diff --git a/README.md b/README.md
index d195a6d..3d7104b 100644
--- a/README.md
+++ b/README.md
@@ -128,6 +128,7 @@ The LSP [specification](https://microsoft.github.io/language-server-protocol/spe
 | ------|:---------:|
 | $/cancelRequest | ✅ |
 | $/progress | ✅ |
+| server-defined | ✅ |
 
 ## Suggestions or Feedback
 
diff --git a/Sources/LanguageServerProtocol/JSONRPCLanguageServer.swift b/Sources/LanguageServerProtocol/JSONRPCLanguageServer.swift
index bce32a3..b9a0079 100644
--- a/Sources/LanguageServerProtocol/JSONRPCLanguageServer.swift
+++ b/Sources/LanguageServerProtocol/JSONRPCLanguageServer.swift
@@ -395,6 +395,8 @@ extension JSONRPCLanguageServer {
             sendRequestWithHandler(params, method: method, handler: completionHandler)
         case .workspaceWillDeleteFiles(let params):
             sendRequestWithHandler(params, method: method, handler: completionHandler)
+        case .custom(let method, let params):
+            sendRequestWithHandler(params, method: method, handler: completionHandler)
         }
     }
 }
diff --git a/Sources/LanguageServerProtocol/LanguageServerProtocol.swift b/Sources/LanguageServerProtocol/LanguageServerProtocol.swift
index 4b8cc86..d0e309a 100644
--- a/Sources/LanguageServerProtocol/LanguageServerProtocol.swift
+++ b/Sources/LanguageServerProtocol/LanguageServerProtocol.swift
@@ -1,4 +1,5 @@
 import JSONRPC
+import AnyCodable
 
 typealias UnusedResult = String?
 typealias UnusedParam = String?
@@ -117,6 +118,7 @@ public enum ClientRequest {
         case textDocumentSemanticTokensFull = "textDocument/semanticTokens/full"
         case textDocumentSemanticTokensFullDelta = "textDocument/semanticTokens/full/delta"
         case textDocumentMoniker = "textDocument/moniker"
+        case custom
     }
 
     case initialize(InitializeParams)
@@ -154,6 +156,7 @@ public enum ClientRequest {
     case semanticTokensFull(SemanticTokensParams)
     case semanticTokensFullDelta(SemanticTokensDeltaParams)
     case semanticTokensRange(SemanticTokensRangeParams)
+    case custom(String, AnyCodable)
 
     public var method: Method {
         switch self {
@@ -227,6 +230,8 @@ public enum ClientRequest {
             return .textDocumentSemanticTokensFullDelta
         case .semanticTokensRange:
             return .textDocumentSemanticTokensRange
+        case .custom:
+            return .custom
         }
     }
 }
diff --git a/Sources/LanguageServerProtocol/Server.swift b/Sources/LanguageServerProtocol/Server.swift
index e8a6a34..d5dedd4 100644
--- a/Sources/LanguageServerProtocol/Server.swift
+++ b/Sources/LanguageServerProtocol/Server.swift
@@ -180,6 +180,10 @@ public extension Server {
     func semanticTokensRange(params: SemanticTokensRangeParams, block: @escaping (ServerResult<SemanticTokensResponse>) -> Void) {
         sendRequest(.semanticTokensRange(params), completionHandler: block)
     }
+
+    func customRequest<Response: Codable>(method: String, params: AnyCodable, block: @escaping (ServerResult<Response>) -> Void) {
+        sendRequest(.custom(method, params), completionHandler: block)
+    }
 }
 
 // Workspace notifications