Hallucination checks

Vela 1.0 Halu reads the context a request carried (tool results, retrieved documents), the user's question and the model's answer, and marks the spans of the answer that the context does not support. The hallucination plugin then adds a warning header, a note in the response body, or only records the result.

It checks support, not truth: an answer can be supported by wrong context, and an answer with no context to check is reported as unverified instead.

Turn it on

Declare the observation as a signal and enforce it with the plugin on the routes that should be checked. Vela FactCheck decides first whether a request makes claims worth checking.

routing:
  signals:
    fact_check:
      - name: needs_fact_check
        description: Requests that make factual claims.
    hallucination:
      - name: ungrounded_claims
        description: Claims the context does not support.
  decisions:
    - name: grounded-answers
      priority: 100
      rules:
        operator: AND
        conditions:
          - type: fact_check
            name: needs_fact_check
      modelRefs:
        - model: answer-model
      plugins:
        - type: hallucination
          configuration:
            enabled: true
            hallucination_action: header
            unverified_factual_action: header
            include_hallucination_details: true
global:
  model_catalog:
    modules:
      hallucination_mitigation:
        enabled: true

The router runs FactCheck and Halu on the CPU. Halu reads up to 8,192 tokens of context, question and answer together; longer context is shortened from its end so the answer is always checked.

Choose where it runs

The binding is hallucination_detector and reads text spans of the answer (token_spans.v1):

global:
  model_catalog:
    deployments:
      vela-halu:
        provider: model_runtime
        artifact: vllm-sr/Vela-1.0-Encoder-307M-Halu
        device: cpu
        input:
          max_tokens: 8192
          overflow: reject
    bindings:
      hallucination_detector:
        deployment: vela-halu
        contract: token_spans.v1

Check it

vllm-sr serve vllm-sr/Vela-1.0-Encoder-307M-Halu --device cpu --port 8100
curl -s localhost:8100/v1/classify -H 'content-type: application/json' -d '{
  "input": [{"context": "The Eiffel Tower is 330 metres tall and stands in Paris.",
             "question": "How tall is the Eiffel Tower?",
             "answer": "The Eiffel Tower is 450 metres tall."}]
}'

The result lists the unsupported spans of the answer, here the height, with their character offsets in the answer. Through the router, the response carries the hallucination warning headers and x-vsr-matched-hallucination.

Earlier releases could add an NLI explanation per span. That explainer is retired; see Migrate.