diff --git a/anthropic-java-mcp/src/main/kotlin/com/anthropic/mcp/BetaMcp.kt b/anthropic-java-mcp/src/main/kotlin/com/anthropic/mcp/BetaMcp.kt
index adaa1220f..a3ee73b5d 100644
--- a/anthropic-java-mcp/src/main/kotlin/com/anthropic/mcp/BetaMcp.kt
+++ b/anthropic-java-mcp/src/main/kotlin/com/anthropic/mcp/BetaMcp.kt
@@ -394,6 +394,9 @@ object BetaMcp {
     private fun betaInputSchema(schema: McpSchema.JsonSchema): BetaTool.InputSchema {
         val builder = BetaTool.InputSchema.builder()
         schema.required()?.let { builder.required(it) }
+        schema.additionalProperties()?.let {
+            builder.putAdditionalProperty("additionalProperties", JsonValue.from(it))
+        }
         schema.properties()?.let { builder.putAdditionalProperty("properties", JsonValue.from(it)) }
         schema.defs()?.let { builder.putAdditionalProperty("\$defs", JsonValue.from(it)) }
         schema.definitions()?.let {
diff --git a/anthropic-java-mcp/src/test/kotlin/com/anthropic/mcp/BetaMcpAdditionalPropertiesTest.kt b/anthropic-java-mcp/src/test/kotlin/com/anthropic/mcp/BetaMcpAdditionalPropertiesTest.kt
new file mode 100644
index 000000000..058ed8b0f
--- /dev/null
+++ b/anthropic-java-mcp/src/test/kotlin/com/anthropic/mcp/BetaMcpAdditionalPropertiesTest.kt
@@ -0,0 +1,82 @@
+package com.anthropic.mcp
+
+import com.fasterxml.jackson.databind.ObjectMapper
+import io.modelcontextprotocol.client.McpSyncClient
+import io.modelcontextprotocol.spec.McpSchema
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.ValueSource
+import org.mockito.kotlin.mock
+
+internal class BetaMcpAdditionalPropertiesTest {
+    private val client: McpSyncClient = mock()
+    private val mapper = ObjectMapper()
+
+    @ParameterizedTest
+    @ValueSource(booleans = [false, true])
+    fun preservesExplicitAdditionalProperties(allowed: Boolean) {
+        val schema =
+            McpSchema.JsonSchema(
+                "object",
+                mapOf("name" to mapOf("type" to "string")),
+                listOf("name"),
+                allowed,
+                null,
+                null,
+            )
+        val before = mapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(schema)
+        val tool = McpSchema.Tool("lookup", null, "Look up a name", schema, null, null, null)
+        val definition = BetaMcp.mcpTool(tool, client).definition
+        val wire = mapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(definition)
+        assertThat(wire.path("input_schema").has("additionalProperties")).isTrue()
+        assertThat(wire.path("input_schema").path("additionalProperties").booleanValue())
+            .isEqualTo(allowed)
+        assertThat(wire.path("input_schema").path("required").get(0).textValue()).isEqualTo("name")
+        assertThat(
+                wire.path("input_schema").path("properties").path("name").path("type").textValue()
+            )
+            .isEqualTo("string")
+        assertThat(mapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(schema))
+            .isEqualTo(before)
+    }
+
+    @Test
+    fun omittedAdditionalPropertiesRemainsAbsent() {
+        val schema = McpSchema.JsonSchema("object", emptyMap(), emptyList(), null, null, null)
+        val tool = McpSchema.Tool("lookup", null, null, schema, null, null, null)
+        val wire =
+            mapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(
+                BetaMcp.mcpTool(tool, client).definition
+            )
+        assertThat(wire.path("input_schema").has("additionalProperties")).isFalse()
+        assertThat(wire.path("input_schema").path("type").textValue()).isEqualTo("object")
+    }
+
+    @Test
+    fun closedSchemaAndDefinitionsSurviveListConversion() {
+        val defs = mapOf<String, Any>("Name" to mapOf("type" to "string"))
+        val schema =
+            McpSchema.JsonSchema(
+                "object",
+                mapOf("name" to mapOf("$" + "ref" to "#/$" + "defs/Name")),
+                listOf("name"),
+                false,
+                defs,
+                defs,
+            )
+        val tool = McpSchema.Tool("lookup", null, null, schema, null, null, null)
+        val converted = BetaMcp.mcpTools(listOf(tool), client)
+        assertThat(converted).hasSize(1)
+        val wire =
+            mapper
+                .valueToTree<com.fasterxml.jackson.databind.JsonNode>(converted[0].definition)
+                .path("input_schema")
+        assertThat(wire.has("additionalProperties")).isTrue()
+        assertThat(wire.path("additionalProperties").booleanValue()).isFalse()
+        assertThat(wire.path("$" + "defs").path("Name").path("type").textValue())
+            .isEqualTo("string")
+        assertThat(wire.path("definitions").path("Name").path("type").textValue())
+            .isEqualTo("string")
+    }
+}