diff --git a/docs/reviews/2026-06-06-NEO-117.md b/docs/reviews/2026-06-06-NEO-117.md
index 04d6c6a..62d6fe9 100644
--- a/docs/reviews/2026-06-06-NEO-117.md
+++ b/docs/reviews/2026-06-06-NEO-117.md
@@ -30,6 +30,12 @@ NEO-117 adds **`QuestStateOperations`** — static accept, step advance, and com
 
 (none)
 
+## Bugbot follow-up (2026-06-06)
+
+1. ~~**TryActivate failure misclassified** — Map store mutation failures from returned snapshots / re-read progress instead of always `unknown_player`.~~ **Done.** `DenyActivateFailure` / `DenyAdvanceFailure` helpers; store replay on `TryMarkComplete` returns idempotent success.
+
+2. ~~**Unknown player wrong deny code** — Check `CanWritePlayer` before prerequisite gate so unknown players get `unknown_player`, not `prerequisite_incomplete`.~~ **Done.** Added `IPlayerQuestStateStore.CanWritePlayer` (in-memory + Postgres); `TryAccept` calls it before prerequisites.
+
 ## Suggestions
 
 1. ~~**Test `ResolveDependencies` pattern** — `QuestStateOperationsTests.ResolveDependencies` creates an `IServiceScope` that is never disposed. **`EncounterCompletionOperationsTests`** resolves from `factory.Services` root (singleton stores). Align to avoid undisposed scopes and match the established encounter test pattern.~~ **Done.** `ResolveDependencies` and host smoke test resolve from `factory.Services`.
diff --git a/server/NeonSprawl.Server.Tests/Game/Quests/QuestStateOperationsTests.cs b/server/NeonSprawl.Server.Tests/Game/Quests/QuestStateOperationsTests.cs
index 2ee2d1e..d4e66a9 100644
--- a/server/NeonSprawl.Server.Tests/Game/Quests/QuestStateOperationsTests.cs
+++ b/server/NeonSprawl.Server.Tests/Game/Quests/QuestStateOperationsTests.cs
@@ -1,4 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
+using NeonSprawl.Server.Game.PositionState;
 using NeonSprawl.Server.Game.Quests;
 using NeonSprawl.Server.Tests;
 using Xunit;
@@ -154,6 +155,25 @@ public sealed class QuestStateOperationsTests
         Assert.Equal(QuestProgressStatus.Completed, result.Snapshot!.Status);
     }
 
+    [Fact]
+    public async Task TryAccept_ShouldDenyUnknownPlayer_WhenPrerequisiteQuestNotWritable()
+    {
+        // Arrange
+        await using var factory = new InMemoryWebApplicationFactory();
+        var deps = ResolveDependencies(factory);
+
+        // Act
+        var result = QuestStateOperations.TryAccept(
+            UnknownPlayerId,
+            RefineQuestId,
+            deps.QuestRegistry,
+            deps.ProgressStore);
+
+        // Assert
+        Assert.False(result.Success);
+        Assert.Equal(QuestStateReasonCodes.UnknownPlayer, result.ReasonCode);
+    }
+
     [Fact]
     public async Task TryAccept_ShouldDenyUnknownPlayer_WhenPlayerNotInStore()
     {
@@ -392,6 +412,30 @@ public sealed class QuestStateOperationsTests
         Assert.Equal(QuestProgressStatus.Completed, complete.Snapshot!.Status);
     }
 
+    [Fact]
+    public void CanWritePlayer_ShouldReturnFalse_ForUnknownPlayer()
+    {
+        // Arrange
+        var store = new InMemoryPlayerQuestStateStore(
+            Microsoft.Extensions.Options.Options.Create(new GamePositionOptions { DevPlayerId = PlayerId }));
+        // Act
+        var canWrite = store.CanWritePlayer(UnknownPlayerId);
+        // Assert
+        Assert.False(canWrite);
+    }
+
+    [Fact]
+    public void CanWritePlayer_ShouldReturnTrue_ForDevPlayer()
+    {
+        // Arrange
+        var store = new InMemoryPlayerQuestStateStore(
+            Microsoft.Extensions.Options.Options.Create(new GamePositionOptions { DevPlayerId = PlayerId }));
+        // Act
+        var canWrite = store.CanWritePlayer(PlayerId);
+        // Assert
+        Assert.True(canWrite);
+    }
+
     private static void AcceptAndComplete(
         (IQuestDefinitionRegistry QuestRegistry, IPlayerQuestStateStore ProgressStore) deps,
         string questId,
diff --git a/server/NeonSprawl.Server/Game/Quests/IPlayerQuestStateStore.cs b/server/NeonSprawl.Server/Game/Quests/IPlayerQuestStateStore.cs
index 53342d4..1b48cd7 100644
--- a/server/NeonSprawl.Server/Game/Quests/IPlayerQuestStateStore.cs
+++ b/server/NeonSprawl.Server/Game/Quests/IPlayerQuestStateStore.cs
@@ -6,6 +6,9 @@ namespace NeonSprawl.Server.Game.Quests;
 /// </summary>
 public interface IPlayerQuestStateStore
 {
+    /// <summary>True when mutations for <paramref name="playerId"/> are allowed (dev bucket or Postgres <c>player_position</c> row).</summary>
+    bool CanWritePlayer(string playerId);
+
     /// <summary>Missing row ⇒ <c>false</c> (callers treat as <c>not_started</c>).</summary>
     bool TryGetProgress(string playerId, string questId, out QuestStepState snapshot);
 
diff --git a/server/NeonSprawl.Server/Game/Quests/InMemoryPlayerQuestStateStore.cs b/server/NeonSprawl.Server/Game/Quests/InMemoryPlayerQuestStateStore.cs
index c32e320..487b2e8 100644
--- a/server/NeonSprawl.Server/Game/Quests/InMemoryPlayerQuestStateStore.cs
+++ b/server/NeonSprawl.Server/Game/Quests/InMemoryPlayerQuestStateStore.cs
@@ -66,6 +66,13 @@ public sealed class InMemoryPlayerQuestStateStore(IOptions<GamePositionOptions>
         return new HashSet<string>(StringComparer.OrdinalIgnoreCase) { id };
     }
 
+    /// <inheritdoc />
+    public bool CanWritePlayer(string playerId)
+    {
+        var player = QuestProgressIds.NormalizePlayerId(playerId);
+        return player.Length > 0 && knownPlayers.Contains(player);
+    }
+
     /// <inheritdoc />
     public bool TryGetProgress(string playerId, string questId, out QuestStepState snapshot)
     {
diff --git a/server/NeonSprawl.Server/Game/Quests/PostgresPlayerQuestStateStore.cs b/server/NeonSprawl.Server/Game/Quests/PostgresPlayerQuestStateStore.cs
index 2a60b37..b8f4631 100644
--- a/server/NeonSprawl.Server/Game/Quests/PostgresPlayerQuestStateStore.cs
+++ b/server/NeonSprawl.Server/Game/Quests/PostgresPlayerQuestStateStore.cs
@@ -10,6 +10,20 @@ public sealed class PostgresPlayerQuestStateStore(Npgsql.NpgsqlDataSource dataSo
         PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
     };
 
+    /// <inheritdoc />
+    public bool CanWritePlayer(string playerId)
+    {
+        var player = QuestProgressIds.NormalizePlayerId(playerId);
+        if (player.Length == 0)
+        {
+            return false;
+        }
+
+        PostgresPlayerQuestProgressBootstrap.EnsureSchema(dataSource);
+        using var conn = dataSource.OpenConnection();
+        return PlayerExists(conn, player);
+    }
+
     /// <inheritdoc />
     public bool TryGetProgress(string playerId, string questId, out QuestStepState snapshot)
     {
diff --git a/server/NeonSprawl.Server/Game/Quests/QuestStateOperations.cs b/server/NeonSprawl.Server/Game/Quests/QuestStateOperations.cs
index 1ca6717..4fd4bf3 100644
--- a/server/NeonSprawl.Server/Game/Quests/QuestStateOperations.cs
+++ b/server/NeonSprawl.Server/Game/Quests/QuestStateOperations.cs
@@ -21,6 +21,11 @@ public static class QuestStateOperations
             return Deny(QuestStateReasonCodes.UnknownQuest);
         }
 
+        if (!progressStore.CanWritePlayer(playerId))
+        {
+            return Deny(QuestStateReasonCodes.UnknownPlayer);
+        }
+
         if (progressStore.TryGetProgress(playerId, normalizedQuestId, out var existing))
         {
             if (existing.Status == QuestProgressStatus.Completed)
@@ -45,7 +50,7 @@ public static class QuestStateOperations
             return Success(snapshot);
         }
 
-        return Deny(QuestStateReasonCodes.UnknownPlayer);
+        return DenyActivateFailure(progressStore, playerId, normalizedQuestId, snapshot);
     }
 
     /// <summary>
@@ -89,7 +94,7 @@ public static class QuestStateOperations
             return Success(snapshot);
         }
 
-        return Deny(QuestStateReasonCodes.UnknownPlayer, existing);
+        return DenyAdvanceFailure(progressStore, playerId, normalizedQuestId, snapshot, existing);
     }
 
     /// <summary>
@@ -128,13 +133,69 @@ public static class QuestStateOperations
             return Success(snapshot);
         }
 
+        if (snapshot?.Status == QuestProgressStatus.Completed)
+        {
+            return Success(snapshot);
+        }
+
         if (progressStore.TryGetProgress(playerId, normalizedQuestId, out var afterAttempt) &&
             afterAttempt.Status == QuestProgressStatus.Completed)
         {
             return Success(afterAttempt);
         }
 
-        return Deny(QuestStateReasonCodes.UnknownPlayer);
+        if (!progressStore.CanWritePlayer(playerId))
+        {
+            return Deny(QuestStateReasonCodes.UnknownPlayer);
+        }
+
+        return DenyFromProgressSnapshot(snapshot, QuestStateReasonCodes.NotActive)
+               ?? Deny(QuestStateReasonCodes.UnknownPlayer, progress);
+    }
+
+    private static QuestStateOperationResult DenyActivateFailure(
+        IPlayerQuestStateStore progressStore,
+        string playerId,
+        string questId,
+        QuestStepState attemptSnapshot) =>
+        DenyFromProgressSnapshot(attemptSnapshot, QuestStateReasonCodes.AlreadyActive)
+        ?? (progressStore.TryGetProgress(playerId, questId, out var reread)
+            ? DenyFromProgressSnapshot(reread, QuestStateReasonCodes.AlreadyActive)
+            : null)
+        ?? Deny(QuestStateReasonCodes.UnknownPlayer);
+
+    private static QuestStateOperationResult DenyAdvanceFailure(
+        IPlayerQuestStateStore progressStore,
+        string playerId,
+        string questId,
+        QuestStepState attemptSnapshot,
+        QuestStepState priorRead) =>
+        DenyFromProgressSnapshot(attemptSnapshot, QuestStateReasonCodes.InvalidStepIndex)
+        ?? (progressStore.TryGetProgress(playerId, questId, out var reread)
+            ? DenyFromProgressSnapshot(reread, QuestStateReasonCodes.InvalidStepIndex)
+            : null)
+        ?? Deny(QuestStateReasonCodes.UnknownPlayer, priorRead);
+
+    private static QuestStateOperationResult? DenyFromProgressSnapshot(
+        QuestStepState? snapshot,
+        string activeOrIntermediateReasonCode)
+    {
+        if (snapshot is null)
+        {
+            return null;
+        }
+
+        if (snapshot.Status == QuestProgressStatus.Completed)
+        {
+            return Deny(QuestStateReasonCodes.AlreadyCompleted, snapshot);
+        }
+
+        if (snapshot.Status == QuestProgressStatus.Active)
+        {
+            return Deny(activeOrIntermediateReasonCode, snapshot);
+        }
+
+        return null;
     }
 
     private static QuestStateOperationResult Success(QuestStepState snapshot) =>
diff --git a/server/README.md b/server/README.md
index c45c63c..b658051 100644
--- a/server/README.md
+++ b/server/README.md
@@ -162,6 +162,7 @@ Per-player quest runtime state lives in **`IPlayerQuestStateStore`**, keyed by *
 
 **Store interface methods:**
 
+- **`CanWritePlayer`** — whether mutations are allowed for the player (dev bucket / Postgres `player_position`).
 - **`TryGetProgress`** — read one row; false when not started.
 - **`TryActivate`** — first accept (`not_started` → `active`).
 - **`TryAdvanceStep`** — bump step index and clear counters for the new step (denies when completed or index does not increase).