diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d012f9111..ffa605359 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -37,6 +37,7 @@ jobs:
       - run: bun run check:no-product-tenancy
       - run: bun run check:browser-safe-subpaths
       - run: bun run check:web-utilities
+      - run: bun run check:tailwind-source
       - run: bun run check:ui-vocabulary
       - run: bun run check:react-ui-drift
       - run: bun run check:react-ui-pin
diff --git a/apps/web/src/tailwind.css b/apps/web/src/tailwind.css
index 8c123f89b..9f0f02424 100644
--- a/apps/web/src/tailwind.css
+++ b/apps/web/src/tailwind.css
@@ -32,4 +32,6 @@
 @source "../../../packages/artifact-ui/src";
 @source "../../../packages/bench-ui/src";
 @source "../../../packages/chat-ui/src";
+@source "../../../packages/plugins-ui/src";
 @source "../../../packages/settings-ui/src";
+@source "../../../packages/tasks-ui/src";
diff --git a/package.json b/package.json
index a3c245b09..8031bda72 100644
--- a/package.json
+++ b/package.json
@@ -29,6 +29,7 @@
     "check:no-product-tenancy": "bun run scripts/checks/no-product-tenancy.ts",
     "check:browser-safe-subpaths": "bun run scripts/checks/browser-safe-subpaths.ts",
     "check:web-utilities": "bun run scripts/checks/web-tailwind-utilities.ts",
+    "check:tailwind-source": "bun run scripts/checks/tailwind-source.ts",
     "check:ui-vocabulary": "bun run scripts/checks/ui-vocabulary.ts",
     "check:react-ui-drift": "bun run scripts/checks/react-ui-drift.ts",
     "check:react-ui-pin": "bun run scripts/checks/react-ui-pin.ts",
diff --git a/scripts/checks/tailwind-source.ts b/scripts/checks/tailwind-source.ts
new file mode 100644
index 000000000..ac2fc30e5
--- /dev/null
+++ b/scripts/checks/tailwind-source.ts
@@ -0,0 +1,77 @@
+// check:tailwind-source — every package whose stylesheet apps/web/src/app.css
+// imports must also be scanned by apps/web/src/tailwind.css's @source list.
+// A workspace UI package is source-only: its .tsx files carry Tailwind
+// utility classes directly, and nothing but @source scanning generates the
+// CSS for them (see tailwind.css's own header comment). Importing a
+// package's prebuilt styles.css says nothing about whether its component
+// tree also leans on Tailwind utilities — CL-6490 found @corbits/plugins-ui
+// imported without a matching @source entry, so `data-[state=connected]:
+// bg-success`, `min-h-16`, and `[&>*:last-child]:border-b-0` never made it
+// into the built CSS. Nothing errored: the classes just silently did not
+// exist, and the failure only showed up as broken layout in production.
+import { readFileSync } from "node:fs";
+import path from "node:path";
+import {
+  emptyReport,
+  reportAndExit,
+  rootFromArgs,
+  type CheckReport,
+} from "./lib/repo";
+
+const APP_CSS = "apps/web/src/app.css";
+const TAILWIND_CSS = "apps/web/src/tailwind.css";
+
+const IMPORT_PATTERN = /@import\s+"@corbits\/([a-z0-9-]+)\/styles\.css";/g;
+const SOURCE_PATTERN =
+  /@source\s+"\.\.\/\.\.\/\.\.\/packages\/([a-z0-9-]+)\/src";/g;
+
+/** Package names behind every `@corbits/<name>/styles.css` import. */
+export function importedStylesheetPackages(css: string): string[] {
+  return [...css.matchAll(IMPORT_PATTERN)].map((match) => match[1] as string);
+}
+
+/** Package names behind every `@source ".../packages/<name>/src"` entry. */
+export function sourcedPackages(css: string): string[] {
+  return [...css.matchAll(SOURCE_PATTERN)].map((match) => match[1] as string);
+}
+
+/**
+ * A package whose stylesheet app.css imports must appear in tailwind.css's
+ * @source list — a package sourced without being imported (e.g. artifact-ui,
+ * which has no prebuilt stylesheet of its own) is not a violation, since
+ * @source scanning is the only thing that package ever relies on.
+ */
+export function auditTailwindSource(
+  imported: readonly string[],
+  sourced: readonly string[],
+): CheckReport {
+  const report = emptyReport();
+  const sourcedSet = new Set(sourced);
+  for (const name of imported) {
+    if (sourcedSet.has(name)) continue;
+    report.violations.push(
+      `${TAILWIND_CSS}: no @source entry for @corbits/${name} even though ` +
+        `${APP_CSS} imports its styles.css — that package's component tree ` +
+        `is scanned for nothing, so any Tailwind utility class it uses ` +
+        `silently does not exist in the built CSS instead of erroring. ` +
+        `Add \`@source "../../../packages/${name}/src";\` to ${TAILWIND_CSS}.`,
+    );
+  }
+  return report;
+}
+
+async function main(): Promise<void> {
+  const root = rootFromArgs(Bun.argv.slice(2));
+  const appCss = readFileSync(path.join(root, APP_CSS), "utf8");
+  const tailwindCss = readFileSync(path.join(root, TAILWIND_CSS), "utf8");
+  const imported = importedStylesheetPackages(appCss);
+  const sourced = sourcedPackages(tailwindCss);
+  const report = auditTailwindSource(imported, sourced);
+  report.notes.push(
+    `${imported.length} stylesheet import(s) in ${APP_CSS}, ` +
+      `${sourced.length} @source entr(y/ies) in ${TAILWIND_CSS}`,
+  );
+  reportAndExit("check:tailwind-source", report);
+}
+
+if (import.meta.main) await main();
diff --git a/scripts/checks/test/tailwind-source.test.ts b/scripts/checks/test/tailwind-source.test.ts
new file mode 100644
index 000000000..a3e31f6cc
--- /dev/null
+++ b/scripts/checks/test/tailwind-source.test.ts
@@ -0,0 +1,44 @@
+import { expect, test } from "bun:test";
+import {
+  auditTailwindSource,
+  importedStylesheetPackages,
+  sourcedPackages,
+} from "../tailwind-source";
+
+test("importedStylesheetPackages reads @corbits/*/styles.css imports only", () => {
+  const css = [
+    '@import "@corbits/chat-ui/styles.css";',
+    '@import "@corbits/plugins-ui/styles.css";',
+    "html { font-size: 15px; }",
+  ].join("\n");
+  expect(importedStylesheetPackages(css)).toEqual(["chat-ui", "plugins-ui"]);
+});
+
+test("sourcedPackages reads @source packages/<name>/src entries only", () => {
+  const css = [
+    '@source "../../../packages/bench-ui/src";',
+    '@source "../../../packages/chat-ui/src";',
+  ].join("\n");
+  expect(sourcedPackages(css)).toEqual(["bench-ui", "chat-ui"]);
+});
+
+test("a stylesheet import with no matching @source entry is a violation", () => {
+  const report = auditTailwindSource(["chat-ui", "plugins-ui"], ["chat-ui"]);
+  expect(report.violations).toHaveLength(1);
+  expect(report.violations[0]).toContain("plugins-ui");
+  expect(report.violations[0]).toContain("tailwind.css");
+  expect(report.violations[0]).toContain("silently");
+});
+
+test("every import matched by a @source entry passes", () => {
+  const report = auditTailwindSource(
+    ["chat-ui", "plugins-ui"],
+    ["chat-ui", "plugins-ui"],
+  );
+  expect(report.violations).toEqual([]);
+});
+
+test("a @source entry with no matching import is not a violation", () => {
+  const report = auditTailwindSource(["chat-ui"], ["artifact-ui", "chat-ui"]);
+  expect(report.violations).toEqual([]);
+});