diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index ccd4968c9d..38fb708983 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -6781,10 +6781,26 @@ func (c *Checker) reportUnusedVariable(location *ast.Node, diagnostic *ast.Diagn
 }
 
 func (c *Checker) reportUnused(location *ast.Node, kind UnusedKind, diagnostic *ast.Diagnostic) {
-	if location.Flags&(ast.NodeFlagsAmbient|ast.NodeFlagsThisNodeOrAnySubNodesHasError) == 0 &&
-		(kind == UnusedKindLocal && c.compilerOptions.NoUnusedLocals.IsTrue() ||
-			(kind == UnusedKindParameter && c.compilerOptions.NoUnusedParameters.IsTrue())) {
-		c.diagnostics.Add(diagnostic)
+	if location.Flags&(ast.NodeFlagsAmbient|ast.NodeFlagsThisNodeOrAnySubNodesHasError) == 0 {
+		isError := c.unusedIsError(kind)
+		if isError {
+			c.diagnostics.Add(diagnostic)
+		} else {
+			suggestion := *diagnostic
+			suggestion.SetCategory(diagnostics.CategorySuggestion)
+			c.suggestionDiagnostics.Add(&suggestion)
+		}
+	}
+}
+
+func (c *Checker) unusedIsError(kind UnusedKind) bool {
+	switch kind {
+	case UnusedKindLocal:
+		return c.compilerOptions.NoUnusedLocals.IsTrue()
+	case UnusedKindParameter:
+		return c.compilerOptions.NoUnusedParameters.IsTrue()
+	default:
+		panic("Unhandled case in unusedIsError")
 	}
 }
 
@@ -13401,18 +13417,30 @@ func (c *Checker) GetSuggestionDiagnostics(ctx context.Context, sourceFile *ast.
 
 func (c *Checker) getDiagnostics(ctx context.Context, sourceFile *ast.SourceFile, collection *ast.DiagnosticsCollection) []*ast.Diagnostic {
 	c.checkNotCanceled()
+	isSuggestionDiagnostics := collection == &c.suggestionDiagnostics
+
+	files := c.files
 	if sourceFile != nil {
-		c.CheckSourceFile(ctx, sourceFile)
-		if c.wasCanceled {
-			return nil
-		}
-		return collection.GetDiagnosticsForFile(sourceFile.FileName())
+		files = []*ast.SourceFile{sourceFile}
 	}
-	for _, file := range c.files {
+
+	for _, file := range files {
 		c.CheckSourceFile(ctx, file)
 		if c.wasCanceled {
 			return nil
 		}
+
+		// Check unused identifiers as suggestions if we're collecting suggestion diagnostics
+		// and they are not configured as errors
+		if isSuggestionDiagnostics && !file.IsDeclarationFile &&
+			!(c.compilerOptions.NoUnusedLocals.IsTrue() || c.compilerOptions.NoUnusedParameters.IsTrue()) {
+			links := c.sourceFileLinks.Get(file)
+			c.checkUnusedIdentifiers(links.identifierCheckNodes)
+		}
+	}
+
+	if sourceFile != nil {
+		return collection.GetDiagnosticsForFile(sourceFile.FileName())
 	}
 	return collection.GetDiagnostics()
 }
diff --git a/internal/fourslash/fourslash.go b/internal/fourslash/fourslash.go
index 842e43d52d..1b78246b62 100644
--- a/internal/fourslash/fourslash.go
+++ b/internal/fourslash/fourslash.go
@@ -282,6 +282,17 @@ func getCapabilitiesWithDefaults(capabilities *lsproto.ClientCapabilities) *lspr
 			},
 		}
 	}
+	if capabilitiesWithDefaults.TextDocument.PublishDiagnostics == nil {
+		capabilitiesWithDefaults.TextDocument.PublishDiagnostics = &lsproto.PublishDiagnosticsClientCapabilities{
+			RelatedInformation: ptrTrue,
+			TagSupport: &lsproto.ClientDiagnosticsTagOptions{
+				ValueSet: []lsproto.DiagnosticTag{
+					lsproto.DiagnosticTagUnnecessary,
+					lsproto.DiagnosticTagDeprecated,
+				},
+			},
+		}
+	}
 	if capabilitiesWithDefaults.Workspace == nil {
 		capabilitiesWithDefaults.Workspace = &lsproto.WorkspaceClientCapabilities{}
 	}
diff --git a/internal/ls/diagnostics.go b/internal/ls/diagnostics.go
index 83c3f1d61d..28df9c80bb 100644
--- a/internal/ls/diagnostics.go
+++ b/internal/ls/diagnostics.go
@@ -20,6 +20,7 @@ func (l *LanguageService) ProvideDiagnostics(ctx context.Context, uri lsproto.Do
 	diagnostics = append(diagnostics, program.GetSemanticDiagnostics(ctx, file))
 	// !!! user preference for suggestion diagnostics; keep only unnecessary/deprecated?
 	// See: https://github.com/microsoft/vscode/blob/3dbc74129aaae102e5cb485b958fa5360e8d3e7a/extensions/typescript-language-features/src/languageFeatures/diagnostics.ts#L114
+	// TODO: also implement reportStyleCheckAsWarnings to rewrite diags with Warning severity
 	diagnostics = append(diagnostics, program.GetSuggestionDiagnostics(ctx, file))
 	if program.Options().GetEmitDeclarations() {
 		diagnostics = append(diagnostics, program.GetDeclarationDiagnostics(ctx, file))
diff --git a/internal/lsp/server.go b/internal/lsp/server.go
index 0f086229b6..cecd426747 100644
--- a/internal/lsp/server.go
+++ b/internal/lsp/server.go
@@ -800,11 +800,7 @@ func (s *Server) handleSetTrace(ctx context.Context, params *lsproto.SetTracePar
 }
 
 func (s *Server) handleDocumentDiagnostic(ctx context.Context, ls *ls.LanguageService, params *lsproto.DocumentDiagnosticParams) (lsproto.DocumentDiagnosticResponse, error) {
-	var diagnosticClientCapabilities *lsproto.DiagnosticClientCapabilities
-	if s.initializeParams != nil && s.initializeParams.Capabilities != nil && s.initializeParams.Capabilities.TextDocument != nil {
-		diagnosticClientCapabilities = s.initializeParams.Capabilities.TextDocument.Diagnostic
-	}
-	return ls.ProvideDiagnostics(ctx, params.TextDocument.Uri, diagnosticClientCapabilities)
+	return ls.ProvideDiagnostics(ctx, params.TextDocument.Uri, getDiagnosticClientCapabilities(s.initializeParams))
 }
 
 func (s *Server) handleHover(ctx context.Context, ls *ls.LanguageService, params *lsproto.HoverParams) (lsproto.HoverResponse, error) {
@@ -1037,3 +1033,28 @@ func getSignatureHelpDocumentationFormat(params *lsproto.InitializeParams) lspro
 	// Return the first (most preferred) format
 	return formats[0]
 }
+
+func getDiagnosticClientCapabilities(params *lsproto.InitializeParams) *lsproto.DiagnosticClientCapabilities {
+	if params == nil || params.Capabilities == nil || params.Capabilities.TextDocument == nil {
+		return nil
+	}
+
+	var caps lsproto.DiagnosticClientCapabilities
+	if params.Capabilities.TextDocument.Diagnostic != nil {
+		caps = *params.Capabilities.TextDocument.Diagnostic
+	}
+
+	// Some clients claim that push and pull diagnostics have different capabilities,
+	// including vscode-languageclient v9. Work around this by defaulting any missing
+	// pull diagnostic caps with the pull diagnostic equivalents.
+	//
+	// TODO: remove when we upgrade to vscode-languageclient v10, which fixes this issue.
+	if publish := params.Capabilities.TextDocument.PublishDiagnostics; publish != nil {
+		caps.RelatedInformation = core.Coalesce(caps.RelatedInformation, publish.RelatedInformation)
+		caps.TagSupport = core.Coalesce(caps.TagSupport, publish.TagSupport)
+		caps.CodeDescriptionSupport = core.Coalesce(caps.CodeDescriptionSupport, publish.CodeDescriptionSupport)
+		caps.DataSupport = core.Coalesce(caps.DataSupport, publish.DataSupport)
+	}
+
+	return &caps
+}