diff --git a/perf/StressTest.cs b/perf/StressTest.cs
index 627ad74..1da38e8 100644
--- a/perf/StressTest.cs
+++ b/perf/StressTest.cs
@@ -46,6 +46,22 @@ public static async Task<ulong> RunAsync(TimeSpan duration, ILogger log) {
         return bytesPerSecond;
     }
 
+    /// <summary>
+    /// Runs a correctness stress test against <paramref name="cache"/> for the specified
+    /// <paramref name="duration"/> using <see cref="Environment.ProcessorCount"/> * 2
+    /// concurrent read/write tasks. Each task writes random blocks and reads them back,
+    /// recomputing the hash to verify data integrity. Throws
+    /// <see cref="HashMismatchException"/> on corruption.
+    /// </summary>
+    public static async Task RunCorrectnessAsync(IContentCache cache, TimeSpan duration) {
+        var timeIsUp = duration.ToCancellation();
+        var tasks = new List<Task>();
+        for (int i = 0; i < Environment.ProcessorCount * 2; i++)
+            tasks.Add(AbuseAsync(cache, timeIsUp));
+
+        await Task.WhenAll(tasks);
+    }
+
     static async Task<long> AbuseAsync(IContentCache cache, CancellationToken cancel) {
         byte[] data = new byte[cache.MaxBlockSize];
         var random = new Random();
@@ -84,7 +100,7 @@ static async Task<long> AbuseAsync(IContentCache cache, CancellationToken cancel
                     throw new HashMismatchException();
                 transmitted += read;
             }
-        } catch (OperationCanceledException e) {
+        } catch (OperationCanceledException) {
             var opTime = accessStart.Elapsed;
             if (!cancel.IsCancellationRequested) {
                 await Console.Error.WriteLineAsync($"last access: {opTime.TotalMilliseconds:N0}ms");
diff --git a/src/BlockCache.cs b/src/BlockCache.cs
index fc3d819..0220b36 100644
--- a/src/BlockCache.cs
+++ b/src/BlockCache.cs
@@ -46,8 +46,13 @@ public async ValueTask<TimeSpan> WriteAsync(ContentHash hash, ReadOnlyMemory<byt
         if (reportPerf)
             this.log.LogDebug("write lock wait: {Microseconds:F0}us",
                               start.Elapsed.TotalMicroseconds);
+
+        // Hoisted outside the try so they survive the finally (indexLock.Release) and are
+        // accessible in the continuation: index is used in CommitWrite and blockWriteLock
+        // in the await-using block, both of which execute after the index lock is dropped.
+        int index = -1;
+        AsyncReaderWriterLock.Releaser blockWriteLock = default;
         try {
-            int index;
             var writeTime = start = StopwatchTimestamp.Now;
             if (this.evictionStrategy.Access(hash, out var evicted)) {
                 this.Evicted?.Invoke(this, evicted);
@@ -61,13 +66,20 @@ public async ValueTask<TimeSpan> WriteAsync(ContentHash hash, ReadOnlyMemory<byt
 
                 start = StopwatchTimestamp.Now;
                 var entryLock = this.blockLocks[index % this.blockLocks.Length];
-                await using (await entryLock.WriteLockAsync(cancel))
-                    await this.storage.WriteAsync(index, content, hash, CancellationToken.None)
-                              .ConfigureAwait(false);
 
-                this.log.Log(
-                    perfLevel, "Set content[{Length}] for {Hash} at {Index} in {Microseconds:F0}us",
-                    content.Length, hash, index, start.Elapsed.TotalMicroseconds);
+                // Acquire the block write lock while still holding the index lock.
+                // This prevents another writer from claiming the same block between the
+                // dictionary update below and the actual data write after the index lock drops.
+                blockWriteLock = await entryLock.WriteLockAsync(cancel);
+                try {
+                    // Update the in-memory hash->index dictionary under the index lock so
+                    // concurrent readers and writers see a consistent view immediately.
+                    this.storage.UpdateIndex(index, newHash: hash, oldHash: evicted);
+                    await this.storage.MarkDirtyAsync().ConfigureAwait(false);
+                } catch {
+                    await blockWriteLock.DisposeAsync().ConfigureAwait(false);
+                    throw;
+                }
             } else {
 #if DEBUG
                 index = this.storage.BlockIndex(hash);
@@ -84,12 +96,26 @@ await this.storage.WriteAsync(index, content, hash, CancellationToken.None)
                 return TimeSpan.Zero;
 #endif
             }
-
-            this.Available?.Invoke(this, hash, content.Span);
-            return start.Elapsed;
         } finally {
+            // Release the index lock before writing block data so other reads and writes
+            // can proceed concurrently while the (potentially large) copy is in progress.
             this.indexLock.Release();
         }
+
+        // Write the persisted index entry and block data under only the block write lock.
+        // The index lock has already been released, allowing other operations to proceed.
+        // index is always valid here: the early-return branches never reach this point.
+        Debug.Assert(index >= 0);
+        await using (blockWriteLock) {
+            this.storage.CommitWrite(index, content, hash);
+        }
+
+        this.log.Log(
+            perfLevel, "Set content[{Length}] for {Hash} at {Index} in {Microseconds:F0}us",
+            content.Length, hash, index, start.Elapsed.TotalMicroseconds);
+
+        this.Available?.Invoke(this, hash, content.Span);
+        return start.Elapsed;
     }
 
     public async ValueTask<int?> ReadAsync(ContentHash hash, long offset, Memory<byte> buffer,
diff --git a/src/BlockIndex.cs b/src/BlockIndex.cs
index f15894e..3587327 100644
--- a/src/BlockIndex.cs
+++ b/src/BlockIndex.cs
@@ -4,7 +4,7 @@ namespace Hash;
 using System.Runtime.InteropServices;
 
 sealed class BlockIndex: IAsyncDisposable {
-    readonly Dictionary<ContentHash, int> positions = new();
+    internal readonly Dictionary<ContentHash, int> positions = new();
     readonly IBlockReader reader;
     readonly IBlockWriter writer;
 
@@ -78,7 +78,7 @@ public bool TrySet(int index, Entry value) {
         return true;
     }
 
-    void SetUnchecked(int index, Entry value) {
+    internal void SetUnchecked(int index, Entry value) {
         var span = MemoryMarshal.CreateSpan(ref value, 1);
         var bytes = MemoryMarshal.Cast<Entry, byte>(span);
         this.writer.Write(bytes, index, offset: 0);
diff --git a/src/BlockStorage.cs b/src/BlockStorage.cs
index a2d9e97..996047e 100644
--- a/src/BlockStorage.cs
+++ b/src/BlockStorage.cs
@@ -116,7 +116,7 @@ public async ValueTask FlushAsync(CancellationToken cancel = default) {
                                 blocksTime, indexTime);
     }
 
-    async ValueTask MarkDirtyAsync() {
+    internal async ValueTask MarkDirtyAsync() {
         if (this.dirty)
             return;
 
@@ -125,6 +125,28 @@ async ValueTask MarkDirtyAsync() {
         this.dirty = true;
     }
 
+    /// <summary>
+    /// Updates only the in-memory hash→index dictionary for the given block.
+    /// Must be called under the index lock, with the block write lock already held.
+    /// </summary>
+    internal void UpdateIndex(int blockIndex, ContentHash newHash, ContentHash oldHash) {
+        if (!this.index.positions.TryAdd(newHash, blockIndex))
+            throw new InvalidOperationException(
+                $"Internal error: hash {newHash} is already mapped to a block (attempted to map to block {blockIndex})");
+        if (!this.index.positions.Remove(oldHash))
+            throw new InvalidOperationException(
+                $"Internal error: evicted hash {oldHash} was not found in the index (block {blockIndex})");
+    }
+
+    /// <summary>
+    /// Writes the persisted index entry and block data.
+    /// Must be called under the block write lock (not the index lock).
+    /// </summary>
+    internal void CommitWrite(int blockIndex, ReadOnlyMemory<byte> block, ContentHash hash) {
+        this.index.SetUnchecked(blockIndex, new(hash, block.Length));
+        this.writer.Write(block.Span, blockIndex, offset: 0);
+    }
+
     public static async Task<BlockStorage> CreateAsync(string indexPath, string blocksPath,
                                                        int blockSize,
                                                        ILogger log,
diff --git a/test/CorrectnessTests.cs b/test/CorrectnessTests.cs
new file mode 100644
index 0000000..add90b9
--- /dev/null
+++ b/test/CorrectnessTests.cs
@@ -0,0 +1,26 @@
+namespace Hash;
+
+using Microsoft.Extensions.Logging.Abstractions;
+
+/// <summary>
+/// Correctness stress tests for <see cref="BlockCache"/>.
+/// Verifies that concurrent reads and writes never produce data corruption,
+/// exercising the locking logic directly without a TCP layer.
+/// </summary>
+public class CorrectnessTests {
+    // Small block count so evictions happen frequently — maximises the chance of
+    // exposing races between eviction, index update, and the data copy.
+    const int BlockCount = 64;
+
+    [Fact]
+    public async Task ConcurrentReadWriteProducesNoDataCorruption() {
+        var cache = new BlockCache(BlockCache.DEFAULT_BLOCK_SIZE, BlockCount,
+                                   NullLogger<BlockCache>.Instance);
+        await using var _ = cache;
+
+        // RunCorrectnessAsync drives ProcessorCount*2 concurrent tasks, each writing
+        // random blocks and reading them back, recomputing the hash to detect corruption.
+        // A HashMismatchException is thrown (and propagated) if any mismatch is found.
+        await StressTest.RunCorrectnessAsync(cache, TimeSpan.FromSeconds(15));
+    }
+}