diff --git a/.github/workflows/design-diff.yml b/.github/workflows/design-diff.yml
new file mode 100644
index 0000000..4972a24
--- /dev/null
+++ b/.github/workflows/design-diff.yml
@@ -0,0 +1,78 @@
+name: DESIGN.md diff
+
+# Runs on PRs that touch DESIGN.md and posts a token-level delta as a
+# comment so reviewers can see added/removed/modified tokens without
+# reading a YAML diff by hand. Uses the upstream `@google/design.md diff`
+# command, which understands token paths and reports a structured report.
+
+on:
+  pull_request:
+    paths:
+      - DESIGN.md
+
+permissions:
+  pull-requests: write
+  contents: read
+
+jobs:
+  diff:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - name: Extract base DESIGN.md
+        run: git show "origin/${{ github.base_ref }}:DESIGN.md" > /tmp/DESIGN.base.md
+
+      - name: Run design.md diff
+        id: diff
+        run: |
+          bunx @google/design.md@latest diff /tmp/DESIGN.base.md DESIGN.md --format=json > /tmp/diff.json || true
+          echo 'report<<__END__' >> "$GITHUB_OUTPUT"
+          cat /tmp/diff.json >> "$GITHUB_OUTPUT"
+          echo '__END__' >> "$GITHUB_OUTPUT"
+
+      - name: Comment token delta
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs')
+            const raw = fs.readFileSync('/tmp/diff.json', 'utf8')
+            let report
+            try { report = JSON.parse(raw) } catch { report = null }
+            if (!report) {
+              console.log('No parseable diff output; skipping comment.')
+              return
+            }
+            const t = report.tokens ?? {}
+            const rows = []
+            for (const group of ['colors', 'typography', 'rounded', 'spacing', 'motion', 'components']) {
+              const g = t[group]
+              if (!g) continue
+              const added = g.added ?? []
+              const removed = g.removed ?? []
+              const modified = g.modified ?? []
+              if (!added.length && !removed.length && !modified.length) continue
+              rows.push(`**${group}** — +${added.length} added, -${removed.length} removed, ~${modified.length} modified`)
+              if (added.length) rows.push(`  added: \`${added.join('`, `')}\``)
+              if (removed.length) rows.push(`  removed: \`${removed.join('`, `')}\``)
+              if (modified.length) rows.push(`  modified: \`${modified.join('`, `')}\``)
+            }
+            const regression = report.regression === true ? '⚠️ **regression detected**' : ''
+            const body = rows.length
+              ? `### DESIGN.md token delta\n\n${regression}\n\n${rows.join('\n')}\n\n<details><summary>Raw JSON</summary>\n\n\`\`\`json\n${JSON.stringify(report, null, 2)}\n\`\`\`\n\n</details>`
+              : '### DESIGN.md token delta\n\nNo token-level changes detected (prose-only edits).'
+            const { owner, repo } = context.repo
+            const issue_number = context.payload.pull_request.number
+            const { data: comments } = await github.rest.issues.listComments({ owner, repo, issue_number })
+            const existing = comments.find(c => c.user?.type === 'Bot' && c.body?.startsWith('### DESIGN.md token delta'))
+            if (existing) {
+              await github.rest.issues.updateComment({ owner, repo, comment_id: existing.id, body })
+            } else {
+              await github.rest.issues.createComment({ owner, repo, issue_number, body })
+            }
diff --git a/DESIGN.md b/DESIGN.md
index 64d92c4..dc4ae92 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -79,6 +79,13 @@ components:
     textColor: "{colors.link}"
   link-hover:
     textColor: "{colors.link-hover}"
+voice:
+  formality: conversational
+  density: concise
+  capitalization: sentence-case
+  emoji: sparingly
+  errorTone: matter-of-fact
+  ctaStyle: short-verb
 variants:
   dark:
     colors:
@@ -144,6 +151,35 @@ Use `easing.standard` for general acceleration; `emphasized` for first-time
 reveals (enter animations). No bounce, no overshoot — streaming UI should feel
 precise, not playful.
 
+## Voice
+
+Stream-UI is rendered by an agent, so copy is part of the design system.
+Component labels, alert text, empty-state prose and button CTAs all inherit
+the tokens below. These apply to every string the agent streams into a spec.
+
+- **formality: conversational** — write like a helpful coworker, not a lawyer
+  and not a TV announcer. Contractions are fine. Second person ("you") is
+  welcome. Never "please click here" — respect the user's time.
+- **density: concise** — one sentence where one sentence works. No preamble
+  ("Here's a button for you!"), no restating the obvious. Agents often want
+  to narrate the spec they just emitted — don't; the UI shows itself.
+- **capitalization: sentence-case** — `Save changes`, not `Save Changes`.
+  Applies to buttons, headings, table headers, menu items. Preserves proper
+  nouns and brand names as written.
+- **emoji: sparingly** — reserved for status (✓ ✗ ⚠) or when the user's own
+  prompt establishes a playful tone. Never in error messages, never in CTAs.
+- **errorTone: matter-of-fact** — "Couldn't save — name is required" beats
+  "Oops! Something went wrong 😔". State the cause, suggest the fix, no
+  apology. Reserve apologies for our failures, not the user's input.
+- **ctaStyle: short-verb** — primary buttons use a single verb when possible
+  (`Save`, `Continue`, `Delete`), verb + object when not (`Add item`, not
+  `Add a new item to the list`). Cancel/dismiss buttons stay `Cancel` — not
+  `Nevermind`, not `Go back`, not `×`.
+
+When a prompt is ambiguous between two voices, pick the one that matches
+these tokens. When the user's prompt explicitly overrides (e.g. "make the
+copy playful"), honor the prompt.
+
 ## Components
 
 Component tokens map to stream-ui's built-in kinds. Variants (hover, pressed,
@@ -153,6 +189,28 @@ disabled) are expressed as sibling entries with a related key name —
 Valid component properties in this theme: `backgroundColor`, `textColor`,
 `rounded`.
 
+## Custom components
+
+stream-ui's `register(kind, renderer)` API lets consumers add domain-specific
+components. Those renderers should honor the design system by reading the
+CSS custom properties this file generates — there's no separate "register
+with DESIGN.md" step. The pattern is just plain CSS:
+
+```css
+.my-custom-card {
+  background: color-mix(in srgb, var(--sui-colors-neutral) 8%, transparent);
+  border: 1px solid var(--sui-colors-neutral);
+  border-radius: var(--sui-rounded-md);
+  padding: var(--sui-spacing-sm);
+}
+.my-custom-card--urgent {
+  border-left: 3px solid var(--sui-colors-error);
+}
+```
+
+Consumers get theme switching, dark mode, and runtime token overrides for
+free. The `kanban-card` example in the playground uses exactly this pattern.
+
 ## Do's and Don'ts
 
 - **Do** reference semantic tokens (`{colors.error}`) in derived themes instead
diff --git a/playground/style.css b/playground/style.css
index e698fca..3be6e90 100644
--- a/playground/style.css
+++ b/playground/style.css
@@ -501,34 +501,41 @@ body.resizing-col {
   font-style: italic;
 }
 
-/* ─── consumer-registered example: kanban-card ─────────────────────── */
+/* ─── consumer-registered example: kanban-card ─────────────────────────
+ * Custom components registered via stream-ui's `register()` API can honor
+ * the design system by reading DESIGN.md tokens directly — no extra spec
+ * surface required. Consumers write plain CSS against `--sui-colors-*`
+ * / `--sui-rounded-*` / `--sui-spacing-*` vars and get theme switching,
+ * dark mode, and per-app overrides for free.
+ */
 
 .kanban-card {
   display: flex;
   flex-direction: column;
-  gap: 0.4rem;
-  border: 1px solid rgba(127, 127, 127, 0.4);
+  gap: var(--sui-spacing-xs, 0.4rem);
+  border: 1px solid
+    color-mix(in srgb, var(--sui-colors-neutral, rgb(127, 127, 127)) 40%, transparent);
   border-left: 3px solid;
-  border-radius: 0.5rem;
-  padding: 0.6rem 0.85rem;
-  background: rgba(127, 127, 127, 0.05);
+  border-radius: var(--sui-rounded-md, 0.5rem);
+  padding: var(--sui-spacing-sm, 0.6rem) var(--sui-spacing-md, 0.85rem);
+  background: color-mix(in srgb, var(--sui-colors-neutral, rgb(127, 127, 127)) 8%, transparent);
 }
 
 .kanban-todo {
-  border-left-color: rgb(180, 180, 180);
+  border-left-color: var(--sui-colors-neutral, rgb(180, 180, 180));
 }
 .kanban-doing {
-  border-left-color: rgb(220, 170, 70);
+  border-left-color: var(--sui-colors-warning, rgb(220, 170, 70));
 }
 .kanban-done {
-  border-left-color: rgb(80, 180, 100);
+  border-left-color: var(--sui-colors-success, rgb(80, 180, 100));
 }
 
 .kanban-header {
   display: flex;
   align-items: center;
   justify-content: space-between;
-  gap: 0.5rem;
+  gap: var(--sui-spacing-sm, 0.5rem);
 }
 
 .kanban-title {
@@ -540,18 +547,18 @@ body.resizing-col {
   letter-spacing: 0.1em;
   font-weight: 700;
   padding: 0.1rem 0.4rem;
-  border-radius: 999px;
-  background: rgba(127, 127, 127, 0.2);
+  border-radius: var(--sui-rounded-pill, 999px);
+  background: color-mix(in srgb, var(--sui-colors-neutral, rgb(127, 127, 127)) 20%, transparent);
 }
 
 .kanban-todo .kanban-status {
-  color: rgb(180, 180, 180);
+  color: var(--sui-colors-neutral, rgb(180, 180, 180));
 }
 .kanban-doing .kanban-status {
-  color: rgb(220, 170, 70);
+  color: var(--sui-colors-warning, rgb(220, 170, 70));
 }
 .kanban-done .kanban-status {
-  color: rgb(80, 180, 100);
+  color: var(--sui-colors-success, rgb(80, 180, 100));
 }
 
 .kanban-assignee {
diff --git a/scripts/lint-design.ts b/scripts/lint-design.ts
index 5a2fbb6..bd4e38c 100644
--- a/scripts/lint-design.ts
+++ b/scripts/lint-design.ts
@@ -1,12 +1,12 @@
 #!/usr/bin/env bun
 /**
  * Runs @google/design.md lint on DESIGN.md, prints a readable summary,
- * and exits non-zero when structural errors are present.
+ * and exits non-zero when errors OR warnings are present.
  *
- * Warnings are surfaced but do NOT fail the build — they typically
- * represent design-quality findings (contrast, unused tokens) that are
- * better treated as review signals than merge gates. Tighten to fail
- * on warnings when the team has cleaned the backlog.
+ * Since the DESIGN.md baseline is currently at 0/0, any new warning is a
+ * regression — unused tokens, failing WCAG contrast, etc. Block those at
+ * PR time instead of letting them accumulate. If a warning becomes
+ * intentional (rare), fix the root cause or relax this script.
  */
 import { spawnSync } from 'node:child_process'
 
@@ -53,7 +53,9 @@ console.log(
   `\nSummary: ${summary.errors} error(s), ${summary.warnings} warning(s), ${summary.infos} info.`,
 )
 
-if (summary.errors > 0) {
-  console.error('\n[lint:design] structural errors — failing build.')
+if (summary.errors > 0 || summary.warnings > 0) {
+  console.error(
+    `\n[lint:design] ${summary.errors} error(s) + ${summary.warnings} warning(s) — failing build.`,
+  )
   process.exit(1)
 }