diff --git a/services/federator/default.nix b/services/federator/default.nix
index fa8eac96bda..e0d3e9d7235 100644
--- a/services/federator/default.nix
+++ b/services/federator/default.nix
@@ -168,6 +168,7 @@ mkDerivation {
   ];
   testHaskellDepends = [
     aeson
+    async
     base
     bytestring
     bytestring-conversion
diff --git a/services/federator/federator.cabal b/services/federator/federator.cabal
index 872f9911236..2a8181f5f18 100644
--- a/services/federator/federator.cabal
+++ b/services/federator/federator.cabal
@@ -380,6 +380,7 @@ test-suite federator-tests
 
   build-depends:
       aeson
+    , async
     , base
     , bytestring
     , bytestring-conversion
diff --git a/services/federator/src/Federator/MockServer.hs b/services/federator/src/Federator/MockServer.hs
index 828a87bcb1a..31094692c8f 100644
--- a/services/federator/src/Federator/MockServer.hs
+++ b/services/federator/src/Federator/MockServer.hs
@@ -151,7 +151,7 @@ mockInternalRequest remoteCalls mock targetDomain component (RPC path) req cont
     if path == "api-version"
       then pure $ MockResponse Wai.status200 "application/json" (Aeson.encode (VersionInfo mock.versions))
       else do
-        modifyIORef remoteCalls (<> [fedRequest])
+        atomicModifyIORef' remoteCalls (\xs -> (xs <> [fedRequest], ()))
         fromException @MockException
           . handle (throw . handleException)
           $ mock.handler fedRequest
diff --git a/services/federator/test/unit/Test/Federator/Client.hs b/services/federator/test/unit/Test/Federator/Client.hs
index 8bc976fe6a9..0660390b393 100644
--- a/services/federator/test/unit/Test/Federator/Client.hs
+++ b/services/federator/test/unit/Test/Federator/Client.hs
@@ -35,6 +35,7 @@
 
 module Test.Federator.Client (tests) where
 
+import Control.Concurrent.Async (replicateConcurrently_)
 import Control.Exception hiding (handle)
 import Control.Monad.Codensity
 import Control.Monad.Except
@@ -92,6 +93,7 @@ tests =
       testGroup
         "HTTP2 client"
         [ testCase "testResponseHeaders" testResponseHeaders,
+          testCase "testConcurrentRequestsAllRecorded" testConcurrentRequestsAllRecorded,
           testCase "testStreaming" testStreaming
         ]
     ]
@@ -248,6 +250,20 @@ testResponseHeaders = do
       responseStatusCode resp @?= HTTP.status200
       lookup "X-Foo" (toList (responseHeaders resp)) @?= Just "bar"
 
+testConcurrentRequestsAllRecorded :: IO ()
+testConcurrentRequestsAllRecorded = do
+  (_, sentRequests) <-
+    withTempMockFederator def $ \port -> do
+      let req =
+            HTTP2.requestBuilder
+              HTTP.methodPost
+              "/rpc/target.example.com/brig/test"
+              [("Wire-Origin-Domain", "origin.example.com"), (federationRequestIdHeaderName, "rid")]
+              "body"
+      mgr <- defaultHttp2Manager
+      replicateConcurrently_ 50 (performHTTP2Request mgr (False, "127.0.0.1", port) req)
+  length sentRequests @?= 50
+
 testStreaming :: IO ()
 testStreaming = withInfiniteMockServer $ \port -> do
   let req = HTTP2.requestBuilder HTTP.methodPost "test" [] mempty