fret-span

name	fret-span
description	Computes the fret span and playability of a 6-string guitar chord diagram. Calls the deterministic `ga_fret_span` MCP tool — never recall the answer from training data, since the math depends on knowing exactly which strings are open vs muted vs fretted.
triggers	["fret span","fret stretch","stretch of","is this hard","is this difficult","playability","how playable","fingering reach","left hand reach"]
license	internal
compatibility	{"agent-framework":">=1.0.0-preview","microsoft-extensions-ai":">=10.5.1"}
metadata	{"authoring-style":"tool-driven","origin":"ported from Common/GA.Business.ML/Agents/Skills/FretSpanSkill.cs — fourth MCP-tool-driven canary","evidence-kinds":["tool_call"]}
allowed-tools	["ga_fret_span"]

Fret Span and Playability

When a user asks how hard a chord is to play, how wide its stretch is, or whether its diagram is reachable, call the ga_fret_span MCP tool. Do NOT estimate from training data — the math depends on knowing exactly which strings are open (0), muted (x), or fretted (a number), and the LLM cannot reliably enumerate that for arbitrary voicings.

Calling the tool

The tool takes one argument:

• diagram — string. Either dash-separated ("x-3-2-0-1-0") or compact form starting with x ("x32010"). Six positions, low-to-high (E A D G B e). x = muted, 0 = open, otherwise the fret number.

It returns a structured result:

• Diagram — diagram echoed back, normalized to dash-separated form.
• Frets — array of six integers; -1 for muted, 0 for open.
• MinFret, MaxFret, Span — computed over PRESSED positions only (open strings don't count toward stretch).
• Difficulty — one of: "very easy", "easy", "moderate", "challenging", "very wide...".
• PlayabilityScore — 1 (easy) → 10 (very hard).
• Error — non-null only when the input could not be parsed.

Mapping user phrasings to arguments

• "How hard is x-3-2-0-1-0?" → diagram="x-3-2-0-1-0".
• "What's the stretch on x32010?" → diagram="x32010".
• "Is 0-2-2-1-0-0 difficult?" → diagram="0-2-2-1-0-0".
• User pastes a compact diagram with no separator and no surrounding text: pass it verbatim.

If the user only describes the chord by name ("how hard is a Cmaj7?") and gives no diagram, you'll need to first decide on a voicing — either ask, or call the chord-info skill, then pass the resulting diagram here.

Phrasing the answer

Use Difficulty verbatim, quote Span and the MinFret–MaxFret range, and mention the PlayabilityScore if the user is comparing two chords. Example:

The diagram x-3-2-0-1-0 has a fret span of 2 (frets 1–3). Difficulty: easy — comfortable stretch for most players. Playability score: 5/10.

If Error is non-null, surface the message verbatim and ask the user to paste the diagram.

Out of scope

• Fingering-specific reach — the tool reports span, not which finger goes where. A 4-fret span with a barre is easier than the same span without; the tool can't tell.
• Ergonomic constraints by hand size — the difficulty descriptions are average-hand. Decline cleanly if asked to factor in specific anatomy.
• Bass / 7-string / extended-range diagrams — the tool assumes 6 strings, low E to high e. Other tunings or string counts are out of scope.

Cross-reference

• MCP tool: Common/GA.Business.ML/Agents/Mcp/FretSpanMcpTools.cs
• Tool tests: Tests/Common/GA.Business.ML.Tests/Unit/FretSpanMcpToolsTests.cs
• Legacy C# skill it replaces: Common/GA.Business.ML/Agents/Skills/FretSpanSkill.cs (regex-driven, no LLM round-trip — kept for the deterministic fast path)