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Agent skill

TeensyROM Build and Flash

by SensoriumEmbedded in SensoriumEmbedded/TeensyROM

Explains how to build the C64 menu programs and the dual-boot TeensyROM firmware from the repo and flash it to one or more boards over USB.

MITAuto-check passedDevelopment

Install TeensyROM Build and Flash

skills CLI
$ npx skills add SensoriumEmbedded/TeensyROM --skill teensyrom-build -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install SensoriumEmbedded/TeensyROM teensyrom-build --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/SensoriumEmbedded/TeensyROM.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.claude/skills/teensyrom-build .claude/skills/teensyrom-build && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
teensyrom-build
GitHub stars
227
Token cost
~1.3k tokens
SKILL.md length
654 words
Files
5 (incl. references)
Skills in repo
4
Repo updated
First seen
Licence
MIT

At a glance

Explains how to build the C64 menu programs and the dual-boot TeensyROM firmware from the repo and flash it to one or more boards over USB.

  • Building firmware for a TeensyROM or TeensyROM+ cartridge
  • SKILL.md covers What needs rebuilding, Easy to get wrong, After a build (for agents) and On real hardware (for agents), plus 1 more section
  • Calls npm
  • Working out what to rebuild after changing C64 or Teensy source

What it does

The build runs in three steps with Node as the main prerequisite, a version pinned by `.nvmrc`, and no npm dependencies. First, `npm run build:c64` produces C headers from the C64 programs. Second, `npm run build:tr` or `npm run build:tr-plus` creates a combined firmware hex file under `build/firmware`. Third, `npm run flash` writes that image to boards and checks it, and `npm test` covers the tools. One reference file per step plus tests and CI is included.

A table says what to rebuild after a change: edits under the C64 sources or shared includes need step one and then step two, other Teensy source needs only step two. Warnings are specific: TeensyROM and TeensyROM+ images are not interchangeable even though both build cleanly, the flasher refuses the wrong one, the build date is the HEAD commit's time so uncommitted edits keep the previous date, and skipping step one fails silently. The excerpt is cut off after those first pitfalls.

When your agent uses it

  • Building firmware for a TeensyROM or TeensyROM+ cartridge
  • Working out what to rebuild after changing C64 or Teensy source
  • Flashing several boards at once and checking which build each runs
  • Fixing a failing firmware build

Example prompts

  • “I changed Menu_Regs.h, what do I need to rebuild before flashing?”
  • “Build the TeensyROM+ firmware and flash it to the board plugged into USB.”
  • “The hex builds fine but the flasher refuses it, which target did I use?”

Requirements

  • Node.js at the version pinned in `.nvmrc`
  • A TeensyROM board connected over USB for flashing

What it can do on your machine

Read from SKILL.md and the folder at commit 28f6aa8. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    Shell commands in SKILL.md call:

    • npm

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md. Its commands use npm, which can reach the network depending on how they are called.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

TeensyROM Build and Flash loads about 1.3k tokens when it runs, and up to ~5.7k if it reads all its reference files. Until then it costs about 146 tokens; SKILL.md has 654 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~146
When it runs · the whole SKILL.md, loaded when a task matches
~1.3k
With references · SKILL.md plus every file in references/, read only if the agent opens them
~5.7k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from SensoriumEmbedded/TeensyROM at commit 28f6aa8, republished under its MIT licence (© SensoriumEmbedded). 654 words, ~1,278 tokens.

Download SKILL.mdSave it as .claude/skills/teensyrom-build/SKILL.md (or your agent's skills folder). This skill also uses 4 other files; get the full folder from GitHub.
name
teensyrom-build
description
How to build and flash TeensyROM firmware from this repo. Covers the C64 menu programs (npm run build:c64), the combined dual-boot firmware .hex for a TeensyROM or a TeensyROM+ (npm run build:tr / build:tr-plus), writing it to one or several boards over USB (npm run flash, tools/flash-firmware.mjs), and the tests and CI around them. Use whenever the user wants to build, compile, rebuild, flash, program or update firmware, make a .hex, find out what needs rebuilding after a change, check which build a board runs, or fix a failing build, for either cartridge type.

Building TeensyROM

Two toolchains and a flasher, run in this order. Node is the only prerequisite for most of it (.nvmrc pins the version), and the repo takes no npm dependencies.

StepCommandProducesHow-to
1. C64 programsnpm run build:c64C headers in Source/Teensy/TRMenuFiles/ROMs/c64-build.md
2. Firmwarenpm run build:tr or npm run build:tr-plusbuild/firmware/TeensyROM[+]_<version>_full.hexfirmware-build.md
3. Flashnpm run flashthat image running on a board, checkedflash.md
Tests and CInpm testtests-and-ci.md

Why the build works this way (the two toolchains, the dual-boot image, the pinned tool versions) is in docs/Architecture/Build-System.md. Read it before changing the build itself.

What needs rebuilding

You changedRun
Anything under Source/C64/: source, a shared include (several projects include c64defs.i, CommonDefs.i and Menu_Regs.i), or a .prg/.bin some project convertsstep 1 (all projects unless you know only one uses the file), then step 2
Source/Teensy/MinimalBoot/Common/Menu_Regs.h (shared with the C64 side)step 1 (all projects), then step 2
Anything else under Source/Teensy/step 2
Nothing: you want a board on this commit's firmwarestep 2 with --force, unless build/firmware already holds this commit's image (its build date is the commit's time, see flash.md); then step 3
The build tools under tools/npm test

Step 2 builds one cartridge type per run. Build both when both kinds of board need it.

Easy to get wrong

  • TR and TR+ images are not interchangeable. --target tr is the TeensyROM (Fab 0.2/0.3), --target tr-plus the TeensyROM+ (Fab 0.4). Both build cleanly. The flasher refuses the wrong one before writing; the SD-card updater refuses it only after the file has been carried over.
  • The build date names a commit, not a build. It is the HEAD commit's time, so a build with uncommitted edits carries the previous commit's date, and rebuilding a commit gives the same hex. Commit first if the date on the board has to prove what it runs.
  • Skipping step 1 is silent. The firmware embeds whatever headers are in the working tree, so a C64 change that was not rebuilt ships the old program.
  • Old images keep their name. An image is named after the firmware version, not the commit, so build/firmware can hold an older build under the current name, and the flasher takes the newest file of either type. Check the build date it prints.
Show full SKILL.md (270 more words)Show less

After a build (for agents)

  • The person does not see the build's output, only your reply. Show them its memory report verbatim, in a code block: for each image built ([minimal], [main], and [extension] when there is one), the [name] Building line, the Memory Usage on Teensy 4.1 block under it and the Hex: line. Do this after every build, a successful one included, and do not summarize or reformat the numbers.
  • Put it in your final reply for the task, even when you already showed it partway through (before flashing, say): messages between tool calls can be collapsed out of sight, and the final reply is the one the person reads.
  • A build that failed before linking has no report for that image; say so, and show the error instead.

On real hardware (for agents)

  • Flashing changes a physical board. Say which board gets which image, and wait for the person's go-ahead; if they want to watch, wait for them.
  • Start with npm run flash -- --check. It identifies every attached board and writes nothing.
  • With more than one Teensy attached, never call teensy_post_compile -port=... yourself: with -port alone it falls back to auto-search and can write the other board. The flasher passes the board's whole address.
  • Stop anything that holds a board's serial port (a TeensyROM app or API, a serial monitor) before flashing; a board it holds cannot answer and cannot be identified.
  • TeensyROM settings survive a USB flash.

Options

npm run flash -- --help and npm run build:c64 -- --help list their options. tools/build-firmware.mjs has no --help; firmware-build.md lists its options, and the top of the file explains them.

© SensoriumEmbedded, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 4 other files (references) in .claude/skills/teensyrom-build of SensoriumEmbedded/TeensyROM.

  • SKILL.md
  • references/c64-build.md
  • references/firmware-build.md
  • references/flash.md
  • references/tests-and-ci.md

Open the folder on GitHubat commit 28f6aa8

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Works with

Categories

Questions about TeensyROM Build and Flash

What does TeensyROM Build and Flash do?

Explains how to build the C64 menu programs and the dual-boot TeensyROM firmware from the repo and flash it to one or more boards over USB. nvmrc`, and no npm dependencies. First, `npm run build:c64` produces C headers from the C64 programs.

When should I use TeensyROM Build and Flash?

TeensyROM Build and Flash fits situations like: building firmware for a TeensyROM or TeensyROM+ cartridge; working out what to rebuild after changing C64 or Teensy source; flashing several boards at once and checking which build each runs; fixing a failing firmware build.

How do I install TeensyROM Build and Flash in Claude Code?

Run `npx skills add SensoriumEmbedded/TeensyROM --skill teensyrom-build -a claude-code`. Or copy the skill folder (.claude/skills/teensyrom-build in SensoriumEmbedded/TeensyROM) into .claude/skills/teensyrom-build in your project. Claude Code loads it when a task matches its description.

How do I install TeensyROM Build and Flash in Codex?

Run `npx skills add SensoriumEmbedded/TeensyROM --skill teensyrom-build -a codex`. Or copy the skill folder (.claude/skills/teensyrom-build in SensoriumEmbedded/TeensyROM) into .agents/skills/teensyrom-build in your project. Codex loads it when a task matches its description.

Can I use TeensyROM Build and Flash in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add SensoriumEmbedded/TeensyROM --skill teensyrom-build -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/teensyrom-build, .gemini/skills/teensyrom-build, .github/skills/teensyrom-build and .opencode/skills/teensyrom-build in your project.

What does TeensyROM Build and Flash need to run?

Going by SKILL.md and its folder, TeensyROM Build and Flash needs the command-line tools its instructions call (npm). Our summary lists: Node.js at the version pinned in `.nvmrc`; A TeensyROM board connected over USB for flashing.

Does TeensyROM Build and Flash access the network?

SKILL.md contains no URLs. Its commands use npm, which can reach the network depending on how they are called. This is read from the text; nothing was executed.

Is TeensyROM Build and Flash safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does TeensyROM Build and Flash use?

TeensyROM Build and Flash is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does TeensyROM Build and Flash use?

About 1.3k tokens (SKILL.md is roughly 5.1k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 4.4k tokens, read only when the agent opens those files.

What are the alternatives to TeensyROM Build and Flash?

Skills that share tags, products or a category with TeensyROM Build and Flash: Library Migration Guide (ailyProject/aily-blockly, 3.8k stars), Install Anti-Slop Oxlint Rules (dmmulroy/anti-slop, 5.3k stars), Nx Run Tasks (nomcopter/react-mosaic, 4.8k stars) and Get API Docs with chub (andrewyng/context-hub, 14k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains TeensyROM Build and Flash?

SensoriumEmbedded (a GitHub user) maintains it in SensoriumEmbedded/TeensyROM, which has 227 GitHub stars. The repository holds 4 skills in this directory. The repository was last updated on October 7, 2026.

Source: SensoriumEmbedded/TeensyROM on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.