fix: support nullable MCP schema types

com.microsoft.copilot.eclipse.core.test/src/com/microsoft/copilot/eclipse/core/lsp/protocol/InputSchemaTests.java

@@ -0,0 +1,84 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+package com.microsoft.copilot.eclipse.core.lsp.protocol;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonSyntaxException;
+
+import com.microsoft.copilot.eclipse.core.lsp.mcp.McpServerToolsCollection;
+
+/**
+ * Tests for input schema deserialization.
+ */
+class InputSchemaTests {
+
+  @Test
+  void testMcpToolInputSchemaAllowsNullableTypeUnion() {
+    String json = """
+        {
+          "name": "nullable-repro",
+          "status": "running",
+          "tools": [
+            {
+              "name": "demo_tool",
+              "description": "Demo tool with one optional nullable parameter.",
+              "_status": "enabled",
+              "inputSchema": {
+                "type": "object",
+                "properties": {
+                  "required_arg": {
+                    "type": "string",
+                    "description": "A required argument."
+                  },
+                  "optional_arg": {
+                    "type": ["string", "null"],
+                    "description": "Optional, nullable."
+                  },
+                  "optional_count": {
+                    "type": ["null", "number"],
+                    "description": "Optional count."
+                  }
+                },
+                "required": ["required_arg"]
+              }
+            }
+          ]
+        }
+        """;
+
+    McpServerToolsCollection server = new Gson().fromJson(json, McpServerToolsCollection.class);
+
+    assertNotNull(server);
+    assertEquals("nullable-repro", server.getName());
+    assertEquals(1, server.getTools().size());
+    InputSchemaPropertyValue optionalArg = server.getTools().get(0).getInputSchema().getProperties()
+        .get("optional_arg");
+    assertNotNull(optionalArg);
+    assertEquals("string", optionalArg.getType());
+    InputSchemaPropertyValue optionalCount = server.getTools().get(0).getInputSchema().getProperties()
+        .get("optional_count");
+    assertNotNull(optionalCount);
+    assertEquals("number", optionalCount.getType());
+  }
+
+  @Test
+  void testInputSchemaAllowsTypeUnion() {
+    InputSchema schema = new Gson().fromJson("{\"type\":[\"object\",\"null\"]}", InputSchema.class);
+
+    assertNotNull(schema);
+    assertEquals("object", schema.getType());
+  }
+
+  @Test
+  void testInputSchemaRejectsMultipleConcreteTypes() {
+    assertThrows(JsonSyntaxException.class,
+        () -> new Gson().fromJson("{\"type\":[\"string\",\"number\"]}", InputSchema.class));
+  }
+}

com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/InputSchema.java

@@ -9,6 +9,7 @@
 import java.util.Map;
 import java.util.Objects;
 
+import com.google.gson.annotations.JsonAdapter;
 import org.apache.commons.lang3.builder.ToStringBuilder;
 
 /**
@@ -17,6 +18,7 @@
  * more information.
  */
 public class InputSchema {
+  @JsonAdapter(JsonSchemaTypeAdapter.class)
   private String type;
   private Map<String, InputSchemaPropertyValue> properties;
   private List<String> required;

com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/InputSchemaPropertyValue.java

@@ -5,12 +5,14 @@
 
 import java.util.Objects;
 
+import com.google.gson.annotations.JsonAdapter;
 import org.apache.commons.lang3.builder.ToStringBuilder;
 
 /**
  * Property value for input schema.
  */
 public class InputSchemaPropertyValue {
+  @JsonAdapter(JsonSchemaTypeAdapter.class)
   private String type;
   private String description;
   private InputSchemaPropertyValue items;

com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/JsonSchemaTypeAdapter.java

@@ -0,0 +1,72 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+package com.microsoft.copilot.eclipse.core.lsp.protocol;
+
+import java.io.IOException;
+
+import com.google.gson.JsonSyntaxException;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
+import com.google.gson.stream.JsonWriter;
+
+/**
+ * Gson adapter for JSON Schema type fields.
+ */
+public class JsonSchemaTypeAdapter extends TypeAdapter<String> {
+
+  @Override
+  public String read(JsonReader in) throws IOException {
+    JsonToken token = in.peek();
+    if (token == JsonToken.NULL) {
+      in.nextNull();
+      return null;
+    }
+    if (token == JsonToken.STRING) {
+      return in.nextString();
+    }
+    if (token == JsonToken.BOOLEAN) {
+      return Boolean.toString(in.nextBoolean());
+    }
+    if (token == JsonToken.BEGIN_ARRAY) {
+      return readTypeArray(in);
+    }
+
+    return in.nextString();
+  }
+
+  @Override
+  public void write(JsonWriter out, String value) throws IOException {
+    if (value == null) {
+      out.nullValue();
+    } else {
+      out.value(value);
+    }
+  }
+
+  private String readTypeArray(JsonReader in) throws IOException {
+    String concreteType = null;
+    boolean nullTypePresent = false;
+
+    in.beginArray();
+    while (in.hasNext()) {
+      JsonToken token = in.peek();
+      if (token == JsonToken.STRING) {
+        String type = in.nextString();
+        if ("null".equals(type)) {
+          nullTypePresent = true;
+        } else if (concreteType == null) {
+          concreteType = type;
+        } else if (!concreteType.equals(type)) {
+          throw new JsonSyntaxException("Multiple non-null JSON Schema types are not supported");
+        }
+      } else {
+        in.skipValue();
+      }
+    }
+    in.endArray();
+
+    return concreteType != null ? concreteType : nullTypePresent ? "null" : null;
+  }
+}