[Excelius-Wang]

Overview

Allow the Rust frontend's vllm.Inference/GenerateStream to return multiple candidates for one prompt. This covers the streaming part of #65841 and reuses #65625.

Claims

• Send prompt_info once, with a separate outputs.index and finish_info for each candidate. Candidates can stream and finish in any order.
• Treat num_sequences=0 as one candidate, and enforce VLLM_MAX_N_SEQUENCES and the existing greedy-sampling restriction.
• Cancel unfinished candidates on a client disconnect or stream error.

Validation

At 3f34cac03 plus this patch, the Rust workspace passed 2582 tests, with 13 existing ignored tests skipped. Clippy, fmt, cargo sort/deny, and full pre-commit checks passed. The three added or extended gRPC tests cover independent completion, validation, and cleanup on disconnect or error through the existing Tonic/scripted-engine fixtures; each also passed 20 runs at the earlier 14d147a7a base.

Real-model validation used 14d147a7a with the identical patch and a matching Python engine wheel: Qwen/Qwen3-0.6B revision c1899de289a04d12100db370d81485cdf75e47ca, one H100 80GB, BF16, TP1, eager mode, TRITON_ATTN, and 512MiB KV cache. It was not rerun after rebasing.

Check	Result
Unpatched GenerateStream, n=2	INVALID_ARGUMENT: num_sequences > 1 is not supported; n=1 generation worked
Patched n=2/3	Three prompts, eight candidates, 32 tokens each; all 256 token IDs and complete texts matched separate requests with seed+i
Compatibility	Streaming n=0 and n=1, plus unary n=1, matched the baseline token IDs and text
Disconnect	After cancelling two running requests, abort count increased by 2 and running/waiting counts returned to zero; the next request succeeded

Both patched model runs passed. The running-to-idle transition was checked in the second run. No throughput benchmark was run.

Check commands, serving configuration, and example request

cargo nextest run --manifest-path rust/Cargo.toml --workspace --all-features --locked --no-fail-fast
cargo clippy --manifest-path rust/Cargo.toml --workspace --all-targets --all-features --locked -- -D warnings
cargo fmt --manifest-path rust/Cargo.toml --all -- --check
cargo sort --workspace --check rust
cargo deny --manifest-path rust/Cargo.toml --config rust/deny.toml check bans
pre-commit run --all-files

With vLLM installed and VLLM_RUST_FRONTEND_PATH exported to the built binary, the equivalent server command is:

VLLM_USE_RUST_FRONTEND=1 VLLM_MAX_N_SEQUENCES=3 \
vllm serve Qwen/Qwen3-0.6B \
  --revision c1899de289a04d12100db370d81485cdf75e47ca \
  --served-model-name qwen-h100 --host 127.0.0.1 \
  --port 18080 --grpc-port 15051 \
  --dtype bfloat16 --enforce-eager --attention-backend TRITON_ATTN \
  --max-model-len 2048 --max-num-seqs 8 --max-num-batched-tokens 2048 \
  --kv-cache-memory-bytes 536870912 --gpu-memory-utilization 0.1 \
  --generation-config vllm

The model checks used a Python gRPC client. An equivalent n=2 request using grpcurl:

grpcurl -plaintext -emit-defaults \
  -import-path rust/proto -proto inference.proto \
  -d '{
    "model": "qwen-h100",
    "text": "The capital of France is",
    "temperature": 0.7,
    "sampling": {"numSequences": 2, "seed": 42, "topP": 0.9},
    "stopping": {"maxNewTokens": 32, "ignoreEos": true},
    "response": {"promptTokenIds": true, "outputTokenIds": true, "outputLogprobs": true}
  }' 127.0.0.1:15051 vllm.Inference/GenerateStream

Read until the stream ends to receive both candidates.

Details

The handler calls TextLlm.generate_n and merges the indexed streams with select_all. The response body owns the streams, so dropping it uses the existing abort path.

#65625 left gRPC at n=1. #56776 adds native sampling parameters and still requires one sequence. Issue and area searches on 2026-11-06 found no open PR for this streaming support. Another contributor has expressed interest in item 1.

Unary Generate still returns one SequenceOutput, so it remains limited to one candidate. Multimodal kwargs from #65841 are a separate item. This uses the existing protobuf fields and adds usage documentation to the serving guide.

AI assistance: Codex.

---

Pull Request Checklist

• I used vLLM's /pr-checklist skill. (Mandatory for agents, optional for humans).

• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

• Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.

• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

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[mergify]

Documentation preview: https://vllm--66078.org.readthedocs.build/en/66078/