IRON LAW #1: NEVER MODIFY CELL DECORATORS OR SIGNATURES

Only edit code INSIDE @app.cell function bodies. This is not negotiable.

NEVER modify:

Cell decorators (@app.cell)
Function signatures (def _(deps):)
Return statements structure (trailing commas required)

ALWAYS verify:

All used variables are in function parameters
All created variables are in return statement
Trailing comma for single returns: return var,

IRON LAW #2: NO EXECUTION CLAIM WITHOUT OUTPUT VERIFICATION

Before claiming ANY marimo notebook works:

VALIDATE syntax and structure: marimo check notebook.py
EXECUTE with outputs: marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
VERIFY using notebook-debug skill's verification checklist
CLAIM success only after verification passes

This is not negotiable. Claiming "notebook works" without executing and inspecting outputs is LYING to the user.

Rationalization Table - STOP If You Think:

Excuse	Reality	Do Instead
"marimo check passed, so it works"	Syntax check ≠ runtime correctness	EXECUTE with --include-outputs and inspect
"Just a small change, can't break anything"	Reactivity means small changes propagate everywhere	VERIFY with full execution
"I'll let marimo handle the dependency tracking"	Verification of correct behavior is still required	CHECK outputs match expectations
"The function signature looks right"	Wrong deps/returns break reactivity silently	VALIDATE all vars are in params AND returns
"I can modify the function signature"	Breaks marimo's dependency detection	ONLY edit inside function bodies
"Variables can be used without returning them"	Will cause NameError in dependent cells	RETURN all created variables
"I can skip the trailing comma for single returns"	Python treats `return var` as returning the value, breaks unpacking	USE `return var,` for single returns

Red Flags - STOP Immediately If You Think:

"Let me add this variable to the function signature" → NO. Marimo manages signatures.
"I'll just run marimo check and call it done" → NO. Execute with outputs required.
"The code looks correct" → NO. Marimo's reactivity must be verified at runtime.
"I can redefine this variable in another cell" → NO. One variable = one cell.

Editing Checklist

Before every marimo edit:

Structure Validation:

Only edit code INSIDE @app.cell function bodies
Do NOT modify decorators or signatures
Verify all used variables are in function parameters
Verify all created variables are in return statement
Ensure trailing comma used for single returns
Ensure no variable redefinitions across cells

Syntax Validation:

Execute marimo check notebook.py
Verify no syntax errors reported
Verify no undefined variable warnings
Verify no redefinition warnings

Runtime Verification:

Execute with marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
Verify export succeeded (exit code 0)
Verify output ipynb exists and is non-empty
Apply notebook-debug verification checklist
Verify no tracebacks in any cell
Verify all cells executed (execution_count not null)
Verify outputs match expectations

Only after ALL checks pass:

Claim "notebook works"

Gate Function: Marimo Verification

Follow this sequence for EVERY marimo task:

1. EDIT     → Modify code inside @app.cell function bodies only
2. CHECK    → marimo check notebook.py
3. EXECUTE  → marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs
4. INSPECT  → Use notebook-debug verification
5. VERIFY   → Outputs match expectations
6. CLAIM    → "Notebook works" only after all gates passed

NEVER skip verification gates. Marimo's reactivity means changes propagate unpredictably.

Honesty Framing

Claiming a marimo notebook works without executing it with --include-outputs and inspecting the results is LYING.

Syntax checks and code inspection prove nothing about reactive execution correctness. The user expects a working notebook where all cells execute correctly with proper dependency tracking.

Key Concepts

Reactive execution: Cells auto-update when dependencies change
No hidden state: Each variable defined in exactly one cell
Pure Python: .py files, version control friendly
Cell structure: @app.cell decorator pattern

Cell Structure

import marimo

app = marimo.App()

@app.cell
def _(pl):  # Dependencies as parameters
    df = pl.read_csv("data.csv")
    return df,  # Trailing comma required for single return

@app.cell
def _(df, pl):
    summary = df.describe()
    filtered = df.filter(pl.col("value") > 0)
    return summary, filtered  # Multiple returns

Editing Rules

Edit code INSIDE @app.cell functions only
Never modify cell decorators or function signatures
Variables cannot be redefined across cells
All used variables must be returned from their defining cell
Markdown cells: Always wrap $ in backticks - mo.md("Cost: $50") not mo.md("Cost: $50")

Core CLI Commands

Command	Purpose
`marimo edit notebook.py`	marimo: Open notebook in browser editor for interactive development
`marimo run notebook.py`	marimo: Run notebook as executable app
`marimo check notebook.py`	marimo: Validate notebook structure and syntax without execution
`marimo convert notebook.ipynb`	marimo: Convert Jupyter notebook to marimo format

Export Commands

# marimo: Export to ipynb with code only
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb

# marimo: Export to ipynb with outputs (runs notebook first)
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs

# marimo: Export to HTML (runs notebook by default)
marimo export html notebook.py -o __marimo__/notebook.html

# marimo: Export to HTML with auto-refresh on changes (live preview)
marimo export html notebook.py -o __marimo__/notebook.html --watch

Key difference: HTML export runs the notebook by default. ipynb export does NOT - use --include-outputs to run and capture outputs.

Tip: Use __marimo__/ folder for all exports (ipynb, html). The editor can auto-save there.

Data and Visualization

Prefer polars over pandas for performance
Use mo.ui for interactive widgets
SQL cells: mo.sql(df, "SELECT * FROM df")
Display markdown: mo.md("# Heading")

Debugging Workflow

1. Pre-execution validation:

# scripts: Validate notebook syntax and cell structure
scripts/check_notebook.sh notebook.py

Runs syntax check, marimo validation, and cell structure overview in one command.

2. Runtime errors: Export with outputs, then use notebook-debug skill:

# marimo: Export to ipynb with outputs for inspection
marimo export ipynb notebook.py -o __marimo__/notebook.ipynb --include-outputs

Common Issues

Issue	Fix
Variable redefinition	Rename one variable or merge cells
Circular dependency	Break cycle by merging or restructuring
Missing return	Add `return var,` with trailing comma
Import not available	Ensure import cell returns the module

Additional Resources

Reference Files

For detailed patterns and advanced techniques, consult:

references/reactivity.md - DAG execution, variable rules, dependency detection patterns
references/debugging.md - Error patterns, runtime debugging, environment-specific issues
references/widgets.md - Interactive UI components and mo.ui patterns
references/sql.md - SQL cells and database integration techniques

Examples

Working examples available in examples/:

examples/basic_notebook.py - Minimal marimo notebook structure
examples/data_analysis.py - Data loading, filtering, and visualization patterns
examples/interactive_widgets.py - Interactive UI component usage

Scripts

Validation utilities in scripts/:

scripts/check_notebook.sh - Primary validation: syntax check, marimo validation, cell structure overview
scripts/get_cell_map.py - Extract cell metadata (invoked by check_notebook.sh)

Related Skills

notebook-debug - Debugging executed ipynb files with tracebacks and output inspection

marimo

Contents

Marimo Reactive Notebooks

Editing and Verification Enforcement