fix(pd): process worker-side media once, on the prefill leg

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Description

Problem

With prefill/decode disaggregation and worker-side media processing, the same image or video is fetched, decoded and encoded twice. The decode leg is built with clone_without_mm_pixels(), which keeps media_refs (its test said so outright: "Both PD legs process the references themselves"), and the servicer runs the media processor whenever media_refs is present with no notion of which leg it serves. Router-side processing already hands the decode leg a pixel-less identity (expanded ids, hashes, placeholders, grid tensors); worker-side mode never produced one, because only the prefill worker knows it after processing. A secondary gap: pd_fanout_width exempted multimodal requests from n>1 fan-out via has_mm_inputs(), which does not see media_refs, so such a request under parallel dispatch became n pairs each processing the media on both legs.

Solution

Worker-side media in PD becomes prefill processes, decode receives identity, reusing the decode contract router-side processing already has:

1. vLLM PD already dispatches sequentially for every request (prefill runs to completion, the router harvests the GenerateComplete, then decode starts), for both Mooncake and NIXL, so the relay needs no dispatch change and is connector-agnostic. The n > 1 case (no KV relay) is the exception: decode recomputes the prompt locally, needs the media itself, and keeps media_refs unchanged; the router gates the identity on the single-sample relay itself, since legacy Mooncake injects its host and port into every decode leg.
2. The prefill servicer, after self._mm_processor.process(...) on a request carrying kv_transfer_params, returns the processed media identity on its GenerateComplete: the expanded prompt ids and, per modality, mm_hashes, mm_placeholders and the inline grid tensors (image_grid_thw, video_grid_thw, second_per_grid_ts), no pixels. It also derives im_token_id from a placeholder's embed mask when the range mixes structural tokens, the way the router does.
3. The router harvests it in the sequential path's prefill drain loop and, once the decode leg actually holds a KV handoff (Mooncake params minted or injected, or NIXL/passthrough params relayed from the prefill completion) and still carries references, builds the decode leg from it: tokenized.input_ids = the expanded ids, mm_inputs / extra_mm_inputs = the pixel-less batches, media_refs cleared. A NIXL prefill that returned an identity but no KV params keeps the references, since a pixel-less leg without KV is refused by the servicer.
4. The decode servicer takes its existing pixel-less branch; no decode-side media processing happens.
5. A prefill completion without an identity leaves the leg as it is; when one was solicited (the prefill request carried KV params) that is an older servicer and one WARN carries the request id: prefill worker returned no media identity; decode leg will reprocess media. A servicer whose installed smg-grpc-proto predates the field logs that the decode leg will reprocess and returns no identity. A grid tensor that is not one row per item also yields no identity (with a warning), so the decode leg falls back rather than receive a layout it cannot read.

pd_fanout_width now also returns None for a media_refs request, so it is never split into n PD pairs on the parallel-dispatch runtimes.

With #2640 (merged after this PR was opened; rebased onto it): a decode worker running --language-model-only no longer advertises mm_processor, so media stay on the router path for such pools under auto, and sequential_pd_decode_form rejects a media_refs request on such a worker before prefill (pd_decode_language_model_only_media_refs). The relayed identity therefore only ever reaches a full-vision decode worker, where it is the IdentityOnly form that worker already accepts. Serving references on a language-model-only pool through the identity (prefill-expanded ids plus hashes, the TextPlusHashes form) is a possible follow-up, not part of this PR.

Compatibility: old servicer + new router falls back with the WARN; new servicer + old router ignores the new field (unknown proto field on the completion); a servicer with smg-grpc-proto 0.4.20 simply cannot set the field and behaves as before.

Changes

• crates/grpc_client/proto/vllm_engine.proto: GenerateComplete.media_identity = 15 (13 and 14 were already taken by the speculative-decoding counters) and message MediaIdentity { prompt_token_ids, mm_inputs, extra_mm_inputs } reusing MultimodalInputs. The Rust and Python stubs are generated at build time (tonic build.rs, setup.py), so the proto edit is the whole schema change; smg-grpc-proto moves to 0.4.21.
• grpc_servicer/smg_grpc_servicer/vllm/media_identity.py (new, engine-free): build_media_identity(prompt) and tensor_to_proto (inline little-endian bytes, the router's wire form; grids go through a Python list so the module needs no numpy).
• grpc_servicer/smg_grpc_servicer/vllm/servicer.py: the identity is built after processing when the request carries KV params and attached to the completion (only when the installed proto package has the field).
• model_gateway/src/routers/grpc/proto_wrapper.rs: ProtoGenerateComplete::media_identity(), ProtoGenerateRequest::apply_media_identity(); the Vllm completion variant is now boxed, because the identity made vllm::GenerateComplete about 2.5x the size of the other variants and ProtoResponseVariant is moved once per streamed token (clippy's large_enum_variant caught it).
• model_gateway/src/routers/grpc/common/stages/request_execution.rs: the fan-out guard, apply_prefill_media_identity (KV relay only, WARN on a missing identity), harvest in the drain loop, the comment block updated.
• e2e_test/router/test_pd_multimodal.py: test_worker_media_is_processed_once_on_the_prefill_leg on the Qwen3-VL PD class (skips on the router-side lane; empty worker logs skip locally with SHOW_WORKER_LOGS=1 and fail under CI, via assert_worker_logs_captured); e2e_test/infra/worker.py: Worker.log_path / read_log() so a test can read a leg's captured log; .github/workflows/pr-test-rust.yml: the e2e-2gpu-pd-mm-worker lane's floor 3 → 4.

Test Plan

Rust (request_execution.rs tests, next to clone_without_mm_pixels_keeps_vllm_media_refs, whose comment now says the sequential path replaces the refs with the prefill's identity):

• media_refs_request_never_fans_out: pd_fanout_width is None for a media_refs request with n = 3 under PdDispatch::Parallel (SGLang).
• n_greater_than_one_keeps_media_refs_on_decode and an_identity_without_a_kv_handoff_keeps_the_references: without a handoff the decode leg keeps media_refs and gets no mm_inputs, even when an identity came back.
• n_greater_than_one_keeps_media_refs_despite_a_legacy_mooncake_handoff: an n>1 leg keeps media_refs even with the legacy Mooncake host/port injected and an identity returned (fails when the relay_kv_params gate is dropped).
• decode_leg_from_media_identity_has_no_refs_and_no_pixels: an identity with 2 hashes, 2 placeholders and one image_grid_thw applied to a media_refs request: media_refs gone, no pixel_values, 2 hashes, input_ids replaced, original_text kept.
• missing_identity_leaves_the_decode_leg_untouched, solicited or not.
• A mutation check (drop the relay guard, drop the fan-out check) fails the two matching tests.
• cargo test -p smg: 29 suites, 2652 passed, 0 failed, 8 ignored; cargo test -p smg-grpc-client: 81 passed; cargo +nightly fmt --all clean; cargo clippy --workspace --all-targets -- -D warnings clean (without --all-features: no OpenCV here).

Python (servicer suite with torch installed: 359 passed, 9 skipped; main on this tree without torch: 338 / 10):

• test_vllm_media_identity.py (torch, engine-free): ids, hashes, placeholders and grids carried with no pixels, im_token_id read off the masked range, keep_on_cpu kept; a second modality lands in extra_mm_inputs; no media → no identity; a grid that is not one row per item → no identity plus a warning; older field spellings read too; tensor_to_proto round-trips through the servicer's own tensor_from_proto, widens int32/float64 to the wire dtypes, and matches the router's byte layout.
• test_vllm_pd_media_identity.py (needs vLLM: runs on the vLLM e2e lane, skipped here): prefill leg (kv_transfer_params_json + media_refs) answers with the identity and the processor is called once; regular leg answers with none; decode leg built from that identity is accepted and never calls the processor.

E2E (e2e-2gpu-pd-mm-worker, E2E_MM_PROCESSING=worker, Qwen3-VL PD pair; cannot run on this machine, result comes from CI): one image_url request answers correctly, smg_mm_processing_total{mode="worker"} goes up by one, the prefill log gains one media_refs=1 line, the decode log gains one preprocessed_mm=True, media_refs=0 line and no media_refs=1 line. The harness CPU gate (PYTHONPATH=e2e_test pytest -q --noconftest e2e_test/infra e2e_test/fixtures) passes: 75 passed, 3 skipped.

Not run here: the live check with a real prefill/decode pair and a redis sidecar (only the prefill sidecar receiving a job).

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets -- -D warnings passes (without --all-features, see above)
• (Optional) Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

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📝 Summary

Summary by CodeRabbit

• New Features

  • Sequential prefill/decode multimodal requests can transfer processed media metadata without resending image or video pixels.
  • Decode requests reuse prefill token, placeholder, hash, and supported grid information.
  • vLLM completion responses can include media identity data.

• Improvements

  • Multimodal processing is coordinated across prefill and decode workers to avoid repeated media handling.
  • Requests with unsupported or incomplete metadata retain standard media processing through improved fallback behavior.
  • Expanded validation helps ensure media is processed once during worker-based execution.

Walkthrough

The change adds pixel-less multimodal media identity data to vLLM completions. The gateway relays this data from sequential PD prefill to decode requests. Tests verify serialization, fallback behavior, request handling, and worker-side processing counts.

Changes

PD multimodal media identity

Layer / File(s)	Summary
Media identity protocol contract crates/grpc_client/proto/vllm_engine.proto, crates/grpc_client/python/pyproject.toml	Adds MediaIdentity and the optional GenerateComplete.media_identity field. Updates the Python package version.
Prefill identity construction grpc_servicer/smg_grpc_servicer/vllm/*, grpc_servicer/tests/test_vllm_media_identity.py, grpc_servicer/tests/test_vllm_pd_media_identity.py	Serializes prompt IDs, hashes, placeholders, grid tensors, and additional multimodal inputs without pixel data. vLLM attaches the identity to supported completion responses. Tests cover serialization, invalid data, compatibility, and PD behavior.
Gateway identity relay model_gateway/src/routers/grpc/proto_wrapper.rs, model_gateway/src/routers/grpc/common/stages/request_execution.rs, model_gateway/src/routers/grpc/common/response_formatting.rs, model_gateway/src/routers/grpc/regular/streaming.rs	Sequential PD captures prefill media identity and applies it to decode requests. Requests with vLLM media references avoid parallel fan-out. Completion variants now use boxed vLLM messages.
End-to-end worker validation e2e_test/infra/worker.py, e2e_test/router/test_pd_multimodal.py, .github/workflows/pr-test-rust.yml	Workers expose captured log contents. Router tests verify one worker-side prefill processing event and identity-based decode handling. The worker test selection threshold increases to four tests.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant PrefillWorker
  participant Gateway
  participant DecodeWorker
  PrefillWorker->>Gateway: return GenerateComplete with MediaIdentity
  Gateway->>Gateway: capture prefill media identity
  Gateway->>DecodeWorker: apply identity and clear media references
  DecodeWorker->>DecodeWorker: process decode without media pixels

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.46% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 61 functions across 10 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: worker-side media is processed once during the prefill leg.
Description check	✅ Passed	The description directly explains the worker-side media duplication problem, the prefill/decode media identity solution, compatibility behavior, implementation changes, and test coverage.

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Inline comments:
In `@grpc_servicer/smg_grpc_servicer/vllm/servicer.py`:
- Around line 316-317: The KV-transfer path must check protobuf support before
calling build_media_identity. In the block guarded by kv_transfer_params,
inspect GenerateComplete.DESCRIPTOR.fields_by_name for media_identity; only then
construct the identity, otherwise log the existing compatibility fallback
warning with request_id and leave media_identity as None.

In `@model_gateway/src/routers/grpc/common/stages/request_execution.rs`:
- Around line 1044-1048: Move apply_prefill_media_identity out of the
unconditional request setup and invoke it only after the KV-transfer branch
confirms valid returned handoff parameters; preserve media_refs when
relay_kv_params is absent or passthrough occurs so decode can recompute the
prompt. Add coverage for an identity-bearing media request with missing KV
parameters.

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• e2e_test/router/test_pd_multimodal.py
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• grpc_servicer/tests/test_vllm_media_identity.py
• grpc_servicer/tests/test_vllm_pd_media_identity.py
• model_gateway/src/routers/grpc/common/response_formatting.rs
• model_gateway/src/routers/grpc/common/stages/request_execution.rs
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Review summary

Solid, well-tested change: worker-side media in PD is now processed once on the prefill leg, with the decode leg served from a pixel-less identity that reuses the router-side decode contract. The proto back-compat story (descriptor field guard on the servicer, WARN fallback on the router, version bump) is carefully done, and the decode-side acceptance paths (grid-carrying → preprocessed, grid-less → cache-salt) both hold up.

Findings:

• 🔴 1 Important — apply_prefill_media_identity runs before the router knows whether prefill actually returned KV params; in the NIXL no-KV-params case the decode leg becomes pixel-less without a handoff and the decode servicer hard-rejects it, replacing a previously graceful local-recompute fallback (request_execution.rs:1044).
• 🟡 2 Nits — the missing-identity WARN fires per-request and blames the servicer in modes (Passthrough, legacy Mooncake) where the router never solicited an identity; the new e2e test's log-count assertions break when ENV_SHOW_WORKER_LOGS routes worker output to the terminal.
• 🟣 0 Pre-existing.

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Inline comments:
In `@e2e_test/router/test_pd_multimodal.py`:
- Around line 174-175: Update the log-path guard in the multimodal router test
to distinguish CI from intentional terminal logging: when running in CI and
either prefill.log_path or decode.log_path is None, fail the test; retain the
existing skip behavior only for non-CI runs where worker output is intentionally
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• grpc_servicer/tests/test_vllm_media_identity.py
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• grpc_servicer/smg_grpc_servicer/vllm/servicer.py
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Inline comments:
In `@model_gateway/src/routers/grpc/common/stages/request_execution.rs`:
- Around line 1117-1122: Update the apply_prefill_media_identity call so its
handoff condition also requires relay_kv_params, preserving media_refs for all n
> 1 requests while retaining existing identity application behavior when relay
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