diff --git a/docs/plans/NEO-23-implementation-plan.md b/docs/plans/NEO-23-implementation-plan.md
index 2ebf334..c791182 100644
--- a/docs/plans/NEO-23-implementation-plan.md
+++ b/docs/plans/NEO-23-implementation-plan.md
@@ -31,10 +31,10 @@
 
 ## Acceptance criteria checklist
 
-- [ ] **Server source of truth:** Client cannot persist a lock the server rejects; every **`POST`** response includes the **authoritative** `targetState` after evaluation (success or denial).
-- [ ] **Denials:** **`selectionApplied`: `false`** ⇒ **required non-empty `reasonCode`**; **HTTP 200** for logical denials; **400** / **404** rules match NEO-9 spirit (unknown player → **404** before body; malformed v1 → **400**).
-- [ ] **Movement coupling:** After **`POST /move`** or **`POST /move-stream`**, the same **`GET`**/ **`POST`** response reflects updated **`validity`** (e.g. walk out of stub radius ⇒ `out_of_range` on read without requiring a new select).
-- [ ] **Contract names** in README + XML match the **illustrative** E1.M3 table ([Key contracts](../decomposition/modules/E1_M3_InteractionAndTargetingLayer.md#key-contracts)); any renames are visible in this plan and in PR text, not only in code.
+- [x] **Server source of truth:** Client cannot persist a lock the server rejects; every **`POST`** response includes the **authoritative** `targetState` after evaluation (success or denial).
+- [x] **Denials:** **`selectionApplied`: `false`** ⇒ **required non-empty `reasonCode`**; **HTTP 200** for logical denials; **400** / **404** rules match NEO-9 spirit (unknown player → **404** before body; malformed v1 → **400**).
+- [x] **Movement coupling:** After **`POST /move`** or **`POST /move-stream`**, the same **`GET`**/ **`POST`** response reflects updated **`validity`** (e.g. walk out of stub radius ⇒ `out_of_range` on read without requiring a new select).
+- [x] **Contract names** in README + XML match the **illustrative** E1.M3 table ([Key contracts](../decomposition/modules/E1_M3_InteractionAndTargetingLayer.md#key-contracts)); any renames are visible in this plan and in PR text, not only in code.
 
 ## Technical approach
 
diff --git a/server/NeonSprawl.Server.Tests/Game/Targeting/PlayerTargetStateReaderTests.cs b/server/NeonSprawl.Server.Tests/Game/Targeting/PlayerTargetStateReaderTests.cs
new file mode 100644
index 0000000..87c9881
--- /dev/null
+++ b/server/NeonSprawl.Server.Tests/Game/Targeting/PlayerTargetStateReaderTests.cs
@@ -0,0 +1,22 @@
+using NeonSprawl.Server.Game.PositionState;
+using NeonSprawl.Server.Game.Targeting;
+using Xunit;
+
+namespace NeonSprawl.Server.Tests.Game.Targeting;
+
+public class PlayerTargetStateReaderTests
+{
+    [Fact]
+    public void ComputeValidity_ShouldReturnInvalidTarget_WhenLockIdNotInRegistry()
+    {
+        var snap = new PositionSnapshot(0, 0, 0, 0);
+        Assert.Equal(TargetValidity.InvalidTarget, PlayerTargetStateReader.ComputeValidity("not_in_registry", in snap));
+    }
+
+    [Fact]
+    public void ComputeValidity_ShouldReturnNone_WhenLockNull()
+    {
+        var snap = new PositionSnapshot(0, 0, 0, 0);
+        Assert.Equal(TargetValidity.None, PlayerTargetStateReader.ComputeValidity(null, in snap));
+    }
+}
diff --git a/server/NeonSprawl.Server.Tests/Game/Targeting/TargetingApiTests.cs b/server/NeonSprawl.Server.Tests/Game/Targeting/TargetingApiTests.cs
new file mode 100644
index 0000000..09b8753
--- /dev/null
+++ b/server/NeonSprawl.Server.Tests/Game/Targeting/TargetingApiTests.cs
@@ -0,0 +1,290 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using NeonSprawl.Server.Game.PositionState;
+using NeonSprawl.Server.Game.Targeting;
+using Xunit;
+
+namespace NeonSprawl.Server.Tests.Game.Targeting;
+
+/// <summary>Integration tests for <see cref="TargetingApi"/> (NEO-23).</summary>
+public class TargetingApiTests
+{
+    [Fact]
+    public async Task GetTarget_ShouldReturnNone_WhenNoLock()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.GetAsync("/game/players/dev-local-1/target");
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<PlayerTargetStateResponse>();
+        Assert.NotNull(body);
+        Assert.Equal(PlayerTargetStateResponse.CurrentSchemaVersion, body!.SchemaVersion);
+        Assert.Equal("dev-local-1", body.PlayerId);
+        Assert.Null(body.LockedTargetId);
+        Assert.Equal(TargetValidity.None, body.Validity);
+        Assert.Equal(0, body.Sequence);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldApplyLock_WhenInRange()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+            });
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.True(body!.SelectionApplied);
+        Assert.Null(body.ReasonCode);
+        Assert.Equal(PrototypeTargetRegistry.PrototypeTargetAlphaId, body.TargetState.LockedTargetId);
+        Assert.Equal(TargetValidity.Ok, body.TargetState.Validity);
+        Assert.Equal(1, body.TargetState.Sequence);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldDenyUnknownTarget_WithReasonCode()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = "not_a_real_target",
+            });
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.False(body!.SelectionApplied);
+        Assert.Equal(TargetingApi.ReasonUnknownTarget, body.ReasonCode);
+        Assert.Null(body.TargetState.LockedTargetId);
+        Assert.Equal(TargetValidity.None, body.TargetState.Validity);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldDenyOutOfRange_WhenBetaFarFromSpawn()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetBetaId,
+            });
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.False(body!.SelectionApplied);
+        Assert.Equal(TargetingApi.ReasonOutOfRange, body.ReasonCode);
+        Assert.Null(body.TargetState.LockedTargetId);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldClearLock_WhenTargetIdOmitted()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        Assert.Equal(
+            HttpStatusCode.OK,
+            (await client.PostAsJsonAsync(
+                "/game/players/dev-local-1/target/select",
+                new TargetSelectRequest
+                {
+                    SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                    TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+                })).StatusCode);
+
+        var clear = await client.PostAsync(
+            "/game/players/dev-local-1/target/select",
+            new StringContent(
+                $"{{\"schemaVersion\":{TargetSelectRequest.CurrentSchemaVersion}}}",
+                Encoding.UTF8,
+                "application/json"));
+
+        Assert.Equal(HttpStatusCode.OK, clear.StatusCode);
+        var body = await clear.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.True(body!.SelectionApplied);
+        Assert.Null(body.TargetState.LockedTargetId);
+        Assert.Equal(TargetValidity.None, body.TargetState.Validity);
+    }
+
+    [Fact]
+    public async Task GetTarget_ShouldShowOutOfRange_AfterMoveWithoutSelect()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        Assert.Equal(
+            HttpStatusCode.OK,
+            (await client.PostAsJsonAsync(
+                "/game/players/dev-local-1/target/select",
+                new TargetSelectRequest
+                {
+                    SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                    TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+                })).StatusCode);
+
+        var move = new MoveCommandRequest
+        {
+            SchemaVersion = MoveCommandRequest.CurrentSchemaVersion,
+            Target = new PositionVector { X = 50, Y = 0, Z = 50 },
+        };
+        Assert.Equal(HttpStatusCode.OK, (await client.PostAsJsonAsync("/game/players/dev-local-1/move", move)).StatusCode);
+
+        var get = await client.GetAsync("/game/players/dev-local-1/target");
+        Assert.Equal(HttpStatusCode.OK, get.StatusCode);
+        var body = await get.Content.ReadFromJsonAsync<PlayerTargetStateResponse>();
+        Assert.NotNull(body);
+        Assert.Equal(PrototypeTargetRegistry.PrototypeTargetAlphaId, body!.LockedTargetId);
+        Assert.Equal(TargetValidity.OutOfRange, body.Validity);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldReturnNotFound_WhenPlayerUnknown()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/missing-player/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+            });
+
+        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldReturnBadRequest_WhenSchemaVersionWrong()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = 999,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+            });
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldReturnBadRequest_WhenTargetIdWhitespaceOnly()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = "  \t ",
+            });
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldBeCaseInsensitive_ForTargetId()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = "  PROTOTYPE_TARGET_ALPHA  ",
+            });
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.True(body!.SelectionApplied);
+        Assert.Equal(PrototypeTargetRegistry.PrototypeTargetAlphaId, body.TargetState.LockedTargetId);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldStayIdempotent_WhenSameTargetReselect()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        var req = new TargetSelectRequest
+        {
+            SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+            TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+        };
+        var first = await client.PostAsJsonAsync("/game/players/dev-local-1/target/select", req);
+        var second = await client.PostAsJsonAsync("/game/players/dev-local-1/target/select", req);
+        Assert.Equal(HttpStatusCode.OK, first.StatusCode);
+        Assert.Equal(HttpStatusCode.OK, second.StatusCode);
+        var b1 = await first.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        var b2 = await second.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.Equal(b1!.TargetState.Sequence, b2!.TargetState.Sequence);
+    }
+
+    [Fact]
+    public async Task PostSelect_ShouldDenyOutOfRange_WhenReselectingSameLockWhileOutOfRange()
+    {
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+            });
+
+        var move = new MoveCommandRequest
+        {
+            SchemaVersion = MoveCommandRequest.CurrentSchemaVersion,
+            Target = new PositionVector { X = 50, Y = 0, Z = 50 },
+        };
+        await client.PostAsJsonAsync("/game/players/dev-local-1/move", move);
+
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/target/select",
+            new TargetSelectRequest
+            {
+                SchemaVersion = TargetSelectRequest.CurrentSchemaVersion,
+                TargetId = PrototypeTargetRegistry.PrototypeTargetAlphaId,
+            });
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadFromJsonAsync<TargetSelectResponse>();
+        Assert.NotNull(body);
+        Assert.False(body!.SelectionApplied);
+        Assert.Equal(TargetingApi.ReasonOutOfRange, body.ReasonCode);
+        Assert.Equal(PrototypeTargetRegistry.PrototypeTargetAlphaId, body.TargetState.LockedTargetId);
+        Assert.Equal(TargetValidity.OutOfRange, body.TargetState.Validity);
+    }
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/IPlayerTargetLockStore.cs b/server/NeonSprawl.Server/Game/Targeting/IPlayerTargetLockStore.cs
new file mode 100644
index 0000000..67be447
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/IPlayerTargetLockStore.cs
@@ -0,0 +1,14 @@
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>In-memory per-player combat target lock for prototype HTTP (NEO-23).</summary>
+public interface IPlayerTargetLockStore
+{
+    /// <summary>Current lock id (lowercase registry key) and sequence; missing player ⇒ <c>(null, 0)</c>.</summary>
+    (string? LockIdLowercase, int Sequence) GetLockState(string playerId);
+
+    /// <summary>Clears lock; bumps sequence only if a lock was present.</summary>
+    (string? LockIdLowercase, int Sequence) ApplyClear(string playerId);
+
+    /// <summary>Sets lock to <paramref name="targetIdLowercase"/>; no sequence bump if already equal (idempotent).</summary>
+    (string? LockIdLowercase, int Sequence) ApplySet(string playerId, string targetIdLowercase);
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/InMemoryPlayerTargetLockStore.cs b/server/NeonSprawl.Server/Game/Targeting/InMemoryPlayerTargetLockStore.cs
new file mode 100644
index 0000000..1926eae
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/InMemoryPlayerTargetLockStore.cs
@@ -0,0 +1,73 @@
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>Thread-safe prototype lock store; player keys are trimmed with ordinal case-insensitive lookup (same spirit as position APIs).</summary>
+public sealed class InMemoryPlayerTargetLockStore : IPlayerTargetLockStore
+{
+    private readonly object mutex = new();
+    private readonly Dictionary<string, PlayerLockRow> rows = new(StringComparer.OrdinalIgnoreCase);
+
+    private readonly record struct PlayerLockRow(string? LockIdLowercase, int Sequence);
+
+    public (string? LockIdLowercase, int Sequence) GetLockState(string playerId)
+    {
+        var key = playerId.Trim();
+        if (key.Length == 0)
+        {
+            return (null, 0);
+        }
+
+        lock (mutex)
+        {
+            return rows.TryGetValue(key, out var row) ? (row.LockIdLowercase, row.Sequence) : (null, 0);
+        }
+    }
+
+    public (string? LockIdLowercase, int Sequence) ApplyClear(string playerId)
+    {
+        var key = playerId.Trim();
+        if (key.Length == 0)
+        {
+            return (null, 0);
+        }
+
+        lock (mutex)
+        {
+            if (!rows.TryGetValue(key, out var prev))
+            {
+                return (null, 0);
+            }
+
+            if (prev.LockIdLowercase is null)
+            {
+                return (null, prev.Sequence);
+            }
+
+            var next = new PlayerLockRow(null, prev.Sequence + 1);
+            rows[key] = next;
+            return (null, next.Sequence);
+        }
+    }
+
+    public (string? LockIdLowercase, int Sequence) ApplySet(string playerId, string targetIdLowercase)
+    {
+        var key = playerId.Trim();
+        if (key.Length == 0)
+        {
+            return (null, 0);
+        }
+
+        lock (mutex)
+        {
+            var prev = rows.TryGetValue(key, out var r) ? r : new PlayerLockRow(null, 0);
+
+            if (string.Equals(prev.LockIdLowercase, targetIdLowercase, StringComparison.Ordinal))
+            {
+                return (prev.LockIdLowercase, prev.Sequence);
+            }
+
+            var next = new PlayerLockRow(targetIdLowercase, prev.Sequence + 1);
+            rows[key] = next;
+            return (next.LockIdLowercase, next.Sequence);
+        }
+    }
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/PlayerTargetStateReader.cs b/server/NeonSprawl.Server/Game/Targeting/PlayerTargetStateReader.cs
new file mode 100644
index 0000000..1d3a800
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/PlayerTargetStateReader.cs
@@ -0,0 +1,38 @@
+using NeonSprawl.Server.Game.PositionState;
+using NeonSprawl.Server.Game.World;
+
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>Computes <see cref="PlayerTargetStateResponse.Validity"/> from authoritative position + persisted lock (NEO-23).</summary>
+public static class PlayerTargetStateReader
+{
+    public static string ComputeValidity(string? lockIdLowercase, in PositionSnapshot snap)
+    {
+        if (string.IsNullOrEmpty(lockIdLowercase))
+        {
+            return TargetValidity.None;
+        }
+
+        if (!PrototypeTargetRegistry.TryGet(lockIdLowercase, out var entry))
+        {
+            return TargetValidity.InvalidTarget;
+        }
+
+        if (!HorizontalReach.IsWithinHorizontalRadius(snap.X, snap.Z, entry.X, entry.Z, entry.LockRadius))
+        {
+            return TargetValidity.OutOfRange;
+        }
+
+        return TargetValidity.Ok;
+    }
+
+    public static PlayerTargetStateResponse Build(string routePlayerId, string? lockIdLowercase, int sequence, in PositionSnapshot snap) =>
+        new()
+        {
+            SchemaVersion = PlayerTargetStateResponse.CurrentSchemaVersion,
+            PlayerId = routePlayerId,
+            LockedTargetId = lockIdLowercase,
+            Validity = ComputeValidity(lockIdLowercase, in snap),
+            Sequence = sequence,
+        };
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/PrototypeTargetRegistry.cs b/server/NeonSprawl.Server/Game/Targeting/PrototypeTargetRegistry.cs
new file mode 100644
index 0000000..f339bdb
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/PrototypeTargetRegistry.cs
@@ -0,0 +1,28 @@
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>Prototype combat target ids → world anchor + horizontal lock radius (NEO-23). Keys are lowercase.</summary>
+public static class PrototypeTargetRegistry
+{
+    /// <summary>Near default dev spawn <c>(-5, -5)</c> on XZ.</summary>
+    public const string PrototypeTargetAlphaId = "prototype_target_alpha";
+
+    /// <summary>Farther stub for out-of-range tests; ascending id order for NEO-24 tab cycle.</summary>
+    public const string PrototypeTargetBetaId = "prototype_target_beta";
+
+    /// <summary>Alpha anchor aligned with spawn horizontal cell; <see cref="PrototypeTargetEntry.LockRadius"/> on XZ only.</summary>
+    public static readonly PrototypeTargetEntry PrototypeTargetAlpha = new(-5, 0.5, -5, 8.0);
+
+    public static readonly PrototypeTargetEntry PrototypeTargetBeta = new(8, 0, 8, 4.0);
+
+    private static readonly Dictionary<string, PrototypeTargetEntry> ById = new(StringComparer.Ordinal)
+    {
+        [PrototypeTargetAlphaId] = PrototypeTargetAlpha,
+        [PrototypeTargetBetaId] = PrototypeTargetBeta,
+    };
+
+    public static bool TryGet(string targetIdLowercase, out PrototypeTargetEntry entry) =>
+        ById.TryGetValue(targetIdLowercase, out entry);
+}
+
+/// <param name="LockRadius">Horizontal reach on X/Z; inclusive boundary (same as NEO-9).</param>
+public readonly record struct PrototypeTargetEntry(double X, double Y, double Z, double LockRadius);
diff --git a/server/NeonSprawl.Server/Game/Targeting/TargetStateDtos.cs b/server/NeonSprawl.Server/Game/Targeting/TargetStateDtos.cs
new file mode 100644
index 0000000..99b865c
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/TargetStateDtos.cs
@@ -0,0 +1,63 @@
+using System.Text.Json.Serialization;
+
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>JSON body for <c>GET /game/players/{{id}}/target</c> and nested <c>targetState</c> on select (NEO-23).</summary>
+public sealed class PlayerTargetStateResponse
+{
+    public const int CurrentSchemaVersion = 1;
+
+    [JsonPropertyName("schemaVersion")]
+    public int SchemaVersion { get; init; } = CurrentSchemaVersion;
+
+    [JsonPropertyName("playerId")]
+    public required string PlayerId { get; init; }
+
+    /// <summary>Lowercase registry id when locked; JSON <c>null</c> when no lock (key always serialized).</summary>
+    [JsonPropertyName("lockedTargetId")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public string? LockedTargetId { get; init; }
+
+    /// <summary>One of <see cref="TargetValidity"/> string constants.</summary>
+    [JsonPropertyName("validity")]
+    public required string Validity { get; init; }
+
+    [JsonPropertyName("sequence")]
+    public int Sequence { get; init; }
+}
+
+/// <summary>POST body for <c>POST /game/players/{{id}}/target/select</c> (NEO-23).</summary>
+/// <remarks>
+/// Omit <c>targetId</c> or send JSON <c>null</c> to clear the lock. Whitespace-only after trim → HTTP 400.
+/// </remarks>
+public sealed class TargetSelectRequest
+{
+    public const int CurrentSchemaVersion = 1;
+
+    [JsonPropertyName("schemaVersion")]
+    public int SchemaVersion { get; init; }
+
+    /// <summary>Registry key after trim + case-insensitive lookup; <c>null</c> or omitted ⇒ clear intent.</summary>
+    [JsonPropertyName("targetId")]
+    public string? TargetId { get; init; }
+}
+
+/// <summary>POST response for target selection (NEO-23).</summary>
+public sealed class TargetSelectResponse
+{
+    public const int CurrentSchemaVersion = 1;
+
+    [JsonPropertyName("schemaVersion")]
+    public int SchemaVersion { get; init; } = CurrentSchemaVersion;
+
+    [JsonPropertyName("selectionApplied")]
+    public bool SelectionApplied { get; init; }
+
+    [JsonPropertyName("targetState")]
+    public required PlayerTargetStateResponse TargetState { get; init; }
+
+    /// <summary>Required when <see cref="SelectionApplied"/> is <c>false</c>; omitted on success.</summary>
+    [JsonPropertyName("reasonCode")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? ReasonCode { get; init; }
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/TargetValidity.cs b/server/NeonSprawl.Server/Game/Targeting/TargetValidity.cs
new file mode 100644
index 0000000..6518886
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/TargetValidity.cs
@@ -0,0 +1,10 @@
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>Serialized <c>validity</c> strings for <see cref="PlayerTargetStateResponse"/> (NEO-23).</summary>
+public static class TargetValidity
+{
+    public const string None = "none";
+    public const string Ok = "ok";
+    public const string OutOfRange = "out_of_range";
+    public const string InvalidTarget = "invalid_target";
+}
diff --git a/server/NeonSprawl.Server/Game/Targeting/TargetingApi.cs b/server/NeonSprawl.Server/Game/Targeting/TargetingApi.cs
new file mode 100644
index 0000000..efca25b
--- /dev/null
+++ b/server/NeonSprawl.Server/Game/Targeting/TargetingApi.cs
@@ -0,0 +1,105 @@
+using NeonSprawl.Server.Game.PositionState;
+using NeonSprawl.Server.Game.World;
+
+namespace NeonSprawl.Server.Game.Targeting;
+
+/// <summary>Maps <c>GET/POST …/target</c> selection APIs (NEO-23).</summary>
+public static class TargetingApi
+{
+    public const string ReasonUnknownTarget = "unknown_target";
+    public const string ReasonOutOfRange = "out_of_range";
+
+    public static WebApplication MapTargetingApi(this WebApplication app)
+    {
+        app.MapGet(
+            "/game/players/{id}/target",
+            (string id, IPositionStateStore positions, IPlayerTargetLockStore locks) =>
+            {
+                if (!positions.TryGetPosition(id, out var snap))
+                {
+                    return Results.NotFound();
+                }
+
+                var (lockId, seq) = locks.GetLockState(id);
+                var body = PlayerTargetStateReader.Build(id, lockId, seq, in snap);
+                return Results.Json(body);
+            });
+
+        app.MapPost(
+            "/game/players/{id}/target/select",
+            (string id, TargetSelectRequest? body, IPositionStateStore positions, IPlayerTargetLockStore locks) =>
+            {
+                if (body is null || body.SchemaVersion != TargetSelectRequest.CurrentSchemaVersion)
+                {
+                    return Results.BadRequest();
+                }
+
+                if (!positions.TryGetPosition(id, out var snap))
+                {
+                    return Results.NotFound();
+                }
+
+                // Clear intent: property omitted or JSON null.
+                if (body.TargetId is null)
+                {
+                    var cleared = locks.ApplyClear(id);
+                    var stateAfter = PlayerTargetStateReader.Build(id, cleared.LockIdLowercase, cleared.Sequence, in snap);
+                    return Results.Json(
+                        new TargetSelectResponse
+                        {
+                            SchemaVersion = TargetSelectResponse.CurrentSchemaVersion,
+                            SelectionApplied = true,
+                            TargetState = stateAfter,
+                        });
+                }
+
+                var raw = body.TargetId;
+                var trimmed = raw.Trim();
+                if (trimmed.Length == 0)
+                {
+                    return Results.BadRequest();
+                }
+
+                var lookupKey = trimmed.ToLowerInvariant();
+                if (!PrototypeTargetRegistry.TryGet(lookupKey, out var entry))
+                {
+                    var (lockUnknown, seqUnknown) = locks.GetLockState(id);
+                    var stateUnknown = PlayerTargetStateReader.Build(id, lockUnknown, seqUnknown, in snap);
+                    return Results.Json(
+                        new TargetSelectResponse
+                        {
+                            SchemaVersion = TargetSelectResponse.CurrentSchemaVersion,
+                            SelectionApplied = false,
+                            TargetState = stateUnknown,
+                            ReasonCode = ReasonUnknownTarget,
+                        });
+                }
+
+                if (!HorizontalReach.IsWithinHorizontalRadius(snap.X, snap.Z, entry.X, entry.Z, entry.LockRadius))
+                {
+                    var (lockFar, seqFar) = locks.GetLockState(id);
+                    var stateFar = PlayerTargetStateReader.Build(id, lockFar, seqFar, in snap);
+                    return Results.Json(
+                        new TargetSelectResponse
+                        {
+                            SchemaVersion = TargetSelectResponse.CurrentSchemaVersion,
+                            SelectionApplied = false,
+                            TargetState = stateFar,
+                            ReasonCode = ReasonOutOfRange,
+                        });
+                }
+
+                var applied = locks.ApplySet(id, lookupKey);
+                var stateOk = PlayerTargetStateReader.Build(id, applied.LockIdLowercase, applied.Sequence, in snap);
+                return Results.Json(
+                    new TargetSelectResponse
+                    {
+                        SchemaVersion = TargetSelectResponse.CurrentSchemaVersion,
+                        SelectionApplied = true,
+                        TargetState = stateOk,
+                    });
+            });
+
+        return app;
+    }
+}
diff --git a/server/NeonSprawl.Server/Program.cs b/server/NeonSprawl.Server/Program.cs
index 5caf2d0..ee66d44 100644
--- a/server/NeonSprawl.Server/Program.cs
+++ b/server/NeonSprawl.Server/Program.cs
@@ -1,8 +1,10 @@
 using NeonSprawl.Server.Game.Interaction;
 using NeonSprawl.Server.Game.PositionState;
+using NeonSprawl.Server.Game.Targeting;
 
 var builder = WebApplication.CreateBuilder(args);
 builder.Services.AddPositionStateStore(builder.Configuration);
+builder.Services.AddSingleton<IPlayerTargetLockStore, InMemoryPlayerTargetLockStore>();
 
 var app = builder.Build();
 
@@ -18,5 +20,6 @@ app.MapGet("/health", () => Results.Json(new
 
 app.MapPositionStateApi();
 app.MapInteractionApi();
+app.MapTargetingApi();
 
 app.Run();
diff --git a/server/README.md b/server/README.md
index 4447ffb..48a1eb4 100644
--- a/server/README.md
+++ b/server/README.md
@@ -125,6 +125,55 @@ When **`allowed` is `true`**, **`reasonCode` is omitted** (clients must not requ
 
 Contract details and PR blurb: [NEO-9 implementation plan](../../docs/plans/NEO-9-implementation-plan.md).
 
+## Targeting (NEO-23)
+
+Authoritative **combat target lock** (prototype): **`GET /game/players/{id}/target`** returns **`PlayerTargetStateResponse`** v1; **`POST /game/players/{id}/target/select`** accepts **`TargetSelectRequest`** v1 and returns **`TargetSelectResponse`** v1. Player id rules match position/interact (trim + ordinal case-insensitive lookup in the in-memory store). **Horizontal lock range** uses **`HorizontalReach`** on **X/Z only** (inclusive radius), same floor-plane policy as NEO-9.
+
+**Soft lock:** the server keeps the persisted **`lockedTargetId`** until clear or a successful swap. If the player moves out of stub range, **`validity`** becomes **`out_of_range`** on **`GET`** and on **`POST`** echoes until they move back in range, clear, or select another valid target (see [NEO-23 implementation plan](../../docs/plans/NEO-23-implementation-plan.md)).
+
+Stub registry: `PrototypeTargetRegistry` in `NeonSprawl.Server/Game/Targeting/` — ids **`prototype_target_alpha`**, **`prototype_target_beta`** (ascending id order for future tab cycle).
+
+**`GET /game/players/{id}/target`**
+
+| Field | Meaning |
+|--------|--------|
+| `schemaVersion` | `1` |
+| `playerId` | Echo of route `{id}` |
+| `lockedTargetId` | Lowercase stub id when locked; JSON **`null`** when no lock (**key always present**) |
+| `validity` | `none` (no lock), `ok`, `out_of_range`, or `invalid_target` (unknown id in store — should not occur in normal use) |
+| `sequence` | Increments when the persisted lock **id** changes (clear or swap); unchanged on idempotent re-select of the same id |
+
+**`POST /game/players/{id}/target/select`** — body `schemaVersion` (`1`), optional **`targetId`**. Omit **`targetId`** or send JSON **`null`** to **clear** the lock. Non-null **`targetId`** is trimmed; **whitespace-only** after trim → **400**.
+
+Examples:
+
+```bash
+curl -s http://localhost:5253/game/players/dev-local-1/target
+```
+
+```bash
+curl -s -X POST http://localhost:5253/game/players/dev-local-1/target/select \
+  -H "Content-Type: application/json" \
+  -d '{"schemaVersion":1,"targetId":"prototype_target_alpha"}'
+```
+
+**HTTP**
+
+| Status | Meaning |
+|--------|---------|
+| **200** | **GET:** `PlayerTargetStateResponse` v1. **POST:** `TargetSelectResponse` v1 with **`targetState`** always set; **`selectionApplied`** `true` or `false`; when `false`, required **`reasonCode`**. |
+| **400** | Invalid v1 body (wrong `schemaVersion`, malformed JSON, or **`targetId`** whitespace-only when provided). |
+| **404** | Unknown **player** only (same as interact). |
+
+**`reasonCode` (POST v1, when `selectionApplied` is `false`)**
+
+| Code | When |
+|------|------|
+| `unknown_target` | Id not in the prototype target registry (after trim + case-insensitive lookup). |
+| `out_of_range` | Horizontal distance on X/Z is **greater than** the target’s `lockRadius` (on the radius is **allowed** — inclusive `<=`). |
+
+When **`selectionApplied` is `true`**, **`reasonCode` is omitted** (clients must not require it on success).
+
 ## Solution
 
 From repo root: `dotnet build NeonSprawl.sln` and `dotnet test NeonSprawl.sln`.