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src/Graph/__Tests/StellaOps.Graph.Api.Tests/DiffServiceTests.cs

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+using System.Collections.Generic;
+using StellaOps.Graph.Api.Contracts;
+using StellaOps.Graph.Api.Services;
+using Xunit;
+
+namespace StellaOps.Graph.Api.Tests;
+
+public class DiffServiceTests
+{
+    [Fact]
+    public async Task DiffAsync_EmitsAddedRemovedChangedAndStats()
+    {
+        var repo = new InMemoryGraphRepository();
+        var service = new InMemoryGraphDiffService(repo);
+
+        var request = new GraphDiffRequest
+        {
+            SnapshotA = "snapA",
+            SnapshotB = "snapB",
+            IncludeEdges = true,
+            IncludeStats = true
+        };
+
+        var lines = new List<string>();
+        await foreach (var line in service.DiffAsync("acme", request))
+        {
+            lines.Add(line);
+        }
+
+        Assert.Contains(lines, l => l.Contains("\"type\":\"node_added\"") && l.Contains("newlib"));
+        Assert.Contains(lines, l => l.Contains("\"type\":\"node_changed\"") && l.Contains("widget"));
+        Assert.Contains(lines, l => l.Contains("\"type\":\"edge_added\""));
+        Assert.Contains(lines, l => l.Contains("\"type\":\"stats\""));
+    }
+
+    [Fact]
+    public async Task DiffAsync_WhenSnapshotMissing_ReturnsError()
+    {
+        var repo = new InMemoryGraphRepository();
+        var service = new InMemoryGraphDiffService(repo);
+
+        var request = new GraphDiffRequest
+        {
+            SnapshotA = "snapA",
+            SnapshotB = "missing"
+        };
+
+        var lines = new List<string>();
+        await foreach (var line in service.DiffAsync("acme", request))
+        {
+            lines.Add(line);
+        }
+
+        Assert.Single(lines);
+        Assert.Contains("GRAPH_SNAPSHOT_NOT_FOUND", lines[0]);
+    }
+}