Reference: the overlay API

Ingest (sparkles.twoslash.ingest)

Symbol	Signature	Notes
parseTwoslash	JsonResult!TwoslashReturn parseTwoslash(scope const(char)[] json)	parse + decode a JSON string
loadTwoslashFile	JsonResult!TwoslashReturn loadTwoslashFile(string path)	read + parse + decode a file
fromTwoslashJson	JsonResult!TwoslashReturn fromTwoslashJson(JSONValue root)	decode a pre-parsed object

Errors are returned (never thrown). Not @nogc (std.json + wired allocate).

Protocol (sparkles.twoslash.protocol)

• enum NodeType : ubyte { hover, query, completion, error, highlight, tag } — lowercase members map the wire type verbatim.
• struct Node — flat POD: type, start, length, line, character, plus the @WireOptional payload fields (text, docs, tags, level, code, id, completions, completionsPrefix, name). tags is string[][] (each inner [name, text?] — hover/query JSDoc tags). end() => start + length.
• struct Completion { string name; string kind; }
• struct TwoslashReturn { string code; Node[] nodes; }

Overlay planner (sparkles.twoslash.overlay)

• TwoslashPlan planTwoslash(in TwoslashReturn tw) — partition into inlineDecorations (sorted outer-first) and belowBlocks (sorted by line). Drops the inline hover on a token that also has a query (the query supersedes it).
• hasInlineDecoration(NodeType) / hasBelowBlock(NodeType) — classification.
• highlightSignature(ref TsConfigCache, sig, ref sink) — reentrant popup re-highlight (degrades to plain text on a missing grammar). @system.
• withoutQuickinfoPrefix(sig) — strips a leading TS quickinfo kind prefix ((property) , …); a real leading paren ((a: number) => void) is preserved.

Renderers

• renderTwoslashHtml(in TwoslashReturn tw, const(HighlightEvent)[] events, in ResolvedTheme theme, ref TsConfigCache cache, ref Writer w, in TwoslashHtmlOptions = …) — content-only HTML overlay. @system.
• renderTwoslashAnsi(… , in TwoslashAnsiOptions = …) — terminal overlay (TwoslashAnsiOptions { ColorDepth depth; bool italics, emitBackground, hovers; }). @system.

TwoslashHtmlOptions controls the HTML fidelity/chrome:

Field	Default	Effect
classPrefix	"syn-"	class prefix for the inner syntax spans
completionIcons	IconStyle.svg	completion-kind icon set (svg | glyph | none)
customCompletionIcon	null	per-kind icon override delegate (non-empty return wins)
tagIcons	IconStyle.svg	// @tag line icon set (svg | glyph | none)
stripQuickinfoPrefix	false	strip (property) etc. from popup signatures
renderDocsMarkdown	true	render docs (block) + @tag values (inline) as markdown

The svg icon sets are the reference @shikijs/twoslash icons, string-imported from views/icons/{completions,tags}/*.svg by sparkles.twoslash.icons.

With renderDocsMarkdown (default on), hover/query docs render as block markdown and each @tag value as inline markdown, via the MdDoc → HTML emitter in sparkles:syntax (renderMarkdownHtml / renderMarkdownInlineHtml, Shiki's renderMarkdown/renderMarkdownInline seam). It degrades to escaped text automatically when the markdown grammars are unavailable, so no grammar bundle is required for correct — if plainer — output.

The 15-markdown-docs fixture is the showcase: hue --twoslash --html fixtures/15-markdown-docs.twoslash.json, then hover the answer token. To exercise that same MdDoc → HTML emitter without twoslash, hue --markdown <file.md> renders any Markdown file straight through extractMarkdown + renderMarkdownHtml.

Both take the snippet already highlighted into events (over tw.code) and the cache used to re-highlight popup signatures. Neither is @nogc.

Stylesheet (sparkles.twoslash.style)

• enum twoslashStyleCss — the ported style-rich.css, compiled in.
• writeTwoslashStyles(ref Writer w) — write it (no <style> wrapper). Styles only the .twoslash-* chrome; syntax token colors come from writeThemeStylesheet (.syn-*).

GUI

hue --gui --twoslash <nodes.json> — the raylib overlay (apps/hue, runGuiTwoslash, version(HueGui)).