fix: serialize ElicitRequest mode discriminator only once

ElicitRequest declares its subtype discriminator with
@JsonTypeInfo(..., property = "mode"), so Jackson already writes the mode
property during serialization. ElicitFormRequest and ElicitUrlRequest also
annotated their mode() accessor with @JsonProperty("mode"), so mode was written
twice, producing a duplicate JSON key.

Mark the overridden mode() accessors @JsonIgnore, mirroring the existing fix on
Content.type(). Deserialization is unaffected (it uses the discriminator and the
@JsonCreator factories). Adds ElicitRequestJsonTests, mirroring ContentJsonTests.

Author: aditya-786

mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java

@@ -4805,7 +4805,7 @@ public record ElicitFormRequest( // @formatter:off
 		}
 
 		@Override
-		@JsonProperty("mode")
+		@JsonIgnore
 		public String mode() {
 			return MODE;
 		}
@@ -4931,7 +4931,7 @@ public record ElicitUrlRequest( // @formatter:off
 		}
 
 		@Override
-		@JsonProperty("mode")
+		@JsonIgnore
 		public String mode() {
 			return MODE;
 		}

mcp-test/src/test/java/io/modelcontextprotocol/spec/ElicitRequestJsonTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2026 - 2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.spec;
+
+import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.IOException;
+import java.util.Map;
+
+import io.modelcontextprotocol.json.McpJsonMapper;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Verifies that every {@link McpSchema.ElicitRequest} subtype serializes with exactly one
+ * {@code mode} property (regression guard for the {@code @JsonIgnore} on the overridden
+ * {@code mode()} accessors, mirroring {@link ContentJsonTests}).
+ */
+class ElicitRequestJsonTests {
+
+	private final McpJsonMapper mapper = JSON_MAPPER;
+
+	@Test
+	void formRequestHasExactlyOneModeProperty() throws IOException {
+		McpSchema.ElicitFormRequest request = McpSchema.ElicitFormRequest.builder("msg", Map.of("type", "object"))
+			.build();
+		String json = mapper.writeValueAsString(request);
+
+		assertExactlyOneModeProperty(json);
+		assertThatJson(json).node("mode").isEqualTo("form");
+	}
+
+	@Test
+	void urlRequestHasExactlyOneModeProperty() throws IOException {
+		McpSchema.ElicitUrlRequest request = McpSchema.ElicitUrlRequest.builder("msg", "https://example.com", "eid")
+			.build();
+		String json = mapper.writeValueAsString(request);
+
+		assertExactlyOneModeProperty(json);
+		assertThatJson(json).node("mode").isEqualTo("url");
+	}
+
+	@Test
+	void formRequestRoundTrip() throws IOException {
+		McpSchema.ElicitFormRequest original = McpSchema.ElicitFormRequest.builder("msg", Map.of("type", "object"))
+			.build();
+		String json = mapper.writeValueAsString(original);
+
+		McpSchema.ElicitRequest decoded = mapper.readValue(json, McpSchema.ElicitRequest.class);
+		assertThat(decoded).isInstanceOf(McpSchema.ElicitFormRequest.class);
+		assertThat(decoded.mode()).isEqualTo("form");
+	}
+
+	@Test
+	void urlRequestRoundTrip() throws IOException {
+		McpSchema.ElicitUrlRequest original = McpSchema.ElicitUrlRequest.builder("msg", "https://example.com", "eid")
+			.build();
+		String json = mapper.writeValueAsString(original);
+
+		McpSchema.ElicitRequest decoded = mapper.readValue(json, McpSchema.ElicitRequest.class);
+		assertThat(decoded).isInstanceOf(McpSchema.ElicitUrlRequest.class);
+		assertThat(decoded.mode()).isEqualTo("url");
+	}
+
+	private static void assertExactlyOneModeProperty(String json) {
+		long count = java.util.Arrays.stream(json.split("\"mode\"")).count() - 1;
+		assertThat(count).as("'mode' property must appear exactly once in: %s", json).isEqualTo(1);
+	}
+
+}