diff --git a/internal/lsp/cache/builtin.go b/internal/lsp/cache/builtin.go
index f9d2fe52d7..eee8297f2a 100644
--- a/internal/lsp/cache/builtin.go
+++ b/internal/lsp/cache/builtin.go
@@ -7,6 +7,7 @@ package cache
 import (
 	"context"
 	"go/ast"
+	"strings"
 
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/internal/lsp/source"
@@ -35,6 +36,10 @@ func (b *builtinPkg) CompiledGoFiles() []source.ParseGoHandle {
 func (v *view) buildBuiltinPackage(ctx context.Context) error {
 	cfg := v.Config(ctx)
 	pkgs, err := packages.Load(cfg, "builtin")
+	// If the error is related to a go.mod parse error, we want to continue loading.
+	if err != nil && strings.Contains(err.Error(), ".mod:") {
+		return nil
+	}
 	if err != nil {
 		return err
 	}
diff --git a/internal/lsp/cache/parse_mod.go b/internal/lsp/cache/parse_mod.go
index 7c358d3db1..07b219f75b 100644
--- a/internal/lsp/cache/parse_mod.go
+++ b/internal/lsp/cache/parse_mod.go
@@ -6,11 +6,16 @@ package cache
 
 import (
 	"context"
+	"regexp"
+	"strconv"
+	"strings"
 
 	"golang.org/x/mod/modfile"
+	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/lsp/telemetry"
 	"golang.org/x/tools/internal/memoize"
+	"golang.org/x/tools/internal/telemetry/log"
 	"golang.org/x/tools/internal/telemetry/trace"
 	errors "golang.org/x/xerrors"
 )
@@ -39,7 +44,7 @@ func (c *cache) ParseModHandle(fh source.FileHandle) source.ParseModHandle {
 	}
 }
 
-func parseMod(ctx context.Context, fh source.FileHandle) (modifle *modfile.File, err error) {
+func parseMod(ctx context.Context, fh source.FileHandle) (*modfile.File, error) {
 	ctx, done := trace.StartSpan(ctx, "cache.parseMod", telemetry.File.Of(fh.Identity().URI.Filename()))
 	defer done()
 
@@ -49,7 +54,25 @@ func parseMod(ctx context.Context, fh source.FileHandle) (modifle *modfile.File,
 	}
 	f, err := modfile.Parse(fh.Identity().URI.Filename(), buf, nil)
 	if err != nil {
-		return nil, err
+		// TODO(golang/go#36486): This can be removed when modfile.Parse returns structured errors.
+		re := regexp.MustCompile(`.*:([\d]+): (.+)`)
+		matches := re.FindStringSubmatch(strings.TrimSpace(err.Error()))
+		if len(matches) < 3 {
+			log.Error(ctx, "could not parse golang/x/mod error message", err)
+			return nil, err
+		}
+		line, e := strconv.Atoi(matches[1])
+		if e != nil {
+			return nil, err
+		}
+		contents := strings.Split(string(buf), "\n")[line-1]
+		return nil, &source.Error{
+			Message: matches[2],
+			Range: protocol.Range{
+				Start: protocol.Position{Line: float64(line - 1), Character: float64(0)},
+				End:   protocol.Position{Line: float64(line - 1), Character: float64(len(contents))},
+			},
+		}
 	}
 	return f, nil
 }
diff --git a/internal/lsp/mod/diagnostics.go b/internal/lsp/mod/diagnostics.go
index eb5ff40a55..2d8b04e7f5 100644
--- a/internal/lsp/mod/diagnostics.go
+++ b/internal/lsp/mod/diagnostics.go
@@ -9,6 +9,7 @@ package mod
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"golang.org/x/mod/modfile"
 	"golang.org/x/tools/internal/lsp/protocol"
@@ -36,10 +37,23 @@ func Diagnostics(ctx context.Context, snapshot source.Snapshot) (source.FileIden
 	cfg := snapshot.View().Config(ctx)
 	args := append([]string{"mod", "tidy"}, cfg.BuildFlags...)
 	if _, err := source.InvokeGo(ctx, snapshot.View().Folder().Filename(), cfg.Env, args...); err != nil {
-		return source.FileIdentity{}, nil, err
+		// Ignore parse errors here. They'll be handled below.
+		if !strings.Contains(err.Error(), "errors parsing go.mod") {
+			return source.FileIdentity{}, nil, err
+		}
 	}
 
 	realMod, err := snapshot.View().Session().Cache().ParseModHandle(realfh).Parse(ctx)
+	if err, ok := err.(*source.Error); ok {
+		return realfh.Identity(), []source.Diagnostic{
+			{
+				Message:  err.Message,
+				Source:   "go mod tidy",
+				Range:    err.Range,
+				Severity: protocol.SeverityError,
+			},
+		}, nil
+	}
 	if err != nil {
 		return source.FileIdentity{}, nil, err
 	}
@@ -48,8 +62,8 @@ func Diagnostics(ctx context.Context, snapshot source.Snapshot) (source.FileIden
 		return source.FileIdentity{}, nil, err
 	}
 
-	reports := []source.Diagnostic{}
 	// Check indirect vs direct, and removal of dependencies.
+	reports := []source.Diagnostic{}
 	realReqs := make(map[string]*modfile.Require, len(realMod.Require))
 	tempReqs := make(map[string]*modfile.Require, len(tempMod.Require))
 	for _, req := range realMod.Require {
@@ -84,7 +98,7 @@ func Diagnostics(ctx context.Context, snapshot source.Snapshot) (source.FileIden
 	return realfh.Identity(), reports, nil
 }
 
-// TODO: Check to see if we need to go through internal/span.
+// TODO: Check to see if we need to go through internal/span (for multiple byte characters).
 func getPos(pos modfile.Position) protocol.Position {
 	return protocol.Position{
 		Line:      float64(pos.Line - 1),
diff --git a/internal/lsp/mod/mod_test.go b/internal/lsp/mod/mod_test.go
index 558bb77f69..8f2e17c194 100644
--- a/internal/lsp/mod/mod_test.go
+++ b/internal/lsp/mod/mod_test.go
@@ -27,6 +27,8 @@ func TestMain(m *testing.M) {
 	os.Exit(m.Run())
 }
 
+// TODO(golang/go#36091): This file can be refactored to look like lsp_test.go
+// when marker support gets added for go.mod files.
 func TestModfileRemainsUnchanged(t *testing.T) {
 	ctx := tests.Context(t)
 	cache := cache.New(nil)
@@ -35,8 +37,6 @@ func TestModfileRemainsUnchanged(t *testing.T) {
 	options.TempModfile = true
 	options.Env = append(os.Environ(), "GOPACKAGESDRIVER=off", "GOROOT=")
 
-	// TODO: Once we refactor this to work with go/packages/packagestest. We do not
-	// need to copy to a temporary directory.
 	// Make sure to copy the test directory to a temporary directory so we do not
 	// modify the test code or add go.sum files when we run the tests.
 	folder, err := copyToTempDir(filepath.Join("testdata", "unchanged"))
@@ -65,6 +65,8 @@ func TestModfileRemainsUnchanged(t *testing.T) {
 	}
 }
 
+// TODO(golang/go#36091): This file can be refactored to look like lsp_test.go
+// when marker support gets added for go.mod files.
 func TestDiagnostics(t *testing.T) {
 	ctx := tests.Context(t)
 	cache := cache.New(nil)
@@ -81,8 +83,10 @@ func TestDiagnostics(t *testing.T) {
 			testdir: "indirect",
 			want: []source.Diagnostic{
 				{
-					Message:  "golang.org/x/tools should not be an indirect dependency.",
-					Source:   "go mod tidy",
+					Message: "golang.org/x/tools should not be an indirect dependency.",
+					Source:  "go mod tidy",
+					// TODO(golang/go#36091): When marker support gets added for go.mod files, we
+					// can remove these hard coded positions.
 					Range:    protocol.Range{Start: getPos(4, 0), End: getPos(4, 61)},
 					Severity: protocol.SeverityWarning,
 				},
@@ -99,10 +103,41 @@ func TestDiagnostics(t *testing.T) {
 				},
 			},
 		},
+		{
+			testdir: "invalidrequire",
+			want: []source.Diagnostic{
+				{
+					Message:  "usage: require module/path v1.2.3",
+					Source:   "go mod tidy",
+					Range:    protocol.Range{Start: getPos(4, 0), End: getPos(4, 17)},
+					Severity: protocol.SeverityError,
+				},
+			},
+		},
+		{
+			testdir: "invalidgo",
+			want: []source.Diagnostic{
+				{
+					Message:  "usage: go 1.23",
+					Source:   "go mod tidy",
+					Range:    protocol.Range{Start: getPos(2, 0), End: getPos(2, 4)},
+					Severity: protocol.SeverityError,
+				},
+			},
+		},
+		{
+			testdir: "unknowndirective",
+			want: []source.Diagnostic{
+				{
+					Message:  "unknown directive: yo",
+					Source:   "go mod tidy",
+					Range:    protocol.Range{Start: getPos(6, 0), End: getPos(6, 2)},
+					Severity: protocol.SeverityError,
+				},
+			},
+		},
 	} {
 		t.Run(tt.testdir, func(t *testing.T) {
-			// TODO: Once we refactor this to work with go/packages/packagestest. We do not
-			// need to copy to a temporary directory.
 			// Make sure to copy the test directory to a temporary directory so we do not
 			// modify the test code or add go.sum files when we run the tests.
 			folder, err := copyToTempDir(filepath.Join("testdata", tt.testdir))
diff --git a/internal/lsp/mod/testdata/indirect/go.mod b/internal/lsp/mod/testdata/indirect/go.mod
index 2e5dc1384a..84e1b537c0 100644
--- a/internal/lsp/mod/testdata/indirect/go.mod
+++ b/internal/lsp/mod/testdata/indirect/go.mod
@@ -1,4 +1,4 @@
-module indirect
+module invalidrequire
 
 go 1.12
 
diff --git a/internal/lsp/mod/testdata/invalidgo/go.mod b/internal/lsp/mod/testdata/invalidgo/go.mod
new file mode 100644
index 0000000000..ca06b60146
--- /dev/null
+++ b/internal/lsp/mod/testdata/invalidgo/go.mod
@@ -0,0 +1,5 @@
+module invalidgo
+
+go 1
+
+require golang.org/x/tools v0.0.0-20191219192050-56b0b28a00f7 // indirect
diff --git a/internal/lsp/mod/testdata/invalidgo/main.go b/internal/lsp/mod/testdata/invalidgo/main.go
new file mode 100644
index 0000000000..5577a36e55
--- /dev/null
+++ b/internal/lsp/mod/testdata/invalidgo/main.go
@@ -0,0 +1,10 @@
+// Package invalidgo does something
+package invalidgo
+
+import (
+	"golang.org/x/tools/go/packages"
+)
+
+func Yo() {
+	var _ packages.Config
+}
diff --git a/internal/lsp/mod/testdata/invalidrequire/go.mod b/internal/lsp/mod/testdata/invalidrequire/go.mod
new file mode 100644
index 0000000000..6829b9c21a
--- /dev/null
+++ b/internal/lsp/mod/testdata/invalidrequire/go.mod
@@ -0,0 +1,5 @@
+module indirect
+
+go 1.12
+
+require golang.or
diff --git a/internal/lsp/mod/testdata/invalidrequire/main.go b/internal/lsp/mod/testdata/invalidrequire/main.go
new file mode 100644
index 0000000000..dd24341ae8
--- /dev/null
+++ b/internal/lsp/mod/testdata/invalidrequire/main.go
@@ -0,0 +1,10 @@
+// Package invalidrequire does something
+package invalidrequire
+
+import (
+	"golang.org/x/tools/go/packages"
+)
+
+func Yo() {
+	var _ packages.Config
+}
diff --git a/internal/lsp/mod/testdata/unknowndirective/go.mod b/internal/lsp/mod/testdata/unknowndirective/go.mod
new file mode 100644
index 0000000000..4f07729db3
--- /dev/null
+++ b/internal/lsp/mod/testdata/unknowndirective/go.mod
@@ -0,0 +1,7 @@
+module unknowndirective
+
+go 1.12
+
+require golang.org/x/tools v0.0.0-20191219192050-56b0b28a00f7
+
+yo
diff --git a/internal/lsp/mod/testdata/unknowndirective/main.go b/internal/lsp/mod/testdata/unknowndirective/main.go
new file mode 100644
index 0000000000..5ee984e8e4
--- /dev/null
+++ b/internal/lsp/mod/testdata/unknowndirective/main.go
@@ -0,0 +1,10 @@
+// Package unknowndirective does something
+package unknowndirective
+
+import (
+	"golang.org/x/tools/go/packages"
+)
+
+func Yo() {
+	var _ packages.Config
+}