Neovim Core ↔ LSP Client: Resolving overlapping diagnostics from multiple servers

Short answer

Neovim does not assign a deterministic priority to overlapping diagnostics from different LSP clients, and it does not merge or discard them. Each publishDiagnostics notification replaces only that client’s diagnostic set for the buffer. Diagnostics from other clients remain intact, so overlapping ranges are kept side-by-side and shown together unless you filter them yourself.

Confirmed facts (Neovim 0.9+)

• Diagnostics are stored per client namespace; vim.diagnostic.get(bufnr) aggregates all namespaces for display.
• publishDiagnostics updates the emitting client’s namespace only. It does not overwrite diagnostics published by another server.
• Arrival order changes when a namespace refreshes, not which diagnostic survives.
• vim.diagnostic.config({ severity_sort = true }) and virtual-text or sign options affect display order and rendering, not removal of overlaps.
• No built-in "highest severity wins" merge strategy exists in vim.diagnostic or the LSP client options.

Likely explanation

Because each server owns its namespace, late-arriving diagnostics from server B cannot delete server A’s diagnostics for the same range. The result is often visual clutter: multiple virtual texts or signs on one line. This is presentation stacking, not a conflict-resolution policy.

Verification steps

1. Attach two language servers to one buffer and note their client IDs.
2. Run :lua print(vim.inspect(vim.diagnostic.get(0, { namespace = ns_id }))) for each client to confirm independent sets.
3. Run :lua print(vim.inspect(vim.diagnostic.get(0))) to see the combined set.
4. Toggle :lua vim.diagnostic.config({ severity_sort = true }) and observe that duplicates remain but are reordered.

What to do next

If you only want to reorder duplicates, vim.diagnostic.config is sufficient. If you want to suppress or merge overlapping diagnostics, you must implement a custom textDocument/publishDiagnostics handler or post-process with vim.diagnostic.get and vim.diagnostic.set; core provides no configurable merge policy.

Assumption: behavior described applies to Neovim 0.9 and 0.10; diagnostic APIs changed between releases, so verify your version with :version. Uncertain: whether a specific third-party plugin layers its own deduplication on top of core.

Do you need to remove duplicate diagnostics entirely, or only change their display order? That determines whether a custom handler is required.