fix: handle array-returning Vite plugins in mergeConfigDedup

.gitignore

@@ -9,3 +9,6 @@ coverage
 *.tsbuildinfo
 
 .turbo
+
+# AI generated plans from developer loop
+docs/plans

packages/appkit/src/utils/tests/vite-config-merge.test.ts

@@ -0,0 +1,103 @@
+import type { Plugin } from "vite";
+import { describe, expect, test } from "vitest";
+import { mergeConfigDedup } from "../vite-config-merge";
+
+const plugin = (name: string): Plugin => ({ name, enforce: "pre" });
+
+const simpleMerge = (a: Plugin, b: Plugin) => ({ ...a, ...b });
+
+describe("mergeConfigDedup", () => {
+  test("deduplicates plugins by name, keeping the first occurrence", () => {
+    const base = { plugins: [plugin("a"), plugin("b")] };
+    const override = { plugins: [plugin("b"), plugin("c")] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual(["a", "b", "c"]);
+  });
+
+  test("returns merged config when no plugins on either side", () => {
+    const result = mergeConfigDedup({ x: 1 }, { y: 2 }, simpleMerge);
+    expect(result).toEqual({ x: 1, y: 2 });
+    expect(result.plugins).toBeUndefined();
+  });
+
+  test("preserves plugins when only base has them", () => {
+    const base = { plugins: [plugin("a")] };
+    const override = { other: true };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+    // mergeFn merges everything; dedup branch only runs when both have plugins
+    expect(result.plugins).toBeDefined();
+  });
+
+  test("flattens array-returning plugins (e.g. @tailwindcss/vite)", () => {
+    const tailwindPreset = [
+      plugin("tw:base"),
+      plugin("tw:scan"),
+      plugin("tw:generate"),
+    ];
+    const base = { plugins: [tailwindPreset] };
+    const override = { plugins: [plugin("react")] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual([
+      "tw:base",
+      "tw:scan",
+      "tw:generate",
+      "react",
+    ]);
+  });
+
+  test("deduplicates across flattened array plugins and single plugins", () => {
+    const preset = [plugin("shared"), plugin("unique-a")];
+    const base = { plugins: [preset] };
+    const override = { plugins: [plugin("shared"), plugin("unique-b")] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual([
+      "shared",
+      "unique-a",
+      "unique-b",
+    ]);
+  });
+
+  test("handles deeply nested plugin arrays", () => {
+    const deep = [[plugin("deep-a")], [[plugin("deep-b")]]];
+    const base = { plugins: [deep] };
+    const override = { plugins: [plugin("top")] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual([
+      "deep-a",
+      "deep-b",
+      "top",
+    ]);
+  });
+
+  test("filters out false/null/undefined plugin entries", () => {
+    const base = { plugins: [plugin("a"), false, null] };
+    const override = { plugins: [undefined, plugin("b")] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual(["a", "b"]);
+  });
+
+  test("handles mixed falsy and array plugins", () => {
+    const preset = [plugin("tw:base"), plugin("tw:scan")];
+    const base = { plugins: [false, preset, null] };
+    const override = { plugins: [undefined, plugin("react"), false] };
+
+    const result = mergeConfigDedup(base, override, simpleMerge);
+
+    expect(result.plugins.map((p: Plugin) => p.name)).toEqual([
+      "tw:base",
+      "tw:scan",
+      "react",
+    ]);
+  });
+});

packages/appkit/src/utils/vite-config-merge.ts

@@ -1,5 +1,9 @@
 import type { Plugin } from "vite";
 
+function flattenPlugins(plugins: any[]): Plugin[] {
+  return plugins.flat(Infinity).filter(Boolean);
+}
+
 export function mergeConfigDedup(
   base: any,
   override: any,
@@ -8,14 +12,13 @@ export function mergeConfigDedup(
   const merged = mergeFn(base, override);
   if (base.plugins && override.plugins) {
     const seen = new Set<string>();
-    merged.plugins = [...base.plugins, ...override.plugins].filter(
-      (p: Plugin) => {
-        const name = p.name;
-        if (seen.has(name)) return false;
-        seen.add(name);
-        return true;
-      },
-    );
+    const allPlugins = flattenPlugins([...base.plugins, ...override.plugins]);
+    merged.plugins = allPlugins.filter((p) => {
+      const name = p.name;
+      if (seen.has(name)) return false;
+      seen.add(name);
+      return true;
+    });
   }
   return merged;
 }