LSP uncaught exception on receiving MethodNotFound error response from client

I'm trying to use the LSP with the Kakoune editor, using its kak-lsp plugin. kak-lsp appears not to support the "workspace/workspaceFolders" method requested by the LSP, so kak-lsp sends back a "method not found" error (code -32601). The bsc LSP appears not to handle this error response appropriately:

node:internal/process/promises:394
    triggerUncaughtException(err, true /* fromPromise */);
    ^

[ResponseError: Method not found
    at handleResponse (/nix/store/38pg66nsynrsmzcqmr3fcl8kj6d6l7kz-brighterscript-1.0.0-alpha.50/lib/node_modules/brighterscript/node_modules/vscode-languageserver-protocol/node_modules/vscode-jsonrpc/lib/common/connection.js:565:48)
    at handleMessage (/nix/store/38pg66nsynrsmzcqmr3fcl8kj6d6l7kz-brighterscript-1.0.0-alpha.50/lib/node_modules/brighterscript/node_modules/vscode-languageserver-protocol/node_modules/vscode-jsonrpc/lib/common/connection.js:345:13)
    at processMessageQueue (/nix/store/38pg66nsynrsmzcqmr3fcl8kj6d6l7kz-brighterscript-1.0.0-alpha.50/lib/node_modules/brighterscript/node_modules/vscode-languageserver-protocol/node_modules/vscode-jsonrpc/lib/common/connection.js:362:17)
    at Immediate.<anonymous> (/nix/store/38pg66nsynrsmzcqmr3fcl8kj6d6l7kz-brighterscript-1.0.0-alpha.50/lib/node_modules/brighterscript/node_modules/vscode-languageserver-protocol/node_modules/vscode-jsonrpc/lib/common/connection.js:334:13)
    at process.processImmediate (node:internal/timers:485:21)] {
  code: -32601,
  data: undefined
}

Node.js v22.17.0

I've tried digging into the bsc code and the LSP protocol code, but I can't locate the point of failure. Looking at the protocol code in vscode-jsonprc, it appears that some response promise is not having the error/reject case properly handled.

For reference, the kak-lsp logs:

LSP: INFO Starting language server bsc as `bsc --lsp --stdio`
LSP: DEBG lsp_config:
LSP: DEBG Language server(s) bsc are still not initialized, parking request EditorRequest { meta: EditorMeta { session: SessionId("22482"), client: Some(ClientId("client0")), buffile: "/home/pascal/workspaces/bsc/testme.brs", language_id: "brs", filetype: "brs", version: 1, hook: false, sourcing: false, language_server: {}, semantic_tokens: SemanticTokenConfig { faces: [SemanticTokenFace { face: "documentation", token: "comment", modifiers: [SemanticTokenModifier("documentation")] }, SemanticTokenFace { face: "comment", token: "comment", modifiers: [] }, SemanticTokenFace { face: "function", token: "function", modifiers: [] }, SemanticTokenFace { face: "function", token: "method", modifiers: [] }, SemanticTokenFace { face: "keyword", token: "keyword", modifiers: [] }, SemanticTokenFace { face: "module", token: "namespace", modifiers: [] }, SemanticTokenFace { face: "operator", token: "operator", modifiers: [] }, SemanticTokenFace { face: "string", token: "string", modifiers: [] }, SemanticTokenFace { face: "type", token: "type", modifiers: [] }, SemanticTokenFace { face: "default+d", token: "variable", modifiers: [SemanticTokenModifier("readonly")] }, SemanticTokenFace { face: "default+d", token: "variable", modifiers: [SemanticTokenModifier("constant")] }, SemanticTokenFace { face: "variable", token: "variable", modifiers: [] }] }, server: None, word_regex: None, servers: [0], legacy_dynamic_config: "", legacy_server_initialization_options: [] }, response_fifo: None, method: "textDocument/hover", params: EditorParams(Any { .. }) }
LSP: ERRO language servers bsc are still not initialized, parking request textDocument/hover
LSP: DEBG To server bsc: {"jsonrpc":"2.0","method":"initialize","params":{"capabilities":{"general":{"markdown":{"parser":"kakoune-lsp","version":"18.2.0"},"positionEncodings":["utf-8","utf-16"],"regularExpressions":{"engine":"Rust regex"}},"offsetEncoding":["utf-8","utf-16"],"textDocument":{"callHierarchy":{"dynamicRegistration":false},"codeAction":{"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["quickfix","refactor","refactor.extract","refactor.inline","refactor.rewrite","source","source.fixAll","source.organizeImports"]}},"dynamicRegistration":false,"isPreferredSupport":false,"resolveSupport":{"properties":["edit"]}},"codeLens":{"dynamicRegistration":false},"colorProvider":{"dynamicRegistration":false},"completion":{"completionItem":{"commitCharactersSupport":false,"deprecatedSupport":false,"documentationFormat":["markdown","plaintext"],"preselectSupport":false,"resolveSupport":{"properties":["additionalTextEdits","detail","documentation"]},"snippetSupport":true},"completionItemKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]},"contextSupport":false,"dynamicRegistration":false},"declaration":{"dynamicRegistration":false,"linkSupport":false},"definition":{"dynamicRegistration":false,"linkSupport":false},"documentHighlight":{"dynamicRegistration":false},"documentLink":{"dynamicRegistration":false,"tooltipSupport":false},"documentSymbol":{"dynamicRegistration":false,"hierarchicalDocumentSymbolSupport":true,"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"formatting":{"dynamicRegistration":false},"hover":{"contentFormat":["markdown","plaintext"],"dynamicRegistration":false},"implementation":{"dynamicRegistration":false,"linkSupport":false},"inlayHint":{"dynamicRegistration":false},"onTypeFormatting":{"dynamicRegistration":false},"publishDiagnostics":{"relatedInformation":true,"tagSupport":{"valueSet":[1,2]}},"rangeFormatting":{"dynamicRegistration":false},"references":{"dynamicRegistration":false},"rename":{"dynamicRegistration":false,"prepareSupport":false},"selectionRange":{},"semanticTokens":{"dynamicRegistration":true,"formats":["relative"],"requests":{"full":true,"range":false},"serverCancelSupport":true,"tokenModifiers":[],"tokenTypes":[]},"signatureHelp":{"contextSupport":false,"dynamicRegistration":false,"signatureInformation":{"documentationFormat":["plaintext"],"parameterInformation":{"labelOffsetSupport":false}}},"synchronization":{"didSave":true,"dynamicRegistration":false,"willSave":false,"willSaveWaitUntil":false},"typeDefinition":{"dynamicRegistration":false,"linkSupport":false}},"window":{"showMessage":{"messageActionItem":{"additionalPropertiesSupport":true}},"workDoneProgress":true},"workspace":{"applyEdit":true,"codeLens":{"refreshSupport":true},"configuration":true,"didChangeConfiguration":{"dynamicRegistration":false},"executeCommand":{"dynamicRegistration":false},"inlayHint":{"refreshSupport":true},"semanticTokens":{"refreshSupport":true},"symbol":{"dynamicRegistration":false,"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"workspaceEdit":{"changeAnnotationSupport":{},"documentChanges":true,"failureHandling":"abort","normalizesLineEndings":false,"resourceOperations":["create","delete","rename"]},"workspaceFolders":false}},"clientInfo":{"name":"kakoune-lsp","version":"18.2.0"},"processId":22561,"rootPath":"/home/pascal/workspaces/bsc","rootUri":"file:///home/pascal/workspaces/bsc","trace":"off","workspaceFolders":[{"name":"/home/pascal/workspaces/bsc","uri":"file:///home/pascal/workspaces/bsc"}]},"id":0}
LSP: DEBG From server bsc: {"jsonrpc":"2.0","id":0,"result":{"capabilities":{"textDocumentSync":1,"completionProvider":{"resolveProvider":false,"triggerCharacters":["."],"allCommitCharacters":[".","@"]},"documentSymbolProvider":true,"workspaceSymbolProvider":true,"semanticTokensProvider":{"legend":{"tokenTypes":["namespace","type","class","enum","interface","struct","typeParameter","parameter","variable","property","enumMember","event","function","method","macro","keyword","modifier","comment","string","number","regexp","operator"],"tokenModifiers":["declaration","definition","readonly","static","deprecated","abstract","async","modification","documentation","defaultLibrary"]},"full":true},"referencesProvider":true,"codeActionProvider":{"codeActionKinds":["refactor"]},"signatureHelpProvider":{"triggerCharacters":["(",","]},"definitionProvider":true,"hoverProvider":true,"executeCommandProvider":{"commands":["TranspileFile"]}}}}
LSP: DEBG To server bsc: {"jsonrpc":"2.0","method":"initialized","params":{}}
LSP: DEBG Failed to read file /home/pascal/workspaces/bsc/testme.brs to simulate textDocument/didOpen: No such file or directory (os error 2)
LSP: DEBG To server bsc: {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"languageId":"brs","text":"","uri":"file:///home/pascal/workspaces/bsc/testme.brs","version":1}}}
LSP: DEBG To server bsc: {"jsonrpc":"2.0","method":"textDocument/hover","params":{"position":{"character":0,"line":0},"textDocument":{"uri":"file:///home/pascal/workspaces/bsc/testme.brs"}},"id":1}
LSP: DEBG From server bsc: {"jsonrpc":"2.0","method":"window/logMessage","params":{"type":4,"message":"[11:19:57:950 PM][LOG  ] onInitialized"}}
LSP: DEBG To editor `22482`: evaluate-commands -verbatim -try-client 'client0' lsp-show-message-log bsc '[11:19:57:950 PM][LOG  ] onInitialized'
LSP: log from bsc: [11:19:57:950 PM][LOG  ] onInitialized
LSP: DEBG From server bsc: {"jsonrpc":"2.0","id":0,"method":"workspace/workspaceFolders"}
LSP: WARN Unsupported method: workspace/workspaceFolders
LSP: DEBG To server bsc: {"jsonrpc":"2.0","error":{"code":-32601,"message":"Method not found"},"id":0}
LSP: DEBG From server bsc: {"jsonrpc":"2.0","method":"busyStatus","params":{"status":"busy","timestamp":1770506397955,"index":0,"activeRuns":[{"scope":"","label":"getHover","startTime":1770506397955}]}}

I believe this is a server issue rather than a client issue, as the MethodNotFound error response is a valid one according to the protocol.

NixOS 25.11

Start by reproducing the bsc LSP session with kak-lsp and trace handling of the workspace/workspaceFolders response through the LSP protocol layer, including the vscode-jsonrpc connection handling shown in the stack trace. The fix is done when a MethodNotFound response from the client no longer causes an uncaught exception; add or update a regression test if the existing LSP test structure provides one.