VSR Routing Headers

This page documents the public x-vsr-* headers emitted by vLLM Semantic Router for routing observability, replay correlation, and debugging.

Emission Rules

The router splits headers across two surfaces:

• Default surface — rides on every non-cache-hit response: the keystone headers (x-vsr-schema-version, x-vsr-response-path), the final routing facts (x-vsr-selected-recipe, x-vsr-selected-decision, x-vsr-selected-confidence, x-vsr-selected-algorithm, x-vsr-selected-model) and the x-vsr-replay-id entry point. The selected recipe also rides on recipe-scoped immediate responses such as cache hits. The client/upstream protocol markers ride here only on cross-protocol handling, and x-vsr-protocol-warnings only when warnings exist.
• Debug surface — the intermediate decision/classification details, the matched-signal headers, the tool-selection observability headers and the retention directive headers (x-vsr-retention-*) are demoted off the default surface (#2205). They are emitted inline only when the request sets x-vsr-debug: true, and remain recoverable from the replay record via x-vsr-replay-id. The retention directive's runtime effects (cache write skip, TTL override, model-switch stay) are applied internally and unaffected by the header demotion.

Decision and matched-signal headers additionally require all of the following:

1. The upstream response is successful (2xx).
2. The response was not served from semantic cache.
3. The router evaluated a routing decision or signal for the request.

Cache-hit responses can emit cache headers, but they do not re-run routing and therefore do not attach fresh matched-signal headers.

Request Headers

Header	Direction	Description
x-session-id	request	Stable client-provided session identifier for Chat Completions. Router Learning protection uses this, together with the configured conversation identity, to reason about stay-vs-switch decisions across turns.
x-conversation-id	request	Stable client-provided conversation or agent-run identifier. Router Learning protection uses this by default when scope: conversation.
x-vsr-skip-processing	request	Opts a request out of router processing when global.router.skip_processing.enabled is enabled. Use value true.
x-vsr-debug	request	Opts the request into verbose/debug response headers — headers the contract otherwise omits or demotes to replay are emitted inline for that request. Use value true.

Protocol And Replay Headers

Header	Description
x-vsr-client-protocol	Inbound protocol shape seen by the router, for example openai or anthropic. Emitted only on cross-protocol handling (client protocol differs from upstream), or when x-vsr-debug is set.
x-vsr-upstream-protocol	Protocol shape sent to the selected upstream backend. Emitted only on cross-protocol handling, or when x-vsr-debug is set.
x-vsr-protocol-warnings	Comma-separated protocol translation warnings encoded as severity;reason;field. Emitted only when warnings exist.
x-vsr-replay-id	Opaque router replay record identifier for correlating a response with replay/Insights data.

Response Warnings

Header	Description
x-vsr-response-warnings	Comma-separated response-quality warning codes for the completion, in fixed order: hallucination, unverified_factual, response_jailbreak. Emitted only when at least one applies.

This header consolidates the v0.3 x-vsr-hallucination-detected, x-vsr-unverified-factual-response, and x-vsr-response-jailbreak-detected headers. The per-warning detail (hallucination spans, jailbreak type/confidence, fact-check verification context) is kept in the replay record rather than the response headers, recoverable via x-vsr-replay-id; the individual warning and detail headers are removed from the v0.4 contract.

Decision Headers

The final routing facts ride on the default surface; the intermediate details (including the Router Learning observability headers) are demoted to x-vsr-debug (#2205).

Header	Surface	Description	Example
x-vsr-selected-recipe	default	Routing isolation scope selected by an entrypoint or auto/looper alias. Omitted for concrete backend passthrough.	privacy-first
x-vsr-selected-decision	default	Final decision selected by the decision engine.	formal_math_proof
x-vsr-selected-confidence	default	Confidence score for the selected decision.	0.9100
x-vsr-selected-algorithm	default	Model-selection algorithm used after the decision matched.	router_dc
x-vsr-selected-model	default	Logical model alias selected by the router.	qwen/qwen3.5-rocm
x-vsr-selected-category	debug	Domain/category classifier result when domain routing runs.	math
x-vsr-selected-reasoning	debug	Reasoning mode selected for the request.	on
x-vsr-selected-modality	debug	Modality result and optional method.	AR;classifier
x-vsr-session-phase	debug	Protection trace phase from the selected routing policy. Detailed learning actions are exposed through the x-vsr-learning-* headers and Router Replay.	user_turn, tool_loop, provider_state
x-vsr-learning-methods	debug	Router Learning methods summarized by this response. Full score/cache details live in Router Replay.	adaptation,protection
x-vsr-learning-actions	debug	Method-keyed compact learning actions.	adaptation=propose_switch,protection=allow_switch
x-vsr-learning-scopes	debug	Method-keyed identity scopes used by learning.	protection=conversation
x-vsr-learning-reasons	debug	Method-keyed machine-readable reasons for actions.	adaptation=sampled_win,protection=switch_allowed
x-vsr-injected-system-prompt	debug	Whether a system-prompt plugin injected text into the request.	true

For UI display guidance, translate x-vsr-learning-actions into user-facing phrases such as tool/protocol pinned, model switched, or learning bypassed. Fresh conversation or session-start diagnostics are usually useful only in debug views, where they should be shown as neutral status text rather than a primary route state.

Matched Signal Headers

Matched signal headers contain comma-separated rule names. They are demoted to the x-vsr-debug surface (#2205) and omitted when the corresponding signal family did not match.

Header	Signal family
x-vsr-matched-keywords	keyword
x-vsr-matched-embeddings	embedding
x-vsr-matched-domains	domain
x-vsr-matched-fact-check	fact_check
x-vsr-matched-user-feedback	user_feedback
x-vsr-matched-reask	reask
x-vsr-matched-preference	preference
x-vsr-matched-language	language
x-vsr-matched-context	context
x-vsr-context-token-count	Context token estimate used by context
x-vsr-matched-structure	structure
x-vsr-matched-complexity	complexity
x-vsr-matched-modality	modality
x-vsr-matched-authz	authz
x-vsr-matched-jailbreak	jailbreak
x-vsr-matched-pii	pii
x-vsr-matched-kb	kb
x-vsr-matched-conversation	conversation
x-vsr-matched-event	event

Projection Headers

Header	Description
x-vsr-matched-projections	Comma-separated projection mapping outputs that matched the request.

Projection scores and full projection traces are stored in router replay records rather than expanded into response headers. Use x-vsr-replay-id to inspect those details in the dashboard or replay APIs.

Cache And Plugin Headers

x-vsr-cache-hit and x-vsr-fast-response mark how an immediate response was produced and ride on the default surface. The cache-similarity and tool-selection observability headers are demoted to the x-vsr-debug surface (#2205).

Header	Surface	Description
x-vsr-cache-hit	default	Response came from semantic cache.
x-vsr-fast-response	default	Response was generated by the fast_response plugin without an upstream model call.
x-vsr-cache-similarity	debug	Similarity score from the semantic-cache lookup.
x-vsr-tools-strategy	debug	Semantic tool-selection retriever strategy used for the request.
x-vsr-tools-confidence	debug	Highest tool-selection retriever similarity score.
x-vsr-tools-latency-ms	debug	Tool-selection retriever latency in milliseconds.

Example Response

Default surface — keystone headers, final routing facts and the replay-id entry point:

HTTP/1.1 200 OK
Content-Type: application/json
x-vsr-schema-version: 2
x-vsr-response-path: upstream
x-vsr-selected-recipe: default
x-vsr-selected-decision: critical_event_tool_session_route
x-vsr-selected-confidence: 1.0000
x-vsr-selected-algorithm: static
x-vsr-selected-model: anthropic/claude-opus-4.6
x-vsr-replay-id: replay_01J...

With x-vsr-debug: true on the request, the demoted intermediate details and matched signals are emitted inline as well:

HTTP/1.1 200 OK
Content-Type: application/json
x-vsr-schema-version: 2
x-vsr-response-path: upstream
x-vsr-selected-recipe: default
x-vsr-selected-decision: critical_event_tool_session_route
x-vsr-selected-confidence: 1.0000
x-vsr-selected-algorithm: static
x-vsr-selected-model: anthropic/claude-opus-4.6
x-vsr-session-phase: tool_loop
x-vsr-matched-conversation: active_tool_use
x-vsr-matched-event: critical_payment_event
x-vsr-matched-projections: verification_required
x-vsr-replay-id: replay_01J...

Notes

• x-vsr-matched-projections is the v0.3 projection header. The old singular form is not part of the public v0.3 contract.
• Recipe names scope local signal, projection, decision, cache, replay, metric, and learning/session identities. Use x-vsr-selected-recipe together with the local decision/signal names when correlating a response with Insights or metrics.
• event is the public signal type used by decisions and DSL. Canonical YAML stores event rules under routing.signals.events, matching other plural signal containers.
• Router Learning uses router-owned online state internally. Users enable online model-choice learning through global.router.learning.adaptation, enable stability protection through global.router.learning.protection, pass stable identity headers, and optionally set routing.decisions[].adaptations.mode, component modes, or adaptations.adaptation.candidate_set. scope: conversation protects one x-conversation-id; scope: session protects the broader x-session-id. The old routing.decisions[].algorithm.session_aware shape is not part of the public contract.