Honor NO_COLOR in the terminal log handler (closes #402)

The TTY log handler always wrote ANSI color, unlike internal/ui, which
disables color when NO_COLOR is set. Add a color flag to TTYHandler,
set from NO_COLOR with the same test internal/ui uses, and route every
color code through a paint helper that returns plain text when color is
off. The README's color sentence now covers log lines too.

Model: deepseek-v4-pro

README.md

@@ -836,6 +836,8 @@ Work planned after 1.0. Loosely ordered by priority.
 Every command's user-facing output is governed by `internal/ui`, in one
 of two ways. Color is enabled when the stream written to is a TTY and
 the `NO_COLOR` environment variable is unset (https://no-color.org/).
+The same rule governs the colorized log lines `internal/log` writes to a
+terminal.
 
 * **Status, progress, warnings, and errors** go through the `internal/ui`
   message methods below: marker-prefixed, colored on a TTY, and — except

internal/log/tty_handler.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"os"
 	"strconv"
 	"strings"
 	"sync"
@@ -60,6 +61,12 @@ type TTYHandler struct {
 	mu   *sync.Mutex
 	out  io.Writer
 
+	// color reports whether records are written with ANSI color. It is
+	// false when NO_COLOR is set, matching internal/ui's shouldColor
+	// test; the caller already chose this handler for a terminal, so
+	// TTY-ness is not re-checked here.
+	color bool
+
 	// attrs are the attributes accumulated through WithAttrs, emitted
 	// ahead of each record's own attributes. Their keys already carry
 	// the group path that was open when they were added, so no
@@ -72,16 +79,19 @@ type TTYHandler struct {
 	groups []string
 }
 
-// NewTTYHandler creates a new TTY handler with colored output.
+// NewTTYHandler creates a new TTY handler. Color is enabled unless the
+// NO_COLOR environment variable is set to a non-empty value, the same
+// test internal/ui applies to status lines.
 func NewTTYHandler(out io.Writer, opts *slog.HandlerOptions) *TTYHandler {
 	if opts == nil {
 		opts = &slog.HandlerOptions{}
 	}
 
 	return &TTYHandler{
-		out:  out,
+		out:   out,
-		opts: *opts,
+		opts:  *opts,
-		mu:   &sync.Mutex{},
+		mu:    &sync.Mutex{},
+		color: os.Getenv("NO_COLOR") == "",
 	}
 }
 
@@ -124,10 +134,10 @@ func (h *TTYHandler) Handle(_ context.Context, r slog.Record) error {
 	// are written around it: it can carry text from an untrusted source
 	// (a storage error, for one), and a raw control character would
 	// otherwise reach the terminal.
-	_, _ = fmt.Fprintf(h.out, "%s%s%s %s%s%s %s%s%s",
+	_, _ = fmt.Fprintf(h.out, "%s %s %s",
-		colorGray, timestamp, colorReset,
+		h.paint(colorGray, timestamp),
-		levelColor, level, colorReset,
+		h.paint(levelColor, level),
-		colorBold, sanitize(r.Message), colorReset)
+		h.paint(colorBold, sanitize(r.Message)))
 
 	// Attributes carried by the handler come first, then the record's
 	// own. Handler attributes were qualified when they were added; the
@@ -236,6 +246,7 @@ func (h *TTYHandler) clone() *TTYHandler {
 		opts:   h.opts,
 		mu:     h.mu,
 		out:    h.out,
+		color:  h.color,
 		attrs:  make([]slog.Attr, len(h.attrs), len(h.attrs)+1),
 		groups: make([]string, len(h.groups), len(h.groups)+1),
 	}
@@ -273,9 +284,19 @@ func (h *TTYHandler) writeAttr(a slog.Attr) {
 	// the snapshot database — so a control character in one of them must
 	// be rendered as an escape sequence rather than reaching the terminal,
 	// where it could move the cursor or inject its own colours.
-	_, _ = fmt.Fprintf(h.out, " %s%s%s=%s%s%s",
+	_, _ = fmt.Fprintf(h.out, " %s=%s",
-		colorCyan, sanitize(a.Key), colorReset,
+		h.paint(colorCyan, sanitize(a.Key)),
-		colorBlue, sanitize(value), colorReset)
+		h.paint(colorBlue, sanitize(value)))
+}
+
+// paint wraps s in the given ANSI color when color is enabled, and
+// returns s unchanged otherwise.
+func (h *TTYHandler) paint(color, s string) string {
+	if !h.color {
+		return s
+	}
+
+	return color + s + colorReset
 }
 
 // sanitize returns s unchanged when every rune in it is printable, and a

internal/log/tty_handler_test.go

@@ -421,3 +421,44 @@ func TestTTYHandlerEmptyGroupAndAttrsAreNoOps(t *testing.T) {
 	assert.Contains(t, plain, "kept=yes")
 	assert.NotContains(t, plain, " =")
 }
+
+// TestTTYHandlerRespectsNoColor checks that only a non-empty NO_COLOR
+// turns color off, matching internal/ui's shouldColor test. The handler
+// is only ever chosen for a terminal, so this is the one remaining
+// condition that can still leave color in log output.
+func TestTTYHandlerRespectsNoColor(t *testing.T) {
+	cases := []struct {
+		name    string
+		noColor string
+		colored bool
+	}{
+		{
+			name:    "NO_COLOR turns color off",
+			noColor: "1",
+		},
+		{
+			name:    "empty NO_COLOR keeps color on",
+			noColor: "",
+			colored: true,
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Setenv("NO_COLOR", testCase.noColor)
+
+			var buf bytes.Buffer
+
+			logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+			logger.Info("hello", "key", "value")
+
+			if testCase.colored {
+				assert.Contains(t, buf.String(), "\x1b[",
+					"an empty NO_COLOR must leave ANSI color in log output")
+			} else {
+				assert.NotContains(t, buf.String(), "\x1b[",
+					"NO_COLOR must suppress every ANSI escape in log output")
+			}
+		})
+	}
+}