Align capability key to SEP-2133 extensions format (#25)

Both SDKs now use Capabilities.Extensions with key
"io.modelcontextprotocol/interceptors". Previously C# used the short key
"interceptors" and Go used Capabilities.Experimental. SEP capability
example updated to match.

https://modelcontextprotocol.io/seps/2133-extensions

---------

Signed-off-by: Ukjae Jeong <jeongukjae@gmail.com>

csharp/sdk/CLAUDE.md

@@ -13,7 +13,7 @@ dotnet test    # 66 tests across the interceptor test project
 
 **Why message filter, not handlers**: `McpServerHandlers` and `McpServerImpl` are `internal` in the SDK. We can't register handlers for new JSON-RPC methods from outside. Instead we use `McpServerOptions.Filters.Message.IncomingFilters` to intercept `interceptors/list` and `interceptor/invoke`, handle them, send `JsonRpcResponse` via `context.Server.SendMessageAsync()`, and skip calling `next`. See `InterceptorMessageFilter.cs`. Chain execution is no longer a wire method — it's SDK orchestration in `Client/InterceptorChainOrchestrator.cs`.
 
-**Why `ServerCapabilities.Extensions`**: The SDK's intended mechanism for protocol extensions. Requires `#pragma warning disable MCPEXP001`. We advertise `InterceptorsCapability { SupportedEvents }` under `Extensions["interceptors"]`.
+**Why `ServerCapabilities.Extensions`**: The SDK's intended mechanism for protocol extensions. Requires `#pragma warning disable MCPEXP001`. We advertise `InterceptorsCapability { SupportedEvents }` under `Extensions["io.modelcontextprotocol/interceptors"]`.
 
 **Client `SendRequestAsync`**: The public overload (`McpSession.Methods.cs:24`) takes `JsonSerializerOptions`. We pass `InterceptorJsonUtilities.DefaultOptions` which chains `McpJsonUtilities.DefaultOptions` + our `InterceptorJsonContext`. The internal overload takes `JsonTypeInfo<T>` — we can't use it.
 

csharp/sdk/src/ModelContextProtocol.Interceptors/InterceptorProtocolConstants.cs

@@ -2,5 +2,5 @@ namespace ModelContextProtocol.Interceptors;
 
 internal static class InterceptorProtocolConstants
 {
-    internal const string ExtensionCapabilityKey = "interceptors";
+    internal const string ExtensionCapabilityKey = "io.modelcontextprotocol/interceptors";
 }

csharp/sdk/src/ModelContextProtocol.Interceptors/Protocol/InterceptorsCapability.cs

@@ -4,7 +4,7 @@ namespace ModelContextProtocol.Interceptors.Protocol;
 
 /// <summary>
 /// Represents the interceptors capability advertised by a server during initialization.
-/// This is placed in <c>ServerCapabilities.Extensions["interceptors"]</c>.
+/// This is placed in <c>ServerCapabilities.Extensions["io.modelcontextprotocol/interceptors"]</c>.
 /// </summary>
 public sealed class InterceptorsCapability
 {

csharp/sdk/tests/ModelContextProtocol.Interceptors.Tests/McpInterceptorGatewayTests.cs

@@ -272,7 +272,7 @@ public class McpInterceptorGatewayTests
         var caps = fixture.ProxyClient.ServerCapabilities;
         Assert.NotNull(caps.Extensions);
         Assert.True(caps.Extensions!.ContainsKey("com.example/backend"));
-        Assert.False(caps.Extensions.ContainsKey("interceptors"));
+        Assert.False(caps.Extensions.ContainsKey("io.modelcontextprotocol/interceptors"));
 #pragma warning restore MCPEXP001
     }
 
@@ -292,7 +292,7 @@ public class McpInterceptorGatewayTests
 
 #pragma warning disable MCPEXP001
         var caps = fixture.ProxyClient.ServerCapabilities;
-        Assert.False(caps?.Extensions?.ContainsKey("interceptors") ?? false);
+        Assert.False(caps?.Extensions?.ContainsKey("io.modelcontextprotocol/interceptors") ?? false);
 #pragma warning restore MCPEXP001
     }
 
@@ -318,7 +318,7 @@ public class McpInterceptorGatewayTests
 #pragma warning disable MCPEXP001
         var caps = fixture.ProxyClient.ServerCapabilities;
         Assert.NotNull(caps?.Extensions);
-        Assert.True(caps!.Extensions!.ContainsKey("interceptors"));
+        Assert.True(caps!.Extensions!.ContainsKey("io.modelcontextprotocol/interceptors"));
         Assert.True(caps.Extensions.ContainsKey("com.example/backend"));
 #pragma warning restore MCPEXP001
     }

docs/sep.md

@@ -1346,13 +1346,15 @@ During initialization, servers declare interceptor support:
   jsonrpc: "2.0",
   id: 1,
   result: {
-    protocolVersion: "2024-11-05",
+    protocolVersion: "2025-06-18",
     capabilities: {
-      interceptor?: {
-        // Events this server's interceptor can handle
-        supportedEvents: InterceptionEvent[];
-      },
       // ...existing capabilities
+      extensions: {
+        "io.modelcontextprotocol/interceptors": {
+          // Events this server's interceptors can handle
+          supportedEvents: InterceptionEvent[];
+        }
+      }
     },
     serverInfo: {
       name: "example-server",

go/sdk/doc/CONFORMANCE.md

@@ -24,7 +24,7 @@ Status of this Go SDK implementation against the SEP-2624 interceptor proposal.
 | Audit mode | `ModeAudit` records results without blocking; mutated payloads not propagated |
 | Timeout & context | Per-interceptor timeouts via `InvokeParams.TimeoutMs`, chain-level context cancellation, `InvocationContext` with principal/traceId |
 | Receiving direction (client → server) | All server-side method calls intercepted via `AddReceivingMiddleware` |
-| Capability declaration | Interceptor metadata injected into `initialize` response via `Capabilities.Experimental` |
+| Capability declaration | Interceptor metadata injected into `initialize` response via `Capabilities.Extensions` |
 | First-party (in-process) deployment | `Server.LocalChain()` creates an in-memory transport `chain.Chain`; interceptors invoked via JSON-RPC even in-process |
 | Third-party and hybrid deployment | Chain entries can point to any `*mcp.ClientSession` — local (in-memory), stdio, or HTTP transport |
 | `json.RawMessage` payloads | Interceptor handlers receive `json.RawMessage` via `interceptor/invoke`, matching the SEP's JSON-level payload model |

go/sdk/doc/DESIGN.md

@@ -50,7 +50,7 @@ behavior as remote interceptor servers.
 
 During initialization, a lightweight middleware intercepts the `"initialize"`
 response and injects interceptor metadata into
-`Capabilities.Experimental["io.modelcontextprotocol/interceptors"]`. This
+`Capabilities.Extensions["io.modelcontextprotocol/interceptors"]`. This
 follows the same pattern as the variants extension
 (`io.modelcontextprotocol/server-variants`).
 

go/sdk/interceptors/extension/server.go

@@ -15,7 +15,7 @@ import (
 	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/chain"
 )
 
-// extensionID is the capability key used in Capabilities.Experimental
+// extensionID is the capability key used in Capabilities.Extensions
 const extensionID = "io.modelcontextprotocol/interceptors"
 
 // Option configures an Extension.
@@ -163,8 +163,7 @@ func (e *Extension) initMiddleware() mcp.Middleware {
 
 // enrichInitResult injects interceptor capability metadata into the
 // InitializeResult, following the same pattern as the variants extension.
-// The capability is declared under Capabilities.Experimental so it works
-// without upstream go-sdk changes.
+// The capability is declared under Capabilities.Extensions per SEP-2133.
 func (e *Extension) enrichInitResult(result mcp.Result) (mcp.Result, error) {
 	initResult, ok := result.(*mcp.InitializeResult)
 	if !ok {
@@ -174,8 +173,8 @@ func (e *Extension) enrichInitResult(result mcp.Result) (mcp.Result, error) {
 	if initResult.Capabilities == nil {
 		initResult.Capabilities = &mcp.ServerCapabilities{}
 	}
-	if initResult.Capabilities.Experimental == nil {
-		initResult.Capabilities.Experimental = make(map[string]any)
+	if initResult.Capabilities.Extensions == nil {
+		initResult.Capabilities.Extensions = make(map[string]any)
 	}
 
 	all := e.getInterceptors()
@@ -193,7 +192,7 @@ func (e *Extension) enrichInitResult(result mcp.Result) (mcp.Result, error) {
 		events = append(events, ev)
 	}
 
-	initResult.Capabilities.Experimental[extensionID] = map[string]any{
+	initResult.Capabilities.Extensions[extensionID] = map[string]any{
 		"supportedEvents": events,
 	}