diff --git a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
index a08bfd67..cea89bf8 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs
@@ -322,7 +322,12 @@ public class ExternalSystemClient : IExternalSystemClient
         HttpResponseMessage response;
         try
         {
-            response = await client.SendAsync(request, linkedCts.Token);
+            // ResponseHeadersRead returns as soon as the headers are in, so the
+            // body is not eagerly buffered — the bounded read below streams it and
+            // aborts once MaxResponseBodyBytes is exceeded, keeping a hostile or
+            // misbehaving endpoint from inflating the active node's memory.
+            response = await client.SendAsync(
+                request, HttpCompletionOption.ResponseHeadersRead, linkedCts.Token);
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
         {
@@ -347,7 +352,8 @@ public class ExternalSystemClient : IExternalSystemClient
             string body;
             try
             {
-                body = await response.Content.ReadAsStringAsync(linkedCts.Token);
+                body = await ReadBodyBoundedAsync(
+                    response.Content, system.Name, _options.MaxResponseBodyBytes, linkedCts.Token);
             }
             catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
             {
@@ -431,6 +437,51 @@ public class ExternalSystemClient : IExternalSystemClient
         }
     }
 
+    /// <summary>
+    /// Reads an HTTP response body into a string while enforcing an upper size
+    /// bound. When the <c>Content-Length</c> header is present it fails fast;
+    /// otherwise the body is streamed in 16 KiB chunks and the read is aborted the
+    /// moment the accumulated length exceeds <paramref name="maxBytes"/> (cap+1
+    /// pattern), so the whole oversized body is never buffered. Oversize is a
+    /// permanent failure — retrying will not shrink the response, and buffering it
+    /// into store-and-forward would defeat the cap. Cancellation
+    /// (<see cref="OperationCanceledException"/>) is allowed to propagate so the
+    /// caller's ordered timeout/cancellation catch filters classify it.
+    /// </summary>
+    private static async Task<string> ReadBodyBoundedAsync(
+        HttpContent content, string systemName, long maxBytes, CancellationToken token)
+    {
+        // Fast path: a declared Content-Length above the cap is rejected before a
+        // single body byte is read.
+        var declaredLength = content.Headers.ContentLength;
+        if (declaredLength.HasValue && declaredLength.Value > maxBytes)
+        {
+            throw new PermanentExternalSystemException(
+                $"Response from {systemName} exceeded the {maxBytes}-byte limit " +
+                $"(declared Content-Length {declaredLength.Value}).");
+        }
+
+        await using var stream = await content.ReadAsStreamAsync(token);
+
+        using var buffer = new MemoryStream();
+        var chunk = new byte[16 * 1024];
+        int read;
+        while ((read = await stream.ReadAsync(chunk.AsMemory(0, chunk.Length), token)) > 0)
+        {
+            buffer.Write(chunk, 0, read);
+
+            // cap+1: as soon as we hold more than maxBytes we know it is oversized,
+            // without reading the remainder of the stream.
+            if (buffer.Length > maxBytes)
+            {
+                throw new PermanentExternalSystemException(
+                    $"Response from {systemName} exceeded the {maxBytes}-byte limit.");
+            }
+        }
+
+        return Encoding.UTF8.GetString(buffer.GetBuffer(), 0, (int)buffer.Length);
+    }
+
     private static string Truncate(string value, int maxChars)
     {
         if (string.IsNullOrEmpty(value) || value.Length <= maxChars)
diff --git a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemGatewayOptions.cs b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemGatewayOptions.cs
index 5061814c..7e0e4c75 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemGatewayOptions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemGatewayOptions.cs
@@ -10,4 +10,13 @@ public class ExternalSystemGatewayOptions
 
     /// <summary>Maximum number of concurrent HTTP connections per external system.</summary>
     public int MaxConcurrentConnectionsPerSystem { get; set; } = 10;
+
+    /// <summary>
+    /// Upper bound (bytes) on an outbound HTTP response body the gateway will
+    /// buffer into memory. A response whose <c>Content-Length</c> (when present)
+    /// or streamed length exceeds this cap fails permanently — retrying will not
+    /// shrink it, and buffering it into store-and-forward would defeat the cap.
+    /// Default 10 MiB.
+    /// </summary>
+    public long MaxResponseBodyBytes { get; set; } = 10 * 1024 * 1024;
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
index c2913966..13c7b589 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs
@@ -942,6 +942,59 @@ public class ExternalSystemClientTests
         }
     }
 
+    // ── ESG: bound outbound response buffering (MaxResponseBodyBytes) ──
+
+    [Fact]
+    public async Task OversizedResponseBody_FailsPermanently_WithoutBufferingWholeBody()
+    {
+        var system = new ExternalSystemDefinition("TestAPI", "https://api.example.com", "none") { Id = 1 };
+        var method = new ExternalSystemMethod("postData", "POST", "/post") { Id = 1, ExternalSystemDefinitionId = 1 };
+        StubResolution(system, method);
+
+        // The endpoint returns a 2 MiB body — far above the 1 KiB cap below.
+        // Hand out a fresh HttpClient per CreateClient call: the client mutates
+        // HttpClient.Timeout before its first request, so a single shared instance
+        // cannot serve the two calls this test makes.
+        var hugeBody = new string('x', 2 * 1024 * 1024);
+        _httpClientFactory.CreateClient(Arg.Any<string>())
+            .Returns(_ => new HttpClient(new MockHttpMessageHandler(HttpStatusCode.OK, hugeBody)));
+
+        // A real S&F service so we can assert the oversized response is NOT buffered.
+        var dbName = $"EsgOversize_{Guid.NewGuid():N}";
+        var connStr = $"Data Source={dbName};Mode=Memory;Cache=Shared";
+        using var keepAlive = new SqliteConnection(connStr);
+        keepAlive.Open();
+        var storage = new StoreAndForwardStorage(connStr, NullLogger<StoreAndForwardStorage>.Instance);
+        await storage.InitializeAsync();
+        var sfOptions = new StoreAndForwardOptions
+        {
+            DefaultRetryInterval = TimeSpan.FromMinutes(10),
+            RetryTimerInterval = TimeSpan.FromMinutes(10),
+        };
+        var sf = new StoreAndForwardService(storage, sfOptions, NullLogger<StoreAndForwardService>.Instance);
+
+        var options = new ExternalSystemGatewayOptions { MaxResponseBodyBytes = 1024 };
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance,
+            storeAndForward: sf,
+            options: Microsoft.Extensions.Options.Options.Create(options));
+
+        // Sync call: oversize is a permanent failure — retrying will not shrink it.
+        var result = await client.CallAsync("TestAPI", "postData");
+        Assert.False(result.Success);
+        Assert.Contains("exceeded", result.ErrorMessage!, StringComparison.OrdinalIgnoreCase);
+
+        // Cached call of the same shape: a permanent failure must NOT be buffered
+        // into S&F (buffering it would defeat the response-size cap).
+        var cached = await client.CachedCallAsync("TestAPI", "postData");
+        Assert.False(cached.WasBuffered);
+
+        var depth = await storage.GetBufferDepthByCategoryAsync();
+        Assert.False(
+            depth.TryGetValue(ZB.MOM.WW.ScadaBridge.Commons.Types.Enums.StoreAndForwardCategory.ExternalSystem, out var n) && n > 0,
+            "An oversized (permanent-failure) response must not be buffered for retry");
+    }
+
     /// <summary>
     /// Test helper: mock HTTP message handler.
     /// </summary>