diff --git a/Package.swift b/Package.swift
index 9aa83bf5..7894a034 100644
--- a/Package.swift
+++ b/Package.swift
@@ -97,6 +97,7 @@ var targets: [Target] = [
     dependencies: [
       "LanguageServerProtocolTransport",
       "SKLogging",
+      "ToolsProtocolsSwiftExtensions",
       "ToolsProtocolsTestSupport",
     ],
     swiftSettings: globalSwiftSettings
diff --git a/Sources/BuildServerProtocol/Messages.swift b/Sources/BuildServerProtocol/Messages.swift
index 91e30710..521160eb 100644
--- a/Sources/BuildServerProtocol/Messages.swift
+++ b/Sources/BuildServerProtocol/Messages.swift
@@ -42,7 +42,22 @@ private let notificationTypes: [NotificationType.Type] = [
 
 extension MessageRegistry {
   public static let bspProtocol: MessageRegistry =
-    MessageRegistry(requests: requestTypes, notifications: notificationTypes)
+    MessageRegistry(requests: requestTypes, notifications: notificationTypes, legacyNames: bspLegacyNames)
+
+  /// Maps current `sourcekit/`-prefixed BSP method names to the legacy names used before the
+  /// prefix migration. Consumed by `MessageRegistry` (incoming routing) and
+  /// `LegacyNameFallbackConnection` (outgoing retries).
+  ///
+  /// This table is frozen. Do not add new entries for newly introduced methods.
+  public static let bspLegacyNames: [String: String] = [
+    BuildTargetPrepareRequest.method: "buildTarget/prepare",
+    FileOptionsChangedNotification.method: "build/sourceKitOptionsChanged",
+    TextDocumentSourceKitOptionsRequest.method: "textDocument/sourceKitOptions",
+    WorkspaceWaitForBuildSystemUpdatesRequest.method: "workspace/waitForBuildSystemUpdates",
+  ]
+  #if compiler(>=6.6)
+  #warning("Remove the legacy method names")
+  #endif
 }
 
 @available(*, deprecated, message: "use MessageRegistry.bspProtocol instead")
diff --git a/Sources/BuildServerProtocol/Messages/BuildTargetPrepareRequest.swift b/Sources/BuildServerProtocol/Messages/BuildTargetPrepareRequest.swift
index 0381859c..91b7ca43 100644
--- a/Sources/BuildServerProtocol/Messages/BuildTargetPrepareRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/BuildTargetPrepareRequest.swift
@@ -26,7 +26,7 @@ public typealias OriginId = String
 /// The server communicates during the initialize handshake whether this method is supported or not by setting
 /// `prepareProvider: true` in `SourceKitInitializeBuildResponseData`.
 public struct BuildTargetPrepareRequest: BSPRequest, Hashable {
-  public static let method: String = "buildTarget/prepare"
+  public static let method: String = "sourcekit/buildTarget/prepare"
   public typealias Response = BuildTargetPrepareResponse
 
   /// A list of build targets to prepare.
diff --git a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
index b100e168..bd964793 100644
--- a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
@@ -82,9 +82,13 @@ public struct BuildClientCapabilities: Codable, Hashable, Sendable {
   /// it's safe to return classpath in ScalacOptionsItem empty. */
   public var jvmCompileClasspathReceiver: Bool?
 
-  public init(languageIds: [Language], jvmCompileClasspathReceiver: Bool? = nil) {
+  /// Experimental client capabilities.
+  public var experimental: LSPAny?
+
+  public init(languageIds: [Language], jvmCompileClasspathReceiver: Bool? = nil, experimental: LSPAny? = nil) {
     self.languageIds = languageIds
     self.jvmCompileClasspathReceiver = jvmCompileClasspathReceiver
+    self.experimental = experimental
   }
 }
 
diff --git a/Sources/BuildServerProtocol/Messages/RegisterForChangeNotifications.swift b/Sources/BuildServerProtocol/Messages/RegisterForChangeNotifications.swift
index f8c4d9cf..60c1bb98 100644
--- a/Sources/BuildServerProtocol/Messages/RegisterForChangeNotifications.swift
+++ b/Sources/BuildServerProtocol/Messages/RegisterForChangeNotifications.swift
@@ -57,7 +57,7 @@ public struct FileOptionsChangedNotification: BSPNotification {
     public var workingDirectory: String?
   }
 
-  public static let method: String = "build/sourceKitOptionsChanged"
+  public static let method: String = "sourcekit/build/sourceKitOptionsChanged"
 
   /// The URI of the document that has changed settings.
   public var uri: URI
diff --git a/Sources/BuildServerProtocol/Messages/TextDocumentSourceKitOptionsRequest.swift b/Sources/BuildServerProtocol/Messages/TextDocumentSourceKitOptionsRequest.swift
index 8cfb7c94..65835539 100644
--- a/Sources/BuildServerProtocol/Messages/TextDocumentSourceKitOptionsRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/TextDocumentSourceKitOptionsRequest.swift
@@ -20,7 +20,7 @@ public import LanguageServerProtocol
 ///
 /// The request may return `nil` if it doesn't have any build settings for this file in the given target.
 public struct TextDocumentSourceKitOptionsRequest: BSPRequest, Hashable {
-  public static let method: String = "textDocument/sourceKitOptions"
+  public static let method: String = "sourcekit/textDocument/sourceKitOptions"
   public typealias Response = TextDocumentSourceKitOptionsResponse?
 
   /// The URI of the document to get options for
diff --git a/Sources/BuildServerProtocol/Messages/WorkspaceWaitForBuildSystemUpdates.swift b/Sources/BuildServerProtocol/Messages/WorkspaceWaitForBuildSystemUpdates.swift
index 06b72f45..2ebcccd3 100644
--- a/Sources/BuildServerProtocol/Messages/WorkspaceWaitForBuildSystemUpdates.swift
+++ b/Sources/BuildServerProtocol/Messages/WorkspaceWaitForBuildSystemUpdates.swift
@@ -19,7 +19,7 @@ public import LanguageServerProtocol
 public struct WorkspaceWaitForBuildSystemUpdatesRequest: BSPRequest, Hashable {
   public typealias Response = VoidResponse
 
-  public static let method: String = "workspace/waitForBuildSystemUpdates"
+  public static let method: String = "sourcekit/workspace/waitForBuildSystemUpdates"
 
   public init() {}
 }
diff --git a/Sources/LanguageServerProtocol/Connection.swift b/Sources/LanguageServerProtocol/Connection.swift
index 8d743eaf..b1e9a60e 100644
--- a/Sources/LanguageServerProtocol/Connection.swift
+++ b/Sources/LanguageServerProtocol/Connection.swift
@@ -28,8 +28,11 @@ public protocol Connection: Sendable {
   /// Send a request with a pre-defined request ID and (asynchronously) receive a reply.
   ///
   /// The request ID must not conflict with any request ID generated by `nextRequestID()`.
+  /// `method` is the JSON-RPC method string; pass `Request.method` for the normal case or a
+  /// legacy method name when talking to an old peer.
   func send<Request: RequestType>(
     _ request: Request,
+    method: String,
     id: RequestID,
     reply: @escaping @Sendable (LSPResult<Request.Response>) -> Void
   )
@@ -42,7 +45,7 @@ extension Connection {
     reply: @escaping @Sendable (LSPResult<Request.Response>) -> Void
   ) -> RequestID {
     let id = nextRequestID()
-    self.send(request, id: id, reply: reply)
+    self.send(request, method: Request.method, id: id, reply: reply)
     return id
   }
 }
diff --git a/Sources/LanguageServerProtocol/MessageRegistry.swift b/Sources/LanguageServerProtocol/MessageRegistry.swift
index 8639aa83..e38b7de7 100644
--- a/Sources/LanguageServerProtocol/MessageRegistry.swift
+++ b/Sources/LanguageServerProtocol/MessageRegistry.swift
@@ -14,9 +14,26 @@ public final class MessageRegistry: Sendable {
   private let methodToRequest: [String: _RequestType.Type]
   private let methodToNotification: [String: NotificationType.Type]
 
-  public init(requests: [_RequestType.Type], notifications: [NotificationType.Type]) {
-    self.methodToRequest = Dictionary(uniqueKeysWithValues: requests.map { ($0.method, $0) })
-    self.methodToNotification = Dictionary(uniqueKeysWithValues: notifications.map { ($0.method, $0) })
+  public init(
+    requests: [_RequestType.Type],
+    notifications: [NotificationType.Type],
+    legacyNames: [String: String] = [:]
+  ) {
+    var methodToRequest: [String: _RequestType.Type] = Dictionary(
+      uniqueKeysWithValues: requests.map { ($0.method, $0) }
+    )
+    for request in requests {
+      if let legacy = legacyNames[request.method] { methodToRequest[legacy] = request }
+    }
+    self.methodToRequest = methodToRequest
+
+    var methodToNotification: [String: NotificationType.Type] = Dictionary(
+      uniqueKeysWithValues: notifications.map { ($0.method, $0) }
+    )
+    for notification in notifications {
+      if let legacy = legacyNames[notification.method] { methodToNotification[legacy] = notification }
+    }
+    self.methodToNotification = methodToNotification
   }
 
   /// Returns the type of the message named `method`, or nil if it is unknown.
diff --git a/Sources/LanguageServerProtocol/Messages.swift b/Sources/LanguageServerProtocol/Messages.swift
index f76ff971..3ac7a77c 100644
--- a/Sources/LanguageServerProtocol/Messages.swift
+++ b/Sources/LanguageServerProtocol/Messages.swift
@@ -134,7 +134,33 @@ public let builtinNotifications: [NotificationType.Type] = [
 
 extension MessageRegistry {
   public static let lspProtocol: MessageRegistry =
-    MessageRegistry(requests: builtinRequests, notifications: builtinNotifications)
+    MessageRegistry(requests: builtinRequests, notifications: builtinNotifications, legacyNames: lspLegacyNames)
+
+  /// Maps current `sourcekit/`-prefixed method names to the legacy names used before the prefix
+  /// migration. Consumed by `MessageRegistry` (incoming routing) and `LegacyNameFallbackConnection`
+  /// (outgoing retries).
+  ///
+  /// This table is frozen. Do not add new entries for newly introduced methods.
+  public static let lspLegacyNames: [String: String] = [
+    DidChangeActiveDocumentNotification.method: "window/didChangeActiveDocument",
+    DoccDocumentationRequest.method: "textDocument/doccDocumentation",
+    DocumentTestsRequest.method: "textDocument/tests",
+    GetReferenceDocumentRequest.method: "workspace/getReferenceDocument",
+    IsIndexingRequest.method: "sourceKit/_isIndexing",
+    OutputPathsRequest.method: "workspace/_outputPaths",
+    PeekDocumentsRequest.method: "workspace/peekDocuments",
+    SetOptionsRequest.method: "workspace/_setOptions",
+    SourceKitOptionsRequest.method: "workspace/_sourceKitOptions",
+    SynchronizeRequest.method: "workspace/synchronize",
+    TriggerReindexRequest.method: "workspace/triggerReindex",
+    WorkspacePlaygroundsRefreshRequest.method: "workspace/playgrounds/refresh",
+    WorkspacePlaygroundsRequest.method: "workspace/playgrounds",
+    WorkspaceTestsRefreshRequest.method: "workspace/tests/refresh",
+    WorkspaceTestsRequest.method: "workspace/tests",
+  ]
+  #if compiler(>=6.6)
+  #warning("Remove the legacy method names")
+  #endif
 }
 
 // MARK: Miscellaneous Message Types
diff --git a/Sources/LanguageServerProtocol/Notifications/DidChangeActiveDocumentNotification.swift b/Sources/LanguageServerProtocol/Notifications/DidChangeActiveDocumentNotification.swift
index d098483d..4e919d3c 100644
--- a/Sources/LanguageServerProtocol/Notifications/DidChangeActiveDocumentNotification.swift
+++ b/Sources/LanguageServerProtocol/Notifications/DidChangeActiveDocumentNotification.swift
@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 public struct DidChangeActiveDocumentNotification: LSPNotification {
-  public static let method: String = "window/didChangeActiveDocument"
+  public static let method: String = "sourcekit/window/didChangeActiveDocument"
 
   /// The document that is being displayed in the active editor  or `null` to indicate that either no document is active
   /// or that the currently open document is not handled by SourceKit-LSP.
diff --git a/Sources/LanguageServerProtocol/Requests/DoccDocumentationRequest.swift b/Sources/LanguageServerProtocol/Requests/DoccDocumentationRequest.swift
index 58d447cf..81998709 100644
--- a/Sources/LanguageServerProtocol/Requests/DoccDocumentationRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/DoccDocumentationRequest.swift
@@ -44,7 +44,7 @@
 /// This request is an extension to LSP supported by SourceKit-LSP.
 /// The client is expected to display the documentation in an editor using swift-docc-render.
 public struct DoccDocumentationRequest: TextDocumentRequest, Hashable {
-  public static let method: String = "textDocument/doccDocumentation"
+  public static let method: String = "sourcekit/textDocument/doccDocumentation"
   public typealias Response = DoccDocumentationResponse
 
   /// The document in which to lookup the symbol location.
diff --git a/Sources/LanguageServerProtocol/Requests/DocumentTestsRequest.swift b/Sources/LanguageServerProtocol/Requests/DocumentTestsRequest.swift
index c79dffac..f3cfdf2b 100644
--- a/Sources/LanguageServerProtocol/Requests/DocumentTestsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/DocumentTestsRequest.swift
@@ -14,7 +14,7 @@
 ///
 /// **(LSP Extension)**
 public struct DocumentTestsRequest: TextDocumentRequest, Hashable {
-  public static let method: String = "textDocument/tests"
+  public static let method: String = "sourcekit/textDocument/tests"
   public typealias Response = [TestItem]
 
   public var textDocument: TextDocumentIdentifier
diff --git a/Sources/LanguageServerProtocol/Requests/GetReferenceDocumentRequest.swift b/Sources/LanguageServerProtocol/Requests/GetReferenceDocumentRequest.swift
index 646c5911..21184451 100644
--- a/Sources/LanguageServerProtocol/Requests/GetReferenceDocumentRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/GetReferenceDocumentRequest.swift
@@ -24,7 +24,7 @@
 /// Enable the experimental client capability `"workspace/getReferenceDocument"` so that the server responds with
 /// reference document URLs for certain requests or commands whenever possible.
 public struct GetReferenceDocumentRequest: LSPRequest {
-  public static let method: String = "workspace/getReferenceDocument"
+  public static let method: String = "sourcekit/workspace/getReferenceDocument"
   public typealias Response = GetReferenceDocumentResponse
 
   public var uri: DocumentURI
diff --git a/Sources/LanguageServerProtocol/Requests/IsIndexingRequest.swift b/Sources/LanguageServerProtocol/Requests/IsIndexingRequest.swift
index 8666d5e3..c7adecd4 100644
--- a/Sources/LanguageServerProtocol/Requests/IsIndexingRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/IsIndexingRequest.swift
@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 public struct IsIndexingRequest: LSPRequest {
-  public static let method: String = "sourceKit/_isIndexing"
+  public static let method: String = "sourcekit/isIndexing"
   public typealias Response = IsIndexingResponse
 
   public init() {}
diff --git a/Sources/LanguageServerProtocol/Requests/OutputPathsRequest.swift b/Sources/LanguageServerProtocol/Requests/OutputPathsRequest.swift
index f6fed318..30516baa 100644
--- a/Sources/LanguageServerProtocol/Requests/OutputPathsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/OutputPathsRequest.swift
@@ -15,7 +15,7 @@
 ///
 /// **(LSP Extension)**.
 public struct OutputPathsRequest: LSPRequest, Hashable {
-  public static let method: String = "workspace/_outputPaths"
+  public static let method: String = "sourcekit/workspace/outputPaths"
   public typealias Response = OutputPathsResponse
 
   /// The target whose output file paths to get.
diff --git a/Sources/LanguageServerProtocol/Requests/PeekDocumentsRequest.swift b/Sources/LanguageServerProtocol/Requests/PeekDocumentsRequest.swift
index cc586b50..058f9fa4 100644
--- a/Sources/LanguageServerProtocol/Requests/PeekDocumentsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/PeekDocumentsRequest.swift
@@ -29,7 +29,7 @@
 /// It requires the experimental client capability `"workspace/peekDocuments"` to use.
 /// It also needs the client to handle the request and present the "peeked" editor.
 public struct PeekDocumentsRequest: LSPRequest {
-  public static let method: String = "workspace/peekDocuments"
+  public static let method: String = "sourcekit/workspace/peekDocuments"
   public typealias Response = PeekDocumentsResponse
 
   public var uri: DocumentURI
diff --git a/Sources/LanguageServerProtocol/Requests/SetOptionsRequest.swift b/Sources/LanguageServerProtocol/Requests/SetOptionsRequest.swift
index 395fe3a8..b3d4c363 100644
--- a/Sources/LanguageServerProtocol/Requests/SetOptionsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/SetOptionsRequest.swift
@@ -14,7 +14,7 @@
 //
 /// Any options not specified in this request will be left as-is.
 public struct SetOptionsRequest: LSPRequest {
-  public static let method: String = "workspace/_setOptions"
+  public static let method: String = "sourcekit/workspace/setOptions"
   public typealias Response = VoidResponse
 
   /// `true` to pause background indexing or `false` to resume background indexing.
diff --git a/Sources/LanguageServerProtocol/Requests/SourceKitOptionsRequest.swift b/Sources/LanguageServerProtocol/Requests/SourceKitOptionsRequest.swift
index 17f5f184..a10c3300 100644
--- a/Sources/LanguageServerProtocol/Requests/SourceKitOptionsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/SourceKitOptionsRequest.swift
@@ -18,7 +18,7 @@
 ///
 /// **(LSP Extension)**.
 public struct SourceKitOptionsRequest: LSPRequest, Hashable {
-  public static let method: String = "workspace/_sourceKitOptions"
+  public static let method: String = "sourcekit/workspace/sourceKitOptions"
   public typealias Response = SourceKitOptionsResponse
 
   /// The document to get options for
diff --git a/Sources/LanguageServerProtocol/Requests/SynchronizeRequest.swift b/Sources/LanguageServerProtocol/Requests/SynchronizeRequest.swift
index cd7b66d5..89a10c0d 100644
--- a/Sources/LanguageServerProtocol/Requests/SynchronizeRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/SynchronizeRequest.swift
@@ -27,7 +27,7 @@
 /// user a result based on the data that is available and (let the user) re-perform the action if the underlying index
 /// data has changed.
 public struct SynchronizeRequest: LSPRequest {
-  public static let method: String = "workspace/synchronize"
+  public static let method: String = "sourcekit/workspace/synchronize"
   public typealias Response = VoidResponse
 
   /// Wait for the build server to have an up-to-date build graph by sending a `workspace/waitForBuildSystemUpdates` to
diff --git a/Sources/LanguageServerProtocol/Requests/TriggerReindexRequest.swift b/Sources/LanguageServerProtocol/Requests/TriggerReindexRequest.swift
index e1baf628..b165b3a2 100644
--- a/Sources/LanguageServerProtocol/Requests/TriggerReindexRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/TriggerReindexRequest.swift
@@ -18,7 +18,7 @@
 ///
 /// **LSP Extension**
 public struct TriggerReindexRequest: LSPRequest {
-  public static let method: String = "workspace/triggerReindex"
+  public static let method: String = "sourcekit/workspace/triggerReindex"
   public typealias Response = VoidResponse
 
   public init() {}
diff --git a/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRefreshRequest.swift b/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRefreshRequest.swift
index 985a2088..f4135c85 100644
--- a/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRefreshRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRefreshRequest.swift
@@ -19,7 +19,7 @@
 ///
 /// **(LSP Extension)**
 public struct WorkspacePlaygroundsRefreshRequest: LSPRequest {
-  public static let method: String = "workspace/playgrounds/refresh"
+  public static let method: String = "sourcekit/workspace/playgrounds/refresh"
   public typealias Response = VoidResponse
 
   public init() {}
diff --git a/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRequest.swift b/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRequest.swift
index c630a37a..614540a3 100644
--- a/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/WorkspacePlaygroundsRequest.swift
@@ -14,7 +14,7 @@
 ///
 /// **(LSP Extension)**
 public struct WorkspacePlaygroundsRequest: LSPRequest, Hashable {
-  public static let method: String = "workspace/playgrounds"
+  public static let method: String = "sourcekit/workspace/playgrounds"
   public typealias Response = [Playground]
 
   public init() {}
diff --git a/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRefreshRequest.swift b/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRefreshRequest.swift
index 027c5fd1..3ba2f4db 100644
--- a/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRefreshRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRefreshRequest.swift
@@ -19,7 +19,7 @@
 ///
 /// **(LSP Extension)**
 public struct WorkspaceTestsRefreshRequest: LSPRequest {
-  public static let method: String = "workspace/tests/refresh"
+  public static let method: String = "sourcekit/workspace/tests/refresh"
   public typealias Response = VoidResponse
 
   public init() {}
diff --git a/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRequest.swift b/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRequest.swift
index a760b682..75f027d2 100644
--- a/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRequest.swift
+++ b/Sources/LanguageServerProtocol/Requests/WorkspaceTestsRequest.swift
@@ -14,7 +14,7 @@
 ///
 /// **(LSP Extension)**
 public struct WorkspaceTestsRequest: LSPRequest, Hashable {
-  public static let method: String = "workspace/tests"
+  public static let method: String = "sourcekit/workspace/tests"
   public typealias Response = [TestItem]
 
   public init() {}
diff --git a/Sources/LanguageServerProtocolTransport/CMakeLists.txt b/Sources/LanguageServerProtocolTransport/CMakeLists.txt
index ab73cffe..29673b7d 100644
--- a/Sources/LanguageServerProtocolTransport/CMakeLists.txt
+++ b/Sources/LanguageServerProtocolTransport/CMakeLists.txt
@@ -4,6 +4,7 @@ add_library(LanguageServerProtocolTransport
   DisableSigpipe.swift
   DocumentURI+CustomLogStringConvertible.swift
   JSONRPCConnection.swift
+  LegacyNameFallbackConnection.swift
   LocalConnection.swift
   LoggableMessageTypes.swift
   MessageCoding.swift
diff --git a/Sources/LanguageServerProtocolTransport/Connection+Send.swift b/Sources/LanguageServerProtocolTransport/Connection+Send.swift
index eda5db07..c458dc88 100644
--- a/Sources/LanguageServerProtocolTransport/Connection+Send.swift
+++ b/Sources/LanguageServerProtocolTransport/Connection+Send.swift
@@ -14,11 +14,13 @@ public import LanguageServerProtocol
 @_spi(SourceKitLSP) private import ToolsProtocolsSwiftExtensions
 
 extension Connection {
-  public func send<R: RequestType>(_ request: R) async throws -> R.Response {
+  public func send<R: RequestType>(_ request: R, method: String = R.method) async throws -> R.Response {
     return try await withCancellableCheckedThrowingContinuation { continuation in
-      return self.send(request) { result in
+      let id = self.nextRequestID()
+      self.send(request, method: method, id: id) { result in
         continuation.resume(with: result)
       }
+      return id
     } cancel: { requestID in
       self.send(CancelRequestNotification(id: requestID))
     }
diff --git a/Sources/LanguageServerProtocolTransport/JSONRPCConnection.swift b/Sources/LanguageServerProtocolTransport/JSONRPCConnection.swift
index 16ced07e..d94ba0ff 100644
--- a/Sources/LanguageServerProtocolTransport/JSONRPCConnection.swift
+++ b/Sources/LanguageServerProtocolTransport/JSONRPCConnection.swift
@@ -432,7 +432,7 @@ public final class JSONRPCConnection: Connection {
     switch message {
     case .notification(let notification):
       notification._handle(receiveHandler)
-    case .request(let request, let id):
+    case .request(let request, _, let id):
       request._handle(receiveHandler, id: id) { (response, id) in
         self.sendReply(response, id: id)
       }
@@ -514,7 +514,7 @@ public final class JSONRPCConnection: Connection {
           message: "sourcekit-lsp failed to encode a notification to the editor"
         )
         return
-      case .request(_, _):
+      case .request(_, _, _):
         // We want to send a notification to the editor but failed to encode it. We don't know the `reply` handle for
         // the request at this point so we can't synthesize an errorResponse for the request. This means that the
         // request will never receive a reply. Inform the user about it.
@@ -620,6 +620,7 @@ public final class JSONRPCConnection: Connection {
   /// When the receiving end replies to the request, execute `reply` with the response.
   public func send<Request: RequestType>(
     _ request: Request,
+    method: String,
     id: RequestID,
     reply: @escaping @Sendable (LSPResult<Request.Response>) -> Void
   ) {
@@ -638,7 +639,7 @@ public final class JSONRPCConnection: Connection {
             logger.info(
               """
               Received reply for request \(id, privacy: .public) from \(self.name, privacy: .public)
-              \(Request.method, privacy: .public)
+              \(method, privacy: .public)
               \(response.forLogging)
               """
             )
@@ -646,7 +647,7 @@ public final class JSONRPCConnection: Connection {
             logger.error(
               """
               Received error for request \(id, privacy: .public) from \(self.name, privacy: .public)
-              \(Request.method, privacy: .public)
+              \(method, privacy: .public)
               \(error.forLogging)
               """
             )
@@ -660,8 +661,7 @@ public final class JSONRPCConnection: Connection {
         \(request.forLogging)
         """
       )
-
-      self.sendAssumingOnQueue(.request(request, id: id))
+      self.sendAssumingOnQueue(.request(request, method: method, id: id))
     }
   }
 
diff --git a/Sources/LanguageServerProtocolTransport/LegacyNameFallbackConnection.swift b/Sources/LanguageServerProtocolTransport/LegacyNameFallbackConnection.swift
new file mode 100644
index 00000000..a650ecb4
--- /dev/null
+++ b/Sources/LanguageServerProtocolTransport/LegacyNameFallbackConnection.swift
@@ -0,0 +1,86 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+public import LanguageServerProtocol
+@_spi(SourceKitLSP) import ToolsProtocolsSwiftExtensions
+
+/// A `Connection` wrapper that retries requests using a legacy method name when the peer
+/// responds with `methodNotFound` for a `sourcekit/`-prefixed method.
+///
+/// On the first successful response to a legacy name for a given method, the wrapper records that
+/// method and from that point on sends requests for it with the legacy name directly, skipping
+/// the primary method.
+///
+/// Pass `MessageRegistry.lspLegacyNames` or `MessageRegistry.bspLegacyNames` as `legacyNames`
+/// as appropriate.
+public final class LegacyNameFallbackConnection: Connection, Sendable {
+  /// The underlying transport connection.
+  public let inner: any Connection
+
+  /// Maps current method names to legacy names (new → old).
+  private let legacyNames: [String: String]
+
+  /// The set of method names for which the peer has successfully responded using the legacy name.
+  /// Requests for these methods are sent with the legacy name directly, skipping the primary method.
+  private let methodNamesPreferringLegacyName: ThreadSafeBox<Set<String>> = .init(initialValue: [])
+
+  public init(_ inner: any Connection, legacyNames: [String: String]) {
+    self.inner = inner
+    self.legacyNames = legacyNames
+  }
+
+  public func nextRequestID() -> RequestID {
+    inner.nextRequestID()
+  }
+
+  public func send(_ notification: some NotificationType) {
+    inner.send(notification)
+  }
+
+  public func send<R: RequestType>(
+    _ request: R,
+    method: String,
+    id: RequestID,
+    reply: @escaping @Sendable (LSPResult<R.Response>) -> Void
+  ) {
+    guard let legacyName = legacyNames[method] else {
+      inner.send(request, method: method, id: id, reply: reply)
+      return
+    }
+    if methodNamesPreferringLegacyName.value.contains(method) {
+      inner.send(request, method: legacyName, id: id, reply: reply)
+      return
+    }
+    inner.send(request, method: method, id: id) { [weak self] result in
+      guard let self else {
+        reply(result)
+        return
+      }
+      if case .failure(let error) = result, error.code == .methodNotFound {
+        self.inner.send(request, method: legacyName, id: self.inner.nextRequestID()) { [weak self] legacyResult in
+          if case .success = legacyResult {
+            self?.methodNamesPreferringLegacyName.value.insert(method)
+          }
+          reply(legacyResult)
+        }
+      } else {
+        reply(result)
+      }
+    }
+  }
+
+  /// Forward to the inner `JSONRPCConnection.changeReceiveHandler` if the inner connection is a
+  /// `JSONRPCConnection`. No-op for other connection types.
+  public func changeReceiveHandler(_ handler: any MessageHandler) {
+    (inner as? JSONRPCConnection)?.changeReceiveHandler(handler)
+  }
+}
diff --git a/Sources/LanguageServerProtocolTransport/LocalConnection.swift b/Sources/LanguageServerProtocolTransport/LocalConnection.swift
index f910ceb1..1bce428f 100644
--- a/Sources/LanguageServerProtocolTransport/LocalConnection.swift
+++ b/Sources/LanguageServerProtocolTransport/LocalConnection.swift
@@ -105,6 +105,7 @@ public final class LocalConnection: Connection, Sendable {
 
   public func send<Request: RequestType>(
     _ request: Request,
+    method: String,
     id: RequestID,
     reply: @Sendable @escaping (LSPResult<Request.Response>) -> Void
   ) {
diff --git a/Sources/LanguageServerProtocolTransport/MessageCoding.swift b/Sources/LanguageServerProtocolTransport/MessageCoding.swift
index 03353dbc..392db46b 100644
--- a/Sources/LanguageServerProtocolTransport/MessageCoding.swift
+++ b/Sources/LanguageServerProtocolTransport/MessageCoding.swift
@@ -15,7 +15,7 @@ public import LanguageServerProtocol
 
 @_spi(Testing) @frozen public enum JSONRPCMessage {
   case notification(NotificationType)
-  case request(_RequestType, id: RequestID)
+  case request(_RequestType, method: String, id: RequestID)
   case response(ResponseType, id: RequestID)
   case errorResponse(ResponseError, id: RequestID?)
 }
@@ -81,7 +81,7 @@ extension JSONRPCMessage: Codable {
 
         let params = try messageType.init(from: container.superDecoder(forKey: .params))
 
-        self = .request(params, id: id)
+        self = .request(params, method: method, id: id)
 
       case (let id?, nil, true, nil):
         msgKind = .response
@@ -148,8 +148,8 @@ extension JSONRPCMessage: Codable {
       try container.encode(type(of: params).method, forKey: .method)
       try params.encode(to: container.superEncoder(forKey: .params))
 
-    case .request(let params, let id):
-      try container.encode(type(of: params).method, forKey: .method)
+    case .request(let params, let method, let id):
+      try container.encode(method, forKey: .method)
       try container.encode(id, forKey: .id)
       try params.encode(to: container.superEncoder(forKey: .params))
 
diff --git a/Tests/LanguageServerProtocolTransportTests/CodingTests.swift b/Tests/LanguageServerProtocolTransportTests/CodingTests.swift
index 5558ec10..a0d3b96b 100644
--- a/Tests/LanguageServerProtocolTransportTests/CodingTests.swift
+++ b/Tests/LanguageServerProtocolTransportTests/CodingTests.swift
@@ -354,13 +354,14 @@ final class CodingTests: XCTestCase {
     decoder.userInfo = defaultCodingInfo
     let decodedValue = try decoder.decode(JSONRPCMessage.self, from: json.data(using: .utf8)!)
 
-    guard case JSONRPCMessage.request(let decodedValueOpaque, let decodedID) = decodedValue,
+    guard case JSONRPCMessage.request(let decodedValueOpaque, let decodedMethod, let decodedID) = decodedValue,
       let decodedRequest = decodedValueOpaque as? ShutdownRequest
     else {
       XCTFail("decodedValue \(decodedValue) is not a ShutdownRequest")
       return
     }
 
+    XCTAssertEqual(ShutdownRequest.method, decodedMethod)
     XCTAssertEqual(.number(1), decodedID, "expected request ID 1")
     XCTAssertEqual(ShutdownRequest(), decodedRequest)
   }
@@ -377,14 +378,21 @@ private func checkMessageCoding<Request: RequestType & Equatable>(
   file: StaticString = #filePath,
   line: UInt = #line
 ) {
-  checkCoding(JSONRPCMessage.request(value, id: id), json: json, userInfo: defaultCodingInfo, file: file, line: line) {
-    guard case JSONRPCMessage.request(let decodedValueOpaque, let decodedID) = $0,
+  checkCoding(
+    JSONRPCMessage.request(value, method: Request.method, id: id),
+    json: json,
+    userInfo: defaultCodingInfo,
+    file: file,
+    line: line
+  ) {
+    guard case JSONRPCMessage.request(let decodedValueOpaque, let decodedMethod, let decodedID) = $0,
       let decodedValue = decodedValueOpaque as? Request
     else {
       XCTFail("decodedValue \($0) does not match expected \(value)", file: file, line: line)
       return
     }
 
+    XCTAssertEqual(Request.method, decodedMethod, "method decoding", file: file, line: line)
     XCTAssertEqual(id, decodedID, "requestID decoding", file: file, line: line)
     XCTAssertEqual(value, decodedValue, file: file, line: line)
   }
diff --git a/Tests/LanguageServerProtocolTransportTests/LegacyNameFallbackConnectionTests.swift b/Tests/LanguageServerProtocolTransportTests/LegacyNameFallbackConnectionTests.swift
new file mode 100644
index 00000000..6683268a
--- /dev/null
+++ b/Tests/LanguageServerProtocolTransportTests/LegacyNameFallbackConnectionTests.swift
@@ -0,0 +1,235 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import LanguageServerProtocol
+import LanguageServerProtocolTransport
+import Testing
+import ToolsProtocolsSwiftExtensions
+
+// MARK: - Fixture request types
+
+private struct NoLegacyRequest: RequestType {
+  static let method = "sourcekit/noLegacy"
+  typealias Response = VoidResponse
+}
+
+private struct WithLegacyRequest: RequestType {
+  static let method = "sourcekit/one"
+  typealias Response = VoidResponse
+}
+
+private struct WithLegacyRequest2: RequestType {
+  static let method = "sourcekit/two"
+  typealias Response = VoidResponse
+}
+
+private let testLegacyNames: [String: String] = [
+  WithLegacyRequest.method: "sourcekit-lsp/one",
+  WithLegacyRequest2.method: "sourcekit-lsp/two",
+]
+
+// MARK: - Mock connection
+
+/// A controllable `Connection` for unit tests.
+///
+/// Calls are recorded in `calls`. `responses` maps method names to explicit
+/// results; any method not in the dictionary defaults to `.methodNotFound`.
+private final class MockConnection: Connection, Sendable {
+  struct State {
+    var calls: [Call] = []
+    var responses: [String: LSPResult<VoidResponse>] = [:]
+    var nextIDValue: Int = 0
+  }
+
+  struct Call: Equatable {
+    var method: String
+    var id: RequestID
+  }
+
+  let state: ThreadSafeBox<State> = .init(initialValue: State())
+
+  var calls: [Call] { state.value.calls }
+  func clearCalls() { state.value.calls.removeAll() }
+  var responses: [String: LSPResult<VoidResponse>] {
+    get { state.value.responses }
+    set { state.value.responses = newValue }
+  }
+
+  func nextRequestID() -> RequestID {
+    state.withLock {
+      $0.nextIDValue += 1
+      return .number($0.nextIDValue)
+    }
+  }
+
+  func send(_ notification: some NotificationType) {}
+
+  func send<Request: RequestType>(
+    _ request: Request,
+    method: String,
+    id: RequestID,
+    reply: @escaping @Sendable (LSPResult<Request.Response>) -> Void
+  ) {
+    let voidResult: LSPResult<VoidResponse> = state.withLock {
+      $0.calls.append(Call(method: method, id: id))
+      return $0.responses[method] ?? .failure(.methodNotFound(method))
+    }
+    switch voidResult {
+    case .failure(let error):
+      reply(.failure(error))
+    case .success(let voidResponse):
+      if let response = voidResponse as? Request.Response {
+        reply(.success(response))
+      } else {
+        reply(.failure(.internalError("MockConnection: unsupported response type")))
+      }
+    }
+  }
+}
+
+// MARK: - Tests
+
+@Suite struct LegacyNameFallbackConnectionTests {
+
+  // No legacyName: pass through regardless of the reply – no retries, flag never set.
+  @Test func testNoLegacyMethods() async {
+    let mock = MockConnection()  // "sourcekit/noLegacy" → methodNotFound (default)
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    let error = await #expect(throws: ResponseError.self) {
+      try await conn.send(NoLegacyRequest())
+    }
+    #expect(error?.code == .methodNotFound)
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit/noLegacy")
+
+    // Confirm the flag was NOT set: a fresh send still uses the primary name.
+    mock.clearCalls()
+    _ = try? await conn.send(NoLegacyRequest())
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit/noLegacy")
+  }
+
+  // Primary method succeeds: no fallback, flag stays false.
+  @Test func testSuccess() async throws {
+    let mock = MockConnection()
+    mock.responses["sourcekit/one"] = .success(VoidResponse())
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    _ = try await conn.send(WithLegacyRequest())
+
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit/one")
+  }
+
+  // Primary returns methodNotFound: falls back to the single legacy name.
+  @Test func testMethodNotFound() async throws {
+    // "sourcekit/one" not in responses → methodNotFound; legacy succeeds.
+    let mock = MockConnection()
+    mock.responses["sourcekit-lsp/one"] = .success(VoidResponse())
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    _ = try await conn.send(WithLegacyRequest())
+
+    #expect(mock.calls.count == 2)
+    #expect(mock.calls[0].method == "sourcekit/one")
+    #expect(mock.calls[1].method == "sourcekit-lsp/one")
+  }
+
+  // Both primary and legacy return methodNotFound: error is propagated, flag stays false.
+  @Test func testLegacyMethodNotFound() async {
+    let mock = MockConnection()  // all methods default to methodNotFound
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    let error = await #expect(throws: ResponseError.self) {
+      try await conn.send(WithLegacyRequest())
+    }
+    #expect(error?.code == .methodNotFound)
+    #expect(mock.calls.count == 2)
+
+    // Flag should NOT have been set: next request still tries the primary first.
+    mock.clearCalls()
+    _ = try? await conn.send(WithLegacyRequest())
+    #expect(mock.calls.count == 2)
+    #expect(mock.calls[0].method == "sourcekit/one")
+    #expect(mock.calls[1].method == "sourcekit-lsp/one")
+  }
+
+  // After the flag is set, subsequent requests skip the primary method entirely.
+  @Test func testFlagSticksAfterFirstMethodNotFound() async throws {
+    let mock = MockConnection()
+    mock.responses["sourcekit-lsp/one"] = .success(VoidResponse())
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    // First request: triggers flag.
+    _ = try await conn.send(WithLegacyRequest())
+    #expect(mock.calls.count == 2)
+
+    // Second request: should go directly to legacy name, bypassing the primary.
+    mock.clearCalls()
+    _ = try await conn.send(WithLegacyRequest())
+
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit-lsp/one")
+  }
+
+  // A non-methodNotFound error does not flip the flag; the next request still tries primary first.
+  @Test func testNonMethodNotFoundError() async {
+    let mock = MockConnection()
+    mock.responses["sourcekit/one"] = .failure(.internalError("unexpected error"))
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    let error = await #expect(throws: ResponseError.self) {
+      try await conn.send(WithLegacyRequest())
+    }
+    #expect(error?.code == .internalError)
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit/one")
+
+    // Flag should NOT have been set: next request still tries the primary method.
+    mock.responses["sourcekit/one"] = .success(VoidResponse())
+    mock.clearCalls()
+    _ = try? await conn.send(WithLegacyRequest())
+    #expect(mock.calls.count == 1)
+    #expect(mock.calls[0].method == "sourcekit/one")
+  }
+
+  // Falling back for one method must not affect other methods: each method tracks its own state.
+  @Test func testFallbackIsPerMethod() async throws {
+    let mock = MockConnection()
+    mock.responses["sourcekit-lsp/one"] = .success(VoidResponse())
+    mock.responses["sourcekit/two"] = .success(VoidResponse())
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+
+    // Trigger legacy fallback for "sourcekit/one".
+    _ = try await conn.send(WithLegacyRequest())
+    #expect(mock.calls.map(\.method) == ["sourcekit/one", "sourcekit-lsp/one"])
+
+    // "sourcekit/two" succeeds on the primary — its fallback state is independent.
+    mock.clearCalls()
+    _ = try await conn.send(WithLegacyRequest2())
+    #expect(mock.calls.map(\.method) == ["sourcekit/two"])
+
+    // "sourcekit/one" now goes directly to legacy because its flag was set.
+    mock.clearCalls()
+    _ = try await conn.send(WithLegacyRequest())
+    #expect(mock.calls.map(\.method) == ["sourcekit-lsp/one"])
+  }
+
+  // Notifications are forwarded to the inner connection unchanged.
+  @Test func testNotification() {
+    let mock = MockConnection()
+    let conn = LegacyNameFallbackConnection(mock, legacyNames: testLegacyNames)
+    conn.send(CancelRequestNotification(id: .number(42)))
+    // Notifications don't appear in `calls`; just ensure it doesn't crash.
+  }
+}