diff --git a/docs/plans/NEO-146-implementation-plan.md b/docs/plans/NEO-146-implementation-plan.md
index 2083e50..78be83c 100644
--- a/docs/plans/NEO-146-implementation-plan.md
+++ b/docs/plans/NEO-146-implementation-plan.md
@@ -68,8 +68,8 @@ Durable per-player **`ContractInstance`** rows (active/completed) and append-onl
 - **Types:** `ContractInstanceStatus`, `ContractInstanceState`, `ContractInstanceIds`, `ContractInstanceReasonCodes`, `ContractOutcomeRow`, `ContractOutcomeIds`.
 - **Stores:** `IContractInstanceStore` / `IContractOutcomeStore`; in-memory + Postgres (`V011`, `V012`) when connection string set; partial unique index + app-level one-active enforcement.
 - **DI:** `AddContractInstanceStores` in `Program.cs`; in-memory override in `InMemoryWebApplicationFactory`.
-- **Tests:** `InMemoryContractInstanceStoreTests`, `InMemoryContractOutcomeStoreTests`, Postgres persistence integration tests; host DI smoke (`867` tests green).
-- **In-memory outcome guard:** `InMemoryContractOutcomeStore.TryAppend` denies when **`IContractInstanceStore.TryGet`** is false (Postgres FK parity).
+- **Tests:** `InMemoryContractInstanceStoreTests`, `InMemoryContractOutcomeStoreTests`, Postgres persistence integration tests; host DI smoke (`871` tests green).
+- **In-memory outcome guard:** `InMemoryContractOutcomeStore.TryAppend` denies when **`IContractInstanceStore.TryGet`** is false (Postgres FK + player/instance SQL guard parity).
 - **Docs:** `server/README.md` contract instance + outcome store sections.
 - **Bruno:** `bruno/neon-sprawl-server/contract-stores/` health smoke (startup-only; no contract HTTP in E7M4-03).
 
diff --git a/server/NeonSprawl.Server.Tests/Game/Contracts/ContractOutcomePersistenceIntegrationTests.cs b/server/NeonSprawl.Server.Tests/Game/Contracts/ContractOutcomePersistenceIntegrationTests.cs
index f8aaae4..3bd6c0a 100644
--- a/server/NeonSprawl.Server.Tests/Game/Contracts/ContractOutcomePersistenceIntegrationTests.cs
+++ b/server/NeonSprawl.Server.Tests/Game/Contracts/ContractOutcomePersistenceIntegrationTests.cs
@@ -129,6 +129,104 @@ public sealed class ContractOutcomePersistenceIntegrationTests(PostgresIntegrati
         Assert.False(appended);
     }
 
+    [RequirePostgresFact]
+    public async Task TryAppend_ShouldReturnFalse_WhenPlayerIdDoesNotMatchInstanceOwner()
+    {
+        // Arrange
+        await ResetContractTablesAsync();
+        const string otherPlayerId = "other-player-pg-001";
+        var idempotencyKey = ContractOutcomeIds.MakeIdempotencyKey(otherPlayerId, InstanceId);
+        var outcomeRow = new ContractOutcomeRow(
+            OutcomeRowId,
+            InstanceId,
+            otherPlayerId,
+            idempotencyKey,
+            [new RewardItemGrantApplied("scrap_metal_bulk", 5)],
+            [],
+            [],
+            RecordedAt);
+        using (var scope = Factory.Services.CreateScope())
+        {
+            var instanceStore = scope.ServiceProvider.GetRequiredService<IContractInstanceStore>();
+            var outcomeStore = scope.ServiceProvider.GetRequiredService<IContractOutcomeStore>();
+            Assert.True(instanceStore.TryCreateActive(
+                PlayerId,
+                InstanceId,
+                TemplateId,
+                SeedBucket,
+                IssuedAt,
+                out _));
+            await SeedPostgresPlayerAsync(otherPlayerId);
+            // Act
+            var appended = outcomeStore.TryAppend(outcomeRow);
+            // Assert
+            Assert.False(appended);
+            Assert.Empty(outcomeStore.GetOutcomesForInstance(InstanceId));
+        }
+    }
+
+    [RequirePostgresFact]
+    public async Task TryAppend_ShouldAllowOnlyOneSuccess_WhenConcurrentDuplicateRowsRace()
+    {
+        // Arrange
+        await ResetContractTablesAsync();
+        var idempotencyKey = ContractOutcomeIds.MakeIdempotencyKey(PlayerId, InstanceId);
+        var outcomeRow = new ContractOutcomeRow(
+            OutcomeRowId,
+            InstanceId,
+            PlayerId,
+            idempotencyKey,
+            [new RewardItemGrantApplied("scrap_metal_bulk", 5)],
+            [],
+            [],
+            RecordedAt);
+        using (var scope = Factory.Services.CreateScope())
+        {
+            var instanceStore = scope.ServiceProvider.GetRequiredService<IContractInstanceStore>();
+            Assert.True(instanceStore.TryCreateActive(
+                PlayerId,
+                InstanceId,
+                TemplateId,
+                SeedBucket,
+                IssuedAt,
+                out _));
+        }
+
+        // Act
+        var results = await Task.WhenAll(
+            Enumerable.Range(0, 8).Select(_ => Task.Run(() =>
+            {
+                using var scope = Factory.Services.CreateScope();
+                var outcomeStore = scope.ServiceProvider.GetRequiredService<IContractOutcomeStore>();
+                return outcomeStore.TryAppend(outcomeRow);
+            })));
+
+        // Assert
+        Assert.Equal(1, results.Count(static r => r));
+        Assert.Equal(7, results.Count(static r => !r));
+    }
+
+    private static async Task SeedPostgresPlayerAsync(string playerId)
+    {
+        var cs = Environment.GetEnvironmentVariable("ConnectionStrings__NeonSprawl");
+        if (string.IsNullOrWhiteSpace(cs))
+        {
+            throw new InvalidOperationException("ConnectionStrings__NeonSprawl is not set.");
+        }
+
+        await using var conn = new NpgsqlConnection(cs);
+        await conn.OpenAsync();
+        await using var cmd = new NpgsqlCommand(
+            """
+            INSERT INTO player_position (player_id, pos_x, pos_y, pos_z, sequence)
+            VALUES (@pid, 0, 0, 0, 0)
+            ON CONFLICT (player_id) DO NOTHING;
+            """,
+            conn);
+        cmd.Parameters.AddWithValue("pid", playerId);
+        await cmd.ExecuteNonQueryAsync();
+    }
+
     private async Task ResetContractTablesAsync()
     {
         var cs = Environment.GetEnvironmentVariable("ConnectionStrings__NeonSprawl");
diff --git a/server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractOutcomeStoreTests.cs b/server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractOutcomeStoreTests.cs
index 9acbaf2..ac1783c 100644
--- a/server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractOutcomeStoreTests.cs
+++ b/server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractOutcomeStoreTests.cs
@@ -93,6 +93,20 @@ public sealed class InMemoryContractOutcomeStoreTests
         Assert.Empty(store.GetOutcomesForInstance(InstanceId));
     }
 
+    [Fact]
+    public void TryAppend_ShouldReturnFalse_WhenPlayerIdDoesNotMatchInstanceOwner()
+    {
+        // Arrange
+        var (instanceStore, store) = CreateStores();
+        SeedActiveInstance(instanceStore);
+        var row = CreatePrototypeOutcome() with { PlayerId = "other-player-xyz" };
+        // Act
+        var appended = store.TryAppend(row);
+        // Assert
+        Assert.False(appended);
+        Assert.Empty(store.GetOutcomesForInstance(InstanceId));
+    }
+
     private static (InMemoryContractInstanceStore InstanceStore, InMemoryContractOutcomeStore OutcomeStore) CreateStores()
     {
         var options = Options.Create(new GamePositionOptions { DevPlayerId = PlayerId });
diff --git a/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeBootstrap.cs b/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeBootstrap.cs
index 3e56365..ef6f253 100644
--- a/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeBootstrap.cs
+++ b/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeBootstrap.cs
@@ -13,6 +13,8 @@ public static class PostgresContractOutcomeBootstrap
 
     public static void EnsureSchema(Npgsql.NpgsqlDataSource dataSource)
     {
+        PostgresContractInstanceBootstrap.EnsureSchema(dataSource);
+
         if (Volatile.Read(ref _schemaReady) != 0)
         {
             return;
diff --git a/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeStore.cs b/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeStore.cs
index fcd694d..f9bab91 100644
--- a/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeStore.cs
+++ b/server/NeonSprawl.Server/Game/Contracts/PostgresContractOutcomeStore.cs
@@ -29,7 +29,10 @@ public sealed class PostgresContractOutcomeStore(Npgsql.NpgsqlDataSource dataSou
             SELECT @id, @instance_id, @pid, @key, @items::jsonb, @skill_xp::jsonb, @rep::jsonb, @recorded
             WHERE NOT EXISTS (
                 SELECT 1 FROM contract_outcome
-                WHERE id = @id OR idempotency_key = @key);
+                WHERE id = @id OR idempotency_key = @key)
+            AND EXISTS (
+                SELECT 1 FROM contract_instance
+                WHERE contract_instance_id = @instance_id AND player_id = @pid);
             """,
             conn);
         cmd.Parameters.AddWithValue("id", normalized.Id);
@@ -44,7 +47,9 @@ public sealed class PostgresContractOutcomeStore(Npgsql.NpgsqlDataSource dataSou
         {
             return cmd.ExecuteNonQuery() == 1;
         }
-        catch (Npgsql.PostgresException ex) when (ex.SqlState == Npgsql.PostgresErrorCodes.ForeignKeyViolation)
+        catch (Npgsql.PostgresException ex) when (
+            ex.SqlState == Npgsql.PostgresErrorCodes.ForeignKeyViolation ||
+            ex.SqlState == Npgsql.PostgresErrorCodes.UniqueViolation)
         {
             return false;
         }
diff --git a/server/README.md b/server/README.md
index dc42b43..ce67670 100644
--- a/server/README.md
+++ b/server/README.md
@@ -292,7 +292,7 @@ Append-only contract completion audit rows live in **`IContractOutcomeStore`**.
 
 **Store interface methods:**
 
-- **`TryAppend`** — first row id or idempotency key returns `true`; duplicates return `false`. Missing **`contract_instance`** row returns `false` (Postgres FK; in-memory checks **`IContractInstanceStore.TryGet`**) — orchestrators (NEO-149) must create the instance before appending an outcome.
+- **`TryAppend`** — first row id or idempotency key returns `true`; duplicates return `false`. Missing **`contract_instance`** row, or a **`player_id`** that does not match the instance owner, returns `false` (Postgres SQL guard + FK/unique catch; in-memory checks **`IContractInstanceStore.TryGet`**) — orchestrators (NEO-149) must create the instance before appending an outcome.
 - **`TryGetByIdempotencyKey`** — read one outcome for delivery replay / audit tests.
 - **`GetOutcomesForInstance`** — list outcomes for one contract instance id (ordered by recorded time).
 - **`TryClearForInstance`** — dev fixture only.