diff --git a/swh/counters/api/server.py b/swh/counters/api/server.py
--- a/swh/counters/api/server.py
+++ b/swh/counters/api/server.py
@@ -6,7 +6,7 @@
 import os
 from typing import Any, Dict, Optional
 
-from flask import abort
+from flask import abort, jsonify
 
 from swh.core import config
 from swh.core.api import RPCServerApp
@@ -133,6 +133,8 @@
 def get_history_file_content(filename: str):
     assert app is not None
     try:
-        return app.config["history"].get_history(filename)
+        content = app.config["history"].get_history(filename)
     except FileNotFoundError:
         abort(404)
+
+    return jsonify(content)
diff --git a/swh/counters/tests/test_server.py b/swh/counters/tests/test_server.py
--- a/swh/counters/tests/test_server.py
+++ b/swh/counters/tests/test_server.py
@@ -170,7 +170,7 @@
 
     expected_result = {"content": [[1, 1], [2, 2]]}
     mock = mocker.patch("swh.counters.history.History.get_history")
-    mock.return_value = json.dumps(expected_result)
+    mock.return_value = expected_result
 
     r = history_test_client.get("/counters_history/test.json")
 
@@ -180,6 +180,7 @@
     response_json = json.loads(response)
 
     assert response_json == expected_result
+    assert "application/json" == r.headers["Content-Type"]
 
 
 def test_server_counters_history_file_not_found(history_test_client, mocker):