diff --git a/npm-packages/convex/src/bundler/index.test.ts b/npm-packages/convex/src/bundler/index.test.ts
index c074fa19e..d7e935ca2 100644
--- a/npm-packages/convex/src/bundler/index.test.ts
+++ b/npm-packages/convex/src/bundler/index.test.ts
@@ -1,5 +1,6 @@
 import { expect, test, afterEach, vi } from "vitest";
 import { oneoffContext } from "./context.js";
+import { normalizeModulePath } from "./index.js";
 
 // Although these tests are run as ESM by ts-lint, this file is built as both
 // CJS and ESM by TypeScript so normal recipes like `__dirname` for getting the
@@ -180,3 +181,17 @@ test("must use isolate", () => {
   expect(mustBeIsolate("schema2.js")).not.toBeTruthy();
   expect(mustBeIsolate("schema/http.js")).not.toBeTruthy();
 });
+
+test("normalizeModulePath creates valid module identifiers from output paths", () => {
+  // Test typical case: esbuild output in out/ directory
+  expect(normalizeModulePath("out/convex/myFunction.js")).toBe("convex/myFunction.js");
+  expect(normalizeModulePath("out\\actions\\myAction.js")).toBe("actions/myAction.js");
+  
+  // Test Windows absolute paths (typical Windows scenario)
+  expect(normalizeModulePath("C:\\project\\out\\convex\\foo.js")).toBe("convex/foo.js");
+  expect(normalizeModulePath("D:/Users/dev/myapp/out/convex/bar.js")).toBe("convex/bar.js");
+  
+  // Test custom base directory
+  expect(normalizeModulePath("build/convex/test.js", "build")).toBe("convex/test.js");
+  expect(normalizeModulePath("C:/project/dist/actions/upload.js", "dist")).toBe("actions/upload.js");
+});
diff --git a/npm-packages/convex/src/bundler/index.ts b/npm-packages/convex/src/bundler/index.ts
index a9eb58f42..c9f945fcd 100644
--- a/npm-packages/convex/src/bundler/index.ts
+++ b/npm-packages/convex/src/bundler/index.ts
@@ -206,8 +206,8 @@ export async function bundle(
       sourceMaps.set(relPath, outputFile.text);
       continue;
     }
-    const posixRelPath = relPath.split(path.sep).join(path.posix.sep);
-    modules.push({ path: posixRelPath, source: outputFile.text, environment });
+    const normalizedPath = normalizeModulePath(outputFile.path);
+    modules.push({ path: normalizedPath, source: outputFile.text, environment });
   }
   for (const module of modules) {
     const sourceMapPath = module.path + ".map";
@@ -437,6 +437,39 @@ export async function entryPoints(
 // A fallback regex in case we fail to parse the AST.
 export const useNodeDirectiveRegex = /^\s*("|')use node("|');?\s*$/;
 
+export function normalizeModulePath(outputPath: string, baseDir: string = "out"): string {
+  // Normalize both paths to handle mixed separators (Windows/POSIX)
+  const normalizedOutput = outputPath.replace(/\\/g, '/');
+  const normalizedBase = baseDir.replace(/\\/g, '/');
+  
+  // Extract the relative path from the esbuild output directory
+  const relativePath = path.posix.relative(normalizedBase, normalizedOutput);
+  
+  // Ensure we don't have upward traversal in module identifiers
+  if (relativePath.startsWith('../')) {
+    // For absolute paths or paths outside the base, extract just the meaningful part
+    const outputParts = normalizedOutput.split('/');
+    const baseParts = normalizedBase.split('/');
+    
+    // Find where the base directory appears in the output path
+    const baseIndex = outputParts.findIndex((part, index) => {
+      return baseParts.every((basePart, bIndex) => 
+        outputParts[index + bIndex] === basePart
+      );
+    });
+    
+    if (baseIndex >= 0) {
+      // Extract everything after the base directory
+      return outputParts.slice(baseIndex + baseParts.length).join('/');
+    }
+    
+    // Fallback: if we can't find the base, return the filename part
+    return outputParts[outputParts.length - 1];
+  }
+  
+  return relativePath;
+}
+
 function hasUseNodeDirective(ctx: Context, fpath: string): boolean {
   // Do a quick check for the exact string. If it doesn't exist, don't
   // bother parsing.