diff --git a/packages/build/package.json b/packages/build/package.json
index d5ec39213..da42a211f 100644
--- a/packages/build/package.json
+++ b/packages/build/package.json
@@ -74,7 +74,9 @@
     "dev": "tshy --watch",
     "typecheck": "tsc --noEmit -p tsconfig.src.json",
     "update-version": "tsx ../../scripts/updateVersion.ts",
-    "check-exports": "attw --pack ."
+    "check-exports": "attw --pack .",
+    "test": "vitest run",
+    "test:dev": "vitest"
   },
   "dependencies": {
     "@prisma/config": "^6.10.0",
@@ -91,7 +93,8 @@
     "esbuild": "^0.23.0",
     "rimraf": "6.0.1",
     "tshy": "^3.0.2",
-    "tsx": "4.17.0"
+    "tsx": "4.17.0",
+    "vitest": "^2.0.0"
   },
   "engines": {
     "node": ">=18.20.0"
diff --git a/packages/build/src/internal/additionalFiles.ts b/packages/build/src/internal/additionalFiles.ts
index 9bde93f46..c92b848dc 100644
--- a/packages/build/src/internal/additionalFiles.ts
+++ b/packages/build/src/internal/additionalFiles.ts
@@ -1,7 +1,7 @@
 import { BuildManifest } from "@trigger.dev/core/v3";
 import { BuildContext } from "@trigger.dev/core/v3/build";
 import { copyFile, mkdir } from "node:fs/promises";
-import { dirname, isAbsolute, join, posix, relative, resolve } from "node:path";
+import { dirname, isAbsolute, join, posix, relative, resolve, sep } from "node:path";
 import { glob } from "tinyglobby";
 
 export type AdditionalFilesOptions = {
@@ -76,7 +76,8 @@ async function findStaticAssetFiles(
 // Extracts the base directory from a glob pattern (the non-wildcard prefix).
 // For example: "../shared/**" -> "../shared", "./assets/*.txt" -> "./assets"
 // For specific files without globs: "./config/settings.json" -> "./config" (parent dir)
-function getGlobBase(pattern: string): string {
+// For single-part patterns: "file.txt" -> "." (current dir)
+export function getGlobBase(pattern: string): string {
   const parts = pattern.split(/[/\\]/);
   const baseParts: string[] = [];
   let hasGlobCharacters = false;
@@ -92,8 +93,9 @@ function getGlobBase(pattern: string): string {
 
   // If no glob characters were found, the pattern is a specific file path.
   // Return the parent directory so that relative() preserves the filename.
-  if (!hasGlobCharacters && baseParts.length > 1) {
-    baseParts.pop(); // Remove the filename, keep the directory
+  // For single-part patterns (just a filename), return "." to indicate current directory.
+  if (!hasGlobCharacters) {
+    baseParts.pop(); // Remove the filename, keep the directory (or empty for single-part)
   }
 
   return baseParts.length > 0 ? baseParts.join(posix.sep) : ".";
@@ -129,8 +131,9 @@ async function findStaticAssetsForMatcher(
       pathInsideDestinationDir = join(options.destination, relativeToGlobBase);
     } else {
       // Default behavior: compute relative path from cwd and strip ".." segments
+      // Use platform-specific separator for splitting since path.relative() returns platform separators
       pathInsideDestinationDir = relative(cwd, file)
-        .split(posix.sep)
+        .split(sep)
         .filter((p) => p !== "..")
         .join(posix.sep);
     }
diff --git a/packages/build/test/additionalFiles.test.ts b/packages/build/test/additionalFiles.test.ts
new file mode 100644
index 000000000..b8fea20d9
--- /dev/null
+++ b/packages/build/test/additionalFiles.test.ts
@@ -0,0 +1,94 @@
+import { describe, it, expect } from "vitest";
+import { getGlobBase } from "../src/internal/additionalFiles.js";
+
+describe("getGlobBase", () => {
+  describe("glob patterns with wildcards", () => {
+    it("extracts base from parent directory glob pattern", () => {
+      expect(getGlobBase("../shared/**")).toBe("../shared");
+    });
+
+    it("extracts base from relative directory glob pattern", () => {
+      expect(getGlobBase("./assets/*.txt")).toBe("./assets");
+    });
+
+    it("extracts base from nested directory glob pattern", () => {
+      expect(getGlobBase("files/nested/**/*.js")).toBe("files/nested");
+    });
+
+    it("returns current directory for top-level glob", () => {
+      expect(getGlobBase("**/*.js")).toBe(".");
+    });
+
+    it("returns current directory for star pattern", () => {
+      expect(getGlobBase("*.js")).toBe(".");
+    });
+
+    it("handles question mark wildcard", () => {
+      expect(getGlobBase("./src/?/*.ts")).toBe("./src");
+    });
+
+    it("handles bracket patterns", () => {
+      expect(getGlobBase("./src/[abc]/*.ts")).toBe("./src");
+    });
+
+    it("handles brace expansion patterns", () => {
+      expect(getGlobBase("./src/{a,b}/*.ts")).toBe("./src");
+    });
+
+    it("handles deeply nested patterns", () => {
+      expect(getGlobBase("a/b/c/d/**")).toBe("a/b/c/d");
+    });
+  });
+
+  describe("specific file paths without globs", () => {
+    it("returns parent directory for file in subdirectory", () => {
+      expect(getGlobBase("./config/settings.json")).toBe("./config");
+    });
+
+    it("returns parent directory for file in nested subdirectory", () => {
+      expect(getGlobBase("../shared/utils/helpers.ts")).toBe("../shared/utils");
+    });
+
+    it("returns current directory for single-part filename", () => {
+      expect(getGlobBase("file.txt")).toBe(".");
+    });
+
+    it("returns current directory for filename starting with dot", () => {
+      expect(getGlobBase(".env")).toBe(".");
+    });
+
+    it("returns parent directory for explicit relative path to file", () => {
+      expect(getGlobBase("./file.txt")).toBe(".");
+    });
+
+    it("returns parent directories for parent reference to file", () => {
+      expect(getGlobBase("../file.txt")).toBe("..");
+    });
+
+    it("handles multiple parent references", () => {
+      expect(getGlobBase("../../config/app.json")).toBe("../../config");
+    });
+  });
+
+  describe("edge cases", () => {
+    it("returns current directory for empty string", () => {
+      expect(getGlobBase("")).toBe(".");
+    });
+
+    it("handles Windows-style backslashes", () => {
+      expect(getGlobBase("..\\shared\\**")).toBe("../shared");
+    });
+
+    it("handles mixed forward and back slashes", () => {
+      expect(getGlobBase("../shared\\nested/**")).toBe("../shared/nested");
+    });
+
+    it("handles patterns with only dots", () => {
+      expect(getGlobBase("./")).toBe(".");
+    });
+
+    it("handles parent directory reference only", () => {
+      expect(getGlobBase("../")).toBe("..");
+    });
+  });
+});
diff --git a/packages/build/vitest.config.ts b/packages/build/vitest.config.ts
new file mode 100644
index 000000000..7f850a544
--- /dev/null
+++ b/packages/build/vitest.config.ts
@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["test/**/*.test.ts", "src/**/*.test.ts"],
+    globals: true,
+  },
+});