diff --git a/.github/scripts/visual-gate/lib/sources.mjs b/.github/scripts/visual-gate/lib/sources.mjs
index 3695985fec1fd..1945d71926472 100644
--- a/.github/scripts/visual-gate/lib/sources.mjs
+++ b/.github/scripts/visual-gate/lib/sources.mjs
@@ -17,10 +17,49 @@
  * two lists.
  */
 
+import {execFileSync} from 'node:child_process';
 import * as fs from 'node:fs';
 import * as path from 'node:path';
 import {pathToFileURL} from 'node:url';
 
+/** The compiled entry a built theme's own `@astryxdesign/core/theme` import resolves to. */
+const CORE_THEME_ENTRY = 'packages/core/dist/theme/index.js';
+
+const BUILD_TIMEOUT_MS = 300_000;
+
+/**
+ * @param {string} repoRoot
+ * @param {string} pkg
+ */
+function pnpmBuild(repoRoot, pkg) {
+  execFileSync('pnpm', ['-F', pkg, 'build'], {
+    cwd: repoRoot,
+    stdio: 'inherit',
+    timeout: BUILD_TIMEOUT_MS,
+  });
+}
+
+/**
+ * @param {string} file
+ * @param {string} [query] - cache-buster, so a post-build retry is a fresh import
+ * @returns {Promise<{ok: true, module: Record<string, any>} | {ok: false, error: Error}>}
+ */
+async function importBuilt(file, query = '') {
+  try {
+    return {ok: true, module: await import(pathToFileURL(file).href + query)};
+  } catch (error) {
+    return {ok: false, error: /** @type {Error} */ (error)};
+  }
+}
+
+/**
+ * @param {string} dir
+ * @returns {string}
+ */
+function packageName(dir) {
+  return JSON.parse(fs.readFileSync(path.join(dir, 'package.json'), 'utf8')).name;
+}
+
 /**
  * @param {string} repoRoot
  * @returns {Promise<import('../../../../packages/cli/foundation/discovery/theming-targets.mjs').ThemingTarget[]>}
@@ -48,25 +87,59 @@ export async function loadThemingTargets(repoRoot) {
  * probe tier answers in 128 with a set cover. It is a coverage instrument, not
  * a theme someone ships.
  *
+ * Builds what it cannot read. On CI these dists arrive as an artifact from the
+ * job that already built them, and three changes in one day to how that
+ * artifact is named and filled each reddened the gate on every open component
+ * PR with a missing-file error about something no PR had touched. The gate owns its own
+ * prerequisites now, so how they get onto the runner is an optimisation rather
+ * than a correctness dependency.
+ *
  * @param {string} repoRoot
  * @param {string} [probeTheme] - name of the coverage fixture to leave out
+ * @param {(repoRoot: string, pkg: string) => void} [build] - seam for tests
  * @returns {Promise<Record<string, Record<string, string[]>>>}
  */
-export async function loadThemeOverrides(repoRoot, probeTheme = 'probe') {
+export async function loadThemeOverrides(repoRoot, probeTheme = 'probe', build = pnpmBuild) {
   const themesDir = path.join(repoRoot, 'packages/themes');
   /** @type {Record<string, Record<string, string[]>>} */
   const overrides = {};
+  /** @type {string[]} */
+  const rebuilt = [];
+
+  /**
+   * @param {string} pkg
+   * @param {string} why
+   */
+  const buildOnce = (pkg, why) => {
+    if (rebuilt.includes(pkg)) return;
+    rebuilt.push(pkg);
+    console.log(`visual gate: ${why}; building ${pkg} here rather than failing.`);
+    build(repoRoot, pkg);
+  };
 
   for (const entry of fs.readdirSync(themesDir, {withFileTypes: true}).sort()) {
     if (!entry.isDirectory()) continue;
-    const built = path.join(themesDir, entry.name, 'dist/source.mjs');
-    if (!fs.existsSync(built)) {
-      throw new Error(
-        `Theme ${entry.name} is not built (${built} missing) — run pnpm build before the visual gate.`,
-      );
+    const dir = path.join(themesDir, entry.name);
+    const built = path.join(dir, 'dist/source.mjs');
+
+    let loaded = await importBuilt(built);
+    if (!loaded.ok) {
+      if (!fs.existsSync(path.join(repoRoot, CORE_THEME_ENTRY))) {
+        buildOnce(
+          '@astryxdesign/core',
+          `${CORE_THEME_ENTRY} is missing and every built theme imports it`,
+        );
+      }
+      if (!fs.existsSync(built)) {
+        buildOnce(packageName(dir), `${path.relative(repoRoot, built)} is missing`);
+      }
+      loaded = await importBuilt(built, '?rebuilt');
+      if (!loaded.ok) {
+        throw unreadableTheme(repoRoot, entry.name, built, loaded.error, rebuilt);
+      }
     }
-    const module = await import(pathToFileURL(built).href);
-    const theme = Object.values(module).find(value => value?.name && value?.components);
+
+    const theme = Object.values(loaded.module).find(value => value?.name && value?.components);
     if (!theme || theme.name === probeTheme) continue;
     overrides[theme.name] = Object.fromEntries(
       Object.entries(theme.components).map(([key, styles]) => [key, Object.keys(styles ?? {})]),
@@ -76,6 +149,29 @@ export async function loadThemeOverrides(repoRoot, probeTheme = 'probe') {
   return overrides;
 }
 
+/**
+ * @param {string} repoRoot
+ * @param {string} name
+ * @param {string} built
+ * @param {Error} error
+ * @param {string[]} rebuilt
+ * @returns {Error}
+ */
+function unreadableTheme(repoRoot, name, built, error, rebuilt) {
+  const rel = path.relative(repoRoot, built);
+  return new Error(
+    [
+      `The visual gate could not load theme ${name} from ${rel}.`,
+      `  ${error.message}`,
+      rebuilt.length
+        ? `  The gate rebuilt ${rebuilt.join(' and ')} here and the import still fails, so this is a broken build rather than a missing one.`
+        : `  Both ${rel} and ${CORE_THEME_ENTRY} are present, so this is not a missing build.`,
+      `The gate reads each theme's BUILT source because a theme's component map is what defineTheme returns, not a literal in its source.`,
+    ].join('\n'),
+    {cause: error},
+  );
+}
+
 /**
  * @param {string} repoRoot
  * @returns {{excludeStories: Record<string, string>, viewport: {width: number, height: number}, settleMs: number, threshold: number, maxDiffPixels: number, defaultTheme: string, probeTheme: string, tiers: string[]}}
diff --git a/.github/scripts/visual-gate/lib/sources.test.mjs b/.github/scripts/visual-gate/lib/sources.test.mjs
new file mode 100644
index 0000000000000..0e8181dcdf2ad
--- /dev/null
+++ b/.github/scripts/visual-gate/lib/sources.test.mjs
@@ -0,0 +1,94 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+
+import {afterEach, describe, expect, it} from 'vitest';
+
+import {loadThemeOverrides} from './sources.mjs';
+
+/** @type {string[]} */
+const made = [];
+
+/**
+ * A repo root with `packages/core/dist` present and one theme package, whose
+ * built source is written only when `built` is true.
+ *
+ * @param {{built: boolean}} options
+ */
+function fixture({built}) {
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), 'visual-gate-sources-'));
+  made.push(root);
+  fs.mkdirSync(path.join(root, 'packages/core/dist/theme'), {recursive: true});
+  fs.writeFileSync(path.join(root, 'packages/core/dist/theme/index.js'), '');
+  const dir = path.join(root, 'packages/themes/butter');
+  fs.mkdirSync(path.join(dir, 'dist'), {recursive: true});
+  fs.writeFileSync(
+    path.join(dir, 'package.json'),
+    JSON.stringify({name: '@astryxdesign/theme-butter'}),
+  );
+  if (built) writeBuilt(dir);
+  return {root, dir};
+}
+
+/** @param {string} dir */
+function writeBuilt(dir) {
+  fs.writeFileSync(
+    path.join(dir, 'dist/source.mjs'),
+    "export const theme = {name: 'butter', components: {badge: {base: {}, 'variant:info': {}}}};\n",
+  );
+}
+
+afterEach(() => {
+  for (const root of made.splice(0)) fs.rmSync(root, {recursive: true, force: true});
+});
+
+describe('loadThemeOverrides', () => {
+  it('reads a built theme without building anything', async () => {
+    const {root} = fixture({built: true});
+    /** @type {string[]} */
+    const builds = [];
+
+    const overrides = await loadThemeOverrides(root, 'probe', (_root, pkg) => builds.push(pkg));
+
+    expect(overrides).toEqual({butter: {badge: ['base', 'variant:info']}});
+    expect(builds).toEqual([]);
+  });
+
+  it('builds the theme it cannot read, so a missing CI artifact is not a failure', async () => {
+    const {root, dir} = fixture({built: false});
+    /** @type {string[]} */
+    const builds = [];
+
+    const overrides = await loadThemeOverrides(root, 'probe', (_root, pkg) => {
+      builds.push(pkg);
+      writeBuilt(dir);
+    });
+
+    expect(builds).toEqual(['@astryxdesign/theme-butter']);
+    expect(overrides).toEqual({butter: {badge: ['base', 'variant:info']}});
+  });
+
+  it('builds core first when its dist is the thing missing', async () => {
+    const {root, dir} = fixture({built: false});
+    fs.rmSync(path.join(root, 'packages/core/dist'), {recursive: true});
+    /** @type {string[]} */
+    const builds = [];
+
+    await loadThemeOverrides(root, 'probe', (_root, pkg) => {
+      builds.push(pkg);
+      if (pkg === '@astryxdesign/theme-butter') writeBuilt(dir);
+    });
+
+    expect(builds).toEqual(['@astryxdesign/core', '@astryxdesign/theme-butter']);
+  });
+
+  it('says it tried when the rebuild does not produce a loadable theme', async () => {
+    const {root} = fixture({built: false});
+
+    await expect(loadThemeOverrides(root, 'probe', () => {})).rejects.toThrow(
+      /could not load theme butter[\s\S]*rebuilt @astryxdesign\/theme-butter here and the import still fails/,
+    );
+  });
+});
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ef674e8c6c43e..b6a49daf6bf3c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -582,7 +582,12 @@ jobs:
           name: storybook-${{ needs.build-storybook.outputs.short_hash }}
           path: apps/storybook/dist
 
+      # Non-fatal: if this artifact is ever missing, renamed, or produced under
+      # a different key, the gate builds the themes itself. A crash here would
+      # be before the gate ever runs, which is how an artifact-wiring change
+      # reddened every open component PR twice in one day.
       - name: Download built themes and core
+        continue-on-error: true
         uses: actions/download-artifact@v8
         with:
           name: dists-${{ needs.build-storybook.outputs.short_hash }}