diff --git a/RemoteSigner.Tests/FakeKmsClient.cs b/RemoteSigner.Tests/FakeKmsClient.cs
new file mode 100644
index 0000000..0014db7
--- /dev/null
+++ b/RemoteSigner.Tests/FakeKmsClient.cs
@@ -0,0 +1,42 @@
+using Amazon.KeyManagementService;
+using Amazon.KeyManagementService.Model;
+using Amazon.Runtime;
+
+namespace RemoteSigner.Tests;
+
+/// <summary>
+/// Fake KMS whose Encrypt/Decrypt are identity transforms, capturing requests, so the whole
+/// encrypt -> decrypt -> re-derive pipeline runs without AWS credentials
+/// </summary>
+internal class FakeKmsClient : AmazonKeyManagementServiceClient
+{
+    public EncryptRequest? LastEncryptRequest;
+    public DecryptRequest? LastDecryptRequest;
+    public int DecryptCalls;
+
+    public FakeKmsClient() : base(new AnonymousAWSCredentials(),
+        new AmazonKeyManagementServiceConfig { RegionEndpoint = Amazon.RegionEndpoint.EUCentral1 })
+    {
+    }
+
+    public override Task<EncryptResponse> EncryptAsync(EncryptRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        LastEncryptRequest = request;
+        return Task.FromResult(new EncryptResponse
+        {
+            CiphertextBlob = new MemoryStream(request.Plaintext.ToArray())
+        });
+    }
+
+    public override Task<DecryptResponse> DecryptAsync(DecryptRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        LastDecryptRequest = request;
+        Interlocked.Increment(ref DecryptCalls);
+        return Task.FromResult(new DecryptResponse
+        {
+            Plaintext = new MemoryStream(request.CiphertextBlob.ToArray())
+        });
+    }
+}
diff --git a/RemoteSigner.Tests/RoutingTests.cs b/RemoteSigner.Tests/RoutingTests.cs
new file mode 100644
index 0000000..f3ea913
--- /dev/null
+++ b/RemoteSigner.Tests/RoutingTests.cs
@@ -0,0 +1,145 @@
+using System.Text;
+using System.Text.Json;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.Lambda.TestUtilities;
+using FluentAssertions;
+using NBitcoin;
+using NSpark.RemoteSigner;
+using Xunit;
+
+namespace RemoteSigner.Tests;
+
+/// <summary>
+/// The whole Lambda handler with a fake KMS: only the /spark/ prefix may reach Spark code, and every
+/// other path must keep signing PSBTs byte-for-byte as before
+/// </summary>
+[Collection(SignerEnvironment.Collection)]
+public class RoutingTests : IDisposable
+{
+    private const string Mnemonic =
+        "middle teach digital prefer fiscal theory syrup enter crash muffin easily anxiety ill barely eagle swim volume consider dynamic unaware deputy middle into physical";
+
+    private readonly FakeKmsClient _kms = new();
+    private readonly Function _function;
+    private int _kmsClientsCreated;
+
+    public RoutingTests()
+    {
+        // The fake KMS decrypts to the ciphertext itself; seeds are stored with their words joined by @
+        var config = new SignPSBTConfig
+        {
+            AwsKmsKeyId = "mrk-test",
+            EncryptedSeedphrase = Convert.ToBase64String(Encoding.UTF8.GetBytes(Mnemonic.Replace(" ", "@")))
+        };
+        Environment.SetEnvironmentVariable("MF_ed0210c8", JsonSerializer.Serialize(config));
+
+        _function = new Function(() =>
+        {
+            _kmsClientsCreated++;
+            return _kms;
+        });
+    }
+
+    public void Dispose() => Environment.SetEnvironmentVariable("MF_ed0210c8", null);
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("/")]
+    [InlineData("/sign")]
+    [InlineData("/spark")]
+    [InlineData("/sparkle/info")]
+    [InlineData("/SPARK/info")]
+    public async Task NonSparkPaths_SignThePsbtExactlyAsBefore(string? path)
+    {
+        var golden = Golden("p2wsh-2of3-all");
+        var body = JsonSerializer.Serialize(new SignPSBTRequest(golden.Psbt, Enum.Parse<SigHash>(golden.SigHash), golden.Network));
+
+        var response = await Handle(path, body);
+
+        response.StatusCode.Should().Be(200);
+        JsonSerializer.Deserialize<SignPSBTResponse>(response.Body)!.Psbt.Should().Be(golden.Expected);
+        _kms.LastDecryptRequest!.KeyId.Should().Be("mrk-test");
+    }
+
+    [Fact]
+    public async Task PsbtErrors_KeepTheirPlainTextResponse()
+    {
+        var response = await Handle("/", "not json");
+
+        response.StatusCode.Should().Be(500);
+        response.Headers.Should().BeNull();
+        response.Body.Should().NotBeNullOrEmpty();
+    }
+
+    [Theory]
+    [InlineData("/spark/info")]
+    [InlineData("//spark/info")]
+    [InlineData("/spark/identity-public-key")]
+    [InlineData("/spark/sign-leaf-frost")]
+    public async Task SparkPaths_NeverReachTheSeeds(string path)
+    {
+        var response = await Handle(path, """{"wallet":"ed0210c8"}""");
+
+        response.StatusCode.Should().Be(403);
+        response.Headers["Content-Type"].Should().Be("application/json");
+        Envelope(response).Error.Should().Contain("not enabled");
+        _kmsClientsCreated.Should().Be(0);
+        _kms.DecryptCalls.Should().Be(0);
+    }
+
+    [Fact]
+    public async Task SparkPath_WithAPsbtRequest_IsABadRequest()
+    {
+        var golden = Golden("p2wsh-2of3-all");
+        var body = JsonSerializer.Serialize(new SignPSBTRequest(golden.Psbt, SigHash.All, golden.Network));
+
+        var response = await Handle("/spark/info", body);
+
+        response.StatusCode.Should().Be(400);
+        Envelope(response).Error.Should().Contain("wallet");
+        _kms.DecryptCalls.Should().Be(0);
+    }
+
+    [Fact]
+    public async Task SparkPath_DecodesABase64Body()
+    {
+        var response = await _function.FunctionHandler(new APIGatewayHttpApiV2ProxyRequest
+        {
+            RawPath = "/spark/info",
+            IsBase64Encoded = true,
+            Body = Convert.ToBase64String("""{"wallet":"ed0210c8"}"""u8.ToArray())
+        }, new TestLambdaContext());
+
+        // Reaching the wallet check means the body was decoded and parsed
+        response.StatusCode.Should().Be(403);
+    }
+
+    [Fact]
+    public async Task SparkPath_WithAnInvalidBase64Body_IsABadRequest()
+    {
+        var response = await _function.FunctionHandler(new APIGatewayHttpApiV2ProxyRequest
+        {
+            RawPath = "/spark/info",
+            IsBase64Encoded = true,
+            Body = "not base64!"
+        }, new TestLambdaContext());
+
+        response.StatusCode.Should().Be(400);
+        Envelope(response).Error.Should().Contain("base64");
+    }
+
+    private Task<APIGatewayHttpApiV2ProxyResponse> Handle(string? path, string body) =>
+        _function.FunctionHandler(new APIGatewayHttpApiV2ProxyRequest { RawPath = path, Body = body },
+            new TestLambdaContext());
+
+    private static SparkSignerEnvelope Envelope(APIGatewayHttpApiV2ProxyResponse response) =>
+        JsonSerializer.Deserialize<SparkSignerEnvelope>(response.Body, SparkSignerContract.Json)!;
+
+    private static GoldenSignatureTests.GoldenCase Golden(string name)
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Fixtures", "golden-psbt-signatures.json");
+        return JsonSerializer.Deserialize<List<GoldenSignatureTests.GoldenCase>>(File.ReadAllText(path),
+            new JsonSerializerOptions(JsonSerializerDefaults.Web))!.Single(c => c.Name == name);
+    }
+}
diff --git a/RemoteSigner.Tests/SeedCeremonyTest.cs b/RemoteSigner.Tests/SeedCeremonyTest.cs
index 588fc64..2cceabc 100644
--- a/RemoteSigner.Tests/SeedCeremonyTest.cs
+++ b/RemoteSigner.Tests/SeedCeremonyTest.cs
@@ -2,7 +2,6 @@
 using System.Text.Json;
 using Amazon.KeyManagementService;
 using Amazon.KeyManagementService.Model;
-using Amazon.Runtime;
 using FluentAssertions;
 using NBitcoin;
 using RemoteSigner.SeedCeremony;
@@ -18,41 +17,6 @@ public class SeedCeremonyTest
     private const string DevMnemonic =
         "middle teach digital prefer fiscal theory syrup enter crash muffin easily anxiety ill barely eagle swim volume consider dynamic unaware deputy middle into physical";
 
-    /// <summary>
-    /// Fake KMS whose Encrypt/Decrypt are identity transforms, capturing requests, so the whole
-    /// encrypt -> decrypt -> re-derive pipeline runs without AWS credentials
-    /// </summary>
-    private class FakeKmsClient : AmazonKeyManagementServiceClient
-    {
-        public EncryptRequest? LastEncryptRequest;
-        public DecryptRequest? LastDecryptRequest;
-
-        public FakeKmsClient() : base(new AnonymousAWSCredentials(),
-            new AmazonKeyManagementServiceConfig { RegionEndpoint = Amazon.RegionEndpoint.EUCentral1 })
-        {
-        }
-
-        public override Task<EncryptResponse> EncryptAsync(EncryptRequest request,
-            CancellationToken cancellationToken = default)
-        {
-            LastEncryptRequest = request;
-            return Task.FromResult(new EncryptResponse
-            {
-                CiphertextBlob = new MemoryStream(request.Plaintext.ToArray())
-            });
-        }
-
-        public override Task<DecryptResponse> DecryptAsync(DecryptRequest request,
-            CancellationToken cancellationToken = default)
-        {
-            LastDecryptRequest = request;
-            return Task.FromResult(new DecryptResponse
-            {
-                Plaintext = new MemoryStream(request.CiphertextBlob.ToArray())
-            });
-        }
-    }
-
     [Theory]
     [InlineData("regtest", "tpub")]
     [InlineData("testnet", "tpub")]
diff --git a/RemoteSigner/Function.cs b/RemoteSigner/Function.cs
index 8af4a26..077e712 100644
--- a/RemoteSigner/Function.cs
+++ b/RemoteSigner/Function.cs
@@ -9,6 +9,7 @@
 using Amazon.Lambda.Serialization.SystemTextJson;
 using Amazon.Runtime.CredentialManagement;
 using NBitcoin;
+using NSpark.RemoteSigner;
 
 [assembly:
     LambdaSerializer(typeof(SourceGeneratorLambdaJsonSerializer<RemoteSigner.HttpApiJsonSerializerContext>))]
@@ -48,8 +49,30 @@ public class SignPSBTConfig
 
 public class Function
 {
+    private readonly Func<IAmazonKeyManagementService> _kmsClientFactory;
+    private readonly SparkRoute _sparkRoute;
+
+    /// <summary>
+    /// Lambda entry point: KMS clients come from the default AWS credential chain
+    /// </summary>
+    public Function() : this(CreateKmsClient)
+    {
+    }
+
     /// <summary>
-    /// A lambda function that takes a psbt and signs it, it it assumed that the psbt inputs come from the same wallet
+    /// Supplies the KMS client the seeds are decrypted with, so tests can run the whole handler
+    /// without AWS
+    /// </summary>
+    /// <param name="kmsClientFactory"></param>
+    public Function(Func<IAmazonKeyManagementService> kmsClientFactory)
+    {
+        _kmsClientFactory = kmsClientFactory ?? throw new ArgumentNullException(nameof(kmsClientFactory));
+        _sparkRoute = new SparkRoute();
+    }
+
+    /// <summary>
+    /// A lambda function that takes a psbt and signs it, it it assumed that the psbt inputs come from the same wallet.
+    /// Requests under /spark/ go to the Spark signer instead; every other path, including none, is the PSBT signer
     /// </summary>MF
     /// <param name="request"></param>
     /// <param name="context"></param>
@@ -57,6 +80,11 @@ public class Function
     public async Task<APIGatewayHttpApiV2ProxyResponse> FunctionHandler(APIGatewayHttpApiV2ProxyRequest request,
         ILambdaContext context)
     {
+        if (SparkSignerHttpHandler.IsSparkPath(request.RawPath))
+        {
+            return await _sparkRoute.HandleAsync(request);
+        }
+
         var response = new APIGatewayHttpApiV2ProxyResponse();
 
         try
@@ -64,14 +92,7 @@ public async Task<APIGatewayHttpApiV2ProxyResponse> FunctionHandler(APIGatewayHt
             var requestBody = JsonSerializer.Deserialize<SignPSBTRequest>(request.Body);
             if (requestBody == null) throw new ArgumentNullException(nameof(requestBody), "Request body not found");
 
-#if DEBUG
-            //AWS SDK v4 removed StoredProfileAWSCredentials; resolve the "default" profile explicitly
-            if (!new CredentialProfileStoreChain().TryGetAWSCredentials("default", out var debugCredentials))
-                throw new InvalidOperationException("AWS profile 'default' was not found");
-            var kmsClient = new AmazonKeyManagementServiceClient(debugCredentials, RegionEndpoint.EUCentral1);
-#else
-            var kmsClient = new AmazonKeyManagementServiceClient();
-#endif
+            var kmsClient = _kmsClientFactory();
 
             Task<string> GetSeed(RootedKeyPath derivationPath) => DecryptSeed(kmsClient, derivationPath);
             var result = await SignPSBT(requestBody.Psbt, requestBody.Network, requestBody.EnforcedSighash, GetSeed);
@@ -253,7 +274,19 @@ private static void EnsureCompromisedSeedOnlyCoSignsMultisig(PSBTInput psbtInput
         }
     }
 
-    private static async Task<string?> DecryptSeed(AmazonKeyManagementServiceClient kmsClient, RootedKeyPath? derivationPath)
+    private static IAmazonKeyManagementService CreateKmsClient()
+    {
+#if DEBUG
+        //AWS SDK v4 removed StoredProfileAWSCredentials; resolve the "default" profile explicitly
+        if (!new CredentialProfileStoreChain().TryGetAWSCredentials("default", out var debugCredentials))
+            throw new InvalidOperationException("AWS profile 'default' was not found");
+        return new AmazonKeyManagementServiceClient(debugCredentials, RegionEndpoint.EUCentral1);
+#else
+        return new AmazonKeyManagementServiceClient();
+#endif
+    }
+
+    private static async Task<string?> DecryptSeed(IAmazonKeyManagementService kmsClient, RootedKeyPath? derivationPath)
     {
         var config = GetConfig(derivationPath.MasterFingerprint);
 
diff --git a/RemoteSigner/SparkRoute.cs b/RemoteSigner/SparkRoute.cs
new file mode 100644
index 0000000..880840b
--- /dev/null
+++ b/RemoteSigner/SparkRoute.cs
@@ -0,0 +1,52 @@
+using System.Text;
+using System.Text.Json;
+using Amazon.Lambda.APIGatewayEvents;
+using NSpark.RemoteSigner;
+using NSpark.Signer;
+
+namespace RemoteSigner;
+
+/// <summary>
+/// The /spark/ routes of the signer, served with the NSpark.RemoteSigner contract: every response is
+/// a JSON envelope (<c>{"result":...}</c> or <c>{"error":"..."}</c>) with a 200/400/403/404/500 status.
+/// No seed is enabled for Spark yet, so every wallet is refused
+/// </summary>
+public class SparkRoute
+{
+    private static readonly SparkSignerPolicy NoPolicy = SparkSignerPolicy.Create([], 0, []);
+
+    public async Task<APIGatewayHttpApiV2ProxyResponse> HandleAsync(APIGatewayHttpApiV2ProxyRequest request)
+    {
+        string? body;
+        try
+        {
+            body = request.IsBase64Encoded && request.Body != null
+                ? Encoding.UTF8.GetString(Convert.FromBase64String(request.Body))
+                : request.Body;
+        }
+        catch (FormatException)
+        {
+            return Respond(400, JsonSerializer.Serialize(new { error = "request body is not valid base64" }, SparkSignerContract.Json));
+        }
+
+        var (status, envelope) = await SparkSignerHttpHandler.HandleAsync(request.RawPath, body, ResolveSigner, NoPolicy);
+
+        if (status != 200)
+        {
+            await Console.Error.WriteLineAsync($"Spark request {request.RawPath} refused with {status}");
+        }
+
+        return Respond(status, envelope);
+    }
+
+    private static Task<ISparkSigner> ResolveSigner(string wallet, CancellationToken ct) =>
+        Task.FromException<ISparkSigner>(new SparkSignerPolicyException("Spark signing is not enabled on this signer"));
+
+    private static APIGatewayHttpApiV2ProxyResponse Respond(int status, string envelope) => new()
+    {
+        StatusCode = status,
+        Headers = new Dictionary<string, string> { ["Content-Type"] = "application/json" },
+        Body = envelope,
+        IsBase64Encoded = false
+    };
+}