diff --git a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
index f1821522..324654a8 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
@@ -614,7 +614,7 @@ public class ExternalSystemClient : IExternalSystemClient
             var consumed = consumedParams;
             var queryString = string.Join("&",
                 parameters.Where(p => p.Value != null && !consumed.Contains(p.Key))
-                    .Select(p => $"{Uri.EscapeDataString(p.Key)}={Uri.EscapeDataString(p.Value?.ToString() ?? "")}"));
+                    .Select(p => $"{Uri.EscapeDataString(p.Key)}={Uri.EscapeDataString(FormatQueryValue(p.Value))}"));
 
             // Only append "?" when the effective query string is non-empty — a method
             // whose parameter values are all null produces no query string, and the
@@ -629,6 +629,31 @@ public class ExternalSystemClient : IExternalSystemClient
         return url;
     }
 
+    /// <summary>
+    /// Renders a GET/DELETE query-string value. Scalars use <c>ToString()</c>; collections,
+    /// dictionaries and other complex objects are JSON-serialised (as the POST/PUT/PATCH
+    /// branch already does) instead of degrading to a CLR type name such as
+    /// <c>System.Int32[]</c> (review 2026-10-21 05/O2).
+    /// </summary>
+    internal static string FormatQueryValue(object? value)
+    {
+        switch (value)
+        {
+            case null:
+                return string.Empty;
+            case string or char or bool or Guid or Uri or DateTime or DateTimeOffset or TimeSpan or Enum or decimal:
+                return value.ToString() ?? string.Empty;
+            case JsonElement { ValueKind: JsonValueKind.Object or JsonValueKind.Array } json:
+                return json.GetRawText();
+            case JsonElement scalar:
+                return scalar.ToString();
+        }
+
+        return value.GetType().IsPrimitive
+            ? value.ToString() ?? string.Empty
+            : JsonSerializer.Serialize(value);
+    }
+
     private void ApplyAuth(HttpRequestMessage request, ExternalSystemDefinition system)
     {
         // Distinguish "intentionally unauthenticated" (AuthType = none)
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
index f01c93fb..f9f48ea5 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
@@ -782,6 +782,35 @@ public class ExternalSystemClientTests : IDisposable
         Assert.Contains("page=2", uri);
     }
 
+    /// <summary>Verifies that non-scalar GET parameters are JSON-serialised rather than rendered as CLR type names.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task Call_GetWithNonScalarParameters_JsonSerialisesThem()
+    {
+        // Review 2026-10-21 05/O2: new[]{1,2,3}.ToString() is "System.Int32[]".
+        var system = new ExternalSystemDefinition("TestAPI", "https://api.example.com", "none") { Id = 1 };
+        var method = new ExternalSystemMethod("search", "GET", "/search") { Id = 1, ExternalSystemDefinitionId = 1 };
+        StubResolution(system, method);
+
+        var handler = new RequestCapturingHandler(HttpStatusCode.OK, "{}");
+        _httpClientFactory.CreateClient(Arg.Any<string>()).Returns(new HttpClient(handler));
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance);
+
+        await client.CallAsync("TestAPI", "search", new Dictionary<string, object?>
+        {
+            ["ids"] = new[] { 1, 2, 3 },
+            ["f"] = new Dictionary<string, object?> { ["a"] = 1 },
+            ["n"] = 7,
+        });
+
+        var uri = handler.LastUri!.AbsoluteUri;
+        Assert.Contains("ids=" + Uri.EscapeDataString("[1,2,3]"), uri);
+        Assert.Contains("f=" + Uri.EscapeDataString("{\"a\":1}"), uri);
+        Assert.Contains("n=7", uri);
+        Assert.DoesNotContain("System.", uri);
+    }
+
     /// <summary>Verifies that a GET call whose parameters are all null does not append a trailing question mark.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]