Building Custom Components

Advanced: This page is for developers who need to go beyond the standard @generative, instruct(), and m.chat() API. If you are getting started with Mellea, see the Quick Start first.

The Component Protocol is the fundamental unit of composition in Mellea. Every high-level API call — m.instruct(), @generative, m.chat() — is backed by a Component that formats its input for the LLM and parses the output into a typed result. This page shows you how to implement the protocol yourself.

When to build a custom component

Use the standard API in most cases. Build a custom Component when:

• You need a domain-specific prompt structure that cannot be expressed as a @generative docstring or an instruct() template.
• You need deterministic, reusable parsing logic across many call sites — not ad-hoc post-processing.
• You want to unit-test prompt formatting and output parsing in isolation, without a real backend.
• You are building a reusable library component that other developers will import.
• You need to feed a ModelOutputThunk from one LLM call directly into the formatted input of another (lazy composition).

If none of these apply, @generative or instruct() covers your use case with less boilerplate.

The Component Protocol

Component is a Protocol generic over S, the return type produced when the component parses LLM output:

from mellea.core import CBlock, Component, ModelOutputThunk

The protocol has three required methods and one public method that wraps _parse:

Method	Signature	Purpose
parts()	-> list[Component | CBlock]	Returns child components and CBlock content blocks
format_for_llm()	-> TemplateRepresentation | str	Formats the component for LLM consumption
_parse()	(computed: ModelOutputThunk) -> S	Parses LLM output into the return type S
parse()	(computed: ModelOutputThunk) -> S	Public wrapper — catches exceptions as ComponentParseError

You implement parts(), format_for_llm(), and _parse(). You do not override parse() — the base implementation calls _parse() and wraps any exception in a ComponentParseError so callers always get a consistent error type.

Type parameter

Component[S] is parameterised by S: the Python type your _parse method returns. For example, Component[str] returns a plain string, while Component[list[str]] returns a list. The type parameter is enforced at static analysis time by mypy.

Minimal example: FeedbackForm

The following component formats a structured feedback request and parses the model's response into a Python dictionary.

import json

from mellea.core import CBlock, Component, ModelOutputThunk


class FeedbackForm(Component[dict[str, str]]):
    """Asks the model to rate content on several dimensions and return JSON."""

    def __init__(self, content: str, dimensions: list[str]) -> None:
        self._content = content
        self._dimensions = dimensions

    def parts(self) -> list[Component | CBlock]:
        return [CBlock(self._content)]

    def format_for_llm(self) -> str:
        dims = ", ".join(self._dimensions)
        return (
            f"Rate the following content on these dimensions: {dims}.\n"
            f"Respond with a JSON object mapping each dimension to a score "
            f'between 1 and 5 and a one-sentence reason. Use the format:\n'
            f'{{"dimension": {{"score": 3, "reason": "..."}}}}\n\n'
            f"Content:\n{self._content}"
        )

    def _parse(self, computed: ModelOutputThunk) -> dict[str, str]:
        raw = computed.value or ""
        # Strip markdown fences if the model wraps the JSON
        if raw.startswith("```"):
            raw = raw.split("```")[1]
            if raw.startswith("json"):
                raw = raw[4:]
        return json.loads(raw.strip())

Pass the component to m.act() to get a result:

import mellea.stdlib.functional as mfuncs
from mellea.backends.ollama import OllamaModelBackend
from mellea.stdlib.context import SimpleContext

backend = OllamaModelBackend("granite4:latest")
ctx = SimpleContext()

form = FeedbackForm(
    content="The onboarding flow was confusing and took too long.",
    dimensions=["clarity", "tone", "actionability"],
)

thunk, _ = mfuncs.act(action=form, context=ctx, backend=backend)
result = form.parse(thunk)
print(result)
# {"clarity": {"score": 2, "reason": "..."}, ...}

You can also use MelleaSession.act() — the session method is a thin wrapper around the same functional API:

from mellea import start_session

with start_session() as m:
    thunk = m.act(form)
    result = form.parse(thunk)

Using TemplateRepresentation for Jinja2-based rendering

For components that need model-specific prompt formatting, return a TemplateRepresentation from format_for_llm() instead of a plain string. TemplateRepresentation is a dataclass with these fields:

Field	Type	Purpose
obj	Any	The component instance (typically self)
args	dict	Variables passed to the Jinja2 template
tools	dict | None	Tool definitions available in the template
template	str | None	Inline Jinja2 template string
template_order	list[str] | None	Template file names to look up; "*" means the class name
images	list | None	Image blocks to include

The formatter resolves template files from a templates/prompts/ directory, traversing subdirectories that match the model ID before falling back to default/. See Mellea Core Internals for the full lookup order.

from mellea.core import CBlock, Component, ModelOutputThunk, TemplateRepresentation


class FeedbackFormTemplate(Component[dict]):
    """FeedbackForm variant using a Jinja2 template for rendering."""

    def __init__(self, content: str, dimensions: list[str]) -> None:
        self._content = content
        self._dimensions = dimensions

    def parts(self) -> list[Component | CBlock]:
        return [CBlock(self._content)]

    def format_for_llm(self) -> TemplateRepresentation:
        return TemplateRepresentation(
            obj=self,
            args={
                "content": self._content,
                "dimensions": self._dimensions,
            },
            template_order=["*"],  # looks up FeedbackFormTemplate.jinja2
        )

    def _parse(self, computed: ModelOutputThunk) -> dict:
        import json

        raw = computed.value or ""
        return json.loads(raw.strip())

Place the template file at mellea/templates/prompts/default/FeedbackFormTemplate.jinja2:

Rate the following content on these dimensions: {{ dimensions | join(", ") }}.
Respond with a JSON object mapping each dimension to a score between 1 and 5
and a one-sentence reason.

Content:
{{ content }}

Use inline template= for one-off components where a separate file is unnecessary:

from mellea.core import CBlock, Component, ModelOutputThunk, TemplateRepresentation

TEMPLATE = """\
Summarise in {{ max_words }} words or fewer:

{{ text }}
"""


class SummaryComponent(Component[str]):
    """Summarises text to a word limit."""

    def __init__(self, text: str, max_words: int = 50) -> None:
        self._text = text
        self._max_words = max_words

    def parts(self) -> list[Component | CBlock]:
        return [CBlock(self._text)]

    def format_for_llm(self) -> TemplateRepresentation:
        return TemplateRepresentation(
            obj=self,
            args={"text": self._text, "max_words": self._max_words},
            template=TEMPLATE,
        )

    def _parse(self, computed: ModelOutputThunk) -> str:
        return (computed.value or "").strip()

Registering with act()

You do not need to register or annotate a custom component. Pass it directly to m.act() or mfuncs.act():

import mellea.stdlib.functional as mfuncs
from mellea.backends.ollama import OllamaModelBackend
from mellea.stdlib.context import SimpleContext

backend = OllamaModelBackend("granite4:latest")
ctx = SimpleContext()

component = SummaryComponent("Long article text here...", max_words=30)
thunk, _ = mfuncs.act(action=component, context=ctx, backend=backend)
result = component.parse(thunk)
print(result)

For async workflows, use mfuncs.aact():

import asyncio
import mellea.stdlib.functional as mfuncs
from mellea.backends.ollama import OllamaModelBackend
from mellea.stdlib.context import SimpleContext


async def main() -> None:
    backend = OllamaModelBackend("granite4:latest")
    ctx = SimpleContext()
    component = SummaryComponent("Long article text here...", max_words=30)
    thunk, _ = await mfuncs.aact(action=component, context=ctx, backend=backend)
    print(component.parse(thunk))


asyncio.run(main())

Testing custom components

Because Component is a Protocol, you can test formatting and parsing without a real backend. Create a ModelOutputThunk with a known value to exercise _parse directly.

import json
import pytest
from mellea.core import CBlock, ModelOutputThunk


def make_thunk(value: str) -> ModelOutputThunk:
    """Return a pre-computed thunk containing value."""
    thunk = ModelOutputThunk(value=value)
    return thunk


class TestFeedbackForm:
    def test_format_for_llm_contains_dimensions(self):
        form = FeedbackForm(
            content="Great product.",
            dimensions=["clarity", "tone"],
        )
        rendered = form.format_for_llm()
        assert "clarity" in rendered
        assert "tone" in rendered

    def test_parts_returns_cblock(self):
        form = FeedbackForm(content="Great product.", dimensions=["clarity"])
        parts = form.parts()
        assert len(parts) == 1
        assert isinstance(parts[0], CBlock)
        assert parts[0].value == "Great product."

    def test_parse_valid_json(self):
        form = FeedbackForm(content="x", dimensions=["clarity"])
        payload = json.dumps({"clarity": {"score": 4, "reason": "Clear."}})
        thunk = make_thunk(payload)
        result = form._parse(thunk)
        assert result["clarity"]["score"] == 4

    def test_parse_raises_component_parse_error_on_bad_json(self):
        from mellea.core import ComponentParseError

        form = FeedbackForm(content="x", dimensions=["clarity"])
        thunk = make_thunk("this is not json")
        with pytest.raises(ComponentParseError):
            form.parse(thunk)

Note: ModelOutputThunk accepts a value keyword argument in tests. Check the current constructor signature in mellea/core/base.py if the import path changes in a future release.

Tip: Keep _parse pure — no I/O, no side effects. This makes it trivial to unit test and means failures are always the model's fault, not your parsing code.

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Next steps

• Mellea Core Internals — understand CBlock, ModelOutputThunk, and the full abstraction stack that custom components plug into.
• Write Custom Verifiers — combine custom components with requirement validation to build structured output pipelines with automatic retry.