Make McpTransportContext.metadata immutable (#1076)

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

mcp-core/src/main/java/io/modelcontextprotocol/common/DefaultMcpTransportContext.java

@@ -20,7 +20,7 @@ class DefaultMcpTransportContext implements McpTransportContext {
 
 	DefaultMcpTransportContext(Map<String, Object> metadata) {
 		Assert.notNull(metadata, "The metadata cannot be null");
-		this.metadata = metadata;
+		this.metadata = Map.copyOf(metadata);
 	}
 
 	@Override

mcp-core/src/test/java/io/modelcontextprotocol/common/McpTransportContextTests.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.common;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link McpTransportContext#create(Map)}, which is documented to return an
+ * unmodifiable context.
+ */
+class McpTransportContextTests {
+
+	@Test
+	void createdContextShouldNotSeeLaterWritesToTheSourceMap() {
+		Map<String, Object> metadata = new HashMap<>();
+		metadata.put("tenant", "acme");
+
+		McpTransportContext context = McpTransportContext.create(metadata);
+		metadata.put("tenant", "other");
+
+		assertThat(context.get("tenant")).isEqualTo("acme");
+	}
+
+	@Test
+	void createdContextShouldNotSeeLaterAdditionsToTheSourceMap() {
+		Map<String, Object> metadata = new HashMap<>();
+		metadata.put("tenant", "acme");
+
+		McpTransportContext context = McpTransportContext.create(metadata);
+		metadata.put("added-after-the-fact", "surprise");
+
+		assertThat(context.get("added-after-the-fact")).isNull();
+	}
+
+	@Test
+	void createdContextShouldNotBeEmptiedByClearingTheSourceMap() {
+		Map<String, Object> metadata = new HashMap<>();
+		metadata.put("tenant", "acme");
+
+		McpTransportContext context = McpTransportContext.create(metadata);
+		metadata.clear();
+
+		assertThat(context.get("tenant")).isEqualTo("acme");
+	}
+
+	@Test
+	void createdContextShouldRemainUsableAsAMapKey() {
+		Map<String, Object> metadata = new HashMap<>();
+		metadata.put("tenant", "acme");
+		McpTransportContext context = McpTransportContext.create(metadata);
+
+		Map<McpTransportContext, String> byContext = new HashMap<>();
+		byContext.put(context, "value");
+		metadata.put("tenant", "other");
+
+		assertThat(byContext.get(context)).isEqualTo("value");
+	}
+
+	@Test
+	void twoContextsCreatedFromEqualMapsShouldStayEqual() {
+		Map<String, Object> first = new HashMap<>();
+		first.put("tenant", "acme");
+		Map<String, Object> second = new HashMap<>();
+		second.put("tenant", "acme");
+
+		McpTransportContext firstContext = McpTransportContext.create(first);
+		McpTransportContext secondContext = McpTransportContext.create(second);
+		assertThat(firstContext).isEqualTo(secondContext);
+
+		first.put("tenant", "other");
+
+		assertThat(firstContext).isEqualTo(secondContext);
+	}
+
+	@Test
+	void createdContextFromAnImmutableMapIsAlreadyCorrect() {
+		McpTransportContext context = McpTransportContext.create(Map.of("tenant", "acme"));
+
+		assertThat(context.get("tenant")).isEqualTo("acme");
+	}
+
+}