diff --git a/SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs b/SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs
index 62d63bf..ba26878 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/RankBattleController.cs
@@ -157,11 +157,66 @@ public sealed class RankBattleController : ControllerBase
         });
     }
 
-    // Filled in by Task 10.
-    private Task<IActionResult> AiStartInternal(Format format, CancellationToken ct)
+    private async Task<IActionResult> AiStartInternal(Format format, CancellationToken ct)
     {
-        if (!TryGetViewerId(out var _)) return Task.FromResult<IActionResult>(Unauthorized());
-        // Placeholder; real impl arrives in Task 10.
-        return Task.FromResult<IActionResult>(Ok(new AiBattleStartResponseDto { AiId = -1 }));
+        if (!TryGetViewerId(out var vid)) return Unauthorized();
+
+        MatchContext selfCtx;
+        try
+        {
+            selfCtx = await _ctxBuilder.BuildForRankBattleAsync(vid, format);
+        }
+        catch (InvalidOperationException ex)
+        {
+            // No deck → can't build a self profile. Surface as the "no AI assigned"
+            // sentinel; the client treats ai_id=-1 as a fallback/error condition.
+            _log.LogWarning(ex, "AiStart failed for viewer {Vid} format {Fmt}; returning ai_id=-1.", vid, format);
+            return Ok(new AiBattleStartResponseDto { AiId = -1 });
+        }
+
+        var bot = _botRoster.Pick(selfCtx);
+
+        // Per spec, ai-start.md TODO: turnState semantics unclear. Default 0 (player first).
+        return Ok(new AiBattleStartResponseDto
+        {
+            AiId = bot.AiId,
+            TurnState = 0,
+            SelfInfo = new AiBattlePlayerInfo
+            {
+                CountryCode = selfCtx.CountryCode,
+                UserName = selfCtx.UserName,
+                SleeveId = int.TryParse(selfCtx.SleeveId, out var sId) ? sId : -1,
+                EmblemId = int.TryParse(selfCtx.EmblemId, out var eId) ? eId : -1,
+                DegreeId = int.TryParse(selfCtx.DegreeId, out var dId) ? dId : -1,
+                FieldId = selfCtx.FieldId,
+                IsOfficial = selfCtx.IsOfficial,
+                OppoId = bot.AiId,
+                Seed = 0,
+                Rank = 0,
+                BattlePoint = 0,
+                ClassId = int.TryParse(selfCtx.ClassId, out var cId) ? cId : -1,
+                CharaId = int.TryParse(selfCtx.CharaId, out var chId) ? chId : -1,
+                IsMasterRank = 0,
+                MasterPoint = 0,
+            },
+            OppoInfo = new AiBattlePlayerInfo
+            {
+                CountryCode = bot.CountryCode,
+                UserName = bot.UserName,
+                SleeveId = bot.SleeveId,
+                EmblemId = bot.EmblemId,
+                DegreeId = bot.DegreeId,
+                FieldId = bot.FieldId,
+                IsOfficial = bot.IsOfficial,
+                OppoId = (int)vid,
+                Seed = 0,
+                Rank = bot.Rank,
+                BattlePoint = bot.BattlePoint,
+                ClassId = bot.ClassId,
+                CharaId = bot.CharaId,
+                IsMasterRank = bot.IsMasterRank,
+                MasterPoint = bot.MasterPoint,
+            },
+        });
     }
 }
diff --git a/SVSim.UnitTests/Controllers/RankBattleControllerTests.cs b/SVSim.UnitTests/Controllers/RankBattleControllerTests.cs
index 7a0f953..a9603f5 100644
--- a/SVSim.UnitTests/Controllers/RankBattleControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/RankBattleControllerTests.cs
@@ -66,6 +66,76 @@ public class RankBattleControllerTests
         Assert.That(j3.GetProperty("battle_id").GetString(), Is.EqualTo(j2.GetProperty("battle_id").GetString()));
     }
 
+    [Test]
+    public async Task AiStart_rotation_returns_ai_id_plus_self_oppo_info_camelCase_keys()
+    {
+        await using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync(displayName: "TestViewer");
+        await factory.SeedGlobalsAsync();
+        await factory.SeedDeckAsync(viewerId, Format.Rotation, 1);
+        var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var resp = await client.PostAsJsonAsync("/ai_rotation_rank_battle/start", new { });
+        var raw = await resp.Content.ReadAsStringAsync();
+
+        using var doc = JsonDocument.Parse(raw);
+        var data = doc.RootElement;
+        Assert.That(data.GetProperty("ai_id").GetInt32(), Is.InRange(4001, 4008));
+        Assert.That(data.GetProperty("turnState").GetInt32(), Is.EqualTo(0));
+
+        // Literal camelCase wire-key checks — these MUST be present verbatim
+        // (client uses JsonData.Keys.Contains).
+        Assert.That(raw, Does.Contain("\"userName\""), "Wire key must be camelCase, not snake_case.");
+        Assert.That(raw, Does.Contain("\"sleeveId\""));
+        Assert.That(raw, Does.Contain("\"emblemId\""));
+        Assert.That(raw, Does.Contain("\"degreeId\""));
+        Assert.That(raw, Does.Contain("\"fieldId\""));
+        Assert.That(raw, Does.Contain("\"isOfficial\""));
+        Assert.That(raw, Does.Contain("\"classId\""));
+        Assert.That(raw, Does.Contain("\"charaId\""));
+        Assert.That(raw, Does.Contain("\"isMasterRank\""));
+        Assert.That(raw, Does.Contain("\"battlePoint\""));
+        Assert.That(raw, Does.Contain("\"masterPoint\""));
+        // self_info / oppo_info / country_code stay snake_case (the outliers per ai-start.md).
+        Assert.That(raw, Does.Contain("\"self_info\""));
+        Assert.That(raw, Does.Contain("\"oppo_info\""));
+        Assert.That(raw, Does.Contain("\"country_code\""));
+    }
+
+    [Test]
+    public async Task AiStart_self_info_reflects_caller_user_name()
+    {
+        await using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync(displayName: "Alice");
+        await factory.SeedGlobalsAsync();
+        await factory.SeedDeckAsync(viewerId, Format.Rotation, 1);
+        var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var resp = await client.PostAsJsonAsync("/ai_rotation_rank_battle/start", new { });
+        using var doc = JsonDocument.Parse(await resp.Content.ReadAsStringAsync());
+        var selfInfo = doc.RootElement.GetProperty("self_info");
+
+        Assert.That(selfInfo.GetProperty("userName").GetString(), Is.EqualTo("Alice"));
+    }
+
+    [Test]
+    public async Task AiStart_oppo_info_reflects_roster_pick()
+    {
+        await using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync(displayName: "PlayerA");
+        await factory.SeedGlobalsAsync();
+        await factory.SeedDeckAsync(viewerId, Format.Rotation, 1);
+        var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var resp = await client.PostAsJsonAsync("/ai_rotation_rank_battle/start", new { });
+        using var doc = JsonDocument.Parse(await resp.Content.ReadAsStringAsync());
+        var oppoInfo = doc.RootElement.GetProperty("oppo_info");
+
+        // BotRoster's stub names contain "AI" — verify the roster was consulted.
+        Assert.That(oppoInfo.GetProperty("userName").GetString(), Does.Contain("AI"));
+        Assert.That(oppoInfo.GetProperty("classId").GetInt32(), Is.InRange(1, 8));
+    }
+
     [Test]
     public async Task Finish_emits_stubbed_zeros_with_battle_result_echo()
     {