Neovim LSP client ↔ external language servers: formatting selection with overlapping capabilities

Resolution of Client Ordering and Capabilities

No user-facing priority attribute determines the ordering of clients for formatting. When no filter is provided to vim.lsp.buf.format(), Neovim iterates through the internal client list based on client ID, which is assigned sequentially as servers start. Consequently, the first server that attached to the buffer and supports textDocument/formatting is typically selected. This is an implementation detail and should not be relied upon for deterministic configuration.

Capability merging is not performed across different servers; it is per-client. Each client merges its own server-declared capabilities with the client-side settings during initialization. When multiple servers offer the same feature, Neovim resolves this at the request level (e.g., picking the first available formatter or aggregating all completion items). This dispatch logic has evolved between versions 0.9 and 0.11, particularly regarding how client.supports_method is evaluated.

Diagnostics from multiple servers are not merged or deduplicated. Each server publishes diagnostics into its own unique namespace. If two servers report an error for the same range, both will be stored and displayed, resulting in overlapping signs or duplicate virtual text.

Implementation Details and Versioning

Confirmed Behavior:

• vim.lsp.buf.format() supports a filter function to explicitly select a server.
• Diagnostics are stored per-namespace via the vim.diagnostic API.

Version-Specific Variance:

The method for disabling overlapping capabilities in on_attach varies. In versions 0.9 and 0.10, users commonly set client.server_capabilities.documentFormattingProvider = false. In version 0.11, the internal handling of capabilities and the supports_method check have been refined, making explicit filters the preferred approach over manual capability mutation.

Verification and Mitigation Steps

1. Inspect current client order:

   :lua print(vim.inspect(vim.lsp.get_clients({bufnr=0})))

   (Use vim.lsp.get_active_clients() for Neovim 0.9).

1. Force a specific formatter via filter:

   vim.lsp.buf.format({ filter = function(c) return c.name == 'lua_ls' end })

1. Verify diagnostic namespaces:

   :lua print(vim.inspect(vim.diagnostic.get(0)))

   Check the namespace field to confirm multiple servers are contributing to the same range.

Required Diagnostic Detail

To provide the exact syntax for disabling capabilities in your on_attach function, please specify your exact Neovim version (e.g., 0.10.2), as the API for overriding server_capabilities has shifted toward supports_method in the latest releases.