diff --git a/.changeset/additional-files-windows.md b/.changeset/additional-files-windows.md
new file mode 100644
index 00000000000..07b11122a0e
--- /dev/null
+++ b/.changeset/additional-files-windows.md
@@ -0,0 +1,5 @@
+---
+"@trigger.dev/build": patch
+---
+
+`additionalFiles` now strips leading `..` segments on Windows too, so a pattern like `../shared/**` copies files inside the build instead of outside it.
diff --git a/packages/build/src/internal/copyFiles.ts b/packages/build/src/internal/copyFiles.ts
index 6fd3ede9545..79a29706324 100644
--- a/packages/build/src/internal/copyFiles.ts
+++ b/packages/build/src/internal/copyFiles.ts
@@ -1,5 +1,5 @@
 import { cp, copyFile, mkdir } from "node:fs/promises";
-import { dirname, join, posix, relative } from "node:path";
+import { dirname, join, posix, relative, sep } from "node:path";
 import { glob } from "tinyglobby";
 
 /**
@@ -44,7 +44,7 @@ export async function findFilesByMatchers(
 
     const assets: CopyPair[] = files.map((file) => {
       const pathInsideDestinationDir = relative(cwd, file)
-        .split(posix.sep)
+        .split(sep)
         .filter((p) => p !== "..")
         .join(posix.sep);
       return {