diff --git a/cmd/vet/lowerell/analyzer.go b/cmd/vet/lowerell/analyzer.go
new file mode 100644
index 000000000..a62f79bdc
--- /dev/null
+++ b/cmd/vet/lowerell/analyzer.go
@@ -0,0 +1,132 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package lowerell forbids variables named "l" (lowercase ell) or "I"
+// (uppercase i), because they are hard to distinguish from the digit
+// "1" and from each other in too many fonts.
+package lowerell
+
+import (
+	"go/ast"
+	"go/token"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+// Analyzer reports variables named "l" (lowercase ell) or "I" (uppercase i).
+var Analyzer = &analysis.Analyzer{
+	Name: "lowerell",
+	Doc:  `forbid variables named "l" (lowercase ell) or "I" (uppercase i), which are hard to distinguish from "1"`,
+	Run:  run,
+}
+
+// messages maps a banned identifier name to the diagnostic shown to users.
+// Each message names the specific symbol that triggered it, so the
+// reader does not have to guess which of "l" or "I" they typed.
+var messages = map[string]string{
+	"l": `do not use "l" (lowercase ell) as a variable name; it is hard to distinguish from "1" and "I" in too many fonts; see https://github.com/tailscale/tailscale/issues/19631`,
+	"I": `do not use "I" (uppercase i) as a variable name; it is hard to distinguish from "1" and "l" in too many fonts; see https://github.com/tailscale/tailscale/issues/19631`,
+}
+
+// reported tracks identifier positions already reported, to avoid duplicate
+// diagnostics when the same declaration is reachable from multiple AST nodes.
+type reportedSet map[token.Pos]bool
+
+func (rs reportedSet) check(pass *analysis.Pass, ident *ast.Ident) {
+	if ident == nil {
+		return
+	}
+	msg, ok := messages[ident.Name]
+	if !ok {
+		return
+	}
+	if rs[ident.Pos()] {
+		return
+	}
+	rs[ident.Pos()] = true
+	pass.Reportf(ident.Pos(), "%s", msg)
+}
+
+func (rs reportedSet) checkFieldList(pass *analysis.Pass, fl *ast.FieldList) {
+	if fl == nil {
+		return
+	}
+	for _, f := range fl.List {
+		for _, n := range f.Names {
+			rs.check(pass, n)
+		}
+	}
+}
+
+func run(pass *analysis.Pass) (any, error) {
+	rs := reportedSet{}
+
+	for _, file := range pass.Files {
+		ast.Inspect(file, func(n ast.Node) bool {
+			switch n := n.(type) {
+			case *ast.FuncDecl:
+				// Receiver name.
+				rs.checkFieldList(pass, n.Recv)
+				// Parameters, results, and type parameters
+				// are checked via the FuncType case below.
+
+			case *ast.FuncType:
+				rs.checkFieldList(pass, n.TypeParams)
+				rs.checkFieldList(pass, n.Params)
+				rs.checkFieldList(pass, n.Results)
+
+			case *ast.StructType:
+				rs.checkFieldList(pass, n.Fields)
+
+			case *ast.GenDecl:
+				if n.Tok != token.VAR && n.Tok != token.CONST {
+					return true
+				}
+				for _, spec := range n.Specs {
+					vs, ok := spec.(*ast.ValueSpec)
+					if !ok {
+						continue
+					}
+					for _, name := range vs.Names {
+						rs.check(pass, name)
+					}
+				}
+
+			case *ast.AssignStmt:
+				if n.Tok != token.DEFINE {
+					return true
+				}
+				for _, lhs := range n.Lhs {
+					if id, ok := lhs.(*ast.Ident); ok {
+						rs.check(pass, id)
+					}
+				}
+
+			case *ast.RangeStmt:
+				if n.Tok != token.DEFINE {
+					return true
+				}
+				if id, ok := n.Key.(*ast.Ident); ok {
+					rs.check(pass, id)
+				}
+				if id, ok := n.Value.(*ast.Ident); ok {
+					rs.check(pass, id)
+				}
+
+			case *ast.TypeSwitchStmt:
+				// switch l := x.(type) { ... }
+				as, ok := n.Assign.(*ast.AssignStmt)
+				if !ok || as.Tok != token.DEFINE {
+					return true
+				}
+				for _, lhs := range as.Lhs {
+					if id, ok := lhs.(*ast.Ident); ok {
+						rs.check(pass, id)
+					}
+				}
+			}
+			return true
+		})
+	}
+	return nil, nil
+}
diff --git a/cmd/vet/lowerell/analyzer_test.go b/cmd/vet/lowerell/analyzer_test.go
new file mode 100644
index 000000000..c566c2ec4
--- /dev/null
+++ b/cmd/vet/lowerell/analyzer_test.go
@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lowerell
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+)
+
+func TestAnalyzer(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, Analyzer, "example")
+}
diff --git a/cmd/vet/lowerell/testdata/src/example/example.go b/cmd/vet/lowerell/testdata/src/example/example.go
new file mode 100644
index 000000000..c67c19781
--- /dev/null
+++ b/cmd/vet/lowerell/testdata/src/example/example.go
@@ -0,0 +1,100 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package example
+
+import "sync"
+
+// Bad: var declarations.
+var l int // want `do not use "l"`
+var I int // want `do not use "I"`
+
+// OK: variables named "ll", "II", "i" are fine.
+var (
+	ll int
+	II int
+	i  int
+)
+
+// Bad: const declaration in a function scope.
+func F0() {
+	const l = 3 // want `do not use "l"`
+	const I = 4 // want `do not use "I"`
+	_ = l
+	_ = I
+}
+
+// Bad: function parameters.
+func F1a(l int) {} // want `do not use "l"`
+func F1b(I int) {} // want `do not use "I"`
+
+// Bad: named return values.
+func F2a() (l int) { return } // want `do not use "l"`
+func F2b() (I int) { return } // want `do not use "I"`
+
+// Bad: receiver names.
+type T struct{}
+
+func (l *T) Ml() {} // want `do not use "l"`
+func (I *T) MI() {} // want `do not use "I"`
+
+// Bad: struct fields.
+type S struct {
+	l int // want `do not use "l"`
+	I int // want `do not use "I"`
+}
+
+// Bad: short variable declarations.
+func F3() {
+	l := 1 // want `do not use "l"`
+	I := 2 // want `do not use "I"`
+	_ = l
+	_ = I
+}
+
+// Bad: var statement inside a function.
+func F4() {
+	var l int // want `do not use "l"`
+	var I int // want `do not use "I"`
+	_ = l
+	_ = I
+}
+
+// Bad: range key/value.
+func F5(xs []int) {
+	for l, v := range xs { // want `do not use "l"`
+		_ = l
+		_ = v
+	}
+	for _, I := range xs { // want `do not use "I"`
+		_ = I
+	}
+}
+
+// Bad: type parameters.
+func F6a[l any](x l) l { return x } // want `do not use "l"`
+func F6b[I any](x I) I { return x } // want `do not use "I"`
+
+// Bad: type switch guards.
+func F7(x any) {
+	switch l := x.(type) { // want `do not use "l"`
+	case int:
+		_ = l
+	}
+	switch I := x.(type) { // want `do not use "I"`
+	case int:
+		_ = I
+	}
+}
+
+// OK: clean code with no banned variables.
+func F8() {
+	count := 0
+	for i := 0; i < 10; i++ {
+		count++
+	}
+	_ = count
+}
+
+// OK: sync.Mutex named "mu".
+var mu sync.Mutex
diff --git a/cmd/vet/vet.go b/cmd/vet/vet.go
index 4087f7073..38ffdb6d0 100644
--- a/cmd/vet/vet.go
+++ b/cmd/vet/vet.go
@@ -9,6 +9,7 @@ import (
 
 	"golang.org/x/tools/go/analysis/unitchecker"
 	"tailscale.com/cmd/vet/jsontags"
+	"tailscale.com/cmd/vet/lowerell"
 	"tailscale.com/cmd/vet/subtestnames"
 )
 
@@ -21,5 +22,5 @@ func init() {
 }
 
 func main() {
-	unitchecker.Main(jsontags.Analyzer, subtestnames.Analyzer)
+	unitchecker.Main(jsontags.Analyzer, lowerell.Analyzer, subtestnames.Analyzer)
 }
diff --git a/drive/driveimpl/drive_test.go b/drive/driveimpl/drive_test.go
index 8f9b43d6b..185ae2a9c 100644
--- a/drive/driveimpl/drive_test.go
+++ b/drive/driveimpl/drive_test.go
@@ -239,7 +239,7 @@ func TestLOCK(t *testing.T) {
 	}
 
 	u := fmt.Sprintf("http://%s/%s/%s/%s/%s",
-		s.local.l.Addr(),
+		s.local.ln.Addr(),
 		url.PathEscape(domain),
 		url.PathEscape(remote1),
 		url.PathEscape(share11),
@@ -365,7 +365,7 @@ func TestUNLOCK(t *testing.T) {
 	}
 
 	u := fmt.Sprintf("http://%s/%s/%s/%s/%s",
-		s.local.l.Addr(),
+		s.local.ln.Addr(),
 		url.PathEscape(domain),
 		url.PathEscape(remote1),
 		url.PathEscape(share11),
@@ -428,12 +428,12 @@ func TestUNLOCK(t *testing.T) {
 }
 
 type local struct {
-	l  net.Listener
+	ln net.Listener
 	fs *FileSystemForLocal
 }
 
 type remote struct {
-	l           net.Listener
+	ln          net.Listener
 	fs          *FileSystemForRemote
 	fileServer  *FileServer
 	shares      map[string]string
@@ -487,7 +487,7 @@ func newSystem(t *testing.T) *system {
 	client.SetTransport(&http.Transport{DisableKeepAlives: true})
 	s := &system{
 		t:       t,
-		local:   &local{l: ln, fs: fs},
+		local:   &local{ln: ln, fs: fs},
 		client:  client,
 		remotes: make(map[string]*remote),
 	}
@@ -510,7 +510,7 @@ func (s *system) addRemote(name string) string {
 	s.t.Logf("FileServer for %v listening at %s", name, fileServer.Addr())
 
 	r := &remote{
-		l:           ln,
+		ln:          ln,
 		fileServer:  fileServer,
 		fs:          NewFileSystemForRemote(log.Printf),
 		shares:      make(map[string]string),
@@ -524,7 +524,7 @@ func (s *system) addRemote(name string) string {
 	for name, r := range s.remotes {
 		remotes = append(remotes, &drive.Remote{
 			Name: name,
-			URL:  func() string { return fmt.Sprintf("http://%s", r.l.Addr()) },
+			URL:  func() string { return fmt.Sprintf("http://%s", r.ln.Addr()) },
 		})
 	}
 	s.local.fs.SetRemotes(
@@ -683,7 +683,7 @@ func (s *system) stop() {
 		s.t.Fatalf("failed to Close fs: %s", err)
 	}
 
-	err = s.local.l.Close()
+	err = s.local.ln.Close()
 	if err != nil {
 		s.t.Fatalf("failed to Close listener: %s", err)
 	}
@@ -694,7 +694,7 @@ func (s *system) stop() {
 			s.t.Fatalf("failed to Close remote fs: %s", err)
 		}
 
-		err = r.l.Close()
+		err = r.ln.Close()
 		if err != nil {
 			s.t.Fatalf("failed to Close remote listener: %s", err)
 		}
diff --git a/metrics/multilabelmap_test.go b/metrics/multilabelmap_test.go
index 70554c63e..0fa730992 100644
--- a/metrics/multilabelmap_test.go
+++ b/metrics/multilabelmap_test.go
@@ -86,10 +86,10 @@ metricname{foo="si",bar="si"} 5
 
 func TestMultiLabelMapTypes(t *testing.T) {
 	type LabelTypes struct {
-		S string
-		B bool
-		I int
-		U uint
+		S   string
+		B   bool
+		Int int
+		U   uint
 	}
 
 	m := new(MultiLabelMap[LabelTypes])
@@ -100,7 +100,7 @@ func TestMultiLabelMapTypes(t *testing.T) {
 	m.WritePrometheus(&buf, "metricname")
 	const want = `# TYPE metricname counter
 # HELP metricname some good stuff
-metricname{s="a",b="true",i="-1",u="2"} 3
+metricname{s="a",b="true",int="-1",u="2"} 3
 `
 	if got := buf.String(); got != want {
 		t.Errorf("got %q; want %q", got, want)
diff --git a/pkgdoc_test.go b/pkgdoc_test.go
index c5e50ee63..d0f0d66bd 100644
--- a/pkgdoc_test.go
+++ b/pkgdoc_test.go
@@ -41,6 +41,9 @@ func TestPackageDocs(t *testing.T) {
 		if fi.Mode().IsDir() && path != "." && strings.HasPrefix(filepath.Base(path), ".") {
 			return filepath.SkipDir // No documentation lives in dot directories (.git, .claude, etc)
 		}
+		if fi.Mode().IsDir() && filepath.Base(path) == "testdata" {
+			return filepath.SkipDir // testdata is ignored by the go tool; not real packages
+		}
 		if fi.Mode().IsRegular() && strings.HasSuffix(path, ".go") {
 			if strings.HasSuffix(path, "_test.go") {
 				return nil
diff --git a/util/deephash/deephash_test.go b/util/deephash/deephash_test.go
index 7ea83566f..a82203d50 100644
--- a/util/deephash/deephash_test.go
+++ b/util/deephash/deephash_test.go
@@ -59,7 +59,7 @@ func TestHash(t *testing.T) {
 		I16  int16
 		I32  int32
 		I64  int64
-		I    int
+		Int  int
 		U8   uint8
 		U16  uint16
 		U32  uint32
@@ -92,7 +92,7 @@ func TestHash(t *testing.T) {
 		{in: tuple{scalars{I16: math.MinInt16}, scalars{I16: math.MinInt16 / 2}}, wantEq: false},
 		{in: tuple{scalars{I32: math.MinInt32}, scalars{I32: math.MinInt32 / 2}}, wantEq: false},
 		{in: tuple{scalars{I64: math.MinInt64}, scalars{I64: math.MinInt64 / 2}}, wantEq: false},
-		{in: tuple{scalars{I: -1234}, scalars{I: -1234 / 2}}, wantEq: false},
+		{in: tuple{scalars{Int: -1234}, scalars{Int: -1234 / 2}}, wantEq: false},
 		{in: tuple{scalars{U8: math.MaxUint8}, scalars{U8: math.MaxUint8 / 2}}, wantEq: false},
 		{in: tuple{scalars{U16: math.MaxUint16}, scalars{U16: math.MaxUint16 / 2}}, wantEq: false},
 		{in: tuple{scalars{U32: math.MaxUint32}, scalars{U32: math.MaxUint32 / 2}}, wantEq: false},