[ryanolson]

Overview

The frontend mints its own request_id (#17032), and that is the key trace consumers should use to tie a request's rows together inside Dynamo. Two things were still missing on request_end for a plain request (one with no agent context):

• model. A consumer that keeps separate state per model, such as one that follows KV block reuse per model, needs it to route the row to the right state without first looking up the request.
• x_request_id. It is not a key inside Dynamo. It is the client's own id, from the x-request-id header, for joining a trace row to records kept outside Dynamo, such as a load generator's results or the client's logs.

Both were written only when the request had agent context. With this change every request_end row carries model, and x_request_id when the client sent the header.

Key Changes

About 25 lines of core logic, plus a test and a docs paragraph.

• lib/llm/src/request_trace/agent_context.rs: one helper, x_request_id_from_context, reads x-request-id from the distributed tracing context, else from the request context. The agent path and the plain path both use it, so they cannot disagree.
• lib/llm/src/request_trace/integration.rs and lib/llm/src/request_trace/record.rs: the request-end state keeps model and x_request_id, and emit_request_end writes them on the plain path.
• docs/fern/pages/reference/observability/request-traces.mdx: says that every row carries model, and x_request_id when the header is sent.

Validation

Case	Before	After
Plain request with an x-request-id header	request_end has no x_request_id, no model	both present
Request with agent context	both present	unchanged
Request with no x-request-id header	field absent	field absent

Test added: plain_request_end_carries_x_request_id_and_model in request_trace/integration.rs. It fails without the change.

cargo test --locked -p dynamo-llm --lib passes: 4,001 passed, 0 failed.

No new environment variables. The fields appear when request tracing is on (DYN_REQUEST_TRACE).

Issues and Ongoing Work

• Closes WCE-87 (part of WCE-75)
• Builds on fix(frontend): mint internal request ids on the server for HTTP and gRPC #17032 (merged), which made the frontend mint its own request_id.
• Two other open PRs touch request_trace/integration.rs for different reasons: fix(frontend): emit request_end trace records on native generate endpoints #17036 (request_end rows on native generate endpoints) and feat(request-trace): retain generated sequence hashes #14636 (output sequence hashes). No overlap in behavior.
• Two follow-up PRs stack on this one: feat(request-trace): record the engine's reused tokens on request_end rows #18104 (the engine_reused_tokens field on request_end) and feat(request-trace): add an opt-in request_start row with routing hashes #18105 (the opt-in request_start row).

Pull Request Checklist

• I used Dynamo's /pr-checklist skill.

[github-actions]

🌿 Fern Docs Preview: https://nvidia-preview-01a1a80f-1a90-70ef-b965-7a9b78513bff.docs.buildwithfern.com/dynamo/dev

[dynamo-ops]

/ok to test 967a494

[devin-ai-integration]

✅ Dynamo PR CI passed — run 41897439797 (attempt 1) on 967a494dc0

Gate checks: ✅ backend-status-check · ✅ deploy-status-check · ✅ dynamo-status-check

_{Posted automatically by Devin for run 41897439797. Updated on every full-CI run of this PR.}