Stale errors remain in the stats and buffers, even when there are no errors

[ranjeethmahankali]

Thank you for the bug report

• I am using the latest version of lsp-mode related packages.
• I checked FAQ and Troubleshooting sections
• You may also try reproduce the issue using clean environment using the following command: M-x lsp-start-plain

Bug description

Errors, linter warnings occur while I am typing code, when I typed something incomplete or incorrect. But once I fix them, they should go away. This used to work fine until recently. After a recent update however, even after I fix all the errors and warnings, the title bar at the top of the file still has a red underline, and when I open LSP Error List buffer, it shows me, for example 9 errors (number may vary) but the list itself is empty. The code compiles fine. It's just hanging on to the errors that once were.

Steps to reproduce

Reproduction:**

1. Open a file with LSP (needs a workflow that triggers lsp-after-apply-edits-hook, e.g. lsp-format-buffer on save)
2. Introduce errors, then fix them and save
3. Headerline breadcrumb still shows error face on the filename
4. *LSP Errors List* reports N errors but lists none
5. Restarting LSP server does not fix it — only restarting Emacs does

Expected behavior

When I fix the errors, the red underlines and the error statistics should be cleared.

Which Language Server did you use?

C++, clangd

OS

Linux

Error callstack

Anything else?

The following is the analysis, fix suggestion and workaround that Claude Code gave me. I sort of understand what it is saying at a high level, and the workaround does seem to do the trick in my config, but it may be all BS. If it doesn't help, please ignore the stuff below:

Root cause:**

lsp-diagnostics--clear-after-edit does remhash on (lsp--workspace-diagnostics workspace) without calling lsp-diagnostics--convert-and-update-path-stats first. This means:

• The global lsp-diagnostic-stats is never decremented for the removed diagnostics
• When the server next publishes diagnostics, the delta calculation in lsp-diagnostics--convert-and-update-path-stats finds no old diagnostics to subtract (already removed), so the counts only grow
• lsp-diagnostics--workspace-cleanup also can't fix it, since it iterates the workspace hash which no longer contains the removed entries
• The counts accumulate with each edit cycle and persist until Emacs restarts

Suggested fix:

Call lsp-diagnostics--convert-and-update-path-stats with empty diagnostics before the remhash:

(defun lsp-diagnostics--clear-after-edit (&rest _)
  (when-let* ((file-path (buffer-file-name))
              (workspaces (lsp-workspaces)))
    (let ((path (lsp--fix-path-casing file-path)))
      (dolist (workspace workspaces)
        (lsp-diagnostics--convert-and-update-path-stats workspace path nil)
        (-let [diagnostics (lsp--workspace-diagnostics workspace)]
          (remhash path diagnostics))))
    (run-hooks 'lsp-diagnostics-updated-hook)))

Workaround:

(remove-hook 'lsp-after-apply-edits-hook #'lsp-diagnostics--clear-after-edit)

This reverts to the pre-#3888 behavior where stale diagnostics may briefly appear at wrong line numbers after workspace edits, but avoids the permanent ghost error accumulation.