diff --git a/models/unittest/mock_http.go b/models/unittest/mock_http.go
index c2c12e55ee..3c4ddd8e27 100644
--- a/models/unittest/mock_http.go
+++ b/models/unittest/mock_http.go
@@ -91,8 +91,6 @@ func NewMockWebServer(t *testing.T, liveServerBaseURL, testDataDir string, liveM
 		fixture, err := os.ReadFile(fixturePath)
 		require.NoError(t, err, "missing mock HTTP response: "+fixturePath)
 
-		w.WriteHeader(http.StatusOK)
-
 		// replace any mention of the live HTTP service by the mocked host
 		stringFixture := strings.ReplaceAll(string(fixture), liveServerBaseURL, mockServerBaseURL)
 		if isGh {
@@ -104,10 +102,16 @@ func NewMockWebServer(t *testing.T, liveServerBaseURL, testDataDir string, liveM
 		for idx, line := range lines {
 			colonIndex := strings.Index(line, ": ")
 			if colonIndex != -1 {
-				w.Header().Set(line[0:colonIndex], line[colonIndex+2:])
+				// Because we modified the body with ReplaceAll() above, we need to
+				// remove Content-Length. w.Write() should add it back.
+				header := line[0:colonIndex]
+				if !strings.EqualFold(header, "Content-Length") {
+					w.Header().Set(line[0:colonIndex], line[colonIndex+2:])
+				}
 			} else {
 				// we reached the end of the headers (empty line), so what follows is the body
 				responseBody := strings.Join(lines[idx+1:], "\n")
+				w.WriteHeader(http.StatusOK)
 				_, err := w.Write([]byte(responseBody))
 				require.NoError(t, err, "writing the body of the HTTP response failed")
 				break
diff --git a/models/unittest/mock_http_test.go b/models/unittest/mock_http_test.go
new file mode 100644
index 0000000000..9ae4592aab
--- /dev/null
+++ b/models/unittest/mock_http_test.go
@@ -0,0 +1,24 @@
+// Copyright 2026 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package unittest
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// NOTE: This is a test of the unittest helper itself
+func TestMockWebServer(t *testing.T) {
+	server := NewMockWebServer(t, "https://example.com", "testdata", false)
+	defer server.Close()
+	request, err := http.NewRequest("GET", server.URL+"/", nil)
+	require.NoError(t, err)
+	response, err := server.Client().Do(request)
+	require.NoError(t, err)
+	assert.Len(t, response.Header["Header"], 1)
+	assert.Equal(t, "value", response.Header["Header"][0])
+}
diff --git a/models/unittest/testdata/GET_%2F b/models/unittest/testdata/GET_%2F
new file mode 100644
index 0000000000..8b79a2cd25
--- /dev/null
+++ b/models/unittest/testdata/GET_%2F
@@ -0,0 +1,3 @@
+Header: value
+
+bodydata