Handling Exceptions and Failures

Handle SamplingResult failures, PreconditionException, and parse errors gracefully in Mellea programs.

Prerequisites: The Requirements System, Quick Start complete, pip install mellea.

Mellea programs encounter two categories of failure: expected failures (IVR exhaustion, precondition violations) that are part of normal operation, and unexpected errors (backend connectivity, parse failures) that indicate configuration or implementation problems.

Expected failures

IVR loop exhaustion: SamplingResult.success = False

When instruct() is called with return_sampling_results=True and the IVR loop exhausts its budget without satisfying all requirements, SamplingResult.success is False. This is not a Python exception — it is a normal return value that your code should handle.

# Requires: mellea
# Returns: SamplingResult
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 haiku about the ocean.",
    requirements=[
        req(
            "Must have exactly 17 syllables (5-7-5).",
            validation_fn=simple_validate(
                lambda x: (
                    len(x.split()) <= 20,  # rough proxy; replace with a real syllable counter
                    "Syllable count does not match the 5-7-5 pattern.",
                )
            ),
        ),
    ],
    strategy=RejectionSamplingStrategy(loop_budget=5),
    return_sampling_results=True,
)

if result.success:
    print(str(result.result))
else:
    # All attempts failed — decide what to do
    print("Could not generate a valid haiku after 5 attempts.")
    print("Best attempt:", str(result.sample_generations[0].value))

Common fallback patterns when success is False:

• Use the best attempt anyway — result.sample_generations[0].value gives the first (often the best) generation, even if requirements were not fully satisfied.
• Lower the bar — retry with reduced requirements or a higher loop_budget.
• Return an error indicator — tell the caller the operation could not be completed to spec, and let it decide.
• Log and alert — if this should rarely fail, log the attempts and notify.

Inspecting failure reasons

SamplingResult.sample_validations gives per-attempt validation details. Use them to understand which requirements are failing and why:

# Requires: None
# Returns: None
if not result.success:
    for attempt_idx, validations in enumerate(result.sample_validations):
        print(f"Attempt {attempt_idx + 1}:")
        for requirement, val_result in validations:
            if not val_result:
                print(f"  FAIL: {requirement.description}")
                print(f"    Reason: {val_result.reason}")

A requirement that fails on every attempt usually indicates one of:

• The model cannot satisfy this constraint with the current prompt and model.
• The validation_fn has a bug (returns False unconditionally or has a logic error).
• The requirement is genuinely contradictory with the instruction.

Precondition failures: PreconditionException

When precondition_requirements are attached to a @generative call, Mellea validates the inputs before calling the model. If any precondition fails, PreconditionException is raised immediately — no model call is made:

# Requires: mellea
# Returns: str
from typing import Literal
from mellea import generative, start_session
from mellea.core import Requirement
from mellea.stdlib.components.genstub import PreconditionException
from mellea.stdlib.requirements import simple_validate

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

m = start_session()

try:
    result = classify_sentiment(
        m,
        text="I love this!",
        precondition_requirements=[
            Requirement(
                "Input must be fewer than 500 characters.",
                validation_fn=simple_validate(
                    lambda x: (
                        len(x) < 500,
                        f"Input is {len(x)} characters; must be under 500.",
                    )
                ),
            )
        ],
    )
    print(result)
except PreconditionException as e:
    print(f"Invalid input: {e}")
    for val_result in e.validation:
        print(f"  - {val_result.reason}")
    # Handle gracefully: sanitize input, reject the request, etc.

PreconditionException.validation is a list of ValidationResult objects for the requirements that failed. Each .reason field explains what was wrong.

Use preconditions to:

• Validate untrusted inputs before they reach the model
• Enforce interface contracts between pipeline stages
• Fail fast on inputs that are guaranteed to produce bad output

Unexpected errors

Backend connection errors

If Ollama is not running, or a cloud API key is invalid, the backend raises an exception on the first model call:

# Requires: mellea
# Returns: str
import mellea

try:
    m = mellea.start_session()
    result = m.instruct("Hello.")
    print(str(result))
except Exception as e:
    # Backend errors are not Mellea-specific exceptions — they come from the
    # underlying HTTP client or the backend constructor.
    print(f"Backend error: {e}")
    # Handle: check connectivity, validate credentials, fall back to another backend

For production code, wrap session creation and the first call together:

# Requires: mellea
# Returns: MelleaSession | None
import mellea

def create_session_or_none():
    try:
        m = mellea.start_session()
        # Probe the connection with a cheap call
        m.chat("ping")
        return m
    except Exception as e:
        print(f"Could not connect to backend: {e}")
        return None

Parse failures: ComponentParseError

When @generative or instruct(format=...) is used with a Pydantic model or Literal return type, Mellea parses the raw model output into the declared type. If parsing fails, a ComponentParseError is raised.

This typically means the model produced output that does not conform to the schema. The IVR loop retries on parse failure automatically — ComponentParseError surfaces only if all retries are exhausted.

# Requires: mellea
# Returns: str
from typing import Literal
from mellea import generative, start_session
from mellea.core.base import ComponentParseError

@generative
def classify(text: str) -> Literal["a", "b", "c"]:
    """Classify the text into category a, b, or c."""

m = start_session()

try:
    result = classify(m, text="...")
except ComponentParseError as e:
    print(f"Model output could not be parsed: {e}")
    # Fall back to a raw string extraction or a default value

If ComponentParseError occurs in practice, check:

• Whether the model is large enough to follow the output format instructions.
• Whether the instruction and docstring are clear about the expected format.
• Whether the backend supports constrained decoding for the return type.

Fallback and retry patterns

Fallback to a simpler call

If a structured call fails, fall back to a plain instruct():

# Requires: mellea, pydantic
# Returns: None
from pydantic import BaseModel
from mellea import generative, start_session
from mellea.core.base import ComponentParseError

class ExtractedData(BaseModel):
    name: str
    email: str

@generative
def extract(text: str) -> ExtractedData:
    """Extract name and email from the text."""

m = start_session()
try:
    data = extract(m, text="Contact Alice at [e-mail].")
    print(data.name, data.email)
except ComponentParseError:
    # Fall back: get the raw text and parse manually
    raw = m.instruct("Extract the name and email from: {{text}}",
                     user_variables={"text": "Contact Alice at [e-mail]."})
    print("Raw fallback:", str(raw))

Fallback to a different model

For calls that require higher capability, escalate to a stronger model on failure:

# Requires: mellea
# Returns: str
from mellea import MelleaSession
from mellea.backends.ollama import OllamaModelBackend
from mellea.backends import model_ids
from mellea.stdlib.sampling import RejectionSamplingStrategy

def instruct_with_fallback(text: str) -> str:
    m_fast = MelleaSession(OllamaModelBackend(model_ids.IBM_GRANITE_4_1_3B))
    result = m_fast.instruct(
        text,
        strategy=RejectionSamplingStrategy(loop_budget=3),
        return_sampling_results=True,
    )
    if result.success:
        return str(result.result)

    # Escalate to a larger model
    m_strong = MelleaSession(OllamaModelBackend(model_ids.IBM_GRANITE_3_3_8B))
    return str(m_strong.instruct(text))

This is the basis of the SOFAI (System 1 / System 2) pattern — fast model first, strong model only when needed. Mellea provides SOFAISamplingStrategy as a built-in implementation. See Inference-Time Scaling.

Logging failures

Use Python’s standard logging module to record failures alongside generation details:

# Requires: mellea
# Returns: None
import logging
from mellea import start_session
from mellea.stdlib.sampling import RejectionSamplingStrategy

logger = logging.getLogger(__name__)

m = start_session()
result = m.instruct(
    "Classify: {{text}}",
    strategy=RejectionSamplingStrategy(loop_budget=3),
    user_variables={"text": "..."},
    return_sampling_results=True,
)

if not result.success:
    logger.warning(
        "instruct() failed after %d attempts",
        len(result.sample_generations),
        extra={
            "attempts": len(result.sample_generations),
            "first_output": str(result.sample_generations[0].value),
        },
    )

For structured telemetry across all calls, see Telemetry.

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See also: The Requirements System | Write Custom Verifiers