fix(acp): route derived declared roots through the McpServer converter

src/SalmonEgg.Acp/Mcp/McpServerConfig.cs

@@ -34,6 +34,11 @@ public enum McpServerTransport
     /// Configuration for a stdio MCP server.
     /// Communicates with the server over standard input/output.
     /// </summary>
+    // The base [JsonConverter] only applies when a contract is *declared* as McpServer, so a derived
+    // declared root (AcpJsonContext lists one per transport) would otherwise serialize raw properties
+    // with no "type" discriminator and no v1/v2 gate. Repeating the converter on each derived record
+    // closes that bypass: every root routes through the same polymorphic writer.
+    [JsonConverter(typeof(McpServerJsonConverter))]
     public sealed record StdioMcpServer : McpServer
     {
         /// <summary>
@@ -85,6 +90,11 @@ public StdioMcpServer(
     /// Configuration for an HTTP MCP server.
     /// Communicates with the server over HTTP requests.
     /// </summary>
+    // The base [JsonConverter] only applies when a contract is *declared* as McpServer, so a derived
+    // declared root (AcpJsonContext lists one per transport) would otherwise serialize raw properties
+    // with no "type" discriminator and no v1/v2 gate. Repeating the converter on each derived record
+    // closes that bypass: every root routes through the same polymorphic writer.
+    [JsonConverter(typeof(McpServerJsonConverter))]
     public sealed record HttpMcpServer : McpServer
     {
         /// <summary>
@@ -124,6 +134,11 @@ public HttpMcpServer(string name, string url, List<McpHttpHeader>? headers = nul
     /// Configuration for an SSE (Server-Sent Events) MCP server.
     /// Communicates with the server over an SSE stream.
     /// </summary>
+    // The base [JsonConverter] only applies when a contract is *declared* as McpServer, so a derived
+    // declared root (AcpJsonContext lists one per transport) would otherwise serialize raw properties
+    // with no "type" discriminator and no v1/v2 gate. Repeating the converter on each derived record
+    // closes that bypass: every root routes through the same polymorphic writer.
+    [JsonConverter(typeof(McpServerJsonConverter))]
     public sealed record SseMcpServer : McpServer
     {
         /// <summary>
@@ -165,6 +180,11 @@ public SseMcpServer(string name, string url, List<McpHttpHeader>? headers = null
     /// stdio/http/sse): the spec requires a receiver to "preserve the raw payload" for a transport it does not
     /// recognize, leaving it to the Agent rather than the client to accept or reject it.
     /// </summary>
+    // The base [JsonConverter] only applies when a contract is *declared* as McpServer, so a derived
+    // declared root (AcpJsonContext lists one per transport) would otherwise serialize raw properties
+    // with no "type" discriminator and no v1/v2 gate. Repeating the converter on each derived record
+    // closes that bypass: every root routes through the same polymorphic writer.
+    [JsonConverter(typeof(McpServerJsonConverter))]
     public sealed record CustomMcpServer : McpServer
     {
         /// <summary>
@@ -384,6 +404,11 @@ internal sealed class McpServerJsonConverter : JsonConverter<McpServer>
         internal const string SseV1OnlyMessage =
             "ACP MCP server transport sse is only available in protocolVersion 1.";
 
+        // The generated context validates a converter against its declared root with an exact-type
+        // CanConvert check; derived records reuse this converter for their own roots, so any type in
+        // the McpServer family is accepted and dispatch still happens on the runtime value in Write.
+        public override bool CanConvert(Type typeToConvert) => typeof(McpServer).IsAssignableFrom(typeToConvert);
+
         public override McpServer? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
             using var document = JsonDocument.ParseValue(ref reader);

src/SalmonEgg.Acp/Serialization/AcpJsonContext.cs

@@ -187,6 +187,11 @@ namespace SalmonEgg.Acp.Serialization;
 [JsonSerializable(typeof(CustomMcpServer))]
 [JsonSerializable(typeof(McpHttpHeader))]
 [JsonSerializable(typeof(McpEnvVariable))]
+// The derived McpServer records carry their own converter, so the generator no longer walks their
+// properties and these two List<> infos must be declared explicitly: they are public members of the
+// shipped context (AcpJsonContext.ListMcpHttpHeader / ListMcpEnvVariable).
+[JsonSerializable(typeof(List<McpHttpHeader>))]
+[JsonSerializable(typeof(List<McpEnvVariable>))]
 [JsonSerializable(typeof(ContentBlock))]
 [JsonSerializable(typeof(TextContentBlock))]
 [JsonSerializable(typeof(ImageContentBlock))]

tests/SalmonEgg.Acp.Tests/Protocol/V2WireContractTests.cs

@@ -240,6 +240,26 @@ public void McpServerV2_TypedSse_RefusesToSerializeAtEveryRoot()
         Assert.Equal("events", stableSetup.RootElement.GetProperty("mcpServers")[0].GetProperty("name").GetString());
     }
 
+    [Fact]
+    public void McpServerV2_DeclaredDerivedRoots_RouteThroughTheVersionGatedConverter()
+    {
+        // Arrange
+        var sse = new SseMcpServer("events", "https://example.test/events");
+        var stdio = new StdioMcpServer { Name = "tools", Command = "tool" };
+
+        // Act
+        var gated = Assert.Throws<JsonException>(() => JsonSerializer.Serialize(sse, Wire.V2<SseMcpServer>()));
+        using var v1Root = JsonDocument.Parse(JsonSerializer.Serialize(sse, Wire.V1<SseMcpServer>()));
+        using var v2StdioRoot = JsonDocument.Parse(JsonSerializer.Serialize(stdio, Wire.V2<StdioMcpServer>()));
+
+        // Assert
+        Assert.Contains(McpServerJsonConverter.SseV1OnlyMessage, gated.Message);
+        Assert.Equal("sse", v1Root.RootElement.GetProperty("type").GetString());
+        // The discriminator must survive at every declared root too, or a derived-root payload
+        // could not round-trip through the v2 read path (which rejects a type-less object).
+        Assert.Equal("stdio", v2StdioRoot.RootElement.GetProperty("type").GetString());
+    }
+
     [Theory]
     [InlineData("")]
     [InlineData(",\"type\":null")]