diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Revision history for wai-handler-hal
 
+## 0.4.0.2 -- 2025-08-14
+
+- Union single and multi-value query parameters and headers when
+  converting a `hal` `ProxyRequest` to a `wai` `Request`
+
 ## 0.4.0.1 -- 2025-02-19
 
 - Use a `NonEmpty` list when consuming the result of `getAddrInfo`.
diff --git a/src/Network/Wai/Handler/Hal.hs b/src/Network/Wai/Handler/Hal.hs
--- a/src/Network/Wai/Handler/Hal.hs
+++ b/src/Network/Wai/Handler/Hal.hs
@@ -216,6 +216,15 @@
 -- | Convert the request sent to a Lambda serving an API Gateway proxy
 -- integration into a WAI request.
 --
+-- When processing multiple values for the same
+-- [HTTP header|query parameter], API Gateway will put the final such
+-- value into ['HalRequest.headers'|'HalRequest.queryStringParameters']
+-- and all such values into
+-- ['HalRequest.multiValueHeaders'|'HalRequest.multiValueQueryStringParameters'].
+-- We merge both but exclusively use the multi-value version if it exists,
+-- to ensure that [headers|query parameters] defined in hand-crafted
+-- 'HalRequest.ProxyRequest's always appear in the 'WaiRequest.Request'.
+--
 -- __Note:__ We aren't told the HTTP version the client is using, so
 -- we assume HTTP 1.1.
 toWaiRequest ::
@@ -227,7 +236,9 @@
       pathSegments =
         Text.splitOn "/" . Text.dropWhile (== '/') $ HalRequest.path req
       query =
-        sort . constructQuery $ HalRequest.multiValueQueryStringParameters req
+        sort . constructQuery $
+          HalRequest.multiValueQueryStringParameters req
+            `HashMap.union` ((: []) <$> HalRequest.queryStringParameters req)
       hints =
         NS.defaultHints
           { NS.addrFlags = [NS.AI_NUMERICHOST],
@@ -275,7 +286,8 @@
                     (CI.map Text.encodeUtf8 hName, Text.encodeUtf8 hValue)
               )
             . HashMap.toList
-            $ HalRequest.multiValueHeaders req,
+            $ HalRequest.multiValueHeaders req
+              `HashMap.union` ((: []) <$> HalRequest.headers req),
         Wai.isSecure = True,
         Wai.remoteHost = sourceHost,
         Wai.pathInfo = pathSegments,
diff --git a/test/data/ProxyRequest.json b/test/data/ProxyRequest.json
--- a/test/data/ProxyRequest.json
+++ b/test/data/ProxyRequest.json
@@ -9,12 +9,17 @@
         "X-Amzn-Trace-Id": "Root=1-11111111-111111111111111111111111",
         "X-Forwarded-For": "123.123.123.123",
         "X-Forwarded-Port": "443",
-        "X-Forwarded-Proto": "https"
+        "X-Forwarded-Proto": "https",
+        "Content-Type": "application/json"
     },
     "multiValueHeaders": {
         "accept": [
             "application/json"
         ],
+        "Accept-Language": [
+            "en",
+            "fr"
+        ],
         "Host": [
             "ffffffffff.execute-api.us-east-1.amazonaws.com"
         ],
@@ -37,7 +42,8 @@
     "queryStringParameters": {
         "MaxResultsPerPage": "100",
         "PostedAfter": "2022-05-31T22:16:43Z",
-        "PostedBefore": "2022-05-31T23:16:43Z"
+        "PostedBefore": "2022-05-31T23:16:43Z",
+        "language": "en"
     },
     "multiValueQueryStringParameters": {
         "MaxResultsPerPage": [
@@ -48,6 +54,10 @@
         ],
         "PostedBefore": [
             "2022-05-31T23:16:43Z"
+        ],
+        "country_code": [
+            "US",
+            "CA"
         ]
     },
     "pathParameters": {
diff --git a/test/golden/WaiRequest.txt b/test/golden/WaiRequest.txt
--- a/test/golden/WaiRequest.txt
+++ b/test/golden/WaiRequest.txt
@@ -2,13 +2,25 @@
     { requestMethod = "GET"
     , httpVersion = HTTP/1.1
     , rawPathInfo = "/foo/bar/baz"
-    , rawQueryString = "?MaxResultsPerPage=100&PostedAfter=2022-05-31T22%3A16%3A43Z&PostedBefore=2022-05-31T23%3A16%3A43Z"
+    , rawQueryString = "?MaxResultsPerPage=100&PostedAfter=2022-05-31T22%3A16%3A43Z&PostedBefore=2022-05-31T23%3A16%3A43Z&country_code=CA&country_code=US&language=en"
     , requestHeaders =
         [
             ( "accept"
             , "application/json"
             )
         ,
+            ( "Accept-Language"
+            , "en"
+            )
+        ,
+            ( "Accept-Language"
+            , "fr"
+            )
+        ,
+            ( "Content-Type"
+            , "application/json"
+            )
+        ,
             ( "Host"
             , "ffffffffff.execute-api.us-east-1.amazonaws.com"
             )
@@ -52,6 +64,18 @@
         ,
             ( "PostedBefore"
             , Just "2022-05-31T23:16:43Z"
+            )
+        ,
+            ( "country_code"
+            , Just "CA"
+            )
+        ,
+            ( "country_code"
+            , Just "US"
+            )
+        ,
+            ( "language"
+            , Just "en"
             )
         ]
     , requestBody = <IO ByteString>
diff --git a/wai-handler-hal.cabal b/wai-handler-hal.cabal
--- a/wai-handler-hal.cabal
+++ b/wai-handler-hal.cabal
@@ -1,6 +1,6 @@
 cabal-version:      2.2
 name:               wai-handler-hal
-version:            0.4.0.1
+version:            0.4.0.2
 synopsis:           Wrap WAI applications to run on AWS Lambda
 description:
   This library provides a function 'Network.Wai.Handler.Hal.run' to
@@ -37,6 +37,7 @@
    || ==9.6.6
    || ==9.8.2
    || ==9.10.1
+   || ==9.12.1
 
 common opts
   default-language: Haskell2010
@@ -49,7 +50,7 @@
 
 common deps
   build-depends:
-    , base                  >=4.12      && <4.21
+    , base                  >=4.12      && <4.22
     , base64-bytestring     >=1.0.0.0   && <1.3
     , bytestring            >=0.10.8    && <0.13
     , case-insensitive      ^>=1.2.0.0