System One Model Selection

Overview

systemone asks a System One model which of a routing decision's modelRefs should answer the request. You describe each candidate in a sentence; the model reads the request and gives every candidate a probability, and the most likely one answers.

Key Advantages

• Chooses with a model that reads the whole request.
• Reuses the Router's decision model by default, so choosing loads no other model.
• Reports a probability per candidate, visible in selection traces.
• Falls back to the first modelRef whenever the model cannot answer in time.

What Problem Does It Solve?

A fixed order ignores the request, and asking a chat model to pick needs a generation round trip and output parsing. A System One model answers the same question in one pass, with probabilities and no text generation.

When to Use

Use it when a decision has two or more candidates whose strengths you can describe in a sentence each, and the right choice depends on what the request asks. Prefer static when the order never changes and multi_factor when the choice is about cost, latency or load.

Configuration

Without a deployment, the Router's decision model chooses (global.model_catalog.system.decision_model, Vela 3.0 vela3-fast unless you choose a size). It is the model that already answers the request's built-in signals and its systemone questions that name no deployment, so choosing the model needs no second copy of it:

global:
  model_catalog:
    deployments:
      primary:
        provider: model_runtime
        artifact: vllm-sr/Vela-2.0-4B
        device: rocm
    system:
      decision_model:
        deployment: primary
recipes:
  - name: default
    signals:
      systemone:
        - name: request_kind
          question:
            type: choice
            instructions: What kind of request is this?
            choices:
              - key: code
                description: Writing, reviewing or debugging code
              - key: chat
                description: Anything else
    decisions:
      - name: code-route
        priority: 100
        rules:
          operator: AND
          conditions:
            - type: systemone
              name: request_kind
              label: code
        modelRefs:
          - model: qwen3-8b
            use_reasoning: false
          - model: qwen3-32b
            use_reasoning: true
        algorithm:
          type: systemone
          systemone:
            instructions: Which model should answer this request?
            candidates:
              qwen3-8b: Fast general model for routine code
              qwen3-32b: Strong reasoning model for hard code
            timeout_ms: 1000

The selector asks after the decision matches, in its own call to the same deployment. The default binding may select Vela or Decision 1.0/2.0; the selected resource must support the choice task. An explicit deployment on the selector overrides the default binding.

Configurations written for earlier releases name this selector decision (algorithm.type: decision with an algorithm.decision block). They still load, with a renamed_algorithm_type warning, and vllm-sr config migrate rewrites them to systemone.

To let another model choose, such as a Decision 2.0 model, name a model_runtime deployment:

global:
  model_catalog:
    deployments:
      decision-kai:
        provider: model_runtime
        artifact: vllm-sr/Decision-2.0-Kai-0.6B
recipes:
  - name: default
    signals:
      systemone:
        - name: request_kind
          deployment: decision-kai
          question:
            type: choice
            instructions: What kind of request is this?
            choices:
              - key: code
                description: Writing, reviewing or debugging code
              - key: chat
                description: Anything else
    decisions:
      - name: code-route
        priority: 100
        rules:
          operator: AND
          conditions:
            - type: systemone
              name: request_kind
              label: code
        modelRefs:
          - model: qwen3-8b
            use_reasoning: false
          - model: qwen3-32b
            use_reasoning: true
        algorithm:
          type: systemone
          systemone:
            deployment: decision-kai
            instructions: Which model should answer this request?
            candidates:
              qwen3-8b: Fast general model for routine code
              qwen3-32b: Strong reasoning model for hard code
            timeout_ms: 1000

A named deployment must use provider: model_runtime, or provider: systemone for a remote System One API; a selector without its own timeout_ms then takes the deployment's. The decision needs 2–255 different modelRefs, and candidates may describe only those models; a model without a description uses its configured description. When the model is not ready, answers late, is overloaded or returns an invalid answer, the router records a selection fallback and uses the first modelRef. The chosen model is reported in the x-vsr-selected-model response header. Routing Preview asks the System One model the same question and reports its choice as the selected model, without calling a backend.

See Choose a deployment to choose a model and where it runs.