servicer reports supports_vision = is_multimodal_model, which stays true under --language-model-only. Mirror the Dynamo P/D contract: when the selected decode worker reports supports_vision=false, the decode leg carries the prefill-expanded input_ids, the relayed kv_transfer_params and only the per-image content hashes (folded into cache_salt servicer-side, so different images behind identical placeholder-token runs cannot alias in the decode prefix cache). The decode engine then sees a pure-text TokensPrompt and never schedules its zero-budget encoder cache. Carve-outs, each a clear non-retryable 400 instead of a deep engine failure: M-RoPE grid tensors (a language-model-only decode leg cannot derive positions), media references (each leg expands its own anchors), and n>1 (no KV handoff, no local recompute without a vision encoder). Refs: #2483 Signed-off-by: Roger Wang <hey@rogerw.io>
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WalkthroughPD setup accepts separate prefill and decode engine flags. The vLLM servicer reports whether engines accept multimodal inputs. The gateway shapes sequential PD decode requests for language-model-only workers, checks KV handoff, and rejects unsupported requests. ChangesLanguage-model-only PD decode
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~30 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant Client
participant Gateway
participant PrefillWorker
participant DecodeWorker
Client->>Gateway: Submit multimodal request
Gateway->>PrefillWorker: Send request for prefill
PrefillWorker-->>Gateway: Return KV parameters
Gateway->>DecodeWorker: Send selected decode request with KV parameters
DecodeWorker-->>Gateway: Return decode response
Gateway-->>Client: Return response or HTTP error
Merge Risk: 🔵 Low · up to The post-prefill handoff guard is present, but no caller-level test protects the case where prefill completes without KV parameters. A focused regression test is a bounded follow-up for this multimodal behavior. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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| /// reach it. A multimodal model reports `"true"`; a text-only model also | ||
| /// reports `"false"`, which is harmless — its requests carry no mm payload | ||
| /// to strip in the first place. | ||
| pub(crate) const SUPPORTS_VISION_LABEL: &str = "supports_vision"; |
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🟡 Nit: SUPPORTS_VISION_LABEL now documents this label as the router-side contract, but the other consumer — model-card inference in create_worker.rs (labels.get("supports_vision")) — still uses a raw string literal. Since the two must stay in sync for the "a full-vision prefill leg still marks the model vision-capable" behavior described in the PR to hold, consider having create_worker.rs reference this constant (it's in the same crate) so a future rename can't silently split the contract.
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Fixed in c31beae — create_worker.rs now imports SUPPORTS_VISION_LABEL from routers::grpc::multimodal instead of the raw "supports_vision" literal, and the constant is exported from the module for that consumer.
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In `@grpc_servicer/smg_grpc_servicer/vllm/mm_salt.py`:
- Line 17: Update the multimodal capability check in mm_salt.py so vision
support is not inferred from model_config.is_multimodal_model when
supports_multimodal_inputs is unavailable. Use
MULTIMODAL_REGISTRY.supports_multimodal_inputs(model_config) only as the
fallback capability probe, then map that result to vision support more narrowly
so enable_mm_embeds-only models still report supports_vision=false. Preserve the
existing behavior when ModelConfig.supports_multimodal_inputs is present, and
add coverage for both the property-based and registry-fallback paths.
In `@model_gateway/src/routers/grpc/common/stages/request_execution.rs`:
- Line 1023: Update the SequentialPdDecodeForm::TextPlusHashes path to check the
actual KV handoff after prefill completes; if a multimodal request has no
kv_transfer_params, return an error before dispatching the stripped
proto_request instead of allowing text-only recomputation.
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e2e_test/fixtures/setup_backend.pye2e_test/router/test_pd_multimodal.pygrpc_servicer/smg_grpc_servicer/vllm/mm_salt.pygrpc_servicer/smg_grpc_servicer/vllm/servicer.pygrpc_servicer/tests/test_vllm_mm_salt.pymodel_gateway/src/routers/grpc/common/stages/request_execution.rsmodel_gateway/src/routers/grpc/multimodal/capability.rsmodel_gateway/src/routers/grpc/multimodal/mod.rsmodel_gateway/src/routers/grpc/proto_wrapper.rs
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The is_multimodal_model fallback in engine_accepts_mm_inputs was wrong on the servicer's whole supported range: ModelConfig.supports_multimodal_inputs only exists on vLLM main, while vLLM 0.19-0.20 carry the equivalent check as MULTIMODAL_REGISTRY.supports_multimodal_inputs(model_config). Under --language-model-only the architecture check stays true, so the fallback kept reporting vision for pixel-less decode workers. Probe the property first (main), then the registry (0.19-0.20), mirroring the ValueError-as-text-only treatment of main's own check; unexpected probe failures keep the architecture answer so a healthy full-vision worker is not blinded. On top of vLLM's answer, enable_mm_embeds-only engines (all modality limits 0, or --language-model-only) report supports_vision=false: they ingest embeddings but have no encoder for pixel payloads. Tests cover the property path, the registry fallback (including --language-model-only), the no-processor ValueError, probe failures, the vllm-less engine-free path, and the enable_mm_embeds-only carve-out. Signed-off-by: Roger Wang <hey@rogerw.io>
create_worker.rs read the label via a raw string literal while capability.rs defined SUPPORTS_VISION_LABEL; route both consumers through the constant so the label contract cannot drift. Signed-off-by: Roger Wang <hey@rogerw.io>
… leg The NIXL fallback when prefill returns no kv_transfer_params is a local prompt recompute on the decode worker. That is fine for text, but a stripped multimodal leg cannot recompute the image KV: the decode engine would recompute the prompt as pure text and answer image-blind. After prefill completes, if the decode leg was stripped of its mm payload and carries no kv_transfer_params (either wire form), fail with a clear non-retryable 502 pd_decode_missing_kv_transfer_params instead of dispatching. Text requests strip to a no-op and keep the recompute fallback. Signed-off-by: Roger Wang <hey@rogerw.io>
| return False | ||
| if getattr(mm_config, "language_model_only", False): | ||
| return True | ||
| return bool(limits) and all(get_limit(modality) == 0 for modality in limits) |
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🟡 Nit: _mm_embeds_only iterates only the explicitly configured limit_per_prompt keys, while vLLM's own embeds-only check iterates the model's supported modalities (info.supported_mm_limits). The two diverge when enable_mm_embeds=True and the operator zeroes a subset of modalities, e.g. --limit-mm-per-prompt image=0 on an image+video model: every explicit limit is 0, so this returns True and the worker advertises supports_vision=false, even though the video encoder is still live (default non-zero limit). That misclassifies a healthy encoder-bearing worker — the same "don't blind a healthy worker" direction the except Exception handler in _registry_supports_multimodal_inputs deliberately guards against — and for an M-RoPE model would turn a valid P/D pairing into a hard pd_decode_language_model_only_mrope 400. Niche config, so not blocking, but if the model's supported-modality set (or a fully populated limit_per_prompt) is reachable here, iterating that instead would close the gap.
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Description
Problem
A multimodal request against a vLLM P/D deployment whose decode workers run
--language-model-onlyfails with HTTP 400decode_worker_failed_to_start.That decode shape — no vision encoder, encoder-cache budget 0 — is how
production P/D fleets run (observed on a live MiniMax-M3 OME deployment;
Dynamo pairs prefill/decode the same way).
Failure chain: the router forwards the pixel-less mm identity to the decode
leg (placeholder ranges, mm hashes, M-RoPE grid tensors); the servicer
rebuilds a multimodal prompt from it; vLLM's input preprocessor raises
"exceeds the pre-allocated encoder cache size 0". The router cannot detect
the pairing because the servicer reports
supports_vision = model_config.is_multimodal_model, which staystrueunder
--language-model-only(vLLM keeps the multimodal architecture butzeroes every modality limit).
Solution
Mirror the Dynamo vLLM-handler contract for a pixel-less decode leg:
kv_transfer_paramsso the KV connector pulls the image-token KV fromprefill.
The decode engine then sees a pure-text
TokensPromptand the encoder-cachecheck never fires.
Detection is automatic, no new flags and no proto change:
GetModelInfoResponse.supports_visionand theGetServerInfomm_processoradvertisement now reflectmodel_config.supports_multimodal_inputsinstead ofis_multimodal_model—falseunder--language-model-only. The onlyrouter consumer of the
supports_visionlabel is model-card inference;a full-vision prefill leg still marks the model vision-capable, and a
pixel-less decode worker correctly stops advertising worker-side media
processing (so
--mm-processing=autokeeps media on the router path forthese pools).
supports_vision=false,execute_sequential_pdbuilds the decode legwith the new
ProtoGenerateRequest::clone_without_mm(): pixels,placeholders, grid tensors and media references are dropped; expanded
input_idsand the KV handoff stay.cache_salt protection. Two same-resolution images behind an identical
text prefix expand to identical placeholder-token runs, so a fully stripped
decode leg would let different images alias in the decode-side prefix cache
(the wrong-image-answer incident class). The stripped leg therefore keeps
each batch's
mm_hashes, and the existing servicer logic (#2365) folds theminto
cache_salt: same-image reuse still hits the cache, different imagescannot collide.
M-RoPE carve-out. Grid-dependent models (Qwen-VL family) derive
decode-side positions from the grid tensors: a language-model-only decode
worker rejects them, and stripping them would mis-rotate every generated
token. Instead of silently stripping, the router returns a clear,
non-retryable 400 (
pd_decode_language_model_only_mrope) naming theincompatibility. Grid-less models (MiniMax-M3-style) take the stripped path.
Two further incompatible shapes get the same treatment:
pd_decode_language_model_only_media_refs(media references arrive asunexpanded anchors each leg expands itself — which is also why
ensure_selection_supports_media_refsdeliberately still requires bothlegs to advertise
mm_processor; worker-side processing on a pixel-lessdecode pool is rejected at selection) and
pd_decode_language_model_only_n_samples(n>1 has no KV handoff, so decode must recompute the prompt locally, which a
pixel-less worker cannot do).
Older servicers keep reporting
supports_vision=true, so mixed-versionfleets keep today's behavior until workers upgrade; both sides of this
change are needed for the fix to engage.
Refs: #2483 — PD pairing could grow a vision-compatibility factor so a
language-model-only decode pool is never selected for an M-RoPE model in the
first place.
Changes
model_gateway/src/routers/grpc/proto_wrapper.rs: newclone_without_mm()(full mm strip keeping a hash-only identity perbatch),
has_vllm_mrope_grids(), and thevllm_mm_hash_identityhelper.model_gateway/src/routers/grpc/common/stages/request_execution.rs:SequentialPdDecodeForm+sequential_pd_decode_form()pick thedecode-leg form (full / identity-only / text-plus-hashes) from the decode
worker's capability and the KV-handoff mode, with the three carve-out
400s;
execute_sequential_pduses it.model_gateway/src/routers/grpc/multimodal/capability.rs(+mod.rs):worker_language_model_only()and thesupports_visionlabel constant.grpc_servicer/smg_grpc_servicer/vllm/servicer.py: honestsupports_visionandmm_processoradvertisement via the newengine-free
engine_accepts_mm_inputs()helper inmm_salt.py(fallsback to
is_multimodal_modelon older vLLM).e2e_test/fixtures/setup_backend.py:prefill_args/decode_argsPD-leg options (engine CLI flags for one leg only).
e2e_test/router/test_pd_multimodal.py: two language-model-only decodelanes — Phi-3.5 (grid-less) asserts cross-image KV isolation still holds
and n>1 is rejected clearly; Qwen3-VL (M-RoPE) asserts the pairing is
rejected clearly.
Test Plan
cargo +nightly fmt --all— clean.cargo clippy --workspace --all-targets -- -D warnings— zero warnings.cargo test— all workspace suites pass (smglib: 1932 passed,0 failed), including the new tests:
clone_without_mm_keeps_only_hashes_ids_and_kv_params— no mm fieldsbeyond hashes/modality,
input_idsandkv_transfer_params_jsonpreserved, prefill leg untouched;
clone_without_mm_drops_hashless_batches_whole;has_vllm_mrope_grids_reads_every_batch;sequential_pd_decode_form_picks_the_leg_for_the_worker— full-visionworkers keep today's forms (identity for n=1, full payload for n>1);
sequential_pd_decode_form_rejects_incompatible_pairings— grids,media refs, and n>1 with a pixel-less decode worker each 400.
ruff check/ruff format --checkon the touched Python — clean;pytest grpc_servicer/tests— 255 passed, 11 skipped (thetest_vllm_mm_salt.pycapability tests included; the vLLM-importingsuites were not runnable locally — no vLLM on this machine).
test_pd_multimodal.pyneed a 2-GPU runner(
E2E_RUNTIME=vllm pytest e2e_test/router/test_pd_multimodal.py -v) andwere not run locally. Before/after they encode: before, the Phi-3.5 lane's
first request dies with 400
decode_worker_failed_to_start(encoder-cachebudget 0); after, the three-probe isolation sequence passes, and the
M-RoPE/n>1 pairings fail fast with the new explicit 400s.
Checklist
cargo +nightly fmtpassescargo clippy --all-targets --all-features -- -D warningspasses (workspace-wide without the opencv feature, per CONTRIBUTING)