diff --git a/internal/lsp/diagnostics.go b/internal/lsp/diagnostics.go
index 7a709da1a5..22f056e4a4 100644
--- a/internal/lsp/diagnostics.go
+++ b/internal/lsp/diagnostics.go
@@ -90,6 +90,9 @@ func (s *Server) publishReports(ctx context.Context, reports map[source.FileIden
 		return
 	}
 
+	s.deliveredMu.Lock()
+	defer s.deliveredMu.Unlock()
+
 	for fileID, diagnostics := range reports {
 		// Don't deliver diagnostics if the context has already been canceled.
 		if ctx.Err() != nil {
@@ -99,6 +102,26 @@ func (s *Server) publishReports(ctx context.Context, reports map[source.FileIden
 		if len(diagnostics) == 0 && !publishEmpty {
 			continue
 		}
+		// Pre-sort diagnostics to avoid extra work when we compare them.
+		source.SortDiagnostics(diagnostics)
+		toSend := sentDiagnostics{
+			version:    fileID.Version,
+			identifier: fileID.Identifier,
+			sorted:     diagnostics,
+		}
+
+		if delivered, ok := s.delivered[fileID.URI]; ok {
+			// We only reuse cached diagnostics in two cases:
+			//   1. This file is at a greater version than that of the previously sent diagnostics.
+			//   2. There are no known versions for the file.
+			greaterVersion := fileID.Version > delivered.version && delivered.version > 0
+			noVersions := (fileID.Version == 0 && delivered.version == 0) && delivered.identifier == fileID.Identifier
+			if (greaterVersion || noVersions) && equalDiagnostics(delivered.sorted, diagnostics) {
+				// Update the delivered map even if we reuse cached diagnostics.
+				s.delivered[fileID.URI] = toSend
+				continue
+			}
+		}
 		if err := s.client.PublishDiagnostics(ctx, &protocol.PublishDiagnosticsParams{
 			Diagnostics: toProtocolDiagnostics(ctx, diagnostics),
 			URI:         protocol.NewURI(fileID.URI),
@@ -107,9 +130,25 @@ func (s *Server) publishReports(ctx context.Context, reports map[source.FileIden
 			log.Error(ctx, "failed to deliver diagnostic", err, telemetry.File)
 			continue
 		}
+		// Update the delivered map.
+		s.delivered[fileID.URI] = toSend
 	}
 }
 
+// equalDiagnostics returns true if the 2 lists of diagnostics are equal.
+// It assumes that both a and b are already sorted.
+func equalDiagnostics(a, b []source.Diagnostic) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if source.CompareDiagnostic(a[i], b[i]) != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 func toProtocolDiagnostics(ctx context.Context, diagnostics []source.Diagnostic) []protocol.Diagnostic {
 	reports := []protocol.Diagnostic{}
 	for _, diag := range diagnostics {
diff --git a/internal/lsp/lsp_test.go b/internal/lsp/lsp_test.go
index 3ff4642d80..664279e6c9 100644
--- a/internal/lsp/lsp_test.go
+++ b/internal/lsp/lsp_test.go
@@ -73,7 +73,8 @@ func testLSP(t *testing.T, exporter packagestest.Exporter) {
 	}
 	r := &runner{
 		server: &Server{
-			session: session,
+			session:   session,
+			delivered: map[span.URI]sentDiagnostics{},
 		},
 		data: data,
 		ctx:  ctx,
diff --git a/internal/lsp/server.go b/internal/lsp/server.go
index e388f71b05..e500b48212 100644
--- a/internal/lsp/server.go
+++ b/internal/lsp/server.go
@@ -21,15 +21,18 @@ import (
 func NewClientServer(ctx context.Context, cache source.Cache, client protocol.Client) (context.Context, *Server) {
 	ctx = protocol.WithClient(ctx, client)
 	return ctx, &Server{
-		client:  client,
-		session: cache.NewSession(ctx),
+		client:    client,
+		session:   cache.NewSession(ctx),
+		delivered: make(map[span.URI]sentDiagnostics),
 	}
 }
 
 // NewServer starts an LSP server on the supplied stream, and waits until the
 // stream is closed.
 func NewServer(ctx context.Context, cache source.Cache, stream jsonrpc2.Stream) (context.Context, *Server) {
-	s := &Server{}
+	s := &Server{
+		delivered: make(map[span.URI]sentDiagnostics),
+	}
 	ctx, s.Conn, s.client = protocol.NewServer(ctx, stream, s)
 	s.session = cache.NewSession(ctx)
 	return ctx, s
@@ -85,6 +88,17 @@ type Server struct {
 	// folders is only valid between initialize and initialized, and holds the
 	// set of folders to build views for when we are ready
 	pendingFolders []protocol.WorkspaceFolder
+
+	// delivered is a cache of the diagnostics that the server has sent.
+	deliveredMu sync.Mutex
+	delivered   map[span.URI]sentDiagnostics
+}
+
+// sentDiagnostics is used to cache diagnostics that have been sent for a given file.
+type sentDiagnostics struct {
+	version    float64
+	identifier string
+	sorted     []source.Diagnostic
 }
 
 // General
diff --git a/internal/lsp/source/util.go b/internal/lsp/source/util.go
index a8d7ad21b7..f9b321f50f 100644
--- a/internal/lsp/source/util.go
+++ b/internal/lsp/source/util.go
@@ -13,6 +13,7 @@ import (
 	"go/types"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strings"
 
 	"golang.org/x/tools/internal/lsp/protocol"
@@ -590,3 +591,22 @@ func formatFunction(params []string, results []string, writeResultParens bool) s
 
 	return detail.String()
 }
+
+func SortDiagnostics(d []Diagnostic) {
+	sort.Slice(d, func(i int, j int) bool {
+		return CompareDiagnostic(d[i], d[j]) < 0
+	})
+}
+
+func CompareDiagnostic(a, b Diagnostic) int {
+	if r := protocol.CompareRange(a.Range, b.Range); r != 0 {
+		return r
+	}
+	if a.Message < b.Message {
+		return -1
+	}
+	if a.Message == b.Message {
+		return 0
+	}
+	return 1
+}
diff --git a/internal/lsp/tests/diagnostics.go b/internal/lsp/tests/diagnostics.go
index 6b0c1cf1c5..192cde40f5 100644
--- a/internal/lsp/tests/diagnostics.go
+++ b/internal/lsp/tests/diagnostics.go
@@ -3,7 +3,6 @@ package tests
 import (
 	"bytes"
 	"fmt"
-	"sort"
 	"strings"
 
 	"golang.org/x/tools/internal/lsp/protocol"
@@ -14,8 +13,8 @@ import (
 // DiffDiagnostics prints the diff between expected and actual diagnostics test
 // results.
 func DiffDiagnostics(uri span.URI, want, got []source.Diagnostic) string {
-	sortDiagnostics(want)
-	sortDiagnostics(got)
+	source.SortDiagnostics(want)
+	source.SortDiagnostics(got)
 
 	if len(got) != len(want) {
 		return summarizeDiagnostics(-1, uri, want, got, "different lengths got %v want %v", len(got), len(want))
@@ -47,26 +46,6 @@ func DiffDiagnostics(uri span.URI, want, got []source.Diagnostic) string {
 	return ""
 }
 
-func sortDiagnostics(d []source.Diagnostic) {
-	sort.Slice(d, func(i int, j int) bool {
-		return compareDiagnostic(d[i], d[j]) < 0
-	})
-}
-
-func compareDiagnostic(a, b source.Diagnostic) int {
-	if r := protocol.CompareRange(a.Range, b.Range); r != 0 {
-		return r
-	}
-	if a.Message < b.Message {
-		return -1
-	}
-	if a.Message == b.Message {
-		return 0
-	} else {
-		return 1
-	}
-}
-
 func summarizeDiagnostics(i int, uri span.URI, want []source.Diagnostic, got []source.Diagnostic, reason string, args ...interface{}) string {
 	msg := &bytes.Buffer{}
 	fmt.Fprint(msg, "diagnostics failed")