feat(mcp): Add McpServerInterceptor with scoped hooks

Add hook-based server interceptor for McpService, following the
established ClientInterceptor pattern. Interceptors observe or modify
requests at specific pipeline stages without exposing async dispatch
internals.

Hooks: readBefore/modifyBefore Execution and ToolCall,
readAfter/modifyAfter Execution and ToolCall.

Also makes McpServerProxy's rpc(), start(), and shutdown() protected
to allow custom proxy transport implementations.

Author: Clay Gifford

mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpExecutionHook.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.mcp.server;
+
+import software.amazon.smithy.java.context.Context;
+import software.amazon.smithy.java.mcp.model.JsonRpcRequest;
+import software.amazon.smithy.utils.SmithyUnstableApi;
+
+/**
+ * Hook data available at the execution level. Passed to execution-scoped hooks in
+ * {@link McpServerInterceptor}.
+ *
+ * <p>The {@link #context()} provides a per-request key-value store for passing state
+ * between hooks. For example, a telemetry interceptor can stash a start timestamp in
+ * {@code readBeforeExecution} and retrieve it in {@code readAfterExecution}.
+ */
+@SmithyUnstableApi
+public class McpExecutionHook {
+
+    private final JsonRpcRequest request;
+    private final ProtocolVersion protocolVersion;
+    private final Context context;
+
+    McpExecutionHook(JsonRpcRequest request, ProtocolVersion protocolVersion, Context context) {
+        this.request = request;
+        this.protocolVersion = protocolVersion;
+        this.context = context;
+    }
+
+    /**
+     * The JSON-RPC request being handled.
+     */
+    public JsonRpcRequest request() {
+        return request;
+    }
+
+    /**
+     * Returns a new hook with the given request, or the same hook if unchanged.
+     */
+    public McpExecutionHook withRequest(JsonRpcRequest request) {
+        return this.request == request ? this : new McpExecutionHook(request, protocolVersion, context);
+    }
+
+    /**
+     * The MCP protocol version for this request.
+     */
+    public ProtocolVersion protocolVersion() {
+        return protocolVersion;
+    }
+
+    /**
+     * Per-request context for passing state between hooks.
+     */
+    public Context context() {
+        return context;
+    }
+}

mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServerBuilder.java

@@ -24,6 +24,7 @@ public final class McpServerBuilder {
     OutputStream os;
     Map<String, Service> services = new HashMap<>();
     List<McpServerProxy> proxyList = new ArrayList<>();
+    McpServerInterceptor interceptor;
     String name;
     String version;
     ToolFilter toolFilter = (server, tool) -> true;
@@ -72,6 +73,10 @@ public Server build() {
             builder.version(version);
         }
 
+        if (interceptor != null) {
+            builder.interceptor(interceptor);
+        }
+
         this.mcpService = builder.build();
         return new McpServer(this);
     }
@@ -101,6 +106,17 @@ public McpServerBuilder metricsObserver(McpMetricsObserver observer) {
         return this;
     }
 
+    /**
+     * Sets the server interceptor. Use {@link McpServerInterceptor#chain(List)} to compose
+     * multiple interceptors into one.
+     *
+     * @see McpServerInterceptor for hook descriptions and the execution lifecycle
+     */
+    public McpServerBuilder interceptor(McpServerInterceptor interceptor) {
+        this.interceptor = Objects.requireNonNull(interceptor, "interceptor");
+        return this;
+    }
+
     private void validate() {
         Objects.requireNonNull(is, "MCP server input stream is required");
         Objects.requireNonNull(os, "MCP server output stream is required");

mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServerInterceptor.java

@@ -0,0 +1,217 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.mcp.server;
+
+import java.util.List;
+import software.amazon.smithy.java.mcp.model.JsonRpcRequest;
+import software.amazon.smithy.java.mcp.model.JsonRpcResponse;
+import software.amazon.smithy.utils.SmithyUnstableApi;
+
+/**
+ * Interceptor for MCP server request processing. Interceptors inject code into the
+ * {@link McpService} request execution pipeline via hooks at specific stages.
+ *
+ * <p>Hooks are either "read" hooks (observe in-flight data) or "modify" hooks (transform
+ * in-flight data). All hooks have default no-op implementations; override only the hooks
+ * you need.
+ *
+ * <h2>Execution lifecycle</h2>
+ *
+ * <p>For every request:
+ * <ol>
+ *   <li>{@link #readBeforeExecution} — observe the incoming request</li>
+ *   <li>{@link #modifyBeforeExecution} — optionally transform the request</li>
+ *   <li>For {@code tools/call} requests only:
+ *     <ol>
+ *       <li>{@link #readBeforeToolCall} — observe before tool dispatch</li>
+ *       <li>{@link #modifyBeforeToolCall} — optionally transform the request</li>
+ *       <li>Tool dispatch (local or proxy)</li>
+ *       <li>{@link #readAfterToolCall} — observe the tool result</li>
+ *       <li>{@link #modifyAfterToolCall} — optionally transform the response</li>
+ *     </ol>
+ *   </li>
+ *   <li>{@link #readAfterExecution} — observe the final result (ALWAYS fires)</li>
+ *   <li>{@link #modifyAfterExecution} — optionally transform the final response</li>
+ * </ol>
+ *
+ * <h2>Error handling</h2>
+ *
+ * <p>Any hook may throw a {@link RuntimeException}. When a hook throws, remaining hooks
+ * in that stage are skipped, and execution jumps to the after-execution hooks with the
+ * error. The {@code readAfterExecution} and {@code modifyAfterExecution} hooks ALWAYS fire,
+ * ensuring cleanup and telemetry logic runs regardless of errors.
+ *
+ * <h2>Async tool calls</h2>
+ *
+ * <p>For proxy tool calls, the after-tool-call and after-execution hooks fire on the
+ * thread that receives the proxy response, not the original request thread. Hook
+ * implementations must be thread-safe.
+ *
+ * <h2>Example: telemetry</h2>
+ * <pre>{@code
+ * public class TelemetryInterceptor implements McpServerInterceptor {
+ *     private static final Context.Key<Long> START = Context.key("start");
+ *
+ *     @Override
+ *     public void readBeforeExecution(McpExecutionHook hook) {
+ *         hook.context().put(START, System.nanoTime());
+ *     }
+ *
+ *     @Override
+ *     public void readAfterExecution(McpExecutionHook hook,
+ *             JsonRpcResponse response, RuntimeException error) {
+ *         long duration = System.nanoTime() - hook.context().get(START);
+ *         emitMetrics(hook.request().getMethod(), duration, error == null);
+ *     }
+ * }
+ * }</pre>
+ *
+ * <h2>Example: access control</h2>
+ * <pre>{@code
+ * public class AccessControlInterceptor implements McpServerInterceptor {
+ *     @Override
+ *     public void readBeforeToolCall(McpToolCallHook hook) {
+ *         if (isBlocked(hook.toolName(), hook.serverId())) {
+ *             throw new RuntimeException("Access denied: " + hook.toolName());
+ *         }
+ *     }
+ * }
+ * }</pre>
+ */
+@SmithyUnstableApi
+public interface McpServerInterceptor {
+
+    /**
+     * An interceptor that does nothing.
+     */
+    McpServerInterceptor NOOP = new McpServerInterceptor() {};
+
+    /**
+     * Combines multiple interceptors into a single interceptor that invokes each one
+     * in order. Hooks are called sequentially on each interceptor in list order.
+     *
+     * @param interceptors The interceptors to compose.
+     * @return A single interceptor that delegates to all provided interceptors.
+     */
+    static McpServerInterceptor chain(List<McpServerInterceptor> interceptors) {
+        return switch (interceptors.size()) {
+            case 0 -> NOOP;
+            case 1 -> interceptors.get(0);
+            default -> new McpServerInterceptorChain(List.copyOf(interceptors));
+        };
+    }
+
+    /**
+     * Combines multiple interceptors into a single interceptor that invokes each one
+     * in order. Convenience overload of {@link #chain(List)}.
+     *
+     * @param interceptors The interceptors to compose.
+     * @return A single interceptor that delegates to all provided interceptors.
+     */
+    static McpServerInterceptor chain(McpServerInterceptor... interceptors) {
+        return chain(List.of(interceptors));
+    }
+
+    // --- Execution-level hooks (fire for all requests) ---
+
+    /**
+     * Called when a request is received, before any dispatch logic.
+     *
+     * @param hook Execution hook data containing the request, protocol version, and context.
+     */
+    default void readBeforeExecution(McpExecutionHook hook) {}
+
+    /**
+     * Called before dispatch. Can return a modified request.
+     *
+     * @param hook Execution hook data.
+     * @return The request to dispatch, or {@code hook.request()} to pass through unmodified.
+     */
+    default JsonRpcRequest modifyBeforeExecution(McpExecutionHook hook) {
+        return hook.request();
+    }
+
+    /**
+     * Called when execution completes. ALWAYS fires, even if an earlier hook threw.
+     *
+     * @param hook Execution hook data.
+     * @param response The response, or {@code null} for notifications and async proxy calls
+     *        still in flight.
+     * @param error The error if one occurred, or {@code null} on success.
+     */
+    default void readAfterExecution(McpExecutionHook hook, JsonRpcResponse response, RuntimeException error) {}
+
+    /**
+     * Called when execution completes. Can modify the response or handle errors.
+     * ALWAYS fires, even if an earlier hook threw.
+     *
+     * @param hook Execution hook data.
+     * @param response The response, or {@code null} for notifications.
+     * @param error The error if one occurred, or {@code null} on success.
+     * @return The final response.
+     * @throws RuntimeException to propagate or replace the error.
+     */
+    default JsonRpcResponse modifyAfterExecution(
+            McpExecutionHook hook,
+            JsonRpcResponse response,
+            RuntimeException error
+    ) {
+        if (error != null) {
+            throw error;
+        }
+        return response;
+    }
+
+    // --- Tool-level hooks (fire only for tools/call) ---
+
+    /**
+     * Called before a tool is invoked.
+     *
+     * @param hook Tool call hook data containing tool name, server ID, and proxy status.
+     */
+    default void readBeforeToolCall(McpToolCallHook hook) {}
+
+    /**
+     * Called before a tool is invoked. Can return a modified request.
+     *
+     * @param hook Tool call hook data.
+     * @return The request to use for tool invocation, or {@code hook.request()} to pass
+     *         through unmodified.
+     */
+    default JsonRpcRequest modifyBeforeToolCall(McpToolCallHook hook) {
+        return hook.request();
+    }
+
+    /**
+     * Called after a tool completes. For proxy tools, this fires on the callback thread.
+     *
+     * @param hook Tool call hook data.
+     * @param response The tool call response.
+     * @param error The error if one occurred, or {@code null} on success.
+     */
+    default void readAfterToolCall(McpToolCallHook hook, JsonRpcResponse response, RuntimeException error) {}
+
+    /**
+     * Called after a tool completes. Can modify the response or handle errors.
+     * For proxy tools, this fires on the callback thread.
+     *
+     * @param hook Tool call hook data.
+     * @param response The tool call response.
+     * @param error The error if one occurred, or {@code null} on success.
+     * @return The response to return.
+     * @throws RuntimeException to propagate or replace the error.
+     */
+    default JsonRpcResponse modifyAfterToolCall(
+            McpToolCallHook hook,
+            JsonRpcResponse response,
+            RuntimeException error
+    ) {
+        if (error != null) {
+            throw error;
+        }
+        return response;
+    }
+}