diff --git a/cmd/tui/chat/debug.go b/cmd/tui/chat/debug.go
index 796626240..c61fa1067 100644
--- a/cmd/tui/chat/debug.go
+++ b/cmd/tui/chat/debug.go
@@ -16,9 +16,11 @@ import (
 )
 
 type chatPromptDebug struct {
-	request api.ChatRequest
-	tokens  int
-	scroll  int
+	request    api.ChatRequest
+	tokens     int
+	scroll     int
+	lines      []string
+	linesWidth int
 }
 
 const maxPromptDebugToolResultRunes = 400
@@ -220,10 +222,13 @@ func (m chatModel) previewChatRequest(opts coreagent.RunOptions, messages []api.
 	return req
 }
 
-func (m chatModel) promptDebugLines(width int) []string {
+func (m *chatModel) promptDebugLines(width int) []string {
 	if m.promptDebug == nil {
 		return nil
 	}
+	if m.promptDebug.lines != nil && m.promptDebug.linesWidth == width {
+		return m.promptDebug.lines
+	}
 	req := m.promptDebug.request
 	innerWidth := max(20, width-2)
 	lines := []string{
@@ -259,15 +264,17 @@ func (m chatModel) promptDebugLines(width int) []string {
 	lines = append(lines, "", chatHeaderStyle.Render("Tools"))
 	if len(req.Tools) == 0 {
 		lines = append(lines, chatMetaStyle.Render("none"))
-		return lines
-	}
-	for i, tool := range req.Tools {
-		if i > 0 {
-			lines = append(lines, "")
+	} else {
+		for i, tool := range req.Tools {
+			if i > 0 {
+				lines = append(lines, "")
+			}
+			lines = append(lines, promptDebugToolLines(i+1, tool, innerWidth)...)
 		}
-		lines = append(lines, promptDebugToolLines(i+1, tool, innerWidth)...)
 	}
-	return lines
+	m.promptDebug.lines = lines
+	m.promptDebug.linesWidth = width
+	return m.promptDebug.lines
 }
 
 func promptDebugFieldLine(label, value string, width int) string {
diff --git a/cmd/tui/chat/input_test.go b/cmd/tui/chat/input_test.go
index fb8b90ac7..3138d82a0 100644
--- a/cmd/tui/chat/input_test.go
+++ b/cmd/tui/chat/input_test.go
@@ -370,6 +370,36 @@ func TestChatPromptDebugMouseWheelScrolls(t *testing.T) {
 	}
 }
 
+func TestChatPromptDebugCachesLinesByWidth(t *testing.T) {
+	m := chatModel{
+		promptDebug: &chatPromptDebug{
+			request: api.ChatRequest{
+				Model: "llama3.2",
+				Messages: []api.Message{{
+					Role:    "user",
+					Content: strings.Repeat("a long prompt line ", 20),
+				}},
+			},
+		},
+	}
+
+	first := m.promptDebugLines(80)
+	if len(first) == 0 || m.promptDebug.linesWidth != 80 {
+		t.Fatalf("prompt cache = %#v, want lines cached at width 80", m.promptDebug)
+	}
+	if &first[0] != &m.promptDebugLines(80)[0] {
+		t.Fatal("prompt debug should reuse cached lines at the same width")
+	}
+
+	resized := m.promptDebugLines(120)
+	if m.promptDebug.linesWidth != 120 {
+		t.Fatalf("prompt cache width = %d, want 120", m.promptDebug.linesWidth)
+	}
+	if &first[0] == &resized[0] {
+		t.Fatal("prompt debug should rebuild lines after a width change")
+	}
+}
+
 func TestTruncateInputLineUsesDisplayWidth(t *testing.T) {
 	line := truncateInputLine(strings.Repeat("界", 10), 10)
 	if got := lipgloss.Width(line); got > 10 {