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The Instruction Model

Prerequisites: Quick Start complete, pip install mellea, Ollama running locally.

instruct() is the primary API in Mellea. It builds a structured Instruction component — not a raw chat message — with a description, requirements, user variables, grounding context, few-shot examples, and images. The instruction is rendered through Jinja2 templates and run through an instruct–validate–repair (IVR) loop by default.

Basic instruct()​

# Requires: mellea
# Returns: str
import mellea

m = mellea.start_session()
email = m.instruct("Write an email inviting interns to an office party at 3:30pm.")
print(str(email))
# Output will vary — LLM responses depend on model and temperature.

instruct() returns a ModelOutputThunk. Access the result as a string with str(email) or via email.value.

User variables​

Embed dynamic values in your description using {{double_braces}}. The description is a Jinja2 template; values are injected at generation time via user_variables:

# Requires: mellea
# Returns: str
import mellea

def write_email(m: mellea.MelleaSession, name: str, notes: str) -> str:
    email = m.instruct(
        "Write an email to {{name}} using the notes following: {{notes}}.",
        user_variables={"name": name, "notes": notes},
    )
    return str(email)

m = mellea.start_session()
print(write_email(m, name="Olivia", notes="Organized intern events."))
# Output will vary — LLM responses depend on model and temperature.

Variables work in requirements too — you can use the same {{var}} syntax anywhere in the instruction description or requirement strings.

Requirements​

Requirements are declarative constraints. They serve two purposes:

  1. They are embedded in the prompt so the model knows what to aim for.
  2. They are checked after generation; if any fail, the IVR loop asks the model to repair its output.

Pass plain strings for LLM-checked requirements:

# Requires: mellea
# Returns: str
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.",
        "Use only lower-case letters.",
    ],
)
print(str(email))
# Output will vary — LLM responses depend on model and temperature.

Custom validation functions​

For deterministic checks, attach a validation_fn to a Requirement:

# Requires: mellea
# Returns: str
from mellea import start_session
from mellea.core import Requirement
from mellea.stdlib.requirements import simple_validate

word_limit_req = Requirement(
    "Use fewer than 100 words.",
    validation_fn=simple_validate(lambda output: len(output.split()) < 100),
)

m = start_session()
email = m.instruct(
    "Write an email inviting the team to a meeting.",
    requirements=["Be formal.", word_limit_req],
)
print(str(email))

simple_validate wraps a callable that returns a bool (or a (bool, str) tuple with a failure reason) into a validation function.

Shorthand helpers​

req() and check() are concise constructors for Requirement:

# Requires: mellea
# Returns: str
from mellea import start_session
from mellea.stdlib.requirements import check, req, simple_validate

m = start_session()
email = m.instruct(
    "Write an email to {{name}}.",
    requirements=[
        req("The email should have a salutation."),
        req(
            "Use only lower-case letters.",
            validation_fn=simple_validate(lambda x: x.lower() == x),
        ),
        check("Do not mention purple elephants."),
    ],
    user_variables={"name": "Olivia"},
)
print(str(email))
# Output will vary — LLM responses depend on model and temperature.
  • req(description) — creates a Requirement with an optional validation_fn
  • check(description) — alias for req(), reads naturally for boolean constraints

Sampling strategies and the IVR loop​

By default, instruct() uses RejectionSamplingStrategy(loop_budget=2): it generates once, validates all requirements, and retries up to two times if any fail.

Configure the loop explicitly with strategy:

# 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 an email to {{name}}.",
    requirements=[
        req(
            "Use only lower-case letters.",
            validation_fn=simple_validate(lambda x: x.lower() == x),
        ),
    ],
    strategy=RejectionSamplingStrategy(loop_budget=5),
    user_variables={"name": "Olivia"},
    return_sampling_results=True,
)

if result.success:
    print(str(result.result))
else:
    # All attempts failed — fall back to the first generation
    print(str(result.sample_generations[0].value))

With return_sampling_results=True, instruct() returns a SamplingResult instead of a ModelOutputThunk. This lets you inspect whether validation passed and access all intermediate generations.

Advanced: SOFAI (SOFAISamplingStrategy) is a dual-model strategy that routes between a fast and a slow model based on confidence. See Inference-Time Scaling.

Grounding context​

Attach reference documents to an instruction for retrieval-augmented generation:

from mellea import start_session

m = start_session()
answer = m.instruct(
    "Given the documents in the context, answer: {{query}}",
    user_variables={"query": "What is the capital of France?"},
    grounding_context={"doc0": "France is a country in Western Europe. Its capital is Paris."},
)
print(str(answer))
# Output will vary — LLM responses depend on model and temperature.

grounding_context maps string keys to document text. The keys are arbitrary labels — they appear in the prompt as [key] = value so the model can reference them by name, but there is no required naming convention (e.g. "doc0", "annual_report", "spec" all work). See Working with Data for richer document handling using MObjects and RichDocument.

ICL examples​

In-context learning (ICL) examples provide few-shot demonstrations. They are rendered as input–output pairs inside the Instruction component's Jinja2 template, giving the model concrete examples to follow.

Note (review needed): The instruct() icl_examples parameter API needs verification against the current source before documenting the full signature here.

Images​

Pass images to instruct() with the images parameter. Accepts both Mellea ImageBlock and PIL images:

from PIL import Image
from mellea import start_session
from mellea.core import ImageBlock

m = start_session()  # requires a vision-capable backend and model
pil_image = Image.open("photo.jpg")
img_block = ImageBlock.from_pil_image(pil_image)

response = m.instruct(
    "Describe what is in this image.",
    images=[img_block],
)
print(str(response))
# Output will vary — LLM responses depend on model and temperature.

Backend note: Vision requires a model that supports image inputs (e.g., qwen2.5vl:7b via the OpenAI backend). The default Ollama/Granite setup does not support images.

Multi-turn with ChatContext​

instruct() works with ChatContext for stateful multi-turn conversations:

from mellea import start_session
from mellea.stdlib.context import ChatContext

m = start_session(ctx=ChatContext())
m.chat("Make up a simple math problem.")
m.chat("Now solve the problem you just made up.")

print(str(m.ctx.last_output()))
# Output will vary — LLM responses depend on model and temperature.

ChatContext accumulates turns. SimpleContext (the default) discards the previous turn on each call.

chat() vs instruct()​

chat() is a lighter-weight alternative that sends a plain message with no requirements and no sampling strategy:

from mellea import start_session
from mellea.stdlib.context import ChatContext

m = start_session(ctx=ChatContext())
response = m.chat("What is 2 + 2?")
print(str(response))

Use chat() for conversational back-and-forth where you don't need the IVR machinery. Use instruct() when you want requirements, validation, or structured output.