Why does clangd respond with "method not found" when using textDocument/didOpen? - language-server-protocol

Question

I try to communicate with clangd. The first initialization is fine, but when trying to open a document with "textDocument/didOpen" i get the response "method not found". I cannot find any other method in the specification for opening documents. Is it the wrong method, or am I doing something else wrong?
My request
Content-Length: 1771
{
"id": 2,
"jsonrpc": "2.0",
"method": "textDocument/didOpen",
"params": {
"textDocument": {
"languageId": "cpp",
"text": "file content...",
"uri": "file://src/main.cpp",
"version": 1
}
}
}
The response I get:
Content-Length: 77
{
"error": {
"code": -32601,
"message": "method not found"
},
"id": 2,
"jsonrpc": "2.0"
}

Answer 1

The language server protocol distinguishes between request messages which are client --> server messages to which the server will respond, and notification messages which are client --> server messages which are meant to notify the server about something and do not require a response.
The id field is only expected for request messages (it's used to associate the response with the request). However, textDocument/didOpen is a notification message. Removing the id field should fix the error.
Further reading:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#requestMessage
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#notificationMessage