diff --git a/docs/requirements/Component-InboundAPI.md b/docs/requirements/Component-InboundAPI.md
index d79febb0..be1854a7 100644
--- a/docs/requirements/Component-InboundAPI.md
+++ b/docs/requirements/Component-InboundAPI.md
@@ -95,7 +95,7 @@ Each API method definition includes:
   ]
 }
 ```
-- **Failure (4xx/5xx)**: The response body is an error object:
+- **Failure (4xx/5xx)**: The response body is an error object carrying a human-readable `error` and a stable, machine-readable `code`:
 ```json
 {
   "error": "Site unreachable",
@@ -103,6 +103,21 @@ Each API method definition includes:
 }
 ```
 - HTTP status codes distinguish success from failure — no envelope wrapper.
+- **Error codes.** Every failure body carries a `code` an external integrator can branch on (the `error` message is for humans and may change). The two authorization negatives — unknown method and key-not-in-scope — deliberately share one code (`NOT_APPROVED`) and one body so a caller cannot enumerate which method names exist:
+
+| `code` | HTTP status | Meaning |
+|--------|-------------|---------|
+| `UNAUTHORIZED` | 401 | Missing/invalid API key (every auth-stage failure maps here — no stage leak). |
+| `NOT_APPROVED` | 403 | Method not found **or** key not in scope — indistinguishable by design. |
+| `UNSUPPORTED_MEDIA_TYPE` | 415 | Request carried a body with a non-JSON `Content-Type`. A body with **no** `Content-Type` is still parsed leniently as JSON. |
+| `INVALID_JSON` | 400 | Request body was not valid JSON. |
+| `VALIDATION_FAILED` | 400 | Parameter validation failed (missing required field, wrong type, undeclared field). |
+| `BODY_TOO_LARGE` | 413 | Request body exceeded `MaxRequestBodyBytes`. |
+| `STANDBY_NODE` | 503 | Request hit a standby central node; the inbound API serves only the active node. |
+| `TIMEOUT` | 500 | The method script exceeded its execution timeout. |
+| `SITE_UNREACHABLE` | 500 | A routed `Route.To(...).Call(...)` could not reach the target site. |
+| `SCRIPT_COMPILE_FAILED` | 500 | The method's script failed to compile (returned consistently on every node until a compiling version is saved). |
+| `SCRIPT_ERROR` | 500 | Catch-all for any other script-execution failure (details are logged centrally, never leaked to the caller). |
 
 ### Extended Type System
 - API method parameter and return type definitions support an **extended type system** beyond the four template attribute types (Boolean, Integer, Float, String):
diff --git a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/EndpointExtensions.cs b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/EndpointExtensions.cs
index df822a27..85e5237c 100644
--- a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/EndpointExtensions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/EndpointExtensions.cs
@@ -51,6 +51,21 @@ public static class EndpointExtensions
     /// </summary>
     private const string UnauthorizedMessage = "Invalid or missing API key";
 
+    /// <summary>
+    /// C3: builds the canonical inbound-API error body — <c>{ "error": message, "code":
+    /// code }</c> — at the given HTTP status. Every error return in
+    /// <see cref="HandleInboundApiRequest"/> routes through this helper so the
+    /// machine-readable <c>code</c> field (the audience is external integrators) is
+    /// present and consistent across all failure branches. The <c>error</c> field is
+    /// unchanged, so callers that only read it are unaffected (additive field).
+    /// </summary>
+    /// <param name="code">The machine-readable error code (e.g. <c>UNAUTHORIZED</c>, <c>SITE_UNREACHABLE</c>).</param>
+    /// <param name="message">The human-readable, caller-safe error message.</param>
+    /// <param name="status">The HTTP status code for the response.</param>
+    /// <returns>A JSON result carrying the error + code at the given status.</returns>
+    private static IResult Error(string code, string message, int status) =>
+        Results.Json(new { error = message, code }, statusCode: status);
+
     /// <summary>Registers the <c>POST /api/{methodName}</c> inbound API endpoint with the active-node gate and body-size filter applied.</summary>
     /// <param name="endpoints">The route builder to add the endpoint to.</param>
     /// <returns>The same <paramref name="endpoints"/> instance for chaining.</returns>
@@ -108,9 +123,7 @@ public static class EndpointExtensions
                 "Inbound API auth failure for method {Method}: {Failure} (status 401)",
                 methodName, verification.Failure);
 
-            return Results.Json(
-                new { error = UnauthorizedMessage },
-                statusCode: StatusCodes.Status401Unauthorized);
+            return Error("UNAUTHORIZED", UnauthorizedMessage, StatusCodes.Status401Unauthorized);
         }
 
         var identity = verification.Identity!;
@@ -150,9 +163,7 @@ public static class EndpointExtensions
                 "Inbound API authz failure for method {Method}: not approved (status 403)",
                 methodName);
 
-            return Results.Json(
-                new { error = NotApprovedMessage },
-                statusCode: StatusCodes.Status403Forbidden);
+            return Error("NOT_APPROVED", NotApprovedMessage, StatusCodes.Status403Forbidden);
         }
 
         // Telemetry follow-on: count this inbound request against the resolved,
@@ -168,6 +179,20 @@ public static class EndpointExtensions
         httpContext.Items[AuditWriteMiddleware.AuditActorItemKey] =
             identity.DisplayName;
 
+        // S9: a body sent with an explicit non-JSON Content-Type is a media-type
+        // mismatch, not malformed JSON — reject it with 415 rather than a misleading
+        // 400 "invalid JSON". A body with NO Content-Type header is still parsed
+        // leniently as JSON below (preserving existing callers). The sniff is
+        // ordinal-ignore-case so "application/JSON" etc. are treated as JSON.
+        if (httpContext.Request.ContentLength > 0
+            && !string.IsNullOrEmpty(httpContext.Request.ContentType)
+            && httpContext.Request.ContentType.Contains("json", StringComparison.OrdinalIgnoreCase) == false)
+        {
+            return Error(
+                "UNSUPPORTED_MEDIA_TYPE", "Content-Type must be application/json",
+                StatusCodes.Status415UnsupportedMediaType);
+        }
+
         // Deserialize and validate parameters
         JsonElement? body = null;
         try
@@ -188,9 +213,7 @@ public static class EndpointExtensions
         }
         catch (JsonException)
         {
-            return Results.Json(
-                new { error = "Invalid JSON in request body" },
-                statusCode: 400);
+            return Error("INVALID_JSON", "Invalid JSON in request body", StatusCodes.Status400BadRequest);
         }
 
         // Thread the JSON-Schema $ref resolution seam into parameter
@@ -209,9 +232,9 @@ public static class EndpointExtensions
         var paramResult = ParameterValidator.Validate(body, method.ParameterDefinitions, resolveRef);
         if (!paramResult.IsValid)
         {
-            return Results.Json(
-                new { error = paramResult.ErrorMessage },
-                statusCode: 400);
+            return Error(
+                "VALIDATION_FAILED", paramResult.ErrorMessage ?? "Parameter validation failed",
+                StatusCodes.Status400BadRequest);
         }
 
         // Execute the method's script
@@ -250,9 +273,13 @@ public static class EndpointExtensions
                 "Inbound API script failure for method {Method}: {Error}",
                 methodName, scriptResult.ErrorMessage);
 
-            return Results.Json(
-                new { error = scriptResult.ErrorMessage ?? "Internal server error" },
-                statusCode: 500);
+            // C3: propagate the executor's machine-readable code (TIMEOUT,
+            // SITE_UNREACHABLE, SCRIPT_COMPILE_FAILED, …); an unclassified failure
+            // falls back to the SCRIPT_ERROR catch-all.
+            return Error(
+                scriptResult.ErrorCode ?? "SCRIPT_ERROR",
+                scriptResult.ErrorMessage ?? "Internal server error",
+                StatusCodes.Status500InternalServerError);
         }
 
         // Return the script result as JSON
diff --git a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs
index a99af5c3..4cfc3edf 100644
--- a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs
@@ -56,7 +56,7 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
             _logger.LogWarning(
                 "Inbound API request rejected — this node is a standby (not the active central node)");
             return Results.Json(
-                new { error = "Inbound API is only available on the active central node" },
+                new { error = "Inbound API is only available on the active central node", code = "STANDBY_NODE" },
                 statusCode: StatusCodes.Status503ServiceUnavailable);
         }
 
@@ -70,7 +70,7 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
                 "Inbound API request rejected — body length {Length} exceeds limit {Limit}",
                 declaredLength, maxBytes);
             return Results.Json(
-                new { error = "Request body too large" },
+                new { error = "Request body too large", code = "BODY_TOO_LARGE" },
                 statusCode: StatusCodes.Status413PayloadTooLarge);
         }
 
diff --git a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundScriptExecutor.cs b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundScriptExecutor.cs
index 86e9ab4b..e3fe8806 100644
--- a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundScriptExecutor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundScriptExecutor.cs
@@ -384,7 +384,7 @@ public class InboundScriptExecutor
                 // Fast-fail only when the CURRENT script text is the one already known bad.
                 if (_knownBadMethods.TryGetValue(method.Name, out var badScript)
                     && string.Equals(badScript, method.Script, StringComparison.Ordinal))
-                    return new InboundScriptResult(false, null, "Script compilation failed for this method");
+                    return new InboundScriptResult(false, null, "Script compilation failed for this method", "SCRIPT_COMPILE_FAILED");
 
                 // Lazy compile (handles methods created after startup, and stale/known-bad
                 // recompiles). Compile outside the cache so a failed compile is not stored,
@@ -396,7 +396,7 @@ public class InboundScriptExecutor
                     // Routed through TryRecordBadMethod so the
                     // cache is bounded under a flood of unique method names.
                     TryRecordBadMethod(method.Name, method.Script ?? string.Empty);
-                    return new InboundScriptResult(false, null, "Script compilation failed for this method");
+                    return new InboundScriptResult(false, null, "Script compilation failed for this method", "SCRIPT_COMPILE_FAILED");
                 }
 
                 _knownBadMethods.TryRemove(method.Name, out _);
@@ -448,17 +448,25 @@ public class InboundScriptExecutor
             if (timeoutCts.IsCancellationRequested && !cancellationToken.IsCancellationRequested)
             {
                 _logger.LogWarning("Script execution timed out for method {Method}", method.Name);
-                return new InboundScriptResult(false, null, "Script execution timed out");
+                return new InboundScriptResult(false, null, "Script execution timed out", "TIMEOUT");
             }
 
             _logger.LogDebug("Inbound API request for method {Method} cancelled by client", method.Name);
             return new InboundScriptResult(false, null, "Request cancelled by client");
         }
+        catch (SiteUnreachableException)
+        {
+            // C3: a routed Route.To(...).Call(...) reached a site that could not be
+            // contacted. Surface the machine-readable SITE_UNREACHABLE code (spec
+            // Component-InboundAPI failure body) instead of the generic SCRIPT_ERROR.
+            _logger.LogWarning("Routed site unreachable for method {Method}", method.Name);
+            return new InboundScriptResult(false, null, "Site unreachable", "SITE_UNREACHABLE");
+        }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Script execution failed for method {Method}", method.Name);
             // Safe error message, no internal details
-            return new InboundScriptResult(false, null, "Internal script error");
+            return new InboundScriptResult(false, null, "Internal script error", "SCRIPT_ERROR");
         }
         finally
         {
@@ -537,7 +545,17 @@ public class InboundScriptContext
 /// <summary>
 /// Result of executing an inbound API script.
 /// </summary>
+/// <param name="Success">Whether the script executed successfully.</param>
+/// <param name="ResultJson">The serialized script return value on success; null otherwise.</param>
+/// <param name="ErrorMessage">A sanitized, caller-safe error message on failure; null on success.</param>
+/// <param name="ErrorCode">
+/// C3: an additive, machine-readable error code the endpoint surfaces in the failure
+/// body's <c>code</c> field (e.g. <c>TIMEOUT</c>, <c>SITE_UNREACHABLE</c>,
+/// <c>SCRIPT_COMPILE_FAILED</c>, <c>SCRIPT_ERROR</c>). Null when the executor does not
+/// classify the failure; the endpoint then falls back to <c>SCRIPT_ERROR</c>.
+/// </param>
 public record InboundScriptResult(
     bool Success,
     string? ResultJson,
-    string? ErrorMessage);
+    string? ErrorMessage,
+    string? ErrorCode = null);
diff --git a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/RouteHelper.cs b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/RouteHelper.cs
index d94d9c3e..dca01913 100644
--- a/src/ZB.MOM.WW.ScadaBridge.InboundAPI/RouteHelper.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.InboundAPI/RouteHelper.cs
@@ -4,6 +4,22 @@ using ZB.MOM.WW.ScadaBridge.Commons.Types;
 
 namespace ZB.MOM.WW.ScadaBridge.InboundAPI;
 
+/// <summary>
+/// C3: a routed call failed because the target site could not be reached (no contact /
+/// unreachable). Derives from <see cref="InvalidOperationException"/> so existing
+/// callers that catch the base type are unaffected, while
+/// <see cref="InboundScriptExecutor"/> catches this more specific type to surface the
+/// spec's <c>SITE_UNREACHABLE</c> error code instead of a generic <c>SCRIPT_ERROR</c>.
+/// A routing failure whose message does <em>not</em> indicate unreachability stays a
+/// plain <see cref="InvalidOperationException"/>.
+/// </summary>
+public sealed class SiteUnreachableException : InvalidOperationException
+{
+    /// <summary>Initializes a new <see cref="SiteUnreachableException"/> with the given message.</summary>
+    /// <param name="message">The routing-level failure message that indicated the site was unreachable.</param>
+    public SiteUnreachableException(string message) : base(message) { }
+}
+
 /// <summary>
 /// Route.To() helper for cross-site calls from inbound API scripts.
 /// Resolves instance to site, routes via <see cref="IInstanceRouter"/>, blocks until
@@ -170,7 +186,7 @@ public class RouteTarget
 
         if (!response.Success)
         {
-            throw new InvalidOperationException(
+            throw RoutingFailure(
                 response.ErrorMessage ?? "Remote script call failed");
         }
 
@@ -218,7 +234,7 @@ public class RouteTarget
 
         if (!response.Success)
         {
-            throw new InvalidOperationException(
+            throw RoutingFailure(
                 response.ErrorMessage ?? "Remote attribute read failed");
         }
 
@@ -282,7 +298,7 @@ public class RouteTarget
 
         if (!response.Success)
         {
-            throw new InvalidOperationException(
+            throw RoutingFailure(
                 response.ErrorMessage ?? "Remote attribute wait failed");
         }
 
@@ -333,7 +349,7 @@ public class RouteTarget
 
         if (!response.Success)
         {
-            throw new InvalidOperationException(
+            throw RoutingFailure(
                 response.ErrorMessage ?? "Remote attribute write failed");
         }
     }
@@ -350,10 +366,30 @@ public class RouteTarget
         var siteId = await _instanceLocator.GetSiteIdForInstanceAsync(_instanceCode, cancellationToken);
         if (siteId == null)
         {
-            throw new InvalidOperationException(
+            // Route the site-resolution failure through the same classifier so a
+            // message indicating the site is unreachable surfaces as SITE_UNREACHABLE;
+            // a plain not-found / no-assigned-site stays a generic InvalidOperationException.
+            throw RoutingFailure(
                 $"Instance '{_instanceCode}' not found or has no assigned site");
         }
 
         return siteId;
     }
+
+    /// <summary>
+    /// C3: classifies a routing-level failure message. When the message indicates the
+    /// site could not be reached (unreachable / no contact) the failure is a
+    /// <see cref="SiteUnreachableException"/> — so <see cref="InboundScriptExecutor"/>
+    /// can surface the spec's <c>SITE_UNREACHABLE</c> code — otherwise a plain
+    /// <see cref="InvalidOperationException"/>.
+    /// </summary>
+    private static InvalidOperationException RoutingFailure(string message) =>
+        IsUnreachable(message)
+            ? new SiteUnreachableException(message)
+            : new InvalidOperationException(message);
+
+    private static bool IsUnreachable(string message) =>
+        message.Contains("unreachable", StringComparison.OrdinalIgnoreCase)
+        || message.Contains("no contact", StringComparison.OrdinalIgnoreCase)
+        || message.Contains("no-contact", StringComparison.OrdinalIgnoreCase);
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointContentTypeTests.cs b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointContentTypeTests.cs
index 5c4a3a29..2e5c1cc2 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointContentTypeTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointContentTypeTests.cs
@@ -92,6 +92,59 @@ public sealed class EndpointContentTypeTests : IDisposable
         Assert.Contains("42", body);
     }
 
+    [Fact]
+    public async Task NonJsonContentType_WithBody_Returns415()
+    {
+        // Task 5 (S9): a body with a non-JSON Content-Type must be rejected with 415 +
+        // UNSUPPORTED_MEDIA_TYPE rather than a misleading 400 "invalid JSON". A body
+        // with NO Content-Type header is still parsed leniently as JSON (preserving
+        // existing callers).
+        const string methodName = "echo415";
+
+        var method = new ApiMethod(methodName, "return Parameters[\"value\"];")
+        {
+            Id = 1,
+            TimeoutSeconds = 10,
+            ParameterDefinitions = """[{"name":"value","type":"Integer","required":true}]""",
+        };
+
+        var repo = Substitute.For<IInboundApiRepository>();
+        repo.GetMethodByNameAsync(methodName, Arg.Any<CancellationToken>())
+            .Returns(method);
+
+        using var host = await BuildHostAsync(repo);
+        var token = await SeedKeyAsync(host, methodName);
+        var client = host.GetTestClient();
+
+        // (a) text/plain body → 415 UNSUPPORTED_MEDIA_TYPE
+        var nonJson = new HttpRequestMessage(HttpMethod.Post, "/api/" + methodName)
+        {
+            Content = new ByteArrayContent(Encoding.UTF8.GetBytes("hello")),
+        };
+        nonJson.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("text/plain");
+        nonJson.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
+
+        var nonJsonResponse = await client.SendAsync(nonJson);
+        var nonJsonBody = await nonJsonResponse.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.UnsupportedMediaType, nonJsonResponse.StatusCode);
+        Assert.Contains("UNSUPPORTED_MEDIA_TYPE", nonJsonBody);
+
+        // (b) body with NO Content-Type header → still parses as JSON (lenient), 200.
+        var noContentType = new HttpRequestMessage(HttpMethod.Post, "/api/" + methodName)
+        {
+            Content = new ByteArrayContent(Encoding.UTF8.GetBytes("{\"value\":42}")),
+        };
+        noContentType.Content.Headers.ContentType = null;
+        noContentType.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
+
+        var lenientResponse = await client.SendAsync(noContentType);
+        var lenientBody = await lenientResponse.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.OK, lenientResponse.StatusCode);
+        Assert.Contains("42", lenientBody);
+    }
+
     /// <summary>Seeds a key scoped for <paramref name="methodName"/> and returns its Bearer token.</summary>
     private static async Task<string> SeedKeyAsync(IHost host, string methodName)
     {
diff --git a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
index a34fe3cd..13226054 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/EndpointExtensionsTests.cs
@@ -26,6 +26,7 @@ using System.Diagnostics.Metrics;
 using System.Net;
 using System.Net.Http.Headers;
 using System.Text;
+using System.Text.Json;
 
 namespace ZB.MOM.WW.ScadaBridge.InboundAPI.Tests;
 
@@ -67,8 +68,11 @@ public sealed class EndpointExtensionsTests : IDisposable
     // two bodies are equal-to-each-other) catches a single-branch divergence where
     // BOTH branches change in lockstep but away from the agreed enumeration-safe
     // body — the equal-to-each-other check alone would still pass in that case.
-    // Pinned to EndpointExtensions.NotApprovedMessage's JSON shape.
-    private const string NotApprovedBodyJson = """{"error":"API key not approved for this method"}""";
+    // Pinned to EndpointExtensions.NotApprovedMessage's JSON shape. Task 5 (C3): the
+    // error body now additionally carries the machine-readable "code" field; both 403
+    // branches still emit this identical body, preserving enumeration-safety.
+    private const string NotApprovedBodyJson =
+        """{"error":"API key not approved for this method","code":"NOT_APPROVED"}""";
 
     // Each test gets its own throwaway SQLite database so seeded keys never leak
     // between tests; the file is deleted on Dispose.
@@ -341,6 +345,45 @@ public sealed class EndpointExtensionsTests : IDisposable
         Assert.Equal(NotApprovedBodyJson, notInScopeBody);
     }
 
+    [Fact]
+    public async Task ErrorBodies_CarryMachineReadableCode()
+    {
+        // Task 5 (C3): every error body carries a machine-readable "code" field for
+        // external integrators — UNAUTHORIZED (401), INVALID_JSON (400),
+        // VALIDATION_FAILED (400).
+        var method = SeedMethod(1, "needsParam", "return Parameters[\"value\"];",
+            """[{"name":"value","type":"Integer","required":true}]""");
+
+        using var host = await BuildHostAsync(method);
+        var token = await SeedKeyAsync(host, "key1", "caller", new[] { "needsParam" });
+        var client = host.GetTestClient();
+
+        // (a) missing credential → UNAUTHORIZED
+        var unauth = await client.SendAsync(new HttpRequestMessage(HttpMethod.Post, "/api/needsParam")
+        {
+            Content = new StringContent("{}", Encoding.UTF8, "application/json"),
+        });
+        Assert.Equal(HttpStatusCode.Unauthorized, unauth.StatusCode);
+        Assert.Equal("UNAUTHORIZED", await ReadCodeAsync(unauth));
+
+        // (b) in-scope but invalid JSON → INVALID_JSON
+        var badJson = await client.SendAsync(BuildPost("needsParam", "{ not json", token));
+        Assert.Equal(HttpStatusCode.BadRequest, badJson.StatusCode);
+        Assert.Equal("INVALID_JSON", await ReadCodeAsync(badJson));
+
+        // (c) missing required parameter → VALIDATION_FAILED
+        var missing = await client.SendAsync(BuildPost("needsParam", "{}", token));
+        Assert.Equal(HttpStatusCode.BadRequest, missing.StatusCode);
+        Assert.Equal("VALIDATION_FAILED", await ReadCodeAsync(missing));
+    }
+
+    private static async Task<string?> ReadCodeAsync(HttpResponseMessage response)
+    {
+        var body = await response.Content.ReadAsStringAsync();
+        using var doc = JsonDocument.Parse(body);
+        return doc.RootElement.TryGetProperty("code", out var code) ? code.GetString() : null;
+    }
+
     [Fact]
     public async Task InvalidJsonBody_Returns400()
     {
diff --git a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundScriptExecutorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundScriptExecutorTests.cs
index cb46895c..9a929c4a 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundScriptExecutorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundScriptExecutorTests.cs
@@ -97,6 +97,32 @@ public class InboundScriptExecutorTests
         Assert.Equal("Internal script error", result.ErrorMessage);
     }
 
+    [Fact]
+    public async Task RoutedSiteUnreachable_Returns500WithSiteUnreachableCode()
+    {
+        // C3: a routed call whose site is unreachable must surface the machine-readable
+        // SITE_UNREACHABLE code (spec Component-InboundAPI failure body) rather than
+        // collapsing into a generic SCRIPT_ERROR.
+        var locator = Substitute.For<IInstanceLocator>();
+        locator.GetSiteIdForInstanceAsync("X", Arg.Any<CancellationToken>()).Returns("SiteA");
+        var router = Substitute.For<IInstanceRouter>();
+        router.RouteToCallAsync("SiteA", Arg.Any<RouteToCallRequest>(), Arg.Any<CancellationToken>())
+            .Returns(ci => new RouteToCallResponse(
+                ((RouteToCallRequest)ci[1]).CorrelationId,
+                Success: false, ReturnValue: null, ErrorMessage: "Site unreachable",
+                DateTimeOffset.UtcNow));
+        var route = new RouteHelper(locator, router);
+
+        var method = new ApiMethod("routed", "return await Route.To(\"X\").Call(\"s\");")
+        { Id = 7, TimeoutSeconds = 10 };
+
+        var result = await _executor.ExecuteAsync(
+            method, new Dictionary<string, object?>(), route, TimeSpan.FromSeconds(10));
+
+        Assert.False(result.Success);
+        Assert.Equal("SITE_UNREACHABLE", result.ErrorCode);
+    }
+
     [Fact]
     public async Task HandlerTimesOut_ReturnsTimeoutError()
     {