Guitar chord charts — MCP server
A small, read-only Model Context Protocol server that lets MCP-capable AI clients look up guitar chord charts from Guitar Practice Routine App (GPRA) and show them to people as text chord diagrams.
It serves exactly what guitarpracticeroutine.com/find-a-chord-chart serves: one chord name in, one chart out, from a bundled snapshot of the same chord library. No database, no authentication, no secrets.
Frets run top to bottom from the nut, and string 1 — the highest-pitched string — is the rightmost column, matching standard chord-box convention and the charts on the site.
Tools
| Tool | What it does |
|---|---|
get_chord_chart_by_name | One chord name (G, Am7, D/F#) → the one chart the website shows for it. |
get_chord_chart_by_id | The same chart by its numeric id, for re-rendering something already returned. |
get_chord_of_the_day | Today's Chord of the Day — the same chord GPRA posts to Bluesky and Facebook. |
Each result leads with a direct PNG URL for the chart, then the same chart as a fenced text grid. The URL is the picture in the form that survives every surface: it renders where markdown images work, stays clickable where they don't, and can be dropped into an artifact, an HTML page, or a saved file. The text grid is the fallback for anywhere neither is true.
Inline PNG bytes are opt-in via format: "image" or format: "both". They
cost image tokens and several clients bury them in a collapsed tool drawer, so
they're not worth sending by default now that a URL does the job. format
defaults to "text", which still includes the image URL. Attribution is kept in
every mode.
Every tool also takes a context argument describing why it's being called.
That's injected by PostHog's MCP SDK and populates agent intent in analytics;
nothing in the response depends on it.
How a chart reaches a person
There is no capability negotiation for "can you show an image", so the result carries several layers and lets the host use whichever it understands:
| Layer | Where it lands |
|---|---|
Text grid + Chart image: URL | Everywhere. This is the floor. |
MCP Apps widget (_meta.ui.resourceUri) | Hosts that render MCP Apps — Claude Code, Cowork, ChatGPT, PostHog Desktop. |
Inline PNG bytes (format: "image") | Hosts that surface image content blocks, on request. |
The widget is a ui://gpra-chord-charts/chart/{id} resource resolving to a
self-contained HTML page with the chord already baked in — no JavaScript, no
postMessage bridge, no client bundle. A host that ignores it simply shows no
widget, and the text answer is untouched.
Measured on claude.ai, for anyone tempted to try these again: bare markdown images render as a click-through placeholder that opens a browser tab rather than loading inline; images wrapped in links collapse to the link; raw HTML is escaped to literal text; and the MCP Apps iframe is fetched but never mounted (ext-apps#671). On that surface the URL is the whole story, which is why it leads the text.
Results deliberately carry no structuredContent. A client that understands
it may render it instead of the content blocks — one measurably did, reducing
the answer to three JSON fields and dropping the chart, the attribution and the
call to action. Nothing goes in a result that can displace the text.
What's in the library
12,708 standard-tuning (EADGBE) voicings, exactly one per chord name. This server does no fuzzy matching and no query cleanup — it passes the name through the same way the website does, so the tool description asks the calling assistant to send a plain chord name ("G", not "how do I play G major").
Charts are drawn on a five-fret grid starting at the nut, matching the site — which means a voicing with notes above the fifth fret has them fall outside the grid. The image drops them, exactly as the website does; the text names them underneath ("Also fretted, past this five-fret grid: string 5 (A) fret 6"). So the pair is honest even where the picture alone isn't.
Images use black ink on white rather than the site's white-on-transparent: the site can assume its own dark UI, and a chat client's background is unknown.
Connecting a client
Remote MCP means users add a URL, no install:
For clients configured with a JSON config file, use the streamable-HTTP transport:
For a client that only speaks stdio, bridge with
mcp-remote:
Run locally
Health check: GET /health → status, version, chord count.
Preview charts from the terminal without a client:
No CORS headers are set. Claude and connector directories fetch server-side, so they don't need them — a browser-based MCP client would.
Rebuilding the chord snapshot
Requires access to a GPRA database. The source table changes rarely, so this is a manual step:
build:index prints the library's fret range to the terminal and reports data
anomalies (voicings with no fretted notes, missing finger numbers, notes dropped
as unplayable) rather than quietly normalizing them away.
Privacy
The hosted server records anonymous usage analytics through
@posthog/mcp: which tool was
called, how long it took, whether it errored, the calling client's identity
(name and version on connect, user-agent on each call), and the context string
the agent supplied describing why it called. No personal identifier is stored and
no person profile is created.
Analytics are off entirely unless POSTHOG_API_KEY is set, so running this
yourself — including over stdio — sends nothing anywhere.
Deployment
deploy/ contains a systemd unit and an nginx reverse-proxy config, if you want
to host your own instance.
Credits
The chord library exists because of other people's work:
- SVGuitar-ChordCollection by @TormodKv — the chord database this is built from, itself based on chord-collection by @T-vK.
- SVGuitar by @omnibrain — the chord-diagram renderer whose data format this uses, and which draws the charts on guitarpracticeroutine.com. (This server renders text, not SVG.)
A note on npm audit
npm audit reports three high-severity advisories against image-size, pulled
in transitively by svguitar → svgdom. They are denial-of-service infinite
loops in the ICNS, JXL and HEIF file parsers, reached only through
svgdom's HTMLImageElement, which calls imageSizeFromFile when an <img>
is given a source.
Nothing here constructs one. Chart rendering draws vector primitives from a
chord name and an integer id; the only inputs that cross the boundary are a
string of at most 64 characters and a positive integer, and no code path
accepts, fetches or decodes an image file. There is no upstream fix, so the
advisories will keep showing until svgdom moves off image-size.
License
- Code: MIT — see
LICENSE. - Provenance of the chord data: see
NOTICE. - Chord data: the voicings originate from SVGuitar-ChordCollection, which carries no stated licence, so no licence is asserted over them here. The fingerings themselves are factual descriptions of where fingers go on a fretboard. Please credit the projects above if you reuse the data.
