Dspy Refine

Iterative Self-Improvement with dspy.Refine

Step 1: Understand the use case

Before writing code, clarify:

1. What module are you wrapping? (ChainOfThought, Predict, ReAct, custom module?)
2. What makes a good output? Can you express quality as a numeric score?
3. How many retries are acceptable? (cost/latency budget)
4. Is feedback useful? Would knowing why attempt 1 failed help attempt 2? If not, consider dspy.BestOfN instead.

When to use Refine (and when not to)

Migrating from dspy.Assert / dspy.Suggest? Both were removed in DSPy 3.x. dspy.Refine is their replacement and handles feedback-driven retries natively — no custom retry loop needed.

Use dspy.Refine when:

• Outputs must meet measurable quality criteria (format, length, accuracy)
• The LM can improve with feedback -- writing, format compliance, multi-criteria quality
• Quality is worth 2-5x cost for N attempts

Do not use Refine when:

• You have no clear way to score outputs -- use dspy.ChainOfThought instead
• You need human-in-the-loop feedback -- build a custom module with a human review step
• Speed matters more than quality -- use a single dspy.Predict call
• Attempts are independent and feedback would not help -- use dspy.BestOfN

Basic usage

Three things are needed: a module to wrap, a reward function, and a threshold.

import dspy

dspy.configure(lm=dspy.LM("openai/gpt-4o-mini"))  # or "anthropic/claude-sonnet-4-5-20250929", etc.

# 1. Define the module to refine
qa = dspy.ChainOfThought("question -> answer")

# 2. Define a reward function
# Takes (args_dict, prediction) -> float
def concise_answer(args, pred):
    """Reward one-word answers."""
    return 1.0 if len(pred.answer.split()) == 1 else 0.0

# 3. Wrap with Refine
refined_qa = dspy.Refine(
    module=qa,
    N=3,
    reward_fn=concise_answer,
    threshold=1.0,
)

# Use it -- same interface as the wrapped module
result = refined_qa(question="What is the capital of Belgium?")
print(result.answer)  # "Brussels"

Constructor parameters

dspy.Refine(
    module,          # dspy.Module -- required
    N,               # int -- required, max attempts
    reward_fn,       # Callable[[dict, Prediction], float] -- required
    threshold,       # float -- required, target reward score
    fail_count=None, # int | None -- max failures before error (defaults to N)
)

Writing reward functions

The reward function is the core of Refine. It receives two arguments:

1. args -- a dict of the inputs passed to the module (e.g., {"question": "What is..."})
2. pred -- the module's prediction object (access fields like pred.answer, pred.reasoning)

It must return a float. Higher is better.

Simple binary reward

def valid_json(args, pred):
    """Accept only valid JSON outputs."""
    import json
    try:
        json.loads(pred.output)
        return 1.0
    except (json.JSONDecodeError, TypeError):
        return 0.0

Graduated reward

Return partial scores to help Refine pick the best attempt even when none fully succeed:

def quality_score(args, pred):
    """Score answer quality on multiple criteria."""
    score = 0.0
    answer = pred.answer

    # Criterion 1: not empty
    if answer.strip():
        score += 0.3

    # Criterion 2: reasonable length (20-200 words)
    word_count = len(answer.split())
    if 20 <= word_count <= 200:
        score += 0.4

    # Criterion 3: addresses the question
    if args["question"].split()[0].lower() in answer.lower():
        score += 0.3

    return score

Using external validation

import re

def valid_email_extraction(args, pred):
    """Reward valid email addresses extracted from text."""
    emails = pred.emails if isinstance(pred.emails, list) else []
    if not emails:
        return 0.0
    email_pattern = r'^[\w.-]+@[\w.-]+\.\w+$'
    valid_count = sum(1 for e in emails if re.match(email_pattern, e))
    return valid_count / len(emails)

How iteration count (N) works

Each attempt runs the wrapped module at temperature=1.0 with a different rollout ID, producing diverse outputs. Refine's selection logic:

1. Run the module and score the output with reward_fn
2. If the score meets or exceeds threshold, return immediately
3. If not, generate feedback from the failure and try again
4. After N attempts, return the attempt with the highest reward score

Choosing N:

The sweet spot for most use cases is N=3 to N=5. Beyond 5, diminishing returns are common unless the reward function is very specific.

In practice, Refine with N=3 and a graduated reward function typically moves a pipeline from a 40-60% baseline pass rate (outputs meeting your quality bar on the first try) to 75-90%. The exact gain depends on how achievable the threshold is and how much the feedback mechanism can guide the LM toward improvement.

The feedback mechanism

What makes Refine different from random retries is feedback generation. When an attempt fails to meet the threshold:

1. Refine examines why the attempt scored below the threshold
2. It generates natural-language feedback describing the shortcoming
3. This feedback is included in the prompt for the next attempt
4. The LM uses this feedback to produce a better output

This means later attempts are informed by earlier failures. Attempt 3 knows what went wrong in attempts 1 and 2.

You do not write the feedback logic -- Refine handles it automatically based on your reward function's scores.

Refine vs BestOfN -- when to use which

Both modules run a wrapped module multiple times and select the best output, but they work differently:

Use Refine when the LM can improve with feedback -- writing tasks, format compliance, multi-criteria quality.

Use BestOfN when attempts are independent and feedback would not help -- creative generation, sampling diverse options, simple binary checks.

Wrapping custom modules

Refine works with any dspy.Module, not just built-in ones:

class Summarizer(dspy.Module):
    def __init__(self):
        self.summarize = dspy.ChainOfThought("article -> summary")

    def forward(self, article):
        return self.summarize(article=article)


def good_summary(args, pred):
    """Score summary quality."""
    summary = pred.summary
    article = args["article"]
    score = 0.0

    # Shorter than original
    if len(summary) < len(article) * 0.3:
        score += 0.5

    # At least 2 sentences
    if summary.count('.') >= 2:
        score += 0.5

    return score


refined_summarizer = dspy.Refine(
    module=Summarizer(),
    N=3,
    reward_fn=good_summary,
    threshold=0.8,
)

result = refined_summarizer(article="Long article text here...")
print(result.summary)

Gotchas

• Claude uses binary reward functions (0 or 1) instead of graduated scores. Binary rewards mean Refine cannot distinguish between a near-miss and a total failure when no attempt hits the threshold. Use graduated floats (0.0 to 1.0) with partial credit for each criterion so Refine returns the best near-miss.
• Claude puts LM calls inside reward functions for "smarter" scoring. The reward function runs on every attempt (up to N times), so an LM call inside it doubles or triples your cost. Use deterministic checks (regex, AST parsing, length checks) in reward functions. Reserve LM-as-judge for the final evaluation after Refine returns.
• Claude sets threshold=1.0 with multi-criteria reward functions. If your reward function scores 4 weighted criteria, achieving a perfect 1.0 is unlikely. Refine then burns through all N attempts and returns a near-miss anyway. Set the threshold to 0.8 or the realistic "good enough" score for your criteria.
• Claude confuses Refine with BestOfN and uses them interchangeably. The key difference is feedback: Refine generates feedback from failures and feeds it into subsequent attempts (sequential, improving). BestOfN runs independent attempts with no cross-attempt learning. Use Refine when later attempts can learn from earlier failures; use BestOfN when attempts are inherently independent.
• Claude wraps a Predict module with Refine for tasks that need reasoning. Refine improves outputs through feedback, but dspy.Predict does not expose a reasoning field. Use dspy.ChainOfThought as the inner module so the feedback loop has reasoning to critique and improve.

Cross-references

Install any skill: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill <name>

• BestOfN for independent sampling without feedback -- see /dspy-best-of-n
• Chain of thought reasoning as the inner module -- see /dspy-chain-of-thought
• Checking and validating outputs with assertions -- see /ai-checking-outputs
• Improving accuracy with optimization -- see /ai-improving-accuracy
• Writing content that benefits from iterative refinement -- see /ai-writing-content
• Install /ai-do if you do not have it — it routes any AI problem to the right skill and is the fastest way to work: npx skills add lebsral/DSPy-Programming-not-prompting-LMs-skills --skill ai-do

Additional resources

• dspy.Refine API docs
• For API details, see reference.md
• For worked examples, see examples.md