The Requirements System

How Requirement, ValidationResult, and the IVR loop work together to enforce constraints on generative output.

Looking to use this in code? See Write Custom Verifiers for practical examples and API details.

Requirements are Mellea’s mechanism for enforcing constraints on generative output. They serve two roles simultaneously: they appear in the prompt so the model knows what to aim for, and they are evaluated after generation so Mellea can detect and repair failures automatically.

This page explains the requirements system in depth. For a quick introduction, see The Instruction Model.

What a requirement is

A Requirement is a Component that wraps a natural-language description and an optional validation function. During the instruct–validate–repair (IVR) loop:

1. Mellea renders the requirement descriptions into the prompt alongside the instruction.
2. After the model generates output, each requirement is validated against that output.
3. If any requirement fails, Mellea sends the model a repair request, listing which requirements failed and why.
4. The loop retries up to loop_budget times (default: 2).

from mellea.core import Requirement

# Simplest form: natural-language string.
# Mellea uses LLM-as-a-judge to check it.
r = Requirement("The email should have a salutation.")

Passing plain strings directly to instruct() is equivalent — they are converted to Requirement objects internally:

import mellea

m = mellea.start_session()
email = m.instruct(
    "Write an email inviting the team to a meeting.",
    requirements=["The email should have a salutation.", "Fewer than 150 words."],
)

req() and check() shorthands

req() and check() are concise constructors from mellea.stdlib.requirements:

from mellea.stdlib.requirements import check, req

# req() creates a standard Requirement (description included in the prompt)
r1 = req("The email should have a salutation.")

# check() creates a check-only Requirement (description NOT included in the prompt)
r2 = check("Do not mention purple elephants.")

The difference matters: when check_only=True, the requirement description is evaluated after generation but not embedded in the prompt. This avoids the purple elephant effect — where mentioning something in a negative instruction (e.g., “do not mention purple elephants”) paradoxically increases the chance the model produces it.

Use req() for positive constraints you want the model to aim for. Use check() for negative or hard-to-explain constraints that are better left out of the prompt.

Custom validation functions

For deterministic checks, attach a validation_fn. Mellea skips LLM-as-a-judge and runs your function directly:

from mellea import start_session
from mellea.core import Requirement
from mellea.stdlib.requirements import simple_validate

word_limit = Requirement(
    "Fewer than 100 words.",
    validation_fn=simple_validate(lambda output: len(output.split()) < 100),
)

m = start_session()
email = m.instruct(
    "Write an email to {{name}}.",
    requirements=[word_limit],
    user_variables={"name": "Olivia"},
)

simple_validate is a convenience wrapper. It accepts a function that receives the most recent model output as a string and returns either:

• bool — pass or fail; no reason is captured
• tuple[bool, str] — pass/fail plus a reason string that Mellea includes in the repair request

from mellea.stdlib.requirements import simple_validate

# Boolean return
is_lowercase = simple_validate(lambda x: x.lower() == x)

# Tuple return — the reason is sent to the model on failure
within_limit = simple_validate(
    lambda x: (len(x.split()) < 100, f"Output is {len(x.split())} words; must be < 100.")
)

ValidationResult in depth

simple_validate produces ValidationResult objects automatically. When you write a full validation function directly, you construct ValidationResult yourself:

from mellea.core import Context, ValidationResult


def validate_json(ctx: Context) -> ValidationResult:
    """Accept output only if it is valid JSON."""
    import json

    output = ctx.last_output()
    text = output.value if output is not None else ""
    try:
        json.loads(text)
        return ValidationResult(True)
    except json.JSONDecodeError as exc:
        return ValidationResult(False, reason=f"Invalid JSON: {exc}")

The validation_fn signature is Callable[[Context], ValidationResult]. The Context object gives you access to the full session state if needed — not just the last output.

ValidationResult fields:

Field	Type	Description
result	bool	Whether the requirement passed.
reason	str | None	Human-readable explanation, included in repair requests.
score	float | None	Optional numeric score from your validator.
thunk	ModelOutputThunk | None	The model output used, if your validator ran a backend call.
context	Context | None	The context snapshot at validation time.

The reason field is the most useful in practice — a clear reason string helps the model make a targeted repair rather than regenerating blindly.

Preconditions in generative functions

The @generative decorator supports precondition_requirements alongside the standard requirements. Preconditions are validated against the inputs to the function before generation starts. If they fail, Mellea raises PreconditionException immediately — no generation attempt is made and no IVR loop runs.

from typing import Literal

from mellea import generative, start_session
from mellea.core import Requirement
from mellea.stdlib.components.genslot import PreconditionException
from mellea.stdlib.requirements import simple_validate
from mellea.stdlib.sampling import RejectionSamplingStrategy


@generative
def classify_sentiment(text: str) -> Literal["positive", "negative", "neutral"]:
    """Classify the sentiment of the text."""


m = start_session()

# Precondition: validate inputs before the model is called
try:
    result = classify_sentiment(
        m,
        text="I love this!",
        precondition_requirements=[
            Requirement(
                "Input must be fewer than 200 characters.",
                validation_fn=simple_validate(lambda x: len(x) < 200),
            )
        ],
        requirements=["Avoid returning 'neutral' unless the sentiment is genuinely ambiguous."],
        strategy=RejectionSamplingStrategy(),
    )
    print(result)
except PreconditionException as e:
    print(f"Precondition failed: {e}")
    for val in e.validation:
        print(f"  - {val.reason}")

PreconditionException.validation is a list of ValidationResult objects for every requirement that failed, giving you a complete picture of what went wrong.

Note: precondition_requirements require a strategy to be specified (e.g., RejectionSamplingStrategy()). Without a strategy the precondition check is skipped with a warning.

Inspecting validation results

When you use return_sampling_results=True, instruct() returns a SamplingResult instead of a ModelOutputThunk. This exposes per-attempt validation results:

from mellea import start_session
from mellea.stdlib.requirements import req, simple_validate
from mellea.stdlib.sampling import RejectionSamplingStrategy

m = start_session()
result = m.instruct(
    "Write a short note to {{name}}.",
    requirements=[
        req(
            "Use only lower-case letters.",
            validation_fn=simple_validate(
                lambda x: (x.lower() == x, "Output contains upper-case characters.")
            ),
        ),
    ],
    strategy=RejectionSamplingStrategy(loop_budget=3),
    user_variables={"name": "Olivia"},
    return_sampling_results=True,
)

if result.success:
    print(str(result.result))
else:
    # Inspect why each attempt failed
    for attempt_idx, attempt_validations in enumerate(result.sample_validations):
        print(f"Attempt {attempt_idx + 1}:")
        for requirement, val_result in attempt_validations:
            status = "PASS" if val_result else "FAIL"
            print(f"  [{status}] {requirement.description}: {val_result.reason}")

SamplingResult.sample_validations is a list of attempts, each containing a list of (Requirement, ValidationResult) tuples. SamplingResult.result_validations gives you the same for the final selected output only.

LLM-as-a-judge vs custom validators

Approach	When to use
Plain string requirement	Subjective or hard-to-code constraints (“be polite”, “stay on topic”).
simple_validate(lambda ...)	Simple deterministic checks (length, regex, JSON parse).
Full validation_fn	Multi-step logic, external API calls, or access to session context.
ALoraRequirement	Fine-tuned constraint LoRA — fastest at scale, requires adapter.

LLM-as-a-judge requirements call the backend for each validation, which adds latency. For high-throughput workloads, prefer simple_validate for deterministic checks and reserve LLM-based requirements for subjective criteria that cannot be coded directly.

Advanced: ALoraRequirement (from mellea.stdlib.requirements) uses a fine-tuned LoRA adapter for validation instead of LLM-as-a-judge. It falls back to LLM-as-a-judge if the adapter is unavailable. See LoRA and aLoRA Adapters.

Composing requirements

Requirements are composable: mix strings, req(), check(), and Requirement objects freely in the same list:

from mellea.core import Requirement
from mellea.stdlib.requirements import check, req, simple_validate

requirements = [
    "The email should have a salutation.",          # plain string → LLM-as-a-judge
    req("Use only lower-case letters.",             # req() with custom validator
        validation_fn=simple_validate(lambda x: x.lower() == x)),
    check("Do not mention competitor products."),  # check-only → not in prompt
    Requirement(                                    # explicit Requirement object
        "Fewer than 100 words.",
        validation_fn=simple_validate(
            lambda x: (len(x.split()) < 100, f"Word count: {len(x.split())}")
        ),
    ),
]

All requirements are validated after each generation attempt. The repair request lists every requirement that failed, not just the first one, so the model can address all issues in a single repair pass.