VSR routing headers

The router uses these request and response headers for session continuity, routing observability, replay correlation, and opt-in debugging.

What appears by default

The router splits headers across two surfaces:

• Default surface — every non-cache-hit response includes x-vsr-schema-version and x-vsr-response-path. Successful routed responses can also include the final recipe, decision, confidence, algorithm, model, routing latency, cost, and replay id. Protocol markers appear when translation occurs; protocol warnings appear only when there are warnings.
• Debug surface — intermediate classification details, matched signals, tool-selection metrics, and x-vsr-retention-* directives appear inline only when the request sets x-vsr-debug: true. When replay is enabled, the same diagnostic context remains available through x-vsr-replay-id.

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 the response 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-claude-code-session-id	request	Conversation identifier supplied by Claude Code on Messages API requests. x-session-id takes precedence when both are present.
x-disable-router-memory	request	Set to true when the client already injects memory and router-managed memory would duplicate it.
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.

Per-warning detail, such as hallucination spans or jailbreak confidence, is kept in the replay record instead of being expanded into response headers.

Decision headers

Final routing facts use the default surface. Intermediate details, including Router Learning observability, require x-vsr-debug.

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.	support
x-vsr-selected-decision	default	Final decision selected by the decision engine.	complex-request
x-vsr-selected-confidence	default	Model-derived score for the selected decision. Absent when the decision rests on policy leaves, aggregates several evidence leaves, or resolves through an error policy.	0.9100
x-vsr-applied-unknown-policy	default	Decisions whose unknown result was resolved by rules.on_unknown, as decision=policy pairs. Also set on the fail_request 503.	guarded=no_match
x-vsr-selected-algorithm	default	Model-selection algorithm used after the decision matched.	static
x-vsr-selected-model	default	Logical model alias selected by the router.	reasoning-model
x-vsr-effective-input-tokens	default	Actual selected-backend rendered input tokens for the finalized automatic-output dispatch, including its chat template.	512
x-vsr-effective-max-output-tokens	default	Resolved output token limit sent in that automatic-output dispatch, including reasoning. This is a budget, not consumed tokens.	261632
x-vsr-routing-latency-ms	default	Time the router spent choosing the model, in milliseconds with sub-millisecond precision.	0.412
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.

The two effective-token headers are emitted together on successful upstream responses only when automatic output was resolved. They are available in the initial headers for both streaming and buffered responses. Explicit output limits, cache hits, skipped processing, and Looper responses omit them. Their values come from the final dispatch after request changes and backend rendering; they are not estimates or the model's configured maximum context length.

Matched signal headers

Matched signal headers contain comma-separated rule names. They require x-vsr-debug and are omitted when that 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
x-vsr-matched-input-modality	input_modality

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 through the authenticated Router management API; public inference listeners do not serve replay records.

Retention headers

When a matched decision emits a retention directive, debug responses expose the fields that were set. These headers help operators verify policy wiring; clients should not use them as commands.

Header	Description
x-vsr-retention-drop	Whether the response should be excluded from response-cache retention.
x-vsr-retention-ttl-turns	Decision-level retention lifetime expressed in conversation turns.
x-vsr-retention-keep-current-model	Whether the policy asks later routing to keep the current model.
x-vsr-retention-prefer-prefix	Whether prefix retention is preferred when the runtime supports it.

Unset fields are omitted. Cache hits do not emit these headers because no decision was evaluated for that response.

Cross-model KV transfer headers (issue #2976)

When the router attempts cross-model KV reuse on a model switch, it injects request-side hints on the upstream call. The target vLLM KVConnector plugin reports the outcome on the response. These headers are internal to the router and inference pool; clients should not depend on them.

Request (router → backend):

Header	Description
x-vsr-kv-source-pod	gRPC address of the pod holding the source model's KV cache.
x-vsr-kv-cache-id	Opaque session or cache identifier for the source KV block.
x-vsr-kv-mapper-id	Published ridge-mapper artifact for the source→target model pair.

Response (backend → router):

Header	Description
x-vsr-kv-transfer-status	applied, fallback_reprefill, or unsupported. Absent ⇒ unsupported.

Cost headers

On a buffered (non-streaming) response, the router prices the usage the model reported with the served model's pricing configuration. This is a configured-price figure, not a provider bill. Streamed responses and models without pricing omit both headers.

Header	Surface	Description	Example
x-vsr-cost	default	Usage tokens multiplied by the served model's configured prices.	0.000054
x-vsr-cost-currency	default	Currency of x-vsr-cost, from pricing.currency.	USD

Cache and plugin headers

x-vsr-cache-hit and x-vsr-fast-response identify an immediate response on the default surface. Cache-similarity, tool-selection metrics, and prompt-cache receipts require x-vsr-debug.

Header	Surface	Description
x-vsr-cache-hit	default	Response came from the response 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 response-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.
x-vsr-prompt-cache-action	debug	Outcome of the prompt_cache plugin: inserted, preserved, skipped, or rejected.
x-vsr-prompt-cache-reason	debug	Machine-readable reason for a non-inserted outcome: caller_markers, no_eligible_target, or unsupported_target. Omitted when the action is inserted.
x-vsr-prompt-cache-inserted	debug	Count of router-inserted cache markers, at most 2 (one instruction block, one tool).
x-vsr-prompt-cache-preserved	debug	Count of caller-supplied cache markers found anywhere in the request.

These headers are content-free: they report counts and outcomes, never the cached text itself, and they never claim a cache hit or provider-side savings.

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: complex-request
x-vsr-selected-algorithm: static
x-vsr-selected-model: reasoning-model
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: complex-request
x-vsr-selected-algorithm: static
x-vsr-selected-model: reasoning-model
x-vsr-session-phase: tool_loop
x-vsr-matched-context: long-context
x-vsr-matched-projections: use-reasoning-model
x-vsr-replay-id: replay_01J...

Compatibility and interpretation

• Use x-vsr-schema-version before parsing optional headers; the current value is 2.
• x-vsr-matched-projections is the projection header. The singular form is not part of the public 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.