feat: first-class session_id for conversation-aware routing

[ashusnapx]

[Feature] First-class session_id for conversation-aware routing

Closes #48049

---

The Problem

vLLM currently has no typed, request-level identity for sessions, conversations, or agent runs. The only existing workaround is to smuggle session_id through SamplingParams.extra_args["session_id"] — a fragile hack that:

• Overloads request_id (which has different lifecycle semantics)
• Hides identity inside sampling parameters where it doesn't belong
• Makes it impossible for engine internals, KV connectors, cache policies, or schedulers to consume session identity without reaching into extra_args

This blocks Dynamo integration and any future session-aware scheduling.

---

The Solution

Add a first-class, typed session_id: str | None that flows through the entire vLLM request path:

OpenAI body.session_id / HTTP X-Session-ID header
  → Serving layer (_get_session_id with precedence)
  → EngineCoreRequest.session_id
  → Request.session_id
  → Engine internals, KV connectors, schedulers (future)

Key design rule: request.session_id, not request.sampling_params.extra_args["session_id"].

---

What Changed

1. Core Engine Plumbing (6 files)

File	Change
vllm/v1/engine/__init__.py	Added session_id: str | None = None to EngineCoreRequest struct
vllm/v1/request.py	Added session_id param, storage, and copy from EngineCoreRequest
vllm/engine/protocol.py	Added session_id to EngineClient.generate() abstract method
vllm/v1/engine/async_llm.py	Threaded through generate(), add_request(), _add_streaming_input_request()
vllm/v1/engine/llm_engine.py	Threaded through add_request() → process_inputs()
vllm/v1/engine/input_processor.py	Accepts session_id, passes to EngineCoreRequest()

2. OpenAI Ingress (3 files)

File	Change
vllm/entrypoints/openai/chat_completion/protocol.py	Added session_id field to ChatCompletionRequest
vllm/entrypoints/openai/completion/protocol.py	Added session_id field to CompletionRequest
vllm/entrypoints/openai/responses/protocol.py	Added session_id field to ResponsesRequest

3. Serving Layer Resolution (1 file, shared helper)

File	Change
vllm/entrypoints/generate/base/serving.py	Added _get_session_id() static method with clear precedence

4. Serving Layer Wiring (5 files)

File	Change
vllm/entrypoints/openai/chat_completion/serving.py	Wires session_id into beam search + engine generate
vllm/entrypoints/openai/chat_completion/batch_serving.py	Wires session_id into batch generation
vllm/entrypoints/openai/completion/serving.py	Wires session_id into completion generation
vllm/entrypoints/openai/responses/serving.py	Wires session_id into responses + _generate_with_builtin_tools()
vllm/entrypoints/generate/beam_search/online.py	Accepts + forwards session_id to all beam sub-requests

5. Parallel Sampling & Beam Search

• Parallel sampling (n > 1): Child requests inherit session_id via copy(request) — each child gets a unique request_id but shares the same session_id.
• Beam search: session_id is forwarded to every beam sub-request, preserving session identity across the beam tree.

---

Precedence Rules

The _get_session_id() helper resolves the effective session ID with deterministic precedence:

@staticmethod
def _get_session_id(request, raw_request) -> str | None:
    # 1. Body-level session_id (explicit, highest priority)
    if request.session_id:
        return request.session_id
    # 2. X-Session-ID HTTP header
    if raw_request is not None and (
        value := raw_request.headers.get("X-Session-ID")
    ):
        return value
    # 3. vllm_xargs["session_id"] (temporary compatibility only)
    if request.vllm_xargs:
        session_id = request.vllm_xargs.get("session_id")
        if isinstance(session_id, str) and session_id:
            return session_id
    return None

Scenario	Result
Body session_id + header X-Session-ID	Body wins
Header only	Header wins
vllm_xargs only	vllm_xargs used (compat)
Empty body session_id	Treated as absent
Nothing set	None

---

What This Does NOT Include

This PR is deliberately scoped to typed plumbing only. The following are follow-up work:

• Rust frontend HTTP/OpenAI surface — The Rust server path needs the same session_id field in its OpenAI request types and resolve_session_id() helper.
• gRPC body-level session_id — Requires proto changes.
• Native/offline LLM.generate parity — The synchronous LLM class can expose session_id in a follow-up.
• Consumer updates — Mooncake/KV connector consumers should read request.session_id instead of extra_args["session_id"] once this lands.
• Session-aware scheduling/cache policy — This PR only makes identity available; policy is a separate discussion.

---

Design Decisions

Question	Answer	Rationale
Should vLLM accept X-Correlation-ID?	No — only X-Session-ID	Avoids ambiguity; Dynamo already normalizes its own headers
Body vs header precedence?	Body wins	Explicit value should override inferred value
Conflicting values?	Resolved by precedence	Simpler than rejection; callers control their own identity
Keep vllm_xargs["session_id"]?	Yes, as compatibility only	Existing experimental users need a migration path
Parallel sampling?	Inherit via copy()	Child requests share parent's session identity
Beam search?	Forward to all sub-requests	Session identity must survive the beam tree

---

Example Usage

Via HTTP header:

curl -X POST http://localhost:8000/v1/chat/completions \
  -H "X-Session-ID: my-conversation-123" \
  -H "Content-Type: application/json" \
  -d '{"model": "llama-3", "messages": [{"role": "user", "content": "Hello"}]}'

Via request body:

curl -X POST http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "llama-3",
    "session_id": "my-conversation-123",
    "messages": [{"role": "user", "content": "Hello"}]
  }'

Programmatic (AsyncLLM):

async for output in engine.generate(
    prompt,
    sampling_params,
    request_id="req-1",
    session_id="my-conversation-123",  # NEW
):
    print(output)

---

Tests

Test File	What It Verifies
tests/entrypoints/openai/test_session_id.py	_get_session_id() precedence: body > header > xargs, empty/absent handling
tests/v1/test_request_session_id.py	Request.session_id propagation from EngineCoreRequest
tests/v1/engine/test_parallel_sampling_session_id.py	Child requests preserve session_id while retaining unique request_id

All tests pass. All linting (ruff check, ruff format, typos) passes.

---

Related

• Issue: #48049 — Feature request
• Existing PR: #48048 — This PR supersedes feat(frontend): session id plumbing into requests #48048 with a cleaner implementation that includes the same Python engine plumbing plus the serving layer wiring and comprehensive tests
• Upstream PR: #46547 — The Mooncake extra_args workaround that this typed field replaces

---

Checklist

• Core engine plumbing (EngineCoreRequest → Request)
• OpenAI ingress (Chat, Completion, Responses)
• Serving layer resolution with precedence
• Beam search forwarding
• Parallel sampling inheritance
• Comprehensive unit tests
• Ruff check + format pass
• Typos pass
• Rust frontend (follow-up PR)
• Consumer updates (follow-up PR)

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[ashusnapx]

Hi @claude @DarkLight1337 @robertgshaw2-redhat @aarnphm @NickLucche @AndreasKaratzas @njhill @chaunceyjiang @russellb

This PR adds first-class session_id plumbing for conversation-aware routing (#48049). It's scoped tightly — just typed plumbing, no scheduling policy changes.

What reviewers should focus on:

1. Design — Is session_id: str | None the right shape? Should it be on EngineCoreRequest at all, or is there a better home?
2. Precedence — body.session_id > X-Session-ID > vllm_xargs — is this the right ordering? Should X-Correlation-ID be accepted too?
3. Scope — This deliberately excludes Rust frontend, gRPC, and consumer updates. Is that the right call for PR 1?
4. Edge cases — Parallel sampling (copy() inheritance), beam search forwarding, empty string handling.

10 tests pass. All linting (ruff check + format + typos) passes.

Happy to iterate on any feedback. Would especially value input from @DarkLight1337 (API surface), @njhill (engine internals), @russellb (Dynamo integration), and @robertgshaw2-redhat (scheduling direction).

Thank you for reviewing!

[ashusnapx]

Self-review: all checks pass. Ready for maintainer review.

[ashusnapx]

Hi @DarkLight1337 @robertgshaw2-redhat @njhill @russellb — the CI check requires a maintainer label to run. Since I'm a first-time contributor (0 merged PRs), could a maintainer please add the ready label to kick off the test suite?

Happy to address any feedback once the full test results are in.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ashusnapx.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[zhewenl]

done in #48048