From 08b51bd249a3ba42fb300c129d4e08c5ba06df29 Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Fri, 15 May 2026 10:16:44 -0700
Subject: [PATCH] Only advertise `semanticTokens` if there's a handler

Neovim 0.12 and above will make `semanticTokens` requests on every
cursor movement. Currently, the `lsp` library advertises
`semanticTokens` capabilities, even if the LSP server implemented with
this library doesn't a handler. This means that every time I move my
cursor I get a blocking error in my editor, making it essentially
unusable!

Let's only advertise the capability if there's a handler registered.

Here's a workaround in my Neovim config:

```
vim.lsp.config("hls", {
  cmd = { "static-ls" },

  on_init = function(client)
    -- static-ls (via the `lsp` Haskell library) falsely advertises
    -- semanticTokensProvider but registers no handler, producing a
    -- "no handler for: textDocument/semanticTokens/full" message on
    -- every cursor move. Strip it.
    client.server_capabilities.semanticTokensProvider = nil
  end,
})
```
---
 lsp/src/Language/LSP/Server/Processing.hs | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/lsp/src/Language/LSP/Server/Processing.hs b/lsp/src/Language/LSP/Server/Processing.hs
index 4a2e46e2..5497ee76 100644
--- a/lsp/src/Language/LSP/Server/Processing.hs
+++ b/lsp/src/Language/LSP/Server/Processing.hs
@@ -399,12 +399,18 @@ inferServerCapabilities _clientCaps o h =
         Just cmds -> ExecuteCommandOptions clientInitiatedProgress cmds
         Nothing -> error "executeCommandCommands needs to be set if a executeCommandHandler is set"
 
-  -- Always provide the default legend
+  -- Only advertise semantic tokens if the server has registered a handler for
+  -- at least one of the semantic-token request methods. Otherwise clients that
+  -- honor server capabilities (e.g. Neovim's built-in LSP client) will fire
+  -- requests we can't answer, producing spurious "no handler for" errors.
   -- TODO: allow user-provided legend via 'Options', or at least user-provided types
-  semanticTokensProvider =
-    Just $
-      InL $
-        SemanticTokensOptions clientInitiatedProgress defaultSemanticTokensLegend semanticTokenRangeProvider semanticTokenFullProvider
+  semanticTokensProvider
+    | supported_b SMethod_TextDocumentSemanticTokensFull
+        || supported_b SMethod_TextDocumentSemanticTokensRange =
+        Just $
+          InL $
+            SemanticTokensOptions clientInitiatedProgress defaultSemanticTokensLegend semanticTokenRangeProvider semanticTokenFullProvider
+    | otherwise = Nothing
   semanticTokenRangeProvider
     | supported_b SMethod_TextDocumentSemanticTokensRange = Just $ InL True
     | otherwise = Nothing