Composo delivers deterministic, accurate evaluation for LLM applications through purpose-built generative reward models. Unlike unreliable LLM-as-judge approaches, our specialized models provide consistent, precise scores you can trust—with just a single sentence criteria.

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Why Composo?

Composo is designed to empower engineering & product teams to ship 10x faster, with complete confidence. What we hear from our customers that they’re looking to achieve is:
“Know exactly where our AI fails or hallucinates, and how to fix it”
“Instantly identify how each prompt and model change impacts quality”
“Prove reliability with quantitative metrics our customers trust”
It’s not possible to do this with manual evals which don’t scale, nor LLM based evals which are extremely unreliable. What Composo’s purpose built models provide is evals that are accurate, precise & deterministic. You simply plug in to any LLM generation, retrieval or tool call, and write a single sentence to create any custom criteria. Check out our blog for more on the theory behind it, our detailed validation results & in depth guides for how to evaluate a range of use cases, based on customers we’ve worked with.

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What are Evaluation Criteria?

Evaluation criteria are simple, single-sentence instructions that tell Composo exactly what to evaluate in your LLM outputs. You write them in plain English, starting with either “Reward responses that…” or “Penalize responses that…”. Example:
  • Input: A customer service conversation
  • Criteria: "Reward responses that express appropriate empathy when the user is frustrated"
  • Result: Composo analyzes the response and returns a score from 0-1 based on how well it meets this criteria
This single sentence is all you need - no complex rubrics, no prompt engineering, no unreliable LLM judges. Just describe what good (or bad) looks like, and Composo handles the rest.

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Technical Architecture

Composo uses a generative reward model architecture that combines:
  • A custom-trained reasoning model that analyzes inputs against criteria
  • A specialized scoring model that produces calibrated, deterministic scores
This approach achieves 89% accuracy on real-world evaluation tasks compared to 72% for LLM-as-judge methods, with a 60% reduction in error rate.

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Key Differences from LLM-as-Judge

  1. Deterministic: Same inputs always produce identical scores
  2. Calibrated: Scores meaningfully distributed across 0-1 range
  3. Consistent: Robust to minor wording changes in criteria
  4. Accurate: Trained specifically for evaluation, not general text generation

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Core Capabilities

Composo offers two types of evaluation metrics:
  1. Response Evaluation - Assess the quality of LLM outputs
  2. Tool Call Evaluation - Measure how well LLMs use functions and tools
Each evaluation type supports two scoring methods:
  • Reward: Continuous scoring from 0 to 1
  • Binary: Pass/fail assessment

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How It Works

The Evals API offers two primary evaluation mechanisms:
  1. Reward Score Evaluation: For continuous scoring (recommended for most use cases). Learn more »
  2. Binary Evaluation: Use for simple pass/fail assessments against specific rules or policies. Perfect for content moderation and clear-cut criteria. Learn more »
You specify what to evaluate through your criteria. So for the reward endpoint:
  • "Reward responses that..." - Positive response evaluation
  • "Penalize responses that..." - Negative response evaluation
And for the binary endpoint:
  • "Response passes if..." - Positive response evaluation
  • "Response fails if..." - Negative response evaluation

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Message Format

Both endpoints accept the same message format: 
{
    "messages": [
        {"role": "system", "content": "..."},
        {"role": "user", "content": "..."},
        {"role": "assistant", "content": "..."},
        {"role": "function", "name": "...", "content": "..."},
        {"role": "assistant", "content": "..."}
    ],
    "evaluation_criteria": "Reward responses that..."
}

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Get Started with Composo

Ready to see how Composo compares to your current evaluation approach? Book in for a quick demo here. For any questions or to learn more about how Composo can support your needs, reach out to us at [email protected].