Fix blob serialization in MCP output adaptation

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

@@ -1308,15 +1308,20 @@ private static Document adaptTimestamp(Document doc) {
         return Document.of(EPOCH_SECONDS.readFromNumber(doc.asNumber()));
     }
 
-    private Document adaptOutputDocument(Document doc, Schema schema) {
+    Document adaptOutputDocument(Document doc, Schema schema) {
         if (doc == null) {
             return null;
         }
         var toType = schema.type();
         return switch (toType) {
             case BIG_DECIMAL -> Document.of(doc.asBigDecimal().toString());
             case BIG_INTEGER -> Document.of(doc.asBigInteger().toString());
-            case BLOB -> Document.of(Base64.getEncoder().encodeToString(ByteBufferUtils.getBytes(doc.asBlob())));
+            case BLOB -> switch (doc.type()) {
+                case STRING -> Document.of(doc.asString());
+                case BLOB -> Document.of(
+                        Base64.getEncoder().encodeToString(ByteBufferUtils.getBytes(doc.asBlob())));
+                default -> badType(doc.type(), toType);
+            };
             // Use adaptTimestamp() instead of asTimestamp() because oneOf union members are
             // deserialized as untyped Documents (no schema available). Timestamps in these
             // documents remain as strings or numbers rather than being converted to Timestamp Documents.

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

@@ -0,0 +1,93 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.mcp.server;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Base64;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import software.amazon.smithy.java.core.schema.Schema;
+import software.amazon.smithy.java.core.serde.document.Document;
+import software.amazon.smithy.model.shapes.ShapeId;
+
+class McpServiceTest {
+
+    private McpService service;
+
+    @BeforeEach
+    void setUp() {
+        service = new McpService(
+                Collections.emptyMap(),
+                List.of(),
+                "test",
+                "1.0",
+                null,
+                null,
+                null
+        );
+    }
+
+    @Test
+    void testAdaptOutputBlobFromBinaryDocument() {
+        var schema = Schema.createBlob(ShapeId.from("smithy.test#Blob"));
+        var bytes = "Hello, World!".getBytes(StandardCharsets.UTF_8);
+
+        var result = service.adaptOutputDocument(Document.of(bytes), schema);
+
+        assertEquals(Base64.getEncoder().encodeToString(bytes), result.asString());
+    }
+
+    @Test
+    void testAdaptOutputBlobFromStringDocument() {
+        var schema = Schema.createBlob(ShapeId.from("smithy.test#Blob"));
+        var base64 = Base64.getEncoder().encodeToString("payload".getBytes(StandardCharsets.UTF_8));
+
+        var result = service.adaptOutputDocument(Document.of(base64), schema);
+
+        assertEquals(base64, result.asString());
+    }
+
+    @Test
+    void testAdaptOutputBlobFromUnsupportedDocumentTypeThrows() {
+        var schema = Schema.createBlob(ShapeId.from("smithy.test#Blob"));
+
+        assertThrows(Exception.class, () -> service.adaptOutputDocument(Document.of(42), schema));
+    }
+
+    @Test
+    void testAdaptOutputNullReturnsNull() {
+        var schema = Schema.createBlob(ShapeId.from("smithy.test#Blob"));
+
+        assertNull(service.adaptOutputDocument(null, schema));
+    }
+
+    @Test
+    void testAdaptOutputStructureWithBlobFields() {
+        var blobSchema = Schema.createBlob(ShapeId.from("smithy.test#Blob"));
+        var structSchema = Schema.structureBuilder(ShapeId.from("smithy.test#Output"))
+                .putMember("binaryBlob", blobSchema)
+                .putMember("stringBlob", blobSchema)
+                .build();
+
+        var bytes = "binary content".getBytes(StandardCharsets.UTF_8);
+        var base64 = Base64.getEncoder().encodeToString("string content".getBytes(StandardCharsets.UTF_8));
+        var doc = Document.of(Map.of(
+                "binaryBlob", Document.of(bytes),
+                "stringBlob", Document.of(base64)));
+
+        var result = service.adaptOutputDocument(doc, structSchema);
+
+        assertEquals(Base64.getEncoder().encodeToString(bytes), result.getMember("binaryBlob").asString());
+        assertEquals(base64, result.getMember("stringBlob").asString());
+    }
+}