diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 9270726a0b..bcd64e9cd9 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -87,6 +87,11 @@ LSP
 • `vim.lsp.semantic_tokens.start/stop` now renamed to
   `vim.lsp.semantic_tokens.enable`
 • Missing fields in LSP messages are now represented using |vim.NIL| instead of nil.
+• |vim.lsp.util.convert_signature_help_to_markdown_lines()| activeParameter
+  handling updated:
+    • Values < 0 are now treated as `nil` instead of 0.
+    • Values outside the range of `signatures[activeSignature].parameters`
+      are now treated as `nil` instead of `#signatures[activeSignature].parameters`
 
 LUA
 
@@ -222,6 +227,7 @@ LSP
   https://microsoft.github.io/language-server-protocol/specification/#textDocument_linkedEditingRange
 • Support for related documents in pull diagnostics:
   https://microsoft.github.io/language-server-protocol/specifications/specification-current/#relatedFullDocumentDiagnosticReport
+• |vim.lsp.buf.signature_help()| supports "noActiveParameterSupport".
 
 LUA
 
diff --git a/runtime/lua/vim/lsp/buf.lua b/runtime/lua/vim/lsp/buf.lua
index 6dc90f7184..86b456546e 100644
--- a/runtime/lua/vim/lsp/buf.lua
+++ b/runtime/lua/vim/lsp/buf.lua
@@ -330,8 +330,8 @@ local function process_signature_help_results(results)
       )
       api.nvim_command('redraw')
     else
-      local result = r.result --- @type lsp.SignatureHelp
-      if result and result.signatures and result.signatures[1] then
+      local result = r.result
+      if result and result.signatures then
         for i, sig in ipairs(result.signatures) do
           sig.activeParameter = sig.activeParameter or result.activeParameter
           local idx = #signatures + 1
diff --git a/runtime/lua/vim/lsp/protocol.lua b/runtime/lua/vim/lsp/protocol.lua
index fd3d192431..30b993c1cc 100644
--- a/runtime/lua/vim/lsp/protocol.lua
+++ b/runtime/lua/vim/lsp/protocol.lua
@@ -514,6 +514,7 @@ function protocol.make_client_capabilities()
         dynamicRegistration = false,
         signatureInformation = {
           activeParameterSupport = true,
+          noActiveParameterSupport = true,
           documentationFormat = { constants.MarkupKind.Markdown, constants.MarkupKind.PlainText },
           parameterInformation = {
             labelOffsetSupport = true,
diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 753dbad9c4..6c3641e627 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -829,18 +829,21 @@ function M.convert_signature_help_to_markdown_lines(signature_help, ft, triggers
     M.convert_input_to_markdown_lines(signature.documentation, contents)
   end
   if signature.parameters and #signature.parameters > 0 then
-    -- First check if the signature has an activeParameter. If it doesn't check if the response
-    -- had that property instead. Else just default to 0.
-    --
-    -- NOTE: Using tonumber() as a temporary workaround to handle `vim.NIL` until #34838 is merged
-    local active_parameter = math.max(
-      tonumber(signature.activeParameter) or tonumber(signature_help.activeParameter) or 0,
-      0
-    )
+    local active_parameter = signature.activeParameter or signature_help.activeParameter
 
-    -- If the activeParameter is > #parameters, then set it to the last
-    -- NOTE: this is not fully according to the spec, but a client-side interpretation
-    active_parameter = math.min(active_parameter, #signature.parameters - 1)
+    -- NOTE: We intentionally violate the LSP spec, which states that if `activeParameter`
+    -- is not provided or is out-of-bounds, it should default to 0.
+    -- Instead, we default to `nil`, as most clients do. In practice, 'no active parameter'
+    -- is better default than 'first parameter' and aligns better with user expectations.
+    -- Related discussion: https://github.com/microsoft/language-server-protocol/issues/1271
+    if
+      not active_parameter
+      or active_parameter == vim.NIL
+      or active_parameter < 0
+      or active_parameter >= #signature.parameters
+    then
+      return contents, nil
+    end
 
     local parameter = signature.parameters[active_parameter + 1]
     local parameter_label = parameter.label