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Prerequisites: pip install mellea, Ollama running locally (or an OpenAI-compatible endpoint). When a single prompt grows too long or asks the LLM to do too many things at once, quality degrades. m decompose analyses the prompt, extracts its constraints, and produces a Python script of ordered m.instruct() calls — one per subtask — that you can run immediately or refine with types and requirements.

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When to use m decompose

Use m decompose when:
  • A prompt contains multiple distinct tasks (write, classify, translate, summarise) that you would benefit from separating.
  • You want to add typed return values or @generative wrappers to each step.
  • You need to assign different requirements to different parts of the pipeline.
  • You are prototyping a pipeline and want a structured starting point to edit.
For prompts that fit cleanly in a single m.instruct() call, use instruct() directly.

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Step 1: Write your prompt to a file

Create a plain-text file that describes the full task. Include all constraints and requirements as part of the description — m decompose extracts them: 
Plan a birthday party for a 10-year-old.

The plan must include:
- A theme suggestion with a short explanation
- A list of at least 5 activities suitable for children aged 8-12
- A catering menu with a main dish, two sides, and a birthday cake option
- A 30-word invitation message addressed to the child's classmates

All content must be age-appropriate. The invitation must not exceed 30 words.
The activity list must be ordered from most energetic to least energetic.
Save this as party_plan.txt.
Tip: The more explicit your constraints in the prompt file, the more accurately m decompose assigns them to individual subtasks. Phrases like “must”, “must not”, “at least”, and “ordered by” are reliably extracted as constraints.

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Step 2: Run the decompose command

m decompose run --prompt-file party_plan.txt --out-dir ./output/
This produces two files in ./output/:
  • m_decomp_result.py — a runnable Python script with one m.instruct() call per subtask, in dependency order
  • m_decomp_result.json — the full decomposition: subtask list, extracted constraints, dependency graph, and Jinja2 prompt templates
Note: The --out-dir directory must already exist. m decompose does not create it.

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What the pipeline does

m decompose runs these steps internally, in order:
  1. Parses the prompt into a list of subtasks, each tagged with a short identifier
  2. Extracts all constraints and requirements from the prompt text
  3. Decides for each constraint whether validation should be done with code ("code") or with an LLM judge ("llm")
  4. Generates a Jinja2 prompt template for each subtask
  5. Assigns constraints to the subtasks they apply to
  6. Writes the output Python script with m.instruct() calls in dependency order

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All CLI options

FlagDefaultDescription
--prompt-file(interactive)Path to a text file containing the task prompt. Omit to enter the prompt interactively.
--out-dir(required)Path to the directory for output files. Must exist.
--out-namem_decomp_resultBase name for the output .py and .json files.
--model-idmistral-small3.2:latestModel to use for the decomposition.
--backendollamaInference backend: ollama or openai.
--backend-endpoint—URL endpoint. Required when --backend openai.
--backend-api-key—API key. Required when --backend openai.
--backend-req-timeout300Request timeout in seconds.
--input-var—Repeatable. Declares a user input variable name (uppercase Python identifier).

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Step 3: Review the generated Python file

Open output/m_decomp_result.py. For the birthday party prompt above, the generated script looks roughly like this: 
import textwrap
import mellea

m = mellea.start_session()

# Subtask: suggest_theme
theme = m.instruct(
    textwrap.dedent("""\
        Suggest a birthday party theme for a 10-year-old.
        Provide a theme name and a short explanation of why it suits this age group.
        All content must be age-appropriate.
    """)
)

# Subtask: list_activities
activities = m.instruct(
    textwrap.dedent("""\
        List at least 5 party activities suitable for children aged 8-12.
        Order the activities from most energetic to least energetic.
        Theme context: {{theme}}
        All content must be age-appropriate.
    """),
    user_variables={"theme": str(theme)},
)

# Subtask: catering_menu
menu = m.instruct(
    textwrap.dedent("""\
        Create a catering menu for a children's birthday party.
        Include a main dish, two sides, and a birthday cake option.
        All content must be age-appropriate.
    """)
)

# Subtask: invitation_message
invitation = m.instruct(
    textwrap.dedent("""\
        Write a birthday party invitation message addressed to the child's classmates.
        The message must not exceed 30 words.
        All content must be age-appropriate.
    """)
)

print("Theme:", str(theme))
print("Activities:", str(activities))
print("Menu:", str(menu))
print("Invitation:", str(invitation))
Each subtask is a separate m.instruct() call. Subtasks that depend on earlier outputs receive them through user_variables. The file runs as-is: 
python output/m_decomp_result.py
Note: Generated output varies — LLM responses depend on model and temperature.

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Step 4: Refine the generated code

The generated script is a starting point. Common refinements:

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Add typed returns with @generative

Replace an instruct() call with a @generative function to get typed output and IDE support: 
from typing import Literal
import mellea
from mellea import generative, start_session

@generative
def suggest_theme(age: int) -> str:
    """Suggest a birthday party theme for a child of the given age.
    Return a theme name followed by a one-sentence explanation."""

@generative
def list_activities(theme: str, age_min: int, age_max: int) -> list[str]:
    """List at least 5 party activities suitable for children aged age_min to age_max,
    ordered from most energetic to least energetic. All activities must be age-appropriate."""

m = start_session()
theme = suggest_theme(m, age=10)
activities = list_activities(m, theme=str(theme), age_min=8, age_max=12)

print(str(theme))
for activity in activities:
    print("-", activity)
# Output will vary — LLM responses depend on model and temperature.

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Add requirements to a subtask

Attach plain-English requirements to enforce constraints that m decompose left as prose: 
import textwrap
import mellea
from mellea.stdlib.requirements import req, simple_validate
from mellea.stdlib.sampling import RejectionSamplingStrategy

m = mellea.start_session()

invitation = m.instruct(
    textwrap.dedent("""\
        Write a birthday party invitation addressed to a 10-year-old's classmates.
        The message must not exceed 30 words. All content must be age-appropriate.
    """),
    requirements=[
        req(
            "Must not exceed 30 words.",
            validation_fn=simple_validate(
                lambda x: (
                    len(x.split()) <= 30,
                    f"Invitation is {len(x.split())} words; must be 30 or fewer.",
                )
            ),
        ),
        req("Must be addressed to classmates, not parents."),
    ],
    strategy=RejectionSamplingStrategy(loop_budget=4),
)
print(str(invitation))
# Output will vary — LLM responses depend on model and temperature.

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Step 5: Use —input-var for dynamic variables

When your prompt refers to values that change at runtime (a customer name, a product ID, a date), declare them with --input-var. Variable names must be valid Python identifiers, uppercase, and contain only alphanumeric characters and underscores: 
m decompose run \
  --prompt-file party_plan.txt \
  --out-dir ./output/ \
  --input-var CHILD_NAME \
  --input-var PARTY_DATE
The generated script will include placeholder references to CHILD_NAME and PARTY_DATE as user_variables, ready for you to wire up at call time.
Warning: --input-var names must be uppercase Python identifiers (e.g. CHILD_NAME, not child-name or childName). The command rejects names that contain hyphens, start with a digit, or use mixed case.

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Step 6: Choose the right model for decomposition

The decomposition quality depends heavily on the model. The default, mistral-small3.2:latest, handles most prompts well. For more complex prompts with many interdependent constraints, a larger model produces clearer subtask boundaries: 
m decompose run \
  --prompt-file party_plan.txt \
  --out-dir ./output/ \
  --model-id mistral-large:latest
To use an OpenAI-compatible endpoint: 
m decompose run \
  --prompt-file party_plan.txt \
  --out-dir ./output/ \
  --backend openai \
  --model-id gpt-4o-mini \
  --backend-endpoint https://api.openai.com/v1 \
  --backend-api-key "$OPENAI_API_KEY"
Tip: Run m decompose run --help to see the current defaults and all available flags.

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What the output JSON contains

The .json file gives you the full structured decomposition if you want to process it programmatically: 
{
  "subtask_list": ["suggest_theme", "list_activities", "catering_menu", "invitation_message"],
  "identified_constraints": [
    {"constraint": "All content must be age-appropriate", "validation_strategy": "llm"},
    {"constraint": "Invitation must not exceed 30 words", "validation_strategy": "code"},
    {"constraint": "Activity list must be ordered from most energetic to least energetic", "validation_strategy": "llm"},
    {"constraint": "At least 5 activities", "validation_strategy": "code"},
    {"constraint": "Menu must include a main dish, two sides, and a birthday cake option", "validation_strategy": "llm"}
  ],
  "subtasks": [
    {
      "subtask": "Suggest a birthday party theme for a 10-year-old",
      "tag": "suggest_theme",
      "depends_on": [],
      "prompt_template": "Suggest a birthday party theme for a 10-year-old...",
      "input_vars_required": [],
      "constraints": [
        {"constraint": "All content must be age-appropriate", "validation_strategy": "llm"}
      ]
    }
  ]
}
Each subtask entry includes depends_on (a list of tag values), a ready-to-use prompt_template, and the constraints that apply to it. Each constraint carries a validation_strategy — "code" for deterministic checks (word count, length) and "llm" for quality checks that require LLM-as-a-judge evaluation.

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Next steps