marimo-pair

name	marimo-pair
description	Work inside a running marimo notebook's kernel — execute code, create cells, and build a notebook as an artifact. Use when the user wants to start a marimo notebook or work in an active marimo session.
allowed-tools	Bash(bash **/scripts/discover-servers.sh *), Bash(bash **/scripts/execute-code.sh *), Read

marimo Pair Programming Protocol

This skill gives you full access to a running marimo notebook. You can read cell code, create and edit cells, install packages, run cells, and inspect the reactive graph — all programmatically. The user sees results live in their browser while you work through bundled scripts or MCP.

Philosophy

marimo notebooks are a dataflow graph — cells are the fundamental unit of computation, connected by the variables they define and reference. When a cell runs, marimo automatically re-executes downstream cells. You have full access to the running notebook.

• Cells are your main lever. Use them to break up work and choose how and when to bring the human into the loop. Not every cell needs rich output — sometimes the object itself is enough, sometimes a summary is better. Match the presentation to the intent.
• Understand intent first. When clear, act. When ambiguous, clarify.
• Follow existing signal. Check imports, pyproject.toml, existing cells, and dir(ctx) before reaching for external tools.
• Stay focused. Build first, polish later — cell names, layout, and styling can wait.

Prerequisites

How to invoke marimo

Only servers started with --no-token register in the local server registry and are auto-discoverable — starting without a token makes discovery easier. If a server has a token, set the MARIMO_TOKEN environment variable before calling the execute script (avoids leaking the token in process listings). The right way to invoke marimo depends on context (project tooling, global install, sandbox mode). See finding-marimo.md for the full decision tree.

Do NOT use --headless unless the user asks for it. Omitting it lets marimo auto-open the browser, which is the expected pairing experience. If the user explicitly requests headless, offer to open http://localhost:<port> in their browser (open on macOS, xdg-open on Linux, start on Windows).

Troubleshooting

SyntaxError or ImportError from execute-code.sh

Code runs inside the running marimo kernel — execute-code.sh POSTs it over HTTP and never invokes a local Python. So errors here are not caused by the local Python version, missing venv, or uv vs pip — they're problems with the code being sent. Fix the code (use a heredoc for anything multiline; don't try to one-line compound statements with ;).

execute-code.sh returns EMPTY output (no error)

When execute-code.sh returns empty output with no error message, and multiple marimo servers are running (common in multi-worktree setups), try the other ports first. The user is likely on a different port than the one you assumed — execute-code.sh silently returns nothing when there is no active session on the targeted port.

# List all running servers, then probe each port
bash scripts/discover-servers.sh
bash scripts/execute-code.sh --port 2719 -c "1 + 1"

Do not spend turns debugging the script itself (removing 2>/dev/null, inspecting internals, hypothesizing version mismatches) — empty output means no session responded on that port, not a script bug.

User keeps getting prompted to allow Bash commands

The skill declares allowed-tools in its frontmatter, but Claude Code may still prompt for each Bash call. To fix this, the user should add the absolute paths to the scripts to their .claude/settings.json (project-level) or ~/.claude/settings.json (global):

{
  "permissions": {
    "allow": [
      "Bash(bash /absolute/path/to/skills/marimo-pair/scripts/discover-servers.sh *)",
      "Bash(bash /absolute/path/to/skills/marimo-pair/scripts/execute-code.sh *)"
    ]
  }
}

How to Discover Servers and Execute Code

Two operations: discover servers and execute code.

Operation	Script	MCP
Discover servers	bash scripts/discover-servers.sh	list_sessions() tool
Execute code	bash scripts/execute-code.sh -c "code"	execute_code(code=..., session_id=...) tool
Execute code (multiline)	bash scripts/execute-code.sh <<'EOF'	same
Execute code (by URL)	bash scripts/execute-code.sh --url http://localhost:2718 -c "code"	same (with url param)

Scripts auto-discover sessions from the local server registry. Use --port to target a specific server when multiple are running, --session to target a specific session when multiple notebooks are open on the same server, or --url to skip discovery and connect to a server by URL (e.g. --url http://localhost:2718). On Windows, prefer direct --url when registry discovery is empty — see the next section for why. Set the MARIMO_TOKEN env var to authenticate when the server has token auth enabled (--token flag also works but exposes the token in process listings). If the server was started with --mcp, you'll have MCP tools available as an alternative.

Discovery finds nothing but the user has a server running?

Only --no-token servers are in the registry. If discovery comes up empty, the server likely has token auth — ask the user for the token and set it as the MARIMO_TOKEN environment variable.

On Windows (Git Bash / MSYS2), discovery can also come up empty even for a running --no-token server. If the user confirms marimo is reachable locally, fall back to --url http://127.0.0.1:<port> (ask for the port).

No servers running?

Always discover before starting. Background task "completed" notifications do not mean the server died — check the output or run discover first.

If no servers are found, read the user's intent — if they want a notebook, start one. Always start marimo as a background process so it doesn't block the conversation:

• Claude Code: use run_in_background on the Bash tool.
• OpenCode: use nohup pixi run python -m marimo edit notebooks/ \ --no-sandbox --no-token &>/tmp/marimo.log & — there is no run_in_background parameter, and if marimo runs in the foreground the shell tool will die/timeout, killing the session before you can connect.

See finding-marimo.md for the three critical pixi/marimo startup gotchas (arg mangling, background process, single-file file_not_found).

If there's no .py file yet, pick a descriptive filename based on context (e.g., exploration.py, analysis.py, dashboard.py). Don't ask — just pick something reasonable.

Avoid shell escaping issues. -c works for simple one-liners, but for multiline code or code with quotes/backticks/${}, use a heredoc or a file:

# heredoc (single-quoted delimiter prevents shell interpolation)
bash scripts/execute-code.sh <<'EOF'
import marimo._code_mode as cm

async with cm.get_context() as ctx:
    ctx.create_cell("x = 1")
EOF

# file
bash scripts/execute-code.sh /tmp/code.py

# target a specific port (skips auto-selection when multiple servers run)
bash scripts/execute-code.sh --port 2718 -c "1 + 1"

Executing Code

Every execute-code call runs inside the notebook's kernel. All cell variables are in scope — print(df.head()) just works. Nothing you define persists between calls (variables, imports, side-effects all reset), but you can freely introspect the notebook: inspect variables, test code snippets, check types and shapes. Use this to explore, prototype, and validate before committing anything to the notebook — then create cells to persist state and make results visible to the user.

To mutate the notebook's dataflow graph — create, edit, and delete cells, install packages, and run cells — use marimo._code_mode:

import marimo._code_mode as cm

async with cm.get_context() as ctx:
    cid = ctx.create_cell("x = 1")
    ctx.packages.add("pandas")
    ctx.run_cell(cid)

You must use async with — without it, operations silently do nothing. All ctx.* methods are synchronous — they queue operations and the context manager flushes them on exit. Do not await them.

The kernel supports top-level await, so use async with directly. Do not wrap calls in async def main(): ... + asyncio.run(main()) — it's unnecessary and easy to get wrong (compound statements like async with can't follow def name(): on the same line, so cramming it into a -c one-liner produces a SyntaxError).

Cells are not auto-executed. create_cell and edit_cell are structural changes only — use run_cell to queue execution.

code_mode is a tested, safe API for notebook mutations — prefer it for all structural changes. You also have access to marimo internals from the kernel, but treat that as a last resort and only with high confidence after exploration.

Edit cells through code_mode, never the file system. Direct file writes are silently lost. It is tempting to reach for Edit/Write for a small tweak since edit_cell requires the full new cell body. Don't — without --watch (off by default) the kernel never sees those edits and overwrites them on its next save, so the user sees nothing. (Read on the .py is okay, but content may lag the live kernel; prefer ctx.cells[target].code.)

UI state lives outside the reactive graph. Anywidget traitlets can be read or set directly (e.g., slider.value = 5). For mo.ui.* elements, use ctx.set_ui_value(element, new_value) inside code_mode.

First Step: Explore the API

The code_mode API can change between marimo versions. Explore it at the start of each session — dig deeper into anything you're unsure about.

import marimo._code_mode as cm
help(cm)

Guard Rails

Skip these and the UI breaks:

• Install packages via ctx.packages.add(), not uv add or pip. The code API handles kernel restarts and dependency resolution correctly. Only fall back to external CLIs if the API is unavailable or fails.
• Custom widget = anywidget. For bespoke visual components, use anywidget with HTML/CSS/JS. Composed mo.ui is fine for simple forms and controls. See rich-representations.md.
• NEVER Edit, Write, or NotebookEdit the notebook .py file while a session is running. Direct writes are silently destroyed and never reach the user. marimo only watches the file with --watch, which is off by default. Without it, the kernel doesn't pick up file edits — and on its next save, the kernel writes its own state and clobbers yours. The user sees no change, you think the work landed, and the bug is invisible. Always use ctx.edit_cell(target, code=...) with the full new cell body — even for a one-character change. (Read is allowed, but disk content may lag the live kernel; for the current truth prefer ctx.cells[target].code.)
• No temp-file deps in cells. pathlib.Path("/tmp/...") in cell code is a bug.
• Avoid empty cells. Prefer edit_cell into existing empty cells rather than creating new ones. Clean up any cells that end up empty after edits.
• Don't worry about cell names. Most cells don't need explicit names — see notebook-improvements.md.

Widgets and Reactivity

Anywidget state (traitlets) lives outside marimo's reactive graph. To hook a widget trait into the graph, pick one strategy per widget — never mix them:

• mo.state + .observe() — you pick specific traits to bridge. Default choice.
• mo.ui.anywidget() — wraps all synced traits into one reactive .value. Convenient but coarser.

Read rich-representations.md before wiring either.

Keep in Mind

• The user is editing too. The notebook can change between your calls — re-inspect notebook state if it's been a while since you last looked.
• Deletions are destructive. Deleting a cell removes its variables from kernel memory — restoring means recreating the cell and re-running it and its dependents. If intent seems ambiguous, ask first.
• Installing packages changes the project. ctx.packages.add() adds real dependencies — confirm when it's not obvious from context.

References

• finding-marimo.md — how to find and invoke the right marimo
• gotchas.md — cached module proxies and other traps
• rich-representations.md — custom widgets and visualizations
• notebook-improvements.md — improving existing notebooks