diff --git a/internal/coding/tui/tool_activity.go b/internal/coding/tui/tool_activity.go
index 3f8d49c..13c33cd 100644
--- a/internal/coding/tui/tool_activity.go
+++ b/internal/coding/tui/tool_activity.go
@@ -1423,6 +1423,10 @@ func renderToolHeading(
 	return ansi.Truncate(heading, max(1, width), "…")
 }
 
+// toolActivityRows returns the rows a compact block renders. The caller owns the
+// result: a preview line lives in the cached activity, and a renderer that prefixes
+// or styles rows used to write that back, so every re-render added another branch
+// prefix and kept the first render's colour escapes inside the text.
 func toolActivityRows(
 	class toolActivityClass,
 	activities []toolActivity,
@@ -1434,7 +1438,7 @@ func toolActivityRows(
 
 	switch class {
 	case toolClassShell:
-		return activities[0].preview
+		return slices.Clone(activities[0].preview)
 	case toolClassPatch:
 		if _, parsed := parsePatchDisplayChanges(activities[0]); !parsed {
 			if _, _, single := singlePatchChange(activities[0]); single {
@@ -1445,11 +1449,11 @@ func toolActivityRows(
 			return nil
 		}
 
-		return activities[0].preview
+		return slices.Clone(activities[0].preview)
 	case toolClassGeneric:
 		return append([]string{activities[0].invocation}, activities[0].preview...)
 	case toolClassSubagent, toolClassTaskList:
-		return activities[0].preview
+		return slices.Clone(activities[0].preview)
 	case toolClassExplore, toolClassPlan:
 		return nil
 	default:
diff --git a/internal/coding/tui/tool_activity_render_test.go b/internal/coding/tui/tool_activity_render_test.go
new file mode 100644
index 0000000..994285c
--- /dev/null
+++ b/internal/coding/tui/tool_activity_render_test.go
@@ -0,0 +1,89 @@
+package tui
+
+import (
+	"encoding/json"
+	"image/color"
+	"slices"
+	"strings"
+	"testing"
+
+	"charm.land/lipgloss/v2"
+	"github.com/rsbin1178/pips/ai"
+	"github.com/rsbin1178/pips/internal/coding"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestToolPreviewRenderingKeepsTheActivityReadOnly pins the renderer to treat a
+// cached preview as input. It used to write its prefixed and styled rows back into
+// the activity, so every re-render added another branch prefix and kept the first
+// render's colour escapes inside the text: switching the theme in a session then
+// showed a growing run of └ markers in front of the same diff line.
+func TestToolPreviewRenderingKeepsTheActivityReadOnly(t *testing.T) {
+	t.Parallel()
+
+	patch := "*** Begin Patch\n" +
+		"*** Update File: world_map/selfcheck_reference_points.py\n" +
+		"@@\n" +
+		"-    (13, 13), (31, 61),\n" +
+		"+    (15, 13), (31, 61),\n" +
+		"*** End Patch\n"
+	arguments, err := json.Marshal(map[string]string{"patch": patch})
+	require.NoError(t, err)
+
+	state := coding.State{Tools: []coding.ToolState{{
+		Call: coding.ToolCall{
+			ID: "call-1", Name: "apply_patch", Arguments: ai.JSON(string(arguments)),
+		},
+		Status: coding.ToolStatusCompleted,
+		Result: ai.ToolResultText(
+			"call-1",
+			"apply_patch",
+			`{"schema":"pips.coding.tool_result/v1alpha1","ok":true,"tool":"apply_patch","counts":{"files":1,"additions":1,"deletions":1}}`+
+				"\n\nworld_map/selfcheck_reference_points.py\n",
+		),
+	}}}
+
+	blocks := projectTimeline(state)
+	require.Len(t, blocks, 1)
+	require.NotEmpty(t, blocks[0].tools[0].preview, "the fixture carries a preview to render")
+
+	preview := slices.Clone(blocks[0].tools[0].preview)
+
+	dark := renderTimeline(blocks, newMarkdownRenderer(8), 80, themeDark, false)
+	assert.Equal(t, preview, blocks[0].tools[0].preview, "the first render rewrote the preview")
+
+	light := renderTimeline(blocks, newMarkdownRenderer(8), 80, themeLight, false)
+	assert.Equal(t, preview, blocks[0].tools[0].preview, "the second render rewrote the preview")
+
+	// The branch prefix is applied once per render rather than accumulating.
+	assert.Equal(t, 1, strings.Count(dark, "└"), "one branch prefix for one preview")
+	assert.Equal(t, 1, strings.Count(light, "└"), "the prefix count grew with the render")
+
+	// Each render reads the rows the theme it was given asks for: the removed line
+	// as the error tone, the added line as the added tone, and neither theme's
+	// escapes left inside the other's output.
+	for _, rendered := range []struct {
+		name    string
+		text    string
+		theme   colorTheme
+		foreign colorTheme
+	}{
+		{name: "dark", text: dark, theme: themeDark, foreign: themeLight},
+		{name: "light", text: light, theme: themeLight, foreign: themeDark},
+	} {
+		assert.Contains(t, rendered.text, sgrPrefix(rendered.theme.palette.error), rendered.name)
+		assert.Contains(t, rendered.text, sgrPrefix(rendered.theme.palette.idle), rendered.name)
+		assert.NotContains(t, rendered.text, sgrPrefix(rendered.foreign.palette.error), rendered.name)
+		assert.NotContains(t, rendered.text, sgrPrefix(rendered.foreign.palette.idle), rendered.name)
+	}
+
+	again := renderTimeline(blocks, newMarkdownRenderer(8), 80, themeDark, false)
+	assert.Equal(t, dark, again, "re-rendering one theme is stable")
+}
+
+// sgrPrefix is the escape a style with one foreground colour opens with, which is
+// how a rendered row's tone is read back.
+func sgrPrefix(value color.Color) string {
+	return strings.TrimSuffix(lipgloss.NewStyle().Foreground(value).Render("x"), "x\x1b[m")
+}