Initial

tests/ClaudeMailbox.Tests/RaceTests.cs

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+using System.Net.Http.Json;
+using System.Text.Json;
+
+namespace ClaudeMailbox.Tests;
+
+public sealed class RaceTests
+{
+    [Fact]
+    public async Task Parallel_CheckInbox_Delivers_Each_Message_Exactly_Once()
+    {
+        await using var host = await TestHost.StartAsync();
+        using var sender = host.NewClientFor("sender");
+        using var recipient = host.NewClientFor("recipient");
+
+        const int messageCount = 50;
+        for (var i = 0; i < messageCount; i++)
+        {
+            var res = await sender.PostAsJsonAsync("/v1/send", new { to = "recipient", body = $"msg-{i}" });
+            res.EnsureSuccessStatusCode();
+        }
+
+        // Fire multiple concurrent checks. Each message must appear in exactly one result set.
+        var tasks = Enumerable.Range(0, 8).Select(async _ =>
+        {
+            var res = await recipient.PostAsync("/v1/check-inbox?name=recipient", null);
+            res.EnsureSuccessStatusCode();
+            return await res.Content.ReadFromJsonAsync<JsonElement>();
+        });
+
+        var results = await Task.WhenAll(tasks);
+
+        var ids = new List<long>();
+        foreach (var arr in results)
+            foreach (var m in arr.EnumerateArray())
+                ids.Add(m.GetProperty("id").GetInt64());
+
+        Assert.Equal(messageCount, ids.Count);
+        Assert.Equal(messageCount, ids.Distinct().Count());
+    }
+}