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VSR Decision Tracking Headers

This document describes the VSR (Vector Semantic Router) decision tracking headers that are automatically added to successful responses for debugging and monitoring purposes.

Overview​

The semantic router automatically adds response headers to track VSR decision-making information. These headers help developers and operations teams understand how requests are being processed and routed.

Headers are only added when:

  1. The request is successful (HTTP status 200-299)
  2. The request did not hit the cache
  3. VSR made routing decisions during request processing

Headers Added​

x-vsr-selected-category​

Description: The category selected by VSR during classification.

Example Values:

  • math
  • business
  • biology
  • computer_science

When Added: When VSR successfully classifies the request into a category.

x-vsr-selected-reasoning​

Description: Whether reasoning mode was determined to be used for this request.

Values:

  • on - Reasoning mode was enabled
  • off - Reasoning mode was disabled

When Added: When VSR makes a reasoning mode decision (both for auto and explicit model selection).

x-vsr-selected-model​

Description: The model selected by VSR for processing the request.

Example Values:

  • deepseek-v31
  • phi4
  • gpt-4

When Added: When VSR selects a model (either through auto-routing or explicit model specification).

Use Cases​

Debugging​

These headers help developers understand:

  • Which category VSR classified their request into
  • Whether reasoning mode was applied
  • Which model was ultimately selected

Monitoring​

Operations teams can use these headers to:

  • Track category distribution across requests
  • Monitor reasoning mode usage patterns
  • Analyze model selection patterns

Analytics​

Product teams can analyze:

  • Request categorization accuracy
  • Reasoning mode effectiveness
  • Model performance by category

Example Response​

HTTP/1.1 200 OK
Content-Type: application/json
x-vsr-selected-category: math
x-vsr-selected-reasoning: on
x-vsr-selected-model: deepseek-v31

{
  "id": "chatcmpl-123",
  "object": "chat.completion",
  "created": 1677652288,
  "model": "deepseek-v31",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "The derivative of x^2 + 3x + 1 is 2x + 3."
      },
      "finish_reason": "stop"
    }
  ]
}

When Headers Are NOT Added​

Headers are not added in the following cases:

  1. Cache Hit: When the response comes from cache, no VSR processing occurs
  2. Error Responses: When the upstream returns 4xx or 5xx status codes
  3. Missing VSR Information: When VSR decision information is not available (shouldn't happen in normal operation)