diff --git a/frontend/packages/dashboard/build.mjs b/frontend/packages/dashboard/build.mjs
index 35427bf0..7276a80a 100644
--- a/frontend/packages/dashboard/build.mjs
+++ b/frontend/packages/dashboard/build.mjs
@@ -74,9 +74,9 @@ await build({
   target: ['es2022'],
   sourcemap: true,
   logLevel: 'info',
-  // `@hive/shared/modal.js` imports its shadow-DOM component CSS
-  // (hive-dialog.css, hive-toast.css, component-common.css) as raw text
-  // via a plain `import css from './foo.css'` — the `text` loader turns
+  // `@hive/shared/modal.js` and `hive-btn.js` import their shadow-DOM
+  // component CSS (hive-dialog.css, hive-toast.css, hive-btn.css) as raw
+  // text via a plain `import css from './foo.css'` — the `text` loader turns
   // that into a string constant at bundle time instead of erroring on an
   // unrecognised extension. None of these JS entries import a `.css`
   // file any other way, so this doesn't collide with the separate
diff --git a/frontend/packages/shared/package.json b/frontend/packages/shared/package.json
index cfd034d9..333bcd63 100644
--- a/frontend/packages/shared/package.json
+++ b/frontend/packages/shared/package.json
@@ -17,8 +17,7 @@
     "./chrome.css": "./src/chrome.css",
     "./forms.js": "./src/forms.js",
     "./dom.js": "./src/dom.js",
-    "./modal.js": "./src/modal.js",
-    "./component-styles.js": "./src/component-styles.js"
+    "./modal.js": "./src/modal.js"
   },
   "files": [
     "src/"
diff --git a/frontend/packages/shared/src/component-common.css b/frontend/packages/shared/src/component-common.css
deleted file mode 100644
index 2cc892f0..00000000
--- a/frontend/packages/shared/src/component-common.css
+++ /dev/null
@@ -1,46 +0,0 @@
-/* component-common.css — the shared stylesheet adopted alongside every
-   shadow-DOM custom element's own scoped CSS (`adoptedStyleSheets`), the
-   native equivalent of a Sass `@include`. Loaded as raw text at build time
-   (esbuild's `text` loader — see build.mjs) and turned into a
-   `CSSStyleSheet` via `replaceSync()` in component-styles.js; a real .css
-   file, not a JS template string, so it gets normal editor tooling and
-   stays syntactically obvious as CSS.
-
-   NOTE: `.btn` here is a near-duplicate of the app-wide `.btn` base rule
-   in dashboard/src/common.css and agent/src/agent.css, which already
-   differ slightly from each other. Shadow DOM can't see either — a
-   shadow component that wants the same button chrome needs its own copy,
-   so this is now a third copy. Picked dashboard's version (the fuller
-   one) as canonical for shadow components going forward. Unifying all
-   three into one source of truth is a separate follow-up (see
-   hyperhive's forge issue tracker: the `<hive-btn>` component proposal),
-   not bundled into whichever component conversion first needed a
-   button. */
-
-.btn {
-  font-family: inherit;
-  font-weight: bold;
-  text-transform: uppercase;
-  letter-spacing: 0.1em;
-  background: transparent;
-  -webkit-appearance: none;
-  appearance: none;
-  border: 1px solid;
-  padding: 0.25em 0.8em;
-  cursor: pointer;
-  text-shadow: 0 0 4px currentColor;
-  box-shadow: 0 0 0 0 currentColor;
-  transition: box-shadow 0.15s ease;
-}
-.btn:hover {
-  background: color-mix(in srgb, var(--fg) 6%, transparent);
-  text-shadow: 0 0 10px currentColor;
-  box-shadow: 0 0 10px -2px currentColor;
-}
-.btn:disabled,
-.btn[disabled] {
-  opacity: 0.32;
-  cursor: not-allowed;
-  text-shadow: none;
-  box-shadow: none;
-}
diff --git a/frontend/packages/shared/src/component-styles.js b/frontend/packages/shared/src/component-styles.js
deleted file mode 100644
index a488b36d..00000000
--- a/frontend/packages/shared/src/component-styles.js
+++ /dev/null
@@ -1,30 +0,0 @@
-// Shared, adopted-alongside-component-specific stylesheets for shadow-DOM
-// custom elements — the native equivalent of a Sass `@include`: a
-// `CSSStyleSheet` is parsed once and adopted by reference
-// (`shadowRoot.adoptedStyleSheets = [sharedComponentStyleSheet(), ownSheet]`)
-// into as many shadow roots as want it, rather than each component
-// duplicating the rule text or re-fetching an external stylesheet.
-//
-// The CSS itself lives in `component-common.css` — a real stylesheet, not
-// a JS template string — imported here as raw text via esbuild's `text`
-// loader (see both packages' `build.mjs`) and turned into a
-// `CSSStyleSheet` at module-init time. Component-scoped stylesheets
-// (`hive-dialog.css`, `hive-toast.css` in modal.js) follow the same
-// pattern.
-//
-// Start small: `.btn` is the one rule shadow-DOM components have needed so
-// far (the dialog buttons in modal.js). Add more shared rules to
-// component-common.css as more components need them — don't grow this
-// into a full reset/utility layer preemptively.
-
-import componentCommonCss from './component-common.css';
-
-let sheet = null;
-
-export function sharedComponentStyleSheet() {
-  if (!sheet) {
-    sheet = new CSSStyleSheet();
-    sheet.replaceSync(componentCommonCss);
-  }
-  return sheet;
-}
diff --git a/frontend/packages/shared/src/dom.js b/frontend/packages/shared/src/dom.js
index da314e69..b4b5f4ef 100644
--- a/frontend/packages/shared/src/dom.js
+++ b/frontend/packages/shared/src/dom.js
@@ -5,12 +5,18 @@
 // used).
 //
 // `el(tag, attrs, ...children)` creates an element, applying `attrs` as
-// either the `class`/`html` special cases or plain attributes, and
+// either the `class`/`html`/`is` special cases or plain attributes, and
 // appending `children` (strings become text nodes, `null`/`undefined`
 // entries are skipped so callers can inline conditional children).
+// `is: 'custom-name'` creates a customized built-in element (e.g.
+// `el('button', { is: 'hive-btn' }, 'label')` → `<button is="hive-btn">`)
+// — passed to `document.createElement` itself, since a customized
+// built-in has to be created with its `is` option up front, not upgraded
+// after the fact via `setAttribute`.
 export const el = (tag, attrs = {}, ...children) => {
-  const e = document.createElement(tag);
+  const e = attrs.is ? document.createElement(tag, { is: attrs.is }) : document.createElement(tag);
   for (const [k, v] of Object.entries(attrs)) {
+    if (k === 'is') continue;
     if (k === 'class') e.className = v;
     else if (k === 'html') e.innerHTML = v;
     else e.setAttribute(k, v);
diff --git a/frontend/packages/shared/src/hive-btn.css b/frontend/packages/shared/src/hive-btn.css
new file mode 100644
index 00000000..0d5c96d9
--- /dev/null
+++ b/frontend/packages/shared/src/hive-btn.css
@@ -0,0 +1,40 @@
+/* hive-btn.css — scoped stylesheet for the <hive-btn> customized built-in
+   button element (hive-btn.js). Loaded as raw text at build time
+   (esbuild's `text` loader) and turned into a `CSSStyleSheet` adopted by
+   the element's own shadow root. `:host` styles the button itself (the
+   element *is* a real `<button is="hive-btn">`, so `:host(:hover)` /
+   `:host(:disabled)` select genuine native pseudo-classes, not
+   hand-rolled state tracking) — this is the base look every `<hive-btn>`
+   gets; `variant` is an attribute (not a class) since it's a semantic
+   property of the component, not an arbitrary styling hook. */
+
+:host {
+  font-family: inherit;
+  font-weight: bold;
+  text-transform: uppercase;
+  letter-spacing: 0.1em;
+  background: transparent;
+  -webkit-appearance: none;
+  appearance: none;
+  border: 1px solid;
+  padding: 0.25em 0.8em;
+  cursor: pointer;
+  color: inherit;
+  text-shadow: 0 0 4px currentColor;
+  box-shadow: 0 0 0 0 currentColor;
+  transition: box-shadow 0.15s ease;
+}
+:host(:hover) {
+  background: color-mix(in srgb, var(--fg) 6%, transparent);
+  text-shadow: 0 0 10px currentColor;
+  box-shadow: 0 0 10px -2px currentColor;
+}
+:host(:disabled) {
+  opacity: 0.32;
+  cursor: not-allowed;
+  text-shadow: none;
+  box-shadow: none;
+}
+:host([variant="cancel"]) { color: var(--subtext0); }
+:host([variant="confirm"]) { color: var(--green); }
+:host([variant="danger"]) { color: var(--red); }
diff --git a/frontend/packages/shared/src/hive-btn.js b/frontend/packages/shared/src/hive-btn.js
new file mode 100644
index 00000000..d05b0a48
--- /dev/null
+++ b/frontend/packages/shared/src/hive-btn.js
@@ -0,0 +1,40 @@
+// hive-btn.js — <hive-btn>, a customized built-in `<button>` (`<button
+// is="hive-btn">`) with a shadow root for style encapsulation, replacing
+// the light-DOM `.btn` class as the button primitive for shadow-DOM
+// consumers (starting with modal.js's dialog buttons — see the frontend
+// components-split discussion on the forge issue tracker).
+//
+// Extending `HTMLButtonElement` (a "customized built-in element", not an
+// autonomous one) keeps every native `<button>` behaviour for free —
+// click/keyboard activation, `:disabled`, form participation/submission —
+// instead of re-implementing them on a generic wrapper. The shadow root
+// holds only a `<style>`-equivalent (adopted stylesheet) plus a `<slot>`
+// so the button's light-DOM content (its label, or asyncBtn's swapped-in
+// spinner span) renders through unchanged — slotted content stays styled
+// by the light-DOM cascade (global `.spinner` etc. still apply), only the
+// host element's own box/text styling is shadow-scoped.
+//
+// Customized built-ins aren't supported in Safari/WebKit (a deliberate
+// WebKit-team stance, unlikely to change) — fine here since the project
+// targets recent Firefox only.
+//
+// Usage: `el('button', { type: 'button', is: 'hive-btn', variant: 'danger' }, 'label')`
+// (dom.js's `el()` passes `is` to `document.createElement` so custom-
+// element upgrade happens at creation, not after). `variant` is one of
+// 'cancel' | 'confirm' | 'danger' | (unset, for the neutral/default look)
+// — a plain attribute, not a class, since it's a semantic prop of the
+// component rather than an arbitrary styling hook.
+
+import hiveBtnCss from './hive-btn.css';
+
+class HiveBtn extends HTMLButtonElement {
+  connectedCallback() {
+    if (this.shadowRoot) return; // guard: connectedCallback can re-fire (e.g. re-parenting)
+    const root = this.attachShadow({ mode: 'open' });
+    const sheet = new CSSStyleSheet();
+    sheet.replaceSync(hiveBtnCss);
+    root.adoptedStyleSheets = [sheet];
+    root.append(document.createElement('slot'));
+  }
+}
+customElements.define('hive-btn', HiveBtn, { extends: 'button' });
diff --git a/frontend/packages/shared/src/hive-dialog.css b/frontend/packages/shared/src/hive-dialog.css
index 54576974..ec760ddb 100644
--- a/frontend/packages/shared/src/hive-dialog.css
+++ b/frontend/packages/shared/src/hive-dialog.css
@@ -1,8 +1,10 @@
 /* hive-dialog.css — scoped stylesheet for the <hive-dialog> shadow-DOM
    custom element (modal.js). Loaded as raw text at build time (esbuild's
-   `text` loader) and adopted via `adoptedStyleSheets` alongside
-   component-common.css. `:host` styles the element itself, which *is*
-   the fixed-position backdrop — no separate light-DOM backdrop div. */
+   `text` loader) and adopted via `adoptedStyleSheets`. `:host` styles the
+   element itself, which *is* the fixed-position backdrop — no separate
+   light-DOM backdrop div. Button styling (base look + cancel/confirm/
+   danger variants) lives in hive-btn.css, not here — the dialog's
+   buttons are <hive-btn> elements with their own shadow root. */
 
 :host {
   position: fixed;
@@ -50,9 +52,6 @@
   gap: 0.6em;
   margin-top: 0.2em;
 }
-.cancel { color: var(--subtext0); }
-.confirm { color: var(--green); }
-.confirm.danger { color: var(--red); }
 .promptfield { display: flex; flex-direction: column; gap: 0.4em; }
 .promptlabel { color: var(--subtext0); font-size: 0.9em; }
 .input {
diff --git a/frontend/packages/shared/src/modal.js b/frontend/packages/shared/src/modal.js
index 9837cbc9..5052a6d8 100644
--- a/frontend/packages/shared/src/modal.js
+++ b/frontend/packages/shared/src/modal.js
@@ -5,45 +5,33 @@
 // dialog.
 //
 // Implemented as two shadow-DOM custom elements (`<hive-dialog>`,
-// `<hive-toast>`). Each attaches its own shadow root and adopts a
-// component-scoped `CSSStyleSheet` alongside the shared one in
-// `component-styles.js` (the native equivalent of a Sass `@include`) —
-// real style encapsulation, no global `.tc-*` class namespace needed any
-// more, and no stylesheet to import from every page. The CSS itself
-// lives in real `.css` files (`hive-dialog.css`, `hive-toast.css`,
-// `component-common.css`), imported here as raw text via esbuild's
-// `text` loader and turned into `CSSStyleSheet`s at runtime — not JS
-// template strings, so it stays normal, editor-tooled CSS. Theme vars
-// (`--bg`, `--purple`, …) keep resolving inside the shadow tree
-// automatically: CSS custom properties inherit through shadow
-// boundaries even though plain class rules don't. This started as a
-// light-DOM pilot (see the frontend components-split discussion on the
-// forge issue tracker) — light DOM validated the lifecycle-
-// encapsulation win (`connectedCallback`/`disconnectedCallback` owning
-// the keydown listener / auto-dismiss timer) cheaply; this is the
-// shadow-DOM follow-up once real per-component style scoping was worth
-// the cost.
+// `<hive-toast>`), plus `<hive-btn>` (hive-btn.js) for the dialog's own
+// buttons — real per-component style encapsulation, CSS in real `.css`
+// files imported as raw text (see each component's own header comment
+// for the design rationale: shadow-DOM-vs-light-DOM tradeoffs live atop
+// the `<hive-dialog>`/`<hive-toast>` classes below, the customized-
+// built-in choice lives in hive-btn.js). Other `.btn` consumers across
+// the app stay on the light-DOM `.btn` class for now — migrating them
+// is a separate follow-up.
 //
 // `openDialog` is the general primitive (any title/message/content + a row of
 // buttons); `themedConfirm` is a thin cancel/confirm wrapper with optional
 // checkboxes built on top of it.
 
 import { el } from './dom.js';
-import { sharedComponentStyleSheet } from './component-styles.js';
+import './hive-btn.js'; // registers <hive-btn> — side-effect import, no named export needed
 import dialogCss from './hive-dialog.css';
 import toastCss from './hive-toast.css';
 
-// Attach an open shadow root to `host`, adopt the shared component
-// stylesheet plus `ownCssText` (a scoped stylesheet built fresh per call
-// from a real .css file's contents, imported as raw text — see
-// component-styles.js's header comment; only the shared one is
-// cached/reused across instances), and return the shadow root for the
-// caller to populate.
+// Attach an open shadow root to `host`, adopt `ownCssText` (a scoped
+// stylesheet built fresh per call from a real .css file's contents,
+// imported as raw text), and return the shadow root for the caller to
+// populate.
 function attachShadow(host, ownCssText) {
   const root = host.attachShadow({ mode: 'open' });
-  const own = new CSSStyleSheet();
-  own.replaceSync(ownCssText);
-  root.adoptedStyleSheets = [sharedComponentStyleSheet(), own];
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync(ownCssText);
+  root.adoptedStyleSheets = [sheet];
   return root;
 }
 
@@ -86,9 +74,13 @@ class HiveDialog extends HTMLElement {
     this._done = done; // exposed for close-on-escape-elsewhere callers, if ever needed
 
     const btnEls = buttons.map((b) => {
+      // `b.class` names the variant ('cancel' | 'confirm'); `b.danger`
+      // overrides it to the 'danger' look regardless (a destructive
+      // confirm button reads as danger, not as a plain confirm).
+      const variant = b.danger ? 'danger' : b.class;
       const btn = el('button', {
-        type: 'button',
-        class: 'btn' + (b.class ? ' ' + b.class : '') + (b.danger ? ' danger' : ''),
+        type: 'button', is: 'hive-btn',
+        ...(variant ? { variant } : {}),
       }, b.label);
       btn.addEventListener('click', () => done(b.value));
       return { spec: b, btn };
@@ -207,12 +199,13 @@ export function themedPrompt(opts = {}) {
   // Enter submits (clicks the confirm button mounted by openDialog);
   // Shift+Enter falls through to the textarea's default newline insert.
   // `closest()`/`querySelector()` stay within the shadow tree the input
-  // lives in, so this resolves to the dialog's own `.box`/`.confirm`
-  // without leaking across instances.
+  // lives in, so this resolves to the dialog's own `.box`/confirm button
+  // without leaking across instances. The confirm button is selected by
+  // its `variant` attribute now (hive-btn.js), not a CSS class.
   input.addEventListener('keydown', (e) => {
     if (e.key === 'Enter' && !e.shiftKey) {
       e.preventDefault();
-      input.closest('.box')?.querySelector('.confirm')?.click();
+      input.closest('.box')?.querySelector('[variant="confirm"]')?.click();
     }
   });
   const content = el('div', { class: 'promptfield' },