diff --git a/src/glean/api_client/_hooks/x_glean.py b/src/glean/api_client/_hooks/x_glean.py
index 6aa88575..5ca01721 100644
--- a/src/glean/api_client/_hooks/x_glean.py
+++ b/src/glean/api_client/_hooks/x_glean.py
@@ -61,20 +61,10 @@ def before_request(
             config_experimental,
         )
 
-        # Create new headers dict with existing headers
-        new_headers = dict(request.headers)
-
         if deprecated_value:
-            new_headers["X-Glean-Exclude-Deprecated-After"] = deprecated_value
+            request.headers["X-Glean-Exclude-Deprecated-After"] = deprecated_value
 
         if experimental_value:
-            new_headers["X-Glean-Experimental"] = experimental_value
+            request.headers["X-Glean-Experimental"] = experimental_value
 
-        # Return new request with updated headers
-        return httpx.Request(
-            method=request.method,
-            url=request.url,
-            headers=new_headers,
-            content=request.content,
-            extensions=request.extensions,
-        )
+        return request
diff --git a/tests/test_x_glean_hook.py b/tests/test_x_glean_hook.py
index 14bb4a1f..1c4dcdd5 100644
--- a/tests/test_x_glean_hook.py
+++ b/tests/test_x_glean_hook.py
@@ -239,5 +239,27 @@ def test_preserves_request_method_and_url(self):
         assert str(result.url) == "https://api.example.com/v1/search"
 
 
+    def test_handles_multipart_streaming_request(self):
+        """Should handle multipart requests without raising RequestNotRead.
+
+        Multipart file uploads use a streaming body. The previous implementation
+        reconstructed the request via httpx.Request(..., content=request.content),
+        which raised httpx.RequestNotRead on streaming bodies.
+        """
+        hook = XGlean()
+        request = httpx.Request(
+            "POST",
+            "https://example.com/rest/api/v1/uploadchatfiles",
+            data={"field": "value"},
+            files={"file": ("test.csv", b"a,b\n1,2", "text/csv")},
+        )
+        context = create_mock_context(include_experimental=True)
+
+        result = hook.before_request(context, request)
+
+        assert result.headers.get("X-Glean-Experimental") == "true"
+        assert result.method == "POST"
+
+
 if __name__ == "__main__":
     pytest.main([__file__])