Respond with the latest supported protocol version when initialize requests an unknown one

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

@@ -296,7 +296,11 @@ private JsonRpcResponse handleInitialize(JsonRpcRequest req) {
         String pv = null;
         if (maybeVersion != null) {
             var protocolVersion = ProtocolVersion.version(maybeVersion.asString());
-            if (!(protocolVersion instanceof ProtocolVersion.UnknownVersion)) {
+            if (protocolVersion instanceof ProtocolVersion.UnknownVersion) {
+                // Per the MCP spec, a server that does not support the requested protocol
+                // version must respond with the latest version it supports.
+                pv = ProtocolVersion.latestVersion().identifier();
+            } else {
                 pv = protocolVersion.identifier();
             }
         }

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

@@ -5,6 +5,8 @@
 
 package software.amazon.smithy.java.mcp.server;
 
+import java.util.Collections;
+import java.util.List;
 import software.amazon.smithy.utils.SmithyUnstableApi;
 
 @SmithyUnstableApi
@@ -49,6 +51,20 @@ private UnknownVersion(String identifier) {
         }
     }
 
+    /**
+     * Holder defers initialization until first use so the version subclasses are fully loaded
+     * first — a plain static field on this class would read a still-null INSTANCE whenever a
+     * subclass is the first member of the hierarchy to be initialized.
+     */
+    private static final class SupportedVersions {
+        private static final List<ProtocolVersion> ALL = List.of(
+                v2024_11_05.INSTANCE,
+                v2025_03_26.INSTANCE,
+                v2025_06_18.INSTANCE,
+                v2025_11_25.INSTANCE);
+        private static final ProtocolVersion LATEST = Collections.max(ALL);
+    }
+
     private final String identifier;
 
     private ProtocolVersion(String identifier) {
@@ -72,17 +88,26 @@ public final int compareTo(ProtocolVersion o) {
     }
 
     public static ProtocolVersion version(String identifier) {
-        return switch (identifier) {
-            case null -> v2025_03_26.INSTANCE;
-            case "2024-11-05" -> v2024_11_05.INSTANCE;
-            case "2025-03-26" -> v2025_03_26.INSTANCE;
-            case "2025-06-18" -> v2025_06_18.INSTANCE;
-            case "2025-11-25" -> v2025_11_25.INSTANCE;
-            default -> new UnknownVersion(identifier);
-        };
+        if (identifier == null) {
+            return defaultVersion();
+        }
+        for (var version : SupportedVersions.ALL) {
+            if (version.identifier.equals(identifier)) {
+                return version;
+            }
+        }
+        return new UnknownVersion(identifier);
     }
 
     public static ProtocolVersion defaultVersion() {
         return v2025_03_26.INSTANCE;
     }
+
+    /**
+     * The most recent protocol version this server supports, derived from the supported-version
+     * registry.
+     */
+    public static ProtocolVersion latestVersion() {
+        return SupportedVersions.LATEST;
+    }
 }

mcp/mcp-server/src/test/java/software/amazon/smithy/java/mcp/server/McpServerTest.java

@@ -154,6 +154,29 @@ public void initializeWithV2025_11_25ProtocolVersion() {
         assertTrue(outputSchema.get("properties").asStringMap().containsKey("outputStr"));
     }
 
+    @Test
+    public void initializeWithUnknownProtocolVersionAnswersLatestSupported() {
+        server = McpServer.builder()
+                .name("smithy-mcp-server")
+                .input(input)
+                .output(output)
+                .addService("test-mcp",
+                        ProxyService.builder()
+                                .service(ShapeId.from("smithy.test#TestService"))
+                                .proxyEndpoint("http://localhost")
+                                .model(MODEL)
+                                .build())
+                .build();
+
+        server.start();
+
+        // Per the MCP spec, a server answers a request for a version it does not support with
+        // the latest version it supports — not the oldest.
+        write("initialize", Document.of(Map.of("protocolVersion", Document.of("2026-07-28"))));
+        var pv = read().getResult().getMember("protocolVersion").asString();
+        assertEquals("2025-11-25", pv);
+    }
+
     @Test
     public void noOutputSchemaWithUnsupportedProtocolVersion() {
         server = McpServer.builder()

mcp/mcp-server/src/test/java/software/amazon/smithy/java/mcp/server/ProtocolVersionTest.java

@@ -39,6 +39,12 @@ void defaultVersionIs2025_03_26() {
         assertEquals("2025-03-26", ProtocolVersion.defaultVersion().identifier());
     }
 
+    @Test
+    void latestVersionIs2025_11_25() {
+        assertInstanceOf(ProtocolVersion.v2025_11_25.class, ProtocolVersion.latestVersion());
+        assertEquals("2025-11-25", ProtocolVersion.latestVersion().identifier());
+    }
+
     @Test
     void compareToOrdersChronologically() {
         assertTrue(ProtocolVersion.v2024_11_05.INSTANCE.compareTo(ProtocolVersion.v2025_03_26.INSTANCE) < 0);