diff --git a/src/tui.zig b/src/tui.zig
index 04039c3..579e65d 100644
--- a/src/tui.zig
+++ b/src/tui.zig
@@ -31,6 +31,26 @@ pub const Viewport = struct {
     }
 };
 
+pub const RenderDiagnostics = struct {
+    viewport_width: usize,
+    viewport_height: usize,
+    frame_bytes: usize,
+    frame_lines: usize,
+    max_line_cells: usize,
+    clipped_sources: usize,
+};
+
+pub const RenderResult = struct {
+    frame: []u8,
+    diagnostics: RenderDiagnostics,
+    warning: []u8,
+
+    pub fn deinit(self: RenderResult, allocator: std.mem.Allocator) void {
+        allocator.free(self.frame);
+        allocator.free(self.warning);
+    }
+};
+
 pub fn runTrace(allocator: std.mem.Allocator, viewport: Viewport, trace: []const u8) !TraceResult {
     var client = try Client.init(allocator, viewport);
     defer client.deinit();
@@ -127,6 +147,15 @@ pub const Client = struct {
         return self.renderEditor(allocator, snap);
     }
 
+    pub fn renderWithDiagnostics(self: *const Client, allocator: std.mem.Allocator) !RenderResult {
+        const frame = try self.render(allocator);
+        errdefer allocator.free(frame);
+        var diagnostics = diagnoseFrame(self.viewport, frame);
+        diagnostics.clipped_sources = try self.countClippedSources(allocator);
+        const warning = try renderWarningAlloc(allocator, diagnostics);
+        return .{ .frame = frame, .diagnostics = diagnostics, .warning = warning };
+    }
+
     fn renderEditor(self: *const Client, allocator: std.mem.Allocator, snap: session_mod.Snapshot) ![]u8 {
         var out = std.ArrayList(u8).empty;
         errdefer out.deinit(allocator);
@@ -334,6 +363,51 @@ pub const Client = struct {
         self.saved_bytes = copy;
         self.message = "saved";
     }
+
+    fn countClippedSources(self: *const Client, allocator: std.mem.Allocator) !usize {
+        const snap = try self.session.snapshot();
+        var clipped: usize = 0;
+        if (snap.active_panel_title == null) {
+            var line_iter = std.mem.splitScalar(u8, snap.bytes, '\n');
+            while (line_iter.next()) |line| {
+                if (session_mod.cellWidth(line) > self.viewport.width) clipped += 1;
+            }
+            if (self.message) |message| {
+                if (session_mod.cellWidth(message) > self.viewport.width) clipped += 1;
+            }
+            return clipped;
+        }
+
+        const path = try self.session.panelPathAlloc(allocator);
+        defer allocator.free(path);
+        const header = try std.fmt.allocPrint(allocator, "panel {s}", .{path});
+        defer allocator.free(header);
+        if (session_mod.cellWidth(header) > self.viewport.width) clipped += 1;
+
+        const remaining_rows = self.viewport.height - 1;
+        const rows = self.session.activeListRowsAlloc(allocator, remaining_rows) catch null;
+        if (rows) |list_rows| {
+            defer {
+                for (list_rows) |row| allocator.free(row);
+                allocator.free(list_rows);
+            }
+            for (list_rows) |row| {
+                if (session_mod.cellWidth(row) > self.viewport.width) clipped += 1;
+            }
+        } else if (snap.active_panel_title) |title| {
+            const detail = try std.fmt.allocPrint(allocator, "{s}: no content yet", .{title});
+            defer allocator.free(detail);
+            if (session_mod.cellWidth(detail) > self.viewport.width) clipped += 1;
+        }
+
+        const status = if (self.message) |message|
+            try std.fmt.allocPrint(allocator, "{s}", .{message})
+        else
+            try std.fmt.allocPrint(allocator, "panel {d}/{d}  x close", .{ snap.active_panel_index.? + 1, snap.panel_depth });
+        defer allocator.free(status);
+        if (session_mod.cellWidth(status) > self.viewport.width) clipped += 1;
+        return clipped;
+    }
 };
 
 fn appendVisibleCells(allocator: std.mem.Allocator, out: *std.ArrayList(u8), bytes: []const u8, max_cells: usize) !void {
@@ -369,6 +443,52 @@ fn columnAt(bytes: []const u8, cursor_byte: usize) usize {
     return session_mod.cellWidth(prefix[line_start..]);
 }
 
+fn diagnoseFrame(viewport: Viewport, frame: []const u8) RenderDiagnostics {
+    var diagnostics = RenderDiagnostics{
+        .viewport_width = viewport.width,
+        .viewport_height = viewport.height,
+        .frame_bytes = frame.len,
+        .frame_lines = 0,
+        .max_line_cells = 0,
+        .clipped_sources = 0,
+    };
+    var lines = std.mem.splitScalar(u8, frame, '\n');
+    while (lines.next()) |line| {
+        diagnostics.frame_lines += 1;
+        diagnostics.max_line_cells = @max(diagnostics.max_line_cells, session_mod.cellWidth(line));
+    }
+    return diagnostics;
+}
+
+fn renderWarningAlloc(allocator: std.mem.Allocator, diagnostics: RenderDiagnostics) ![]u8 {
+    if (diagnostics.frame_lines != diagnostics.viewport_height) {
+        return std.fmt.allocPrint(
+            allocator,
+            "render warning: expected {d} lines, rendered {d}",
+            .{ diagnostics.viewport_height, diagnostics.frame_lines },
+        );
+    }
+    if (diagnostics.max_line_cells > diagnostics.viewport_width) {
+        return std.fmt.allocPrint(
+            allocator,
+            "render warning: line width {d} exceeds viewport {d}",
+            .{ diagnostics.max_line_cells, diagnostics.viewport_width },
+        );
+    }
+    if (diagnostics.clipped_sources > 0) {
+        return std.fmt.allocPrint(
+            allocator,
+            "render warning: clipped {d} source line(s) to fit {d} columns",
+            .{ diagnostics.clipped_sources, diagnostics.viewport_width },
+        );
+    }
+    return std.fmt.allocPrint(
+        allocator,
+        "render ok: {d} lines, {d} bytes, max {d}/{d} cells",
+        .{ diagnostics.frame_lines, diagnostics.frame_bytes, diagnostics.max_line_cells, diagnostics.viewport_width },
+    );
+}
+
 fn assertLinesFit(frame: []const u8, width: usize) !void {
     var lines = std.mem.splitScalar(u8, frame, '\n');
     while (lines.next()) |line| {
@@ -737,3 +857,68 @@ test "adversarial: long feedback messages are clipped to viewport width" {
     try std.testing.expect(std.mem.indexOf(u8, frame, "this-message-is-") != null);
     try std.testing.expect(std.mem.indexOf(u8, frame, "phone-width") == null);
 }
+
+test "regular: render diagnostics report ok for representative narrow editor frame" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 32, .height = 5 });
+    defer client.deinit();
+
+    try client.handleTraceLine("open fn main() {}");
+    try client.handleTraceLine("key space");
+    try client.handleTraceLine("key p");
+    try client.handleTraceLine("key c");
+    try client.handleTraceLine("type x");
+    const result = try client.renderWithDiagnostics(std.testing.allocator);
+    defer result.deinit(std.testing.allocator);
+
+    try std.testing.expectEqual(@as(usize, 5), result.diagnostics.frame_lines);
+    try std.testing.expect(result.diagnostics.frame_bytes <= 32 * 5 * 4);
+    try std.testing.expect(result.diagnostics.max_line_cells <= 32);
+    try std.testing.expectEqual(@as(usize, 0), result.diagnostics.clipped_sources);
+    try std.testing.expect(std.mem.indexOf(u8, result.warning, "render ok") != null);
+}
+
+test "regular: render diagnostics report ok for representative list panel frame" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 36, .height = 5 });
+    defer client.deinit();
+
+    try client.handleTraceLine("list_open files src/main.zig|src/panel.zig|README.md");
+    try client.handleTraceLine("list_filter src");
+    try client.handleTraceLine("list_down");
+    const result = try client.renderWithDiagnostics(std.testing.allocator);
+    defer result.deinit(std.testing.allocator);
+
+    try std.testing.expectEqual(@as(usize, 5), result.diagnostics.frame_lines);
+    try std.testing.expect(result.diagnostics.frame_bytes <= 36 * 5 * 4);
+    try std.testing.expect(result.diagnostics.max_line_cells <= 36);
+    try std.testing.expectEqual(@as(usize, 0), result.diagnostics.clipped_sources);
+    try std.testing.expect(std.mem.indexOf(u8, result.warning, "render ok") != null);
+}
+
+test "adversarial: render diagnostics name clipped editor sources" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 12, .height = 4 });
+    defer client.deinit();
+
+    try client.handleTraceLine("open this-line-is-far-too-long-for-phone");
+    const result = try client.renderWithDiagnostics(std.testing.allocator);
+    defer result.deinit(std.testing.allocator);
+
+    try std.testing.expectEqual(@as(usize, 4), result.diagnostics.frame_lines);
+    try std.testing.expect(result.diagnostics.max_line_cells <= 12);
+    try std.testing.expect(result.diagnostics.clipped_sources >= 1);
+    try std.testing.expect(std.mem.indexOf(u8, result.warning, "clipped") != null);
+    try std.testing.expect(std.mem.indexOf(u8, result.warning, "12 columns") != null);
+}
+
+test "adversarial: render diagnostics name clipped panel sources" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 14, .height = 4 });
+    defer client.deinit();
+
+    try client.handleTraceLine("list_open files src/very-long-mobile-render-file-name.zig|b.zig");
+    const result = try client.renderWithDiagnostics(std.testing.allocator);
+    defer result.deinit(std.testing.allocator);
+
+    try std.testing.expectEqual(@as(usize, 4), result.diagnostics.frame_lines);
+    try std.testing.expect(result.diagnostics.max_line_cells <= 14);
+    try std.testing.expect(result.diagnostics.clipped_sources >= 1);
+    try std.testing.expect(std.mem.indexOf(u8, result.warning, "clipped") != null);
+}