diff --git a/toolchain/defs.bzl b/toolchain/defs.bzl
index f6f613e..534e97e 100644
--- a/toolchain/defs.bzl
+++ b/toolchain/defs.bzl
@@ -147,12 +147,16 @@ def _zig_repository_impl(repository_ctx):
 
     cache_prefix = repository_ctx.os.environ.get("HERMETIC_CC_TOOLCHAIN_CACHE_PREFIX", "")
     if cache_prefix == "":
+        # Append the username when available to reduce risks of conflict.
+        username = repository_ctx.os.environ.get("USER", "")
+        if username:
+            username = "-" + username
         if os == "windows":
-            cache_prefix = "C:\\\\Temp\\\\zig-cache"
+            cache_prefix = "C:\\\\Temp\\\\zig-cache" + username
         elif os == "macos":
-            cache_prefix = "/var/tmp/zig-cache"
+            cache_prefix = "/var/tmp/zig-cache" + username
         elif os == "linux":
-            cache_prefix = "/tmp/zig-cache"
+            cache_prefix = "/tmp/zig-cache" + username
         else:
             fail("unknown os: {}".format(os))