diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index da50e2cd..12de86ff 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -9,6 +9,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Hardening
 
+- **Same-window interleaved A/B harness (`--pk-ab`) (P3c)** - new harness mode runs two default-
+  config variants as alternating rep pairs (A1,B1,A2,B2,...) and reports the per-rep paired median
+  ratio B/A per phase, so machine drift affects both arms of a pair and cancels out. Smoke (1 rep,
+  pure default vs `plain`/NoEncryptMode=true): UPDATE 1.41x, DELETE 1.28x, INSERT 1.21x, READ 1.10x
+  in the same window. Use `SHARPCOREDB_PK_AB_ARM_A` / `SHARPCOREDB_PK_AB_ARM_B` /
+  `SHARPCOREDB_BENCH_REPS`; documented in `docs/benchmarks/default-config-pk.md`.
+
 - **Default-config benchmark follow-up: FullSync hypothesis falsified (P3b)** - single-knob
   isolation via `SHARPCOREDB_PK_DEFAULT_VARIANT` (`async`, `bufferedio`, `novalidate`,
   `noadaptive`, `hsinsert`, `plain`, `tuned`) disproved the earlier “FullSync dominates the
diff --git a/docs/benchmarks/default-config-pk.md b/docs/benchmarks/default-config-pk.md
index b1739def..2a61d5c2 100644
--- a/docs/benchmarks/default-config-pk.md
+++ b/docs/benchmarks/default-config-pk.md
@@ -39,13 +39,26 @@ Observed (2026-09-04 afternoon):
 Machine drift is significant: SQLite's own UPDATE varied 275K-315K across these runs. Single-knob
 deltas below ~1.3x are not reliably attributable outside a same-window interleaved A/B.
 
+## Same-window interleaved A/B (--pk-ab)
+
+Run: `dotnet run --project tests/benchmarks/SharpCoreDB.Benchmarks.Comparative -- -c Release -- --pk-ab`
+Arm names: `SHARPCOREDB_PK_AB_ARM_A` / `SHARPCOREDB_PK_AB_ARM_B` (defaults: `""` pure default vs
+`plain`); reps via `SHARPCOREDB_BENCH_REPS` (default 3). Each rep runs A then B back-to-back and
+the per-rep **paired ratio** B/A per phase is reported (median), so slow-machine windows affect
+both arms of a pair and cancel out — this is the reliable way to attribute default-vs-tuned deltas.
+
+Preliminary smoke result (1 rep, 2026-09-04, default vs `plain`): UPDATE **1.41x**, DELETE **1.28x**,
+INSERT 1.21x, READ 1.10x — direction consistent with the earlier medians, now measured inside a
+single window. Re-run with the default 3 reps before quoting final numbers.
+
 ## Honest conclusion
 
 1. The earlier claim that the default `WalDurabilityMode.FullSync` dominates the gap is **wrong**
    (disproven by the `async` variant). Do **not** implement a “FullSync commit-flush optimization”
    based on it.
-2. The default path is correct and engages the fast paths; part of the remaining gap correlates
-   with `NoEncryptMode` (record/at-rest toggles and file-format decisions), part is machine drift.
-3. Next step: add a **same-window interleaved A/B** mode to this harness (arms round-robin within
-   one process) so default-vs-tuned deltas are attributable, then re-open the optimization only on
-   a measured knob.
+2. The default path is correct and engages the fast paths; the `--pk-ab` smoke (same window) shows
+   `plain` (NoEncryptMode=true) ahead by UPDATE 1.41x / DELETE 1.28x, so the remaining gap
+   correlates with `NoEncryptMode`; the exact mechanism (record/at-rest toggles and file-format
+   decisions) still needs a code-level explanation before any change.
+3. The same-window A/B tooling now exists; re-run with 3 reps to quantify the paired ratio, then
+   investigate the `NoEncryptMode` mechanism in code and only then consider a change.
diff --git a/tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs b/tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs
index aa3d5f8a..4fd752b0 100644
--- a/tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs
+++ b/tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs
@@ -79,6 +79,21 @@ static async Task Main(string[] args)
             return;
         }
 
+        // Optional: --pk-ab → same-window interleaved A/B: runs arm A and arm B as alternating
+        // rep pairs (A1,B1,A2,B2,...) and reports the PER-REP median ratio B/A per phase, so
+        // machine drift affects both arms of each pair equally. Arms are config variants named by
+        // SHARPCOREDB_PK_AB_ARM_A / SHARPCOREDB_PK_AB_ARM_B (defaults: pure default vs 'plain').
+        if (args.Any(a => a.Equals("--pk-ab", StringComparison.OrdinalIgnoreCase)))
+        {
+            var engineArgAb = args.FirstOrDefault(a => a.StartsWith("--engine=", StringComparison.OrdinalIgnoreCase));
+            var engineTypeAb = engineArgAb is not null
+                && engineArgAb.Substring("--engine=".Length).Equals("pagebased", StringComparison.OrdinalIgnoreCase)
+                    ? SharpCoreDB.Interfaces.StorageEngineType.PageBased
+                    : SharpCoreDB.Interfaces.StorageEngineType.AppendOnly;
+            RunPkAbComparison(engineTypeAb);
+            return;
+        }
+
         // Optional: --engine=appendonly (default) | --engine=pagebased
         // PageBased is the v2.0 in-place-update engine (WP10-WP13 storage engine roadmap).
         var engineArg = args.FirstOrDefault(a => a.StartsWith("--engine=", StringComparison.OrdinalIgnoreCase));
@@ -395,6 +410,34 @@ static DatabaseConfig BuildConfig(SharpCoreDB.Interfaces.StorageEngineType engin
         };
     }
 
+    /// <summary>
+    /// Builds the variant DatabaseConfig used by the --pk-default arm. The variant name comes from
+    /// SHARPCOREDB_PK_DEFAULT_VARIANT or the --pk-ab arm selector; "" is the pure default config.
+    /// </summary>
+    private static DatabaseConfig BuildPkDefaultVariantConfig(
+        SharpCoreDB.Interfaces.StorageEngineType engineType,
+        string? variant)
+    {
+        return variant switch
+        {
+            "async" => new DatabaseConfig { StorageEngineType = engineType, WalDurabilityMode = SharpCoreDB.Services.DurabilityMode.Async },
+            "bufferedio" => new DatabaseConfig { StorageEngineType = engineType, UseBufferedIO = true },
+            "novalidate" => new DatabaseConfig
+            {
+                StorageEngineType = engineType,
+                SqlValidationMode = SharpCoreDB.Services.SqlQueryValidator.ValidationMode.Disabled,
+                StrictParameterValidation = false,
+            },
+            "noadaptive" => new DatabaseConfig { StorageEngineType = engineType, EnableAdaptiveWalBatching = false },
+            "hsinsert" => new DatabaseConfig { StorageEngineType = engineType, HighSpeedInsertMode = true },
+            // "plain" == the tuned harness config with NoEncryptMode=true (BuildConfig default);
+            // "tuned" == the same knob set but NoEncryptMode=false (isolates that flag).
+            "plain" => BuildConfig(engineType, fixedWidth: true),
+            "tuned" => BuildConfig(engineType, fixedWidth: true, noEncrypt: false),
+            _ => new DatabaseConfig { StorageEngineType = engineType },
+        };
+    }
+
     static BenchmarkResult RunSharpCoreDB(SharpCoreDB.Interfaces.StorageEngineType engineType)
     {
         var dbPath = Path.Combine(Path.GetTempPath(), $"bench-sharpcoredb-{Guid.NewGuid()}");
@@ -862,7 +905,8 @@ data TEXT
     static BenchmarkResult RunSharpCoreDBPk(
         SharpCoreDB.Interfaces.StorageEngineType engineType,
         bool fixedWidth = false,
-        bool useDefaultConfig = false)
+        bool useDefaultConfig = false,
+        string? defaultVariant = null)
     {
         var dbPath = Path.Combine(Path.GetTempPath(), $"bench-sharpcoredb-pk-{Guid.NewGuid()}");
         var result = new BenchmarkResult();
@@ -877,25 +921,9 @@ static BenchmarkResult RunSharpCoreDBPk(
             DatabaseConfig config;
             if (useDefaultConfig)
             {
-                var variant = Environment.GetEnvironmentVariable("SHARPCOREDB_PK_DEFAULT_VARIANT")?.ToLowerInvariant();
-                config = variant switch
-                {
-                    "async" => new DatabaseConfig { StorageEngineType = engineType, WalDurabilityMode = SharpCoreDB.Services.DurabilityMode.Async },
-                    "bufferedio" => new DatabaseConfig { StorageEngineType = engineType, UseBufferedIO = true },
-                    "novalidate" => new DatabaseConfig
-                    {
-                        StorageEngineType = engineType,
-                        SqlValidationMode = SharpCoreDB.Services.SqlQueryValidator.ValidationMode.Disabled,
-                        StrictParameterValidation = false,
-                    },
-                    "noadaptive" => new DatabaseConfig { StorageEngineType = engineType, EnableAdaptiveWalBatching = false },
-                    "hsinsert" => new DatabaseConfig { StorageEngineType = engineType, HighSpeedInsertMode = true },
-                    // "plain" == the tuned harness config with NoEncryptMode=true (BuildConfig default);
-                    // "tuned" == the same knob set but NoEncryptMode=false (isolates that flag).
-                    "plain" => BuildConfig(engineType, fixedWidth: true),
-                    "tuned" => BuildConfig(engineType, fixedWidth: true, noEncrypt: false),
-                    _ => new DatabaseConfig { StorageEngineType = engineType },
-                };
+                var variant = defaultVariant
+                    ?? Environment.GetEnvironmentVariable("SHARPCOREDB_PK_DEFAULT_VARIANT")?.ToLowerInvariant();
+                config = BuildPkDefaultVariantConfig(engineType, variant);
             }
             else
             {
@@ -1127,6 +1155,85 @@ static void RunPkDefaultComparison(SharpCoreDB.Interfaces.StorageEngineType engi
         Console.WriteLine($"\nResults saved to: {path}");
     }
 
+    /// <summary>
+    /// P3c: same-window interleaved A/B comparison of two default-config variants. Each rep runs
+    /// arm A then arm B back-to-back, and the result reports the per-rep paired ratio B/A per phase
+    /// (median), so slow-machine windows affect both arms of a pair and cancel out.
+    /// </summary>
+    static void RunPkAbComparison(SharpCoreDB.Interfaces.StorageEngineType engineType)
+    {
+        var armA = Environment.GetEnvironmentVariable("SHARPCOREDB_PK_AB_ARM_A")?.ToLowerInvariant() ?? string.Empty;
+        var armB = Environment.GetEnvironmentVariable("SHARPCOREDB_PK_AB_ARM_B")?.ToLowerInvariant() ?? "plain";
+        int reps = 3;
+        if (int.TryParse(Environment.GetEnvironmentVariable("SHARPCOREDB_BENCH_REPS"), out int envReps) && envReps > 0)
+        {
+            reps = envReps;
+        }
+
+        Console.WriteLine("╔══════════════════════════════════════════════════════════╗");
+        Console.WriteLine("║  Same-window interleaved A/B (default-config variants)    ║");
+        Console.WriteLine("╚══════════════════════════════════════════════════════════╝");
+        Console.WriteLine();
+        Console.WriteLine($"Arm A variant: '{armA}'   Arm B variant: '{armB}'   reps: {reps}");
+        Console.WriteLine("(each rep: A then B back-to-back; per-rep ratios cancel drift)");
+        Console.WriteLine();
+
+        var listA = new List<BenchmarkResult>(reps);
+        var listB = new List<BenchmarkResult>(reps);
+        for (int r = 0; r < reps; r++)
+        {
+            Console.WriteLine($"── rep {r + 1}/{reps} · A='{armA}' ──");
+            listA.Add(RunSharpCoreDBPk(engineType, useDefaultConfig: true, defaultVariant: armA));
+            Console.WriteLine($"── rep {r + 1}/{reps} · B='{armB}' ──");
+            listB.Add(RunSharpCoreDBPk(engineType, useDefaultConfig: true, defaultVariant: armB));
+        }
+
+        static double Med(IEnumerable<double> xs)
+        {
+            var sorted = xs.Where(x => x > 0).OrderBy(x => x).ToArray();
+            return sorted.Length == 0 ? 0 : sorted[sorted.Length / 2];
+        }
+
+        static int Ops(IEnumerable<BenchmarkResult> runs, Func<BenchmarkResult, int> select)
+        {
+            var arr = runs.Select(select).Where(x => x > 0).OrderBy(x => x).ToArray();
+            return arr.Length == 0 ? 0 : arr[arr.Length / 2];
+        }
+
+        static double Ratio(IEnumerable<BenchmarkResult> a, IEnumerable<BenchmarkResult> b, Func<BenchmarkResult, int> sel)
+        {
+            var ratios = a.Zip(b, (x, y) => sel(x) > 0 ? sel(y) / (double)sel(x) : 0.0);
+            return Med(ratios);
+        }
+
+        var aUpdate = Ops(listA, static r => r.UpdateOpsPerSec);
+        var bUpdate = Ops(listB, static r => r.UpdateOpsPerSec);
+        var aDelete = Ops(listA, static r => r.DeleteOpsPerSec);
+        var bDelete = Ops(listB, static r => r.DeleteOpsPerSec);
+
+        Console.WriteLine("║ phase   │ A median ops/s │ B median ops/s │ median B/A (per rep) ║");
+        Console.WriteLine($"║ UPDATE  │ {aUpdate,13:N0} │ {bUpdate,14:N0} │ {Ratio(listA, listB, static r => r.UpdateOpsPerSec):F2}x");
+        Console.WriteLine($"║ DELETE  │ {aDelete,13:N0} │ {bDelete,14:N0} │ {Ratio(listA, listB, static r => r.DeleteOpsPerSec):F2}x");
+        Console.WriteLine($"║ INSERT  │ {Ops(listA, static r => r.InsertOpsPerSec),13:N0} │ {Ops(listB, static r => r.InsertOpsPerSec),14:N0} │ {Ratio(listA, listB, static r => r.InsertOpsPerSec):F2}x");
+        Console.WriteLine($"║ READ    │ {Ops(listA, static r => r.ReadOpsPerSec),13:N0} │ {Ops(listB, static r => r.ReadOpsPerSec),14:N0} │ {Ratio(listA, listB, static r => r.ReadOpsPerSec):F2}x");
+
+        var summary = new Dictionary<string, object>
+        {
+            ["armA"] = armA,
+            ["armB"] = armB,
+            ["medianUpdateRatio_B_over_A"] = Ratio(listA, listB, static r => r.UpdateOpsPerSec),
+            ["medianDeleteRatio_B_over_A"] = Ratio(listA, listB, static r => r.DeleteOpsPerSec),
+            ["medianInsertRatio_B_over_A"] = Ratio(listA, listB, static r => r.InsertOpsPerSec),
+            ["medianReadRatio_B_over_A"] = Ratio(listA, listB, static r => r.ReadOpsPerSec),
+        };
+
+        var dir = "results";
+        Directory.CreateDirectory(dir);
+        var path = Path.Combine(dir, $"pk_ab_{armA}_{armB}_{DateTime.UtcNow:yyyyMMdd_HHmmss}.json");
+        File.WriteAllText(path, JsonSerializer.Serialize(summary, new JsonSerializerOptions { WriteIndented = true }));
+        Console.WriteLine($"\nResults saved to: {path}");
+    }
+
     // ══════════════════════════════════════
     // LiteDB
     // ══════════════════════════════════════