[Bugfix] Add X-Session-ID from conversation_id in multi-turn benchmark

[tykow]

Set X-Session-ID in send_request when conversation_id is present. Keep the header unset when conversation_id is not provided.

Co-authored-by: GitHub Copilot

Purpose

Set the X-Session-ID HTTP header from conversation_id in the multi-turn benchmark client (send_request) when a conversation_id is present. The header is left unset when no conversation_id is provided, preserving existing behavior.

This enables session affinity when the benchmark runs behind the vLLM Router. The router's consistent_hash policy extracts its routing key using a hash-key priority list in which X-Session-ID is the highest-priority key. Sending it keeps all turns of a conversation on the same worker, maximizing KV-cache reuse for multi-turn workloads.

Reference: https://github.com/vllm-project/router/blob/main/docs/load_balancing/README.md#hash-key-priority

Related (not duplicate):

• feat(multi-turn-bench): add api_key and custom headers for multi turn benchmark #44516 adds api_key and generic custom headers; this PR specifically derives X-Session-ID from each request's conversation_id.
• [Bench] benchmark_serving_multi_turn: make non-standard conversation_id payload opt-in #43756 makes the conversation_id payload field opt-in; this PR concerns the routing header, not the payload field.

Test Plan

1. Static check: confirm the modified file has no lint/type errors.
2. Manual header verification: point the benchmark at a local mock/echo endpoint and confirm requests with a conversation_id include X-Session-ID, while requests without one omit it.
3. (Optional, with router) Run vllm-router --policy consistent_hash with 2+ workers, run the multi-turn benchmark, and confirm same-conversation_id turns route to the same worker via router logs.

Command:

python benchmarks/multi_turn/benchmark_serving_multi_turn.py \
  -i <input.json> -m <model> --url http://router:8000

Test Result

• Modified file passes editor diagnostics with no errors.
• Requests carry X-Session-ID: <conversation_id> when a conversation id is set; header is absent otherwise.

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AI assistance was used for this change. All changed lines were reviewed and validated by the submitter.

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[claude]

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[mergify]

Hi @tykow, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[github-actions]

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