diff --git a/src/diagnostics.zig b/src/diagnostics.zig
index 7924e3d..136a629 100644
--- a/src/diagnostics.zig
+++ b/src/diagnostics.zig
@@ -13,6 +13,99 @@ pub const SourceLabel = struct {
     row: []const u8,
 };
 
+pub const Severity = enum {
+    err,
+    warning,
+    info,
+    hint,
+
+    pub fn parse(text: []const u8) !Severity {
+        if (std.mem.eql(u8, text, "error")) return .err;
+        if (std.mem.eql(u8, text, "err")) return .err;
+        if (std.mem.eql(u8, text, "warning")) return .warning;
+        if (std.mem.eql(u8, text, "info")) return .info;
+        if (std.mem.eql(u8, text, "hint")) return .hint;
+        return error.InvalidDiagnosticRow;
+    }
+
+    pub fn label(self: Severity) []const u8 {
+        return switch (self) {
+            .err => "error",
+            .warning => "warning",
+            .info => "info",
+            .hint => "hint",
+        };
+    }
+};
+
+pub const Diagnostic = struct {
+    provider: []const u8,
+    version: u64,
+    file: []const u8,
+    start: usize,
+    end: usize,
+    severity: Severity,
+    message: []const u8,
+
+    pub fn stale(self: Diagnostic, document_version: u64) bool {
+        return self.version < document_version;
+    }
+};
+
+pub fn rowAlloc(
+    allocator: std.mem.Allocator,
+    provider: []const u8,
+    version: u64,
+    file: []const u8,
+    start: usize,
+    end: usize,
+    severity: Severity,
+    message: []const u8,
+) ![]u8 {
+    if (provider.len == 0 or file.len == 0 or start > end) return error.InvalidDiagnosticRow;
+    return std.fmt.allocPrint(
+        allocator,
+        "diag:{s}:{d}:{s}:{d}:{d}:{s}:{s}",
+        .{ sanitizeToken(provider), version, severity.label(), start, end, sanitizeToken(file), sanitizeToken(message) },
+    );
+}
+
+pub fn parseRow(row: []const u8) !Diagnostic {
+    if (!std.mem.startsWith(u8, row, "diag:")) return error.InvalidDiagnosticRow;
+    var parts = std.mem.splitScalar(u8, row, ':');
+    _ = parts.next() orelse return error.InvalidDiagnosticRow;
+    const provider = parts.next() orelse return error.InvalidDiagnosticRow;
+    const version_text = parts.next() orelse return error.InvalidDiagnosticRow;
+    const severity_text = parts.next() orelse return error.InvalidDiagnosticRow;
+    const start_text = parts.next() orelse return error.InvalidDiagnosticRow;
+    const end_text = parts.next() orelse return error.InvalidDiagnosticRow;
+    const file = parts.next() orelse return error.InvalidDiagnosticRow;
+    const message = parts.rest();
+    const version = std.fmt.parseUnsigned(u64, version_text, 10) catch return error.InvalidDiagnosticRow;
+    const start = std.fmt.parseUnsigned(usize, start_text, 10) catch return error.InvalidDiagnosticRow;
+    const end = std.fmt.parseUnsigned(usize, end_text, 10) catch return error.InvalidDiagnosticRow;
+    if (provider.len == 0 or file.len == 0 or start > end) return error.InvalidDiagnosticRow;
+    return .{ .provider = provider, .version = version, .file = file, .start = start, .end = end, .severity = try Severity.parse(severity_text), .message = message };
+}
+
+pub fn panelRowAlloc(allocator: std.mem.Allocator, row: []const u8, document_version: u64) ![]u8 {
+    const diagnostic = try parseRow(row);
+    const freshness = if (diagnostic.stale(document_version)) "stale" else "fresh";
+    return std.fmt.allocPrint(
+        allocator,
+        "diag:{s}:{s}:{s}:{d}-{d}:{s}:{s}",
+        .{ freshness, diagnostic.provider, diagnostic.severity.label(), diagnostic.start, diagnostic.end, diagnostic.file, diagnostic.message },
+    );
+}
+
+pub fn sanitizeToken(text: []const u8) []const u8 {
+    if (text.len == 0) return "unknown";
+    for (text) |byte| {
+        if (byte <= 0x20 or byte == ':' or byte == '|') return "invalid";
+    }
+    return text;
+}
+
 pub fn sourceLabel(row: []const u8) SourceLabel {
     if (std.mem.startsWith(u8, row, "provider:")) {
         return .{
diff --git a/src/leader.zig b/src/leader.zig
index 2bbb6cb..ebbe172 100644
--- a/src/leader.zig
+++ b/src/leader.zig
@@ -14,6 +14,7 @@ pub const Feature = enum {
     search,
     panel_close,
     lsp,
+    diagnostics,
 };
 
 pub const Action = union(enum) {
@@ -28,6 +29,10 @@ pub const Action = union(enum) {
     hover,
     signature,
     expand_hover,
+    diagnostics_open,
+    diagnostics_next,
+    diagnostics_previous,
+    diagnostics_filter,
     repeat_rail,
     restore_last,
     not_built: Feature,
@@ -46,6 +51,7 @@ const Mode = enum {
     symbol_rail,
     search_rail,
     language_rail,
+    diagnostic_rail,
     open_prompt,
 };
 
@@ -71,6 +77,7 @@ pub const Leader = struct {
             .symbol_rail => return self.handleSymbolRail(event),
             .search_rail => return self.handleSearchRail(event),
             .language_rail => return self.handleLanguageRail(event),
+            .diagnostic_rail => return self.handleDiagnosticRail(event),
             .open_prompt => return self.handleOpenPrompt(event),
         }
     }
@@ -83,6 +90,7 @@ pub const Leader = struct {
             .symbol_rail => symbol_mod.rail_status,
             .search_rail => "search: f current file  p project  s symbols",
             .language_rail => "language: h hover  s signature  o open hover",
+            .diagnostic_rail => "diagnostics: d/open  n next  p previous  f filter-source",
             .open_prompt => "open: type path, Enter opens, Esc cancels",
         };
     }
@@ -131,6 +139,10 @@ pub const Leader = struct {
                     self.mode = .language_rail;
                     return .none;
                 }
+                if (std.mem.eql(u8, text, "d")) {
+                    self.mode = .diagnostic_rail;
+                    return .diagnostics_open;
+                }
                 if (std.mem.eql(u8, text, "q")) {
                     self.mode = .idle;
                     return .quit;
@@ -216,6 +228,50 @@ pub const Leader = struct {
         }
     }
 
+    fn handleDiagnosticRail(self: *Leader, event: input.Event) Action {
+        self.message = null;
+        switch (event) {
+            .text => |text| {
+                if (std.mem.eql(u8, text, "d")) {
+                    self.mode = .idle;
+                    return .diagnostics_open;
+                }
+                if (std.mem.eql(u8, text, "n")) {
+                    self.mode = .idle;
+                    return .diagnostics_next;
+                }
+                if (std.mem.eql(u8, text, "p")) {
+                    self.mode = .idle;
+                    return .diagnostics_previous;
+                }
+                if (std.mem.eql(u8, text, "f")) {
+                    self.mode = .idle;
+                    return .diagnostics_filter;
+                }
+                self.mode = .idle;
+                self.message = "unknown diagnostics key";
+                return .none;
+            },
+            .key => |key| switch (key) {
+                .escape, .backspace => {
+                    self.mode = .idle;
+                    self.message = "diagnostics cancelled";
+                    return .none;
+                },
+                else => {
+                    self.mode = .idle;
+                    self.message = "unknown diagnostics key";
+                    return .none;
+                },
+            },
+            .unknown => {
+                self.mode = .idle;
+                self.message = "unknown diagnostics key";
+                return .none;
+            },
+        }
+    }
+
     fn handleSearchRail(self: *Leader, event: input.Event) Action {
         self.message = null;
         switch (event) {
diff --git a/src/tui.zig b/src/tui.zig
index f597715..c580bc0 100644
--- a/src/tui.zig
+++ b/src/tui.zig
@@ -152,6 +152,11 @@ pub const Client = struct {
     panel_context: PanelContext = .none,
     hover_rows: std.ArrayList([]u8) = .empty,
     signature_rows: std.ArrayList([]u8) = .empty,
+    diagnostic_rows: std.ArrayList([]u8) = .empty,
+    diagnostic_filter: ?[]u8 = null,
+    diagnostic_index: usize = 0,
+    diagnostic_selected: bool = false,
+    document_version: u64 = 1,
     undo_stack: std.ArrayList(EditSnapshot) = .empty,
     redo_stack: std.ArrayList(EditSnapshot) = .empty,
     yank_bytes: ?[]u8 = null,
@@ -182,6 +187,8 @@ pub const Client = struct {
         self.search_matches.deinit(self.allocator);
         self.freeOwnedClientRows(&self.hover_rows);
         self.freeOwnedClientRows(&self.signature_rows);
+        self.freeOwnedClientRows(&self.diagnostic_rows);
+        if (self.diagnostic_filter) |filter| self.allocator.free(filter);
         self.freeSnapshotStack(&self.undo_stack);
         self.freeSnapshotStack(&self.redo_stack);
         self.repo.deinit();
@@ -203,6 +210,8 @@ pub const Client = struct {
         if (std.mem.startsWith(u8, line, "repo_file ")) return self.addRepoFile(line[10..]);
         if (std.mem.startsWith(u8, line, "lsp_hover_fixture ")) return self.addHoverFixture(line[18..]);
         if (std.mem.startsWith(u8, line, "lsp_signature_fixture ")) return self.addSignatureFixture(line[22..]);
+        if (std.mem.startsWith(u8, line, "diagnostic_fixture ")) return self.addDiagnosticFixture(line[19..]);
+        if (std.mem.startsWith(u8, line, "diagnostic_filter ")) return self.setDiagnosticFilter(line[18..]);
         if (std.mem.eql(u8, line, "file_picker")) return self.openRepoList("files", false, false);
         if (std.mem.eql(u8, line, "file_picker all")) return self.openRepoList("files", false, true);
         if (std.mem.eql(u8, line, "file_tree")) return self.openRepoList("tree", true, false);
@@ -514,6 +523,31 @@ pub const Client = struct {
         try self.signature_rows.append(self.allocator, row);
     }
 
+    fn addDiagnosticFixture(self: *Client, payload: []const u8) !void {
+        var parts = std.mem.splitScalar(u8, payload, '|');
+        const provider = parts.next() orelse return Error.ProtocolRejected;
+        const version_text = parts.next() orelse return Error.ProtocolRejected;
+        const file = parts.next() orelse return Error.ProtocolRejected;
+        const start_text = parts.next() orelse return Error.ProtocolRejected;
+        const end_text = parts.next() orelse return Error.ProtocolRejected;
+        const severity_text = parts.next() orelse return Error.ProtocolRejected;
+        const message = parts.rest();
+        const version = std.fmt.parseUnsigned(u64, version_text, 10) catch return Error.ProtocolRejected;
+        const start = std.fmt.parseUnsigned(usize, start_text, 10) catch return Error.ProtocolRejected;
+        const end = std.fmt.parseUnsigned(usize, end_text, 10) catch return Error.ProtocolRejected;
+        const severity = diagnostics_mod.Severity.parse(severity_text) catch return Error.ProtocolRejected;
+        const row = diagnostics_mod.rowAlloc(self.allocator, provider, version, file, start, end, severity, message) catch return Error.ProtocolRejected;
+        try self.diagnostic_rows.append(self.allocator, row);
+    }
+
+    fn setDiagnosticFilter(self: *Client, provider: []const u8) !void {
+        if (self.diagnostic_filter) |old| self.allocator.free(old);
+        self.diagnostic_filter = if (std.mem.eql(u8, provider, "all")) null else try self.allocator.dupe(u8, diagnostics_mod.sanitizeToken(provider));
+        self.diagnostic_index = 0;
+        self.diagnostic_selected = false;
+        try self.openDiagnosticsPanel();
+    }
+
     fn addRepoFile(self: *Client, payload: []const u8) !void {
         const separator = std.mem.indexOfScalar(u8, payload, '=') orelse {
             self.message = "diagnostic:unsupported_file:invalid_repo_file_payload";
@@ -1176,6 +1210,138 @@ pub const Client = struct {
         try self.openListRows("hover", rows, .none);
     }
 
+    const DiagnosticDirection = enum { next, previous };
+
+    fn diagnosticMatchesFilter(self: *const Client, row: []const u8) bool {
+        const filter = self.diagnostic_filter orelse return true;
+        const diagnostic = diagnostics_mod.parseRow(row) catch return false;
+        return std.mem.eql(u8, diagnostic.provider, filter);
+    }
+
+    fn diagnosticPanelRowsAlloc(self: *const Client) ![][]const u8 {
+        var rows = std.ArrayList([]const u8).empty;
+        errdefer freeOwnedRows(self.allocator, rows.items);
+        for (self.diagnostic_rows.items) |row| {
+            if (!self.diagnosticMatchesFilter(row)) continue;
+            const panel_row = diagnostics_mod.panelRowAlloc(self.allocator, row, self.document_version) catch try self.allocator.dupe(u8, "diag:invalid");
+            try rows.append(self.allocator, panel_row);
+        }
+        if (rows.items.len == 0) try rows.append(self.allocator, try self.allocator.dupe(u8, "diag:empty"));
+        return rows.toOwnedSlice(self.allocator);
+    }
+
+    fn openDiagnosticsPanel(self: *Client) !void {
+        const snap = try self.session.snapshot();
+        if (snap.active_panel_title) |title| {
+            if (std.mem.eql(u8, title, "diagnostics")) self.session.closePanel() catch {};
+        }
+        const rows = try self.diagnosticPanelRowsAlloc();
+        defer freeOwnedRows(self.allocator, rows);
+        try self.session.openListPanel("diagnostics", rows);
+        self.message = null;
+    }
+
+    fn filteredDiagnosticCount(self: *const Client) usize {
+        var count: usize = 0;
+        for (self.diagnostic_rows.items) |row| {
+            if (self.diagnosticMatchesFilter(row)) count += 1;
+        }
+        return count;
+    }
+
+    fn filteredDiagnosticRow(self: *const Client, index: usize) ?[]const u8 {
+        var seen: usize = 0;
+        for (self.diagnostic_rows.items) |row| {
+            if (!self.diagnosticMatchesFilter(row)) continue;
+            if (seen == index) return row;
+            seen += 1;
+        }
+        return null;
+    }
+
+    fn gotoDiagnostic(self: *Client, direction: DiagnosticDirection) !void {
+        const count = self.filteredDiagnosticCount();
+        if (count == 0) {
+            self.message = "diag:empty";
+            return;
+        }
+        self.diagnostic_index = if (self.diagnostic_selected) switch (direction) {
+            .next => (self.diagnostic_index + 1) % count,
+            .previous => if (self.diagnostic_index == 0) count - 1 else self.diagnostic_index - 1,
+        } else switch (direction) {
+            .next => 0,
+            .previous => count - 1,
+        };
+        self.diagnostic_selected = true;
+        const row = self.filteredDiagnosticRow(self.diagnostic_index) orelse return;
+        const range = self.diagnosticRangeFromRow(row) catch |err| {
+            self.message = @errorName(err);
+            return;
+        };
+        try self.session.moveToByte(range.start);
+        try self.session.selectRange(range.start, range.end);
+        self.session.clearSelection();
+        try self.session.moveToByte(range.start);
+        self.session.closePanel() catch {};
+        self.message = "diag:jumped";
+    }
+
+    fn filterDiagnosticsFromPanel(self: *Client) !void {
+        const selected = self.session.activeListItem() catch null;
+        if (selected) |row| {
+            if (diagnosticProviderFromPanelRow(row)) |provider| {
+                try self.setDiagnosticFilter(provider);
+                return;
+            }
+        }
+        if (self.diagnostic_filter) |old| {
+            self.allocator.free(old);
+            self.diagnostic_filter = null;
+            self.message = "diag:filter_all";
+        } else self.message = "diag:filter_select_source";
+        try self.openDiagnosticsPanel();
+    }
+
+    fn diagnosticProviderFromPanelRow(row: []const u8) ?[]const u8 {
+        if (!std.mem.startsWith(u8, row, "diag:")) return null;
+        var parts = std.mem.splitScalar(u8, row, ':');
+        _ = parts.next() orelse return null;
+        const freshness_or_empty = parts.next() orelse return null;
+        if (std.mem.eql(u8, freshness_or_empty, "empty") or std.mem.eql(u8, freshness_or_empty, "invalid")) return null;
+        return parts.next();
+    }
+
+    fn diagnosticRangeFromRow(self: *Client, row: []const u8) !ObjectRange {
+        const diagnostic = try diagnostics_mod.parseRow(row);
+        const snap = try self.session.snapshot();
+        if (diagnostic.stale(self.document_version)) return error.StaleDiagnostic;
+        if (diagnostic.end > snap.bytes.len) return error.InvalidDiagnosticRange;
+        return .{ .start = diagnostic.start, .end = diagnostic.end };
+    }
+
+    fn currentDiagnosticRange(self: *Client) !ObjectRange {
+        const count = self.filteredDiagnosticCount();
+        if (count == 0) return error.NoDiagnostics;
+        if (self.diagnostic_index >= count) self.diagnostic_index = 0;
+        const row = self.filteredDiagnosticRow(self.diagnostic_index) orelse return error.NoDiagnostics;
+        return self.diagnosticRangeFromRow(row);
+    }
+
+    fn selectCurrentDiagnosticRange(self: *Client) !void {
+        const range = self.currentDiagnosticRange() catch |err| {
+            self.message = @errorName(err);
+            return;
+        };
+        try self.session.selectRange(range.start, range.end);
+        self.message = "selected diagnostic";
+    }
+
+    fn noteDocumentChanged(self: *Client) void {
+        self.document_version += 1;
+        self.diagnostic_index = 0;
+        self.diagnostic_selected = false;
+    }
+
     fn openSearchPrompt(self: *Client) void {
         self.search_prompt_active = true;
         self.search_prompt.clearRetainingCapacity();
@@ -1257,6 +1423,7 @@ pub const Client = struct {
         const text = eventText(event) orelse return self.unknownPrefixOrInput("normal");
         if (std.mem.eql(u8, text, "a")) return self.moveParameter(.next);
         if (std.mem.eql(u8, text, "A")) return self.moveParameter(.previous);
+        if (std.mem.eql(u8, text, "e")) return self.gotoDiagnostic(.next);
         self.applyKnownRailOrMessage(event, "go rail ready");
     }
 
@@ -1414,6 +1581,7 @@ pub const Client = struct {
             self.dropLastUndoSnapshot();
             return err;
         };
+        self.noteDocumentChanged();
     }
 
     fn changeRange(self: *Client, range: ObjectRange) !void {
@@ -1424,6 +1592,7 @@ pub const Client = struct {
             self.dropLastUndoSnapshot();
             return err;
         };
+        self.noteDocumentChanged();
         self.mode = .insert;
         self.message = "insert";
     }
@@ -1447,6 +1616,7 @@ pub const Client = struct {
             self.dropLastUndoSnapshot();
             return err;
         };
+        self.noteDocumentChanged();
     }
 
     fn yankCurrentLine(self: *Client, include_newline: bool) !void {
@@ -1497,7 +1667,7 @@ pub const Client = struct {
             'i' => try self.indentRange(),
             'p' => try self.parameterRange(),
             'f' => try self.enclosingFormRange(),
-            'd' => try self.lineRange(false),
+            'd' => self.currentDiagnosticRange() catch try self.lineRange(false),
             else => error.UnsupportedObject,
         };
     }
@@ -1599,6 +1769,7 @@ pub const Client = struct {
             self.dropLastUndoSnapshot();
             return err;
         };
+        self.noteDocumentChanged();
     }
 
     fn applyMutatingProtocol(self: *Client, line: []const u8) !void {
@@ -1608,6 +1779,7 @@ pub const Client = struct {
             self.dropLastUndoSnapshot();
             return err;
         };
+        self.noteDocumentChanged();
     }
 
     fn repeatProtocol(self: *Client, line: []const u8, repeat: usize) !void {
@@ -1675,6 +1847,10 @@ pub const Client = struct {
             .hover => try self.showCompactHover(),
             .signature => try self.showCompactSignature(),
             .expand_hover => try self.openExpandedHover(),
+            .diagnostics_open => try self.openDiagnosticsPanel(),
+            .diagnostics_next => try self.gotoDiagnostic(.next),
+            .diagnostics_previous => try self.gotoDiagnostic(.previous),
+            .diagnostics_filter => try self.filterDiagnosticsFromPanel(),
             .repeat_rail => self.openPrefix(.repeat),
             .restore_last => self.restoreLastRail(),
             .not_built => {},
@@ -3720,3 +3896,109 @@ test "regular: go rail parameter movement uses keymap paths" {
     snap = try client.session.snapshot();
     try std.testing.expectEqual(@as(usize, 5), snap.cursor_byte);
 }
+
+test "regular: diagnostics panel preserves providers filters and navigates" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 72, .height = 8 });
+    defer client.deinit();
+    try client.handleTraceLine("open abcdef\nsecond");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|1|3|error|bad_token");
+    try client.handleTraceLine("diagnostic_fixture lint|1|main.zig|1|3|warning|style_duplicate");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|8|13|info|second_line");
+
+    try client.handleInput(" ");
+    try client.handleInput("d");
+    const panel_frame = try client.render(std.testing.allocator);
+    defer std.testing.allocator.free(panel_frame);
+    try std.testing.expect(std.mem.indexOf(u8, panel_frame, "diag:fresh:zls:error:1-3:main.zig:bad_token") != null);
+    try std.testing.expect(std.mem.indexOf(u8, panel_frame, "diag:fresh:lint:warning:1-3:main.zig:style_duplicate") != null);
+
+    try client.handleInput("f");
+    const filtered_frame = try client.render(std.testing.allocator);
+    defer std.testing.allocator.free(filtered_frame);
+    try std.testing.expect(std.mem.indexOf(u8, filtered_frame, "diag:fresh:zls:error:1-3") != null);
+    try std.testing.expect(std.mem.indexOf(u8, filtered_frame, "lint:warning") == null);
+
+    try client.handleInput(" ");
+    try client.handleInput("d");
+    try client.handleInput("n");
+    var snap = try client.session.snapshot();
+    try std.testing.expectEqual(@as(usize, 1), snap.cursor_byte);
+
+    try client.handleInput(" ");
+    try client.handleInput("d");
+    try client.handleInput("n");
+    snap = try client.session.snapshot();
+    try std.testing.expectEqual(@as(usize, 8), snap.cursor_byte);
+}
+
+test "regular: go rail jumps to first diagnostic" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 64, .height = 6 });
+    defer client.deinit();
+    try client.handleTraceLine("open abcdef");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|2|5|error|range");
+
+    try client.handleInput("g");
+    try client.handleInput("e");
+    const snap = try client.session.snapshot();
+    try std.testing.expectEqual(@as(usize, 2), snap.cursor_byte);
+}
+
+test "regular: select diagnostic object range with s d" {
+    var client = try Client.init(std.testing.allocator, .{ .width = 64, .height = 6 });
+    defer client.deinit();
+    try client.handleTraceLine("open abcdef");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|2|5|error|range");
+
+    try client.handleInput("s");
+    try client.handleInput("d");
+    const snap = try client.session.snapshot();
+    try std.testing.expect(snap.selection != null);
+    try std.testing.expectEqual(@as(usize, 2), snap.selection.?.anchor);
+    try std.testing.expectEqual(@as(usize, 5), snap.selection.?.cursor);
+}
+
+test "adversarial: diagnostics stale empty duplicate and invalid ranges are safe" {
+    var empty_client = try Client.init(std.testing.allocator, .{ .width = 72, .height = 8 });
+    defer empty_client.deinit();
+    try empty_client.handleTraceLine("open abcdef");
+    try empty_client.handleInput(" ");
+    try empty_client.handleInput("d");
+    const empty_frame = try empty_client.render(std.testing.allocator);
+    defer std.testing.allocator.free(empty_frame);
+    try std.testing.expect(std.mem.indexOf(u8, empty_frame, "diag:empty") != null);
+
+    var client = try Client.init(std.testing.allocator, .{ .width = 72, .height = 8 });
+    defer client.deinit();
+    try client.handleTraceLine("open abcdef");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|1|3|error|first");
+    try client.handleTraceLine("diagnostic_fixture lint|1|main.zig|1|3|warning|same_range_other_provider");
+    try client.handleTraceLine("diagnostic_fixture zls|1|main.zig|20|25|hint|invalid_range");
+    try client.handleInput("i");
+    try client.handleInput("X");
+    try client.handleTraceLine("key escape");
+
+    try client.handleInput(" ");
+    try client.handleInput("d");
+    const stale_frame = try client.render(std.testing.allocator);
+    defer std.testing.allocator.free(stale_frame);
+    try std.testing.expect(std.mem.indexOf(u8, stale_frame, "diag:stale:zls:error:1-3") != null);
+    try std.testing.expect(std.mem.indexOf(u8, stale_frame, "diag:stale:lint:warning:1-3") != null);
+
+    try client.handleTraceLine("diagnostic_filter all");
+    try client.handleInput("n");
+    const invalid_nav_frame = try client.render(std.testing.allocator);
+    defer std.testing.allocator.free(invalid_nav_frame);
+    try std.testing.expect(std.mem.indexOf(u8, invalid_nav_frame, "StaleDiagnostic") != null or std.mem.indexOf(u8, invalid_nav_frame, "InvalidDiagnosticRange") != null);
+}
+
+test "regular: diagnostic row model sanitizes and parses" {
+    const row = try diagnostics_mod.rowAlloc(std.testing.allocator, "zls", 3, "main.zig", 1, 4, .err, "bad: token");
+    defer std.testing.allocator.free(row);
+    try std.testing.expect(std.mem.indexOf(u8, row, "invalid") != null);
+    const parsed = try diagnostics_mod.parseRow(row);
+    try std.testing.expectEqual(@as(u64, 3), parsed.version);
+    try std.testing.expectEqual(diagnostics_mod.Severity.err, parsed.severity);
+    const panel = try diagnostics_mod.panelRowAlloc(std.testing.allocator, row, 4);
+    defer std.testing.allocator.free(panel);
+    try std.testing.expect(std.mem.indexOf(u8, panel, "diag:stale:zls:error:1-4") != null);
+}