diff --git a/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs b/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs
new file mode 100644
index 0000000..507b163
--- /dev/null
+++ b/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs
@@ -0,0 +1,50 @@
+using System;
+using System.Globalization;
+using System.Linq;
+using System.Security.Cryptography;
+using System.Text;
+
+#nullable enable
+
+namespace GameCult.Eve.Surface
+{
+    /// <summary>Canonical digest for one immutable Eve command invocation.</summary>
+    public static class EveCommandInvocationHash
+    {
+        public static string Compute(EveSurfaceCommandRequest request)
+        {
+            if (request == null) throw new ArgumentNullException(nameof(request));
+            var canonical = new StringBuilder();
+            Append(request.Schema);
+            Append(request.ProviderId);
+            Append(request.SurfaceId);
+            Append(request.OperationRecord.OperationId);
+            Append(request.OperationRecord.SchemaId);
+            Append(request.OperationRecord.RouteKind);
+            Append(request.OperationRecord.RouteDescription);
+            Append(request.OperationRecord.IdempotencyKey);
+            foreach (var field in request.PayloadFields.OrderBy(pair => pair.Key, StringComparer.Ordinal))
+            {
+                Append(field.Key);
+                Append(field.Value);
+            }
+            Append(request.IssuedAt.ToUniversalTime().ToString("O", CultureInfo.InvariantCulture));
+            Append(request.ClientId);
+            Append(request.CommandBoundary);
+            Append(request.ReceiptSchema);
+            using var sha256 = SHA256.Create();
+            var bytes = sha256.ComputeHash(Encoding.UTF8.GetBytes(canonical.ToString()));
+            var digest = new StringBuilder(bytes.Length * 2);
+            foreach (var value in bytes) digest.Append(value.ToString("x2", CultureInfo.InvariantCulture));
+            return digest.ToString();
+
+            void Append(string? value)
+            {
+                value ??= "";
+                canonical.Append(value.Length.ToString(CultureInfo.InvariantCulture));
+                canonical.Append(':');
+                canonical.Append(value);
+            }
+        }
+    }
+}
diff --git a/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs.meta b/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs.meta
new file mode 100644
index 0000000..cf3cc3e
--- /dev/null
+++ b/packages/org.gamecult.eve.surface/Runtime/EveCommandInvocationHash.cs.meta
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 7d5f81f817c44d1ab25d76166354364d
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/tests/GameCult.Eve.Surface.Serialization.Tests/EveSurfaceSerializationCompatibilityTests.cs b/tests/GameCult.Eve.Surface.Serialization.Tests/EveSurfaceSerializationCompatibilityTests.cs
index 30156c3..52d71cc 100644
--- a/tests/GameCult.Eve.Surface.Serialization.Tests/EveSurfaceSerializationCompatibilityTests.cs
+++ b/tests/GameCult.Eve.Surface.Serialization.Tests/EveSurfaceSerializationCompatibilityTests.cs
@@ -68,6 +68,46 @@ public sealed class EveSurfaceSerializationCompatibilityTests
 
     private static readonly MessagePackSerializerOptions Options = MessagePackSerializerOptions.Standard;
 
+    [Test]
+    public void CommandInvocationHashBindsEveryImmutableEnvelopeField()
+    {
+        var request = new EveSurfaceCommandRequest(
+            "provider",
+            "surface",
+            new CultMeshOperationInvocationDescriptor(
+                "launch",
+                "launch.v1",
+                new CultMeshRouteHint(CultMeshLocalityKind.Network, "commands"),
+                "command-1"),
+            CultMesh.OperationPayload(("shipId", "ship:one")),
+            DateTimeOffset.Parse("2026-08-19T00:00:00Z"),
+            "pilot:one",
+            "eve:commands",
+            EveCommandReceiptDocument.SchemaId);
+        var same = new EveSurfaceCommandRequest(
+            request.Schema,
+            request.ProviderId,
+            request.SurfaceId,
+            request.OperationRecord,
+            new Dictionary<string, string>(request.PayloadFields, StringComparer.Ordinal),
+            request.IssuedAt,
+            request.ClientId,
+            request.CommandBoundary,
+            request.ReceiptSchema);
+        var changedPayload = new EveSurfaceCommandRequest(
+            request.ProviderId,
+            request.SurfaceId,
+            request.Operation,
+            CultMesh.OperationPayload(("shipId", "ship:two")),
+            request.IssuedAt,
+            request.ClientId,
+            request.CommandBoundary,
+            request.ReceiptSchema);
+
+        Assert.That(EveCommandInvocationHash.Compute(same), Is.EqualTo(EveCommandInvocationHash.Compute(request)));
+        Assert.That(EveCommandInvocationHash.Compute(changedPayload), Is.Not.EqualTo(EveCommandInvocationHash.Compute(request)));
+    }
+
     [Test]
     public void LegacySevenFieldSurfaceDocumentDeserializes()
     {