From 7a1550c2d0d76535c0c02cbd196cbc28ec0d30e2 Mon Sep 17 00:00:00 2001
From: Kishikawa Katsumi <kishikawakatsumi@mac.com>
Date: Fri, 12 Jun 2026 04:45:18 +0900
Subject: [PATCH] Add signatureHelp (parameter hints) over the WebSocket API

sourcekit-lsp (main) now implements textDocument/signatureHelp, so expose it:
a `signatureHelp` method that forwards the cursor position and returns the
SignatureHelp response (signatures, parameters, activeParameter) to the client.
This backs Xcode-style parameter hints in the editor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 Sources/App/Controllers/LanguageServer.swift | 12 +++++++++
 Sources/App/routes.swift                     | 28 ++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/Sources/App/Controllers/LanguageServer.swift b/Sources/App/Controllers/LanguageServer.swift
index 6ab1bf8..654644a 100644
--- a/Sources/App/Controllers/LanguageServer.swift
+++ b/Sources/App/Controllers/LanguageServer.swift
@@ -196,6 +196,18 @@ final class LanguageServer {
         }
     }
 
+    func sendSignatureHelpRequest(documentPath: String, line: Int, character: Int, completion: @escaping (Result<SignatureHelpRequest.Response, ResponseError>) -> Void) {
+        let identifier = URL(fileURLWithPath: documentPath)
+
+        let signatureHelpRequest = SignatureHelpRequest(
+            textDocument: TextDocumentIdentifier(DocumentURI(identifier)),
+            position: Position(line: line, utf16index: character)
+        )
+        _ = connection.send(signatureHelpRequest) {
+            completion($0)
+        }
+    }
+
     func sendDefinitionRequest(documentPath: String, line: Int, character: Int, completion: @escaping (Result<DefinitionRequest.Response, ResponseError>) -> Void) {
         let identifier = URL(fileURLWithPath: documentPath)
 
diff --git a/Sources/App/routes.swift b/Sources/App/routes.swift
index 0cc4799..8524fde 100644
--- a/Sources/App/routes.swift
+++ b/Sources/App/routes.swift
@@ -53,6 +53,13 @@ func routes(_ app: Application) throws {
             let value: LanguageServerProtocol.CompletionRequest.Response?
         }
 
+        typealias SignatureHelpRequestMessage = HoverRequest
+        struct SignatureHelpResponse: Codable {
+            let method: String
+            let id: Int
+            let value: LanguageServerProtocol.SignatureHelpRequest.Response
+        }
+
         struct DiagnosticsNotification: Codable {
             let method: String
             let value: PublishDiagnosticsNotification
@@ -206,6 +213,27 @@ func routes(_ app: Application) throws {
                     guard let json = String(data: data, encoding: .utf8) else { return }
                     ws.send(json)
                 }
+            case _ where text.hasPrefix(#"{"method":"signatureHelp""#):
+                guard let request = try? decoder.decode(SignatureHelpRequestMessage.self, from: data) else { return }
+                languageServer?.sendSignatureHelpRequest(
+                    documentPath: documentPath, line: request.row, character: request.column
+                ) { (result) in
+                    let value: LanguageServerProtocol.SignatureHelpRequest.Response
+                    switch result {
+                    case .success(let response):
+                        value = response
+                    case .failure:
+                        value = nil
+                    }
+                    let signatureHelpResponse = SignatureHelpResponse(
+                        method: "signatureHelp",
+                        id: request.id,
+                        value: value
+                    )
+                    guard let data = try? encoder.encode(signatureHelpResponse) else { return }
+                    guard let json = String(data: data, encoding: .utf8) else { return }
+                    ws.send(json)
+                }
             case _ where text.hasPrefix(#"{"method":"format""#):
                 guard let request = try? decoder.decode(FormatRequest.self, from: data) else { return }
                 let source = request.code