diff --git a/frontend/src/utils/designTokens.ts b/frontend/src/utils/designTokens.ts
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+/**
+ * Design-token inventory — what tokens exist, and what they actually resolve to.
+ *
+ * Foundation for the design explorer (milestone #251): the gallery renders
+ * against these, and the drift panel compares them to the design rulebook.
+ *
+ * DESIGN NOTE — why this parses NAMES but never VALUES.
+ * Extracting `--foo` from a stylesheet is a trivial, robust regex. Extracting
+ * its VALUE is not: values contain nested parens, commas inside rgba(),
+ * `var()` references to other tokens, multi-part shadows, and gradients — and
+ * `theme.css` has all of those today. So we take the names from the source and
+ * ask the BROWSER for every value.
+ *
+ * That is not just easier, it is more correct. getComputedStyle reports what
+ * actually won the cascade, resolves `var()` chains, and — critically for this
+ * milestone — reflects live overrides set on a container, which is exactly what
+ * the preview surface needs (see #2261). Parsing the source would report what
+ * the file says rather than what the user is looking at.
+ *
+ * It also means this module needs no unit tests to be trustworthy: the only
+ * logic here is a name regex and a group lookup. The frontend has no test
+ * runner today (`vue-tsc --noEmit` is the whole check), so keeping the
+ * error-prone half in the browser rather than in our code is deliberate.
+ */
+import themeCss from "@/assets/theme.css?raw";
+
+export type TokenGroup =
+  | "color"
+  | "radius"
+  | "gradient"
+  | "glow"
+  | "focus"
+  | "layout"
+  | "other";
+
+export type ThemeMode = "light" | "dark";
+
+export interface DesignToken {
+  /** Full custom-property name, including the leading `--`. */
+  name: string;
+  /** Coarse family, derived from the name prefix. */
+  group: TokenGroup;
+  /** Resolved value in the requested context, straight from the browser. */
+  value: string;
+  /** True when the declaration appears inside the dark block in source. */
+  overriddenInDark: boolean;
+}
+
+/**
+ * Matches a custom-property DECLARATION, and never a `var(--name)` use.
+ *
+ * The discriminator is the COLON, not the preceding character. A declaration is
+ * `--name:`; a reference is `var(--name)` or `var(--name, fallback)` — followed
+ * by `)` or `,`, never by `:`. So no anchor is needed, and adding one is
+ * actively wrong: an earlier version required the match to follow `{` or `;`,
+ * which silently dropped every declaration that came after a comment —
+ * including `--color-bg`, the first and most-used token in the file.
+ */
+const DECLARATION = /(--[A-Za-z0-9_-]+)\s*:/g;
+
+/** Comments are stripped first so a commented-out declaration isn't counted. */
+const COMMENT = /\/\*[\s\S]*?\*\//g;
+
+/** The dark block's selector, as written in theme.css. */
+const DARK_SELECTOR = '[data-theme="dark"]';
+
+const GROUP_PREFIXES: ReadonlyArray<[string, TokenGroup]> = [
+  ["--color-", "color"],
+  ["--radius-", "radius"],
+  ["--gradient-", "gradient"],
+  ["--glow-", "glow"],
+  ["--focus-", "focus"],
+  ["--page-", "layout"],
+  ["--sidebar-", "layout"],
+  ["--chat-", "layout"],
+];
+
+export function groupFor(name: string): TokenGroup {
+  for (const [prefix, group] of GROUP_PREFIXES) {
+    if (name.startsWith(prefix)) return group;
+  }
+  return "other";
+}
+
+/** Every custom property declared anywhere in the stylesheet, in source order, deduped. */
+export function tokenNames(css: string = themeCss): string[] {
+  const seen = new Set<string>();
+  const out: string[] = [];
+  for (const match of css.replace(COMMENT, "").matchAll(DECLARATION)) {
+    const name = match[1];
+    if (!seen.has(name)) {
+      seen.add(name);
+      out.push(name);
+    }
+  }
+  return out;
+}
+
+/** The subset re-declared inside the dark block — i.e. tokens that change with mode. */
+export function darkOverriddenNames(css: string = themeCss): Set<string> {
+  const bare = css.replace(COMMENT, "");
+  const start = bare.indexOf(DARK_SELECTOR);
+  if (start === -1) return new Set();
+  const open = bare.indexOf("{", start);
+  const close = bare.indexOf("}", open);
+  if (open === -1 || close === -1) return new Set();
+  return new Set(tokenNames(bare.slice(open, close)));
+}
+
+/**
+ * Read the resolved value of every token in `host`'s context.
+ *
+ * Pass a container to read the tokens as they apply INSIDE it — which is how
+ * the preview surface reads a scoped override without disturbing the page.
+ * Defaults to the document root, i.e. the app-wide values.
+ */
+export function readTokens(host: Element = document.documentElement): DesignToken[] {
+  const computed = getComputedStyle(host);
+  const dark = darkOverriddenNames();
+  return tokenNames().map((name) => ({
+    name,
+    group: groupFor(name),
+    value: computed.getPropertyValue(name).trim(),
+    overriddenInDark: dark.has(name),
+  }));
+}
+
+/**
+ * Read tokens as they would resolve in a given mode, without touching the page.
+ *
+ * Uses an offscreen probe carrying the mode attribute, so the live UI is never
+ * mutated to take a reading.
+ *
+ * KNOWN LIMITATION, and it is a property of the stylesheet rather than of this
+ * function: light is declared on `:root` while dark is declared on
+ * `[data-theme="dark"]`. An attribute selector can ADD the dark values to a
+ * subtree, but there is no `[data-theme="light"]` block to add the light ones
+ * back. So reading "light" from inside a dark page returns the dark values —
+ * the probe has nothing to match.
+ *
+ * Concretely: dark-inside-light previews work, light-inside-dark previews do
+ * not. Introducing a `[data-theme="light"]` block alongside the dark-first flip
+ * (milestone #251 step 6) is what makes this symmetric, and until then callers
+ * should treat a cross-mode read as best-effort.
+ */
+export function readTokensForMode(mode: ThemeMode): DesignToken[] {
+  const probe = document.createElement("div");
+  probe.setAttribute("data-theme", mode);
+  probe.style.display = "none";
+  document.body.appendChild(probe);
+  try {
+    return readTokens(probe);
+  } finally {
+    probe.remove();
+  }
+}
+
+/** Tokens grouped by family, preserving source order within each group. */
+export function groupTokens(tokens: DesignToken[]): Map<TokenGroup, DesignToken[]> {
+  const out = new Map<TokenGroup, DesignToken[]>();
+  for (const token of tokens) {
+    const bucket = out.get(token.group);
+    if (bucket) bucket.push(token);
+    else out.set(token.group, [token]);
+  }
+  return out;
+}
+
+/**
+ * Tokens declared in the stylesheet that nothing references with `var()`.
+ *
+ * Dead tokens are drift too: `--chat-reading-width` and
+ * `--chat-context-sidebar-width` outlived the chat subsystem that was deleted
+ * in the MCP-first pivot, and nothing has referenced them since. Takes the
+ * corpus of source files to search as an argument so the caller decides what
+ * "used" means — this module has no opinion about the project layout.
+ */
+export function unreferencedTokens(tokens: DesignToken[], sources: string[]): DesignToken[] {
+  const haystack = sources.join("\n");
+  return tokens.filter((token) => !haystack.includes(`var(${token.name}`));
+}