diff --git a/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs b/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
index b88cfb762..677606eb6 100644
--- a/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
+++ b/src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs
@@ -6,7 +6,7 @@ namespace ModelContextProtocol.AspNetCore;
 /// <summary>
 /// Configuration options for <see cref="M:McpEndpointRouteBuilderExtensions.MapMcp"/>.
 /// which implements the Streaming HTTP transport for the Model Context Protocol.
-/// See the protocol specification for details on the Streamable HTTP transport. <see href="https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http"/>
+/// See the protocol specification for details on the Streamable HTTP transport. <see href="https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#streamable-http"/>
 /// </summary>
 public class HttpServerTransportOptions
 {
diff --git a/src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs b/src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs
index a65994c44..e7c5f97c5 100644
--- a/src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs
+++ b/src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs
@@ -15,7 +15,7 @@ public static class McpEndpointRouteBuilderExtensions
 {
     /// <summary>
     /// Sets up endpoints for handling MCP Streamable HTTP transport.
-    /// See <see href="https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http">the 2025-03-26 protocol specification</see> for details about the Streamable HTTP transport.
+    /// See <see href="https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#streamable-http">the 2025-06-18 protocol specification</see> for details about the Streamable HTTP transport.
     /// Also maps legacy SSE endpoints for backward compatibility at the path "/sse" and "/message". <see href="https://modelcontextprotocol.io/specification/2024-11-05/basic/transports#http-with-sse">the 2024-11-05 protocol specification</see> for details about the HTTP with SSE transport.
     /// </summary>
     /// <param name="endpoints">The web application to attach MCP HTTP endpoints.</param>
diff --git a/src/ModelContextProtocol.Core/Client/SseClientTransportOptions.cs b/src/ModelContextProtocol.Core/Client/SseClientTransportOptions.cs
index cd522c42d..9b4af6db5 100644
--- a/src/ModelContextProtocol.Core/Client/SseClientTransportOptions.cs
+++ b/src/ModelContextProtocol.Core/Client/SseClientTransportOptions.cs
@@ -39,7 +39,7 @@ public required Uri Endpoint
     /// Streamable HTTP transport and automatically fall back to SSE transport if the server doesn't support it.
     /// </para>
     /// <para>
-    /// <see href="https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http">Streamable HTTP transport specification</see>.
+    /// <see href="https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#streamable-http">Streamable HTTP transport specification</see>.
     /// <see href="https://modelcontextprotocol.io/specification/2024-11-05/basic/transports#http-with-sse">HTTP with SSE transport specification</see>.
     /// </para>
     /// </remarks>
diff --git a/src/ModelContextProtocol.Core/McpSession.cs b/src/ModelContextProtocol.Core/McpSession.cs
index 225971fa6..47c4d212b 100644
--- a/src/ModelContextProtocol.Core/McpSession.cs
+++ b/src/ModelContextProtocol.Core/McpSession.cs
@@ -29,12 +29,13 @@ internal sealed partial class McpSession : IDisposable
         "mcp.server.operation.duration", "Measures the duration of inbound message processing.", longBuckets: false);
 
     /// <summary>The latest version of the protocol supported by this implementation.</summary>
-    internal const string LatestProtocolVersion = "2025-03-26";
+    internal const string LatestProtocolVersion = "2025-06-18";
 
     /// <summary>All protocol versions supported by this implementation.</summary>
     internal static readonly string[] SupportedProtocolVersions =
     [
         "2024-11-05",
+        "2025-03-26",
         LatestProtocolVersion,
     ];
 
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs
index b1c1a89da..f14cc10a6 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs
+++ b/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs
@@ -171,7 +171,10 @@ public async Task StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitia
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        await using var mcpClient = await ConnectAsync();
+        await using var mcpClient = await ConnectAsync(clientOptions: new()
+        {
+            ProtocolVersion = "2025-03-26",
+        });
         await mcpClient.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
 
         // The header should be included in the GET request, the initialized notification, and the tools/list call.