diff --git a/api/config.py b/api/config.py
index e99f69bb..8c2c71e3 100644
--- a/api/config.py
+++ b/api/config.py
@@ -7343,6 +7343,8 @@ _SETTINGS_DEFAULTS = {
     "font_size": "default",  # small | default | large | xlarge
     "session_jump_buttons": False,  # show Start/End transcript jump pills
     "render_user_markdown": False,  # opt-in: render full markdown in user messages (#3870)
+    "structured_code_default_view": "auto",  # JSON/YAML fenced-block default render: auto | on | off (#484 follow-up). auto => Tree when line count >= structured_code_auto_tree_lines, else Raw.
+    "structured_code_auto_tree_lines": 10,  # in 'auto' mode, minimum line count to default a JSON/YAML block to Tree view (preserves the original hardcoded >=10 behavior)
     "session_endless_scroll": False,  # auto-load older transcript pages while scrolling upward
     "chat_activity_display_mode": "compact_worklog",  # compact_worklog | transparent_stream
     "auto_scroll_follow": True,  # follow new output to the bottom while streaming (Codex/Claude-Code-style sticky bottom); the user scrolling up unpins and is respected
@@ -7551,6 +7553,7 @@ _SETTINGS_ENUM_VALUES = {
     "auto_title_refresh_every": {"0", "5", "10", "20"},
     "busy_input_mode": {"queue", "interrupt", "steer"},
     "chat_activity_display_mode": {"compact_worklog", "transparent_stream"},
+    "structured_code_default_view": {"auto", "on", "off"},
 }
 _SETTINGS_INT_RANGES = {
     "pinned_sessions_limit": (1, 99),
@@ -7559,6 +7562,7 @@ _SETTINGS_INT_RANGES = {
     "inflight_state_max_tool_calls": (1, 200),
     "inflight_state_max_string_chars": (1000, 500000),
     "inflight_state_max_json_chars": (100000, 4000000),
+    "structured_code_auto_tree_lines": (1, 1000),
 }
 _SETTINGS_BOOL_KEYS = {
     "onboarding_completed",
diff --git a/static/boot.js b/static/boot.js
index a04cd195..c80d319e 100644
--- a/static/boot.js
+++ b/static/boot.js
@@ -2158,6 +2158,11 @@ window._applyTitlebarProfileVisibility=_applyTitlebarProfileVisibility;
     }
     window._sessionJumpButtonsEnabled=!!s.session_jump_buttons;
     window._renderUserMarkdown=!!s.render_user_markdown;
+    // JSON/YAML structured code-block default view (#484): auto | on | off,
+    // plus the 'auto'-mode line threshold (sanitized int 1..1000, fallback 10).
+    window._structuredCodeDefaultView=['on','off','auto'].includes(s.structured_code_default_view)?s.structured_code_default_view:'auto';
+    const _sctLines=parseInt(s.structured_code_auto_tree_lines,10);
+    window._structuredCodeAutoTreeLines=(Number.isFinite(_sctLines)&&_sctLines>=1&&_sctLines<=1000)?_sctLines:10;
     // Reconcile appearance: prefer localStorage (what the user last saw) over
     // the server.  If they diverge (e.g. a previous autosave POST failed),
     // push the localStorage values back to the server so settings.json stays
@@ -2227,6 +2232,8 @@ window._applyTitlebarProfileVisibility=_applyTitlebarProfileVisibility;
     window._workspaceTodosTab=false;
     if(typeof _applyWorkspaceTodosTabVisibility==='function') _applyWorkspaceTodosTabVisibility();
     window._sessionJumpButtonsEnabled=false;
+    window._structuredCodeDefaultView='auto';
+    window._structuredCodeAutoTreeLines=10;
     window._sidebarDensity='compact';
     window._pinnedSessionsLimit=3;
     window._busyInputMode='queue';
diff --git a/static/i18n.js b/static/i18n.js
index bbe6ec5d..316a170f 100644
--- a/static/i18n.js
+++ b/static/i18n.js
@@ -598,6 +598,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar',
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.',
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -2171,6 +2177,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -3735,6 +3747,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Worklog の詳細を自動的に開く',
     settings_desc_worklog_details_expanded_default: '有効にすると、新しい Worklog の詳細が展開された状態で始まり、ツール、Thinking、進捗カードをクリックなしで確認できます。無効の場合、Worklog の詳細はデフォルトで折りたたまれます。ターンごとの手動折りたたみ/展開は優先されます。',
     settings_label_chat_activity_display_mode: 'アクティビティ表示',
@@ -6000,6 +6018,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -7489,6 +7513,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -8659,6 +8689,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -10502,6 +10538,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -11304,6 +11346,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: '在標題列顯示設定檔切換器',
     settings_desc_show_titlebar_profile: '啟用後，左上角的應用程式標題列會出現設定檔切換按鈕，讓你在任何分頁都能切換設定檔。預設關閉；無論此設定為何，輸入列底部一律保留設定檔切換器。',
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -12764,6 +12812,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -14230,6 +14284,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -15718,6 +15778,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -17300,6 +17366,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Open Worklog details automatically',
     settings_desc_worklog_details_expanded_default: 'When enabled, new Worklog details start expanded so tool, thinking, and progress cards are visible without an extra click. When off, Worklog details stay folded by default; per-turn manual collapse/expand choices still win.',
     settings_label_chat_activity_display_mode: 'Activity display',
@@ -18878,6 +18950,12 @@ const LOCALES = {
     settings_label_show_titlebar_profile: 'Show profile switcher in titlebar', // TODO: translate
     settings_desc_show_titlebar_profile: 'When enabled, a profile switcher button appears in the top-left app titlebar so you can change profiles from any tab. Off by default; the composer footer always has a profile switcher regardless of this setting.', // TODO: translate
     settings_desc_render_user_markdown: 'When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.',
+    settings_label_structured_code: 'JSON/YAML code blocks',
+    settings_option_structured_code_auto: 'Auto: Tree for long blocks',
+    settings_option_structured_code_on: 'Tree by default',
+    settings_option_structured_code_off: 'Raw by default',
+    settings_label_structured_code_auto_lines: 'Auto threshold lines',
+    settings_desc_structured_code: 'When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.',
     settings_label_worklog_details_expanded_default: 'Otwieraj szczegóły Worklog automatycznie',
     settings_desc_worklog_details_expanded_default: 'Gdy ta opcja jest włączona, nowe szczegóły Worklog zaczynają rozwinięte, dzięki czemu karty narzędzi, Thinking i postępu są widoczne bez dodatkowego kliknięcia. Gdy jest wyłączona, szczegóły Worklog pozostają domyślnie zwinięte. Ręczne decyzje o zwinięciu/rozwinięciu w danej turze mają pierwszeństwo.',
     settings_label_chat_activity_display_mode: 'Activity display',
diff --git a/static/index.html b/static/index.html
index a213d508..c01e022f 100644
--- a/static/index.html
+++ b/static/index.html
@@ -1071,6 +1071,19 @@
               </label>
               <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_render_user_markdown">When enabled, bold, italic, links, and other markdown in your own messages are rendered. Off by default; fenced code blocks and math always render regardless of this setting.</div>
             </div>
+            <div class="settings-field" style="margin-top:8px">
+              <label for="settingsStructuredCodeMode" data-i18n="settings_label_structured_code">JSON/YAML code blocks</label>
+              <select id="settingsStructuredCodeMode" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px">
+                <option value="auto" data-i18n="settings_option_structured_code_auto">Auto: Tree for long blocks</option>
+                <option value="on" data-i18n="settings_option_structured_code_on">Tree by default</option>
+                <option value="off" data-i18n="settings_option_structured_code_off">Raw by default</option>
+              </select>
+              <div style="margin-top:8px">
+                <label for="settingsStructuredCodeAutoLines" data-i18n="settings_label_structured_code_auto_lines">Auto threshold lines</label>
+                <input type="number" id="settingsStructuredCodeAutoLines" min="1" max="1000" step="1" inputmode="numeric" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px;font-size:13px">
+              </div>
+              <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_structured_code">When Auto is selected, JSON/YAML blocks with at least this many lines open in Tree view. Tree always opens the structured viewer; Raw keeps the syntax-highlighted text. You can still switch Tree/Raw on each block.</div>
+            </div>
             <div class="settings-field" style="margin-top:8px">
               <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
                 <input type="checkbox" id="settingsShowTitlebarProfile" style="width:15px;height:15px;accent-color:var(--accent)">
diff --git a/static/panels.js b/static/panels.js
index 36140823..672b22cb 100644
--- a/static/panels.js
+++ b/static/panels.js
@@ -7002,6 +7002,44 @@ function _applyTtsEnabled(enabled){
   document.body.classList.toggle('tts-enabled', !!enabled);
 }
 
+// Read + sanitize the JSON/YAML structured code-block default-view controls
+// (#484). mode is one of auto|on|off; lines is clamped to an int 1..1000 with a
+// fallback of 10 (the original hardcoded threshold).
+function _structuredCodeViewFromUi(){
+  const modeSel=$('settingsStructuredCodeMode');
+  const mode=modeSel&&['auto','on','off'].includes(modeSel.value)?modeSel.value:'auto';
+  const linesField=$('settingsStructuredCodeAutoLines');
+  const n=parseInt((linesField||{}).value,10);
+  const lines=(Number.isFinite(n)&&n>=1&&n<=1000)?n:10;
+  return {structured_code_default_view:mode,structured_code_auto_tree_lines:lines};
+}
+
+// Apply the structured code-block settings to runtime globals and re-render the
+// transcript so already-rendered JSON/YAML blocks pick up the new default. The
+// per-block Raw/Tree toggle is unaffected.
+function _applyStructuredCodeViewSettings(mode,lines,rerender){
+  window._structuredCodeDefaultView=['auto','on','off'].includes(mode)?mode:'auto';
+  const n=parseInt(lines,10);
+  window._structuredCodeAutoTreeLines=(Number.isFinite(n)&&n>=1&&n<=1000)?n:10;
+  if(rerender){
+    if(typeof clearMessageRenderCache==='function') clearMessageRenderCache();
+    if(typeof renderMessages==='function') renderMessages({preserveScroll:true});
+  }
+}
+
+// The Auto-threshold input is only meaningful in 'auto' mode; disable it
+// otherwise so the control reads as inactive without hiding it.
+function _syncStructuredCodeLinesEnabled(){
+  const modeSel=$('settingsStructuredCodeMode');
+  const linesField=$('settingsStructuredCodeAutoLines');
+  // Both controls live in the same settings-field and are present together;
+  // if either is missing there's nothing to sync.
+  if(!modeSel||!linesField) return;
+  const isAuto=modeSel.value==='auto';
+  linesField.disabled=!isAuto;
+  linesField.style.opacity=isAuto?'':'0.5';
+}
+
 function _appearancePayloadFromUi(){
   const worklogDetailsExpanded=!!($('settingsWorklogDetailsExpandedDefault')||{}).checked;
   const chatActivityModeSel=$('settingsChatActivityDisplayMode');
@@ -7014,6 +7052,7 @@ function _appearancePayloadFromUi(){
     session_endless_scroll: !!($('settingsSessionEndlessScroll')||{}).checked,
     auto_scroll_follow: !!($('settingsAutoScrollFollow')||{}).checked,
     render_user_markdown: !!($('settingsRenderUserMarkdown')||{}).checked,
+    ..._structuredCodeViewFromUi(),
     show_titlebar_profile: !!($('settingsShowTitlebarProfile')||{}).checked,
     worklog_details_expanded_default: worklogDetailsExpanded,
     activity_feed_expanded_default: worklogDetailsExpanded,
@@ -7102,6 +7141,16 @@ async function _autosaveAppearanceSettings(payload){
     }
     window._sessionEndlessScrollEnabled=!!(saved&&saved.session_endless_scroll);
     window._autoScrollFollow=!saved||saved.auto_scroll_follow!==false;
+    if(saved&&Object.prototype.hasOwnProperty.call(saved,'structured_code_default_view')){
+      // Re-sync from the server-validated/clamped values so the UI and runtime
+      // globals match exactly what was persisted.
+      _applyStructuredCodeViewSettings(saved.structured_code_default_view,saved.structured_code_auto_tree_lines,false);
+      const modeSel=$('settingsStructuredCodeMode');
+      if(modeSel) modeSel.value=window._structuredCodeDefaultView;
+      const linesField=$('settingsStructuredCodeAutoLines');
+      if(linesField) linesField.value=window._structuredCodeAutoTreeLines;
+      _syncStructuredCodeLinesEnabled();
+    }
     if(saved&&payload&&Object.prototype.hasOwnProperty.call(payload,'worklog_details_expanded_default')&&(
       Object.prototype.hasOwnProperty.call(saved,'worklog_details_expanded_default') ||
       Object.prototype.hasOwnProperty.call(saved,'activity_feed_expanded_default')
@@ -7406,6 +7455,34 @@ async function loadSettingsPanel(){
         _scheduleAppearanceAutosave();
       };
     }
+    const structuredCodeModeSel=$('settingsStructuredCodeMode');
+    const structuredCodeLinesField=$('settingsStructuredCodeAutoLines');
+    if(structuredCodeModeSel){
+      const mode=['auto','on','off'].includes(settings.structured_code_default_view)?settings.structured_code_default_view:'auto';
+      structuredCodeModeSel.value=mode;
+      const lines=parseInt(settings.structured_code_auto_tree_lines,10);
+      const safeLines=(Number.isFinite(lines)&&lines>=1&&lines<=1000)?lines:10;
+      if(structuredCodeLinesField) structuredCodeLinesField.value=safeLines;
+      _applyStructuredCodeViewSettings(mode,safeLines,false);
+      _syncStructuredCodeLinesEnabled();
+      structuredCodeModeSel.onchange=function(){
+        const cfg=_structuredCodeViewFromUi();
+        _applyStructuredCodeViewSettings(cfg.structured_code_default_view,cfg.structured_code_auto_tree_lines,true);
+        _syncStructuredCodeLinesEnabled();
+        _scheduleAppearanceAutosave();
+      };
+      if(structuredCodeLinesField){
+        // Commit on change (blur / Enter / spinner) rather than every keystroke,
+        // so a long transcript isn't rebuilt per digit typed. Only re-render in
+        // 'auto' mode, where the threshold actually affects the default view.
+        structuredCodeLinesField.addEventListener('change',function(){
+          const cfg=_structuredCodeViewFromUi();
+          structuredCodeLinesField.value=cfg.structured_code_auto_tree_lines;
+          _applyStructuredCodeViewSettings(cfg.structured_code_default_view,cfg.structured_code_auto_tree_lines,cfg.structured_code_default_view==='auto');
+          _scheduleAppearanceAutosave();
+        },{once:false});
+      }
+    }
     const showTitlebarProfileCb=$('settingsShowTitlebarProfile');
     if(showTitlebarProfileCb){
       showTitlebarProfileCb.checked=!!settings.show_titlebar_profile;
@@ -9091,6 +9168,9 @@ function _applySavedSettingsUi(saved, body, opts){
   window._busyInputMode=body.busy_input_mode||'queue';
   window._sessionEndlessScrollEnabled=!!body.session_endless_scroll;
   window._autoScrollFollow=body.auto_scroll_follow!==false;
+  if(Object.prototype.hasOwnProperty.call(body,'structured_code_default_view')){
+    _applyStructuredCodeViewSettings(body.structured_code_default_view,body.structured_code_auto_tree_lines,false);
+  }
   window._botName=body.bot_name||'Hermes';
   if(typeof applyBotName==='function') applyBotName();
   if(typeof setLocale==='function') setLocale(language);
@@ -9612,6 +9692,7 @@ async function saveSettings(andClose){
   body.chat_activity_display_mode=(($('settingsChatActivityDisplayMode')||{}).value==='transparent_stream')?'transparent_stream':'compact_worklog';
   body.auto_scroll_follow=!!($('settingsAutoScrollFollow')||{}).checked;
   body.render_user_markdown=!!($('settingsRenderUserMarkdown')||{}).checked;
+  Object.assign(body,_structuredCodeViewFromUi());
   body.language=language;
   body.show_token_usage=showTokenUsage;
   body.show_quota_chip=showQuotaChip===true;
diff --git a/static/ui.js b/static/ui.js
index cedaca31..139f15fb 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -13437,6 +13437,34 @@ function _loadJsyamlThen(cb){
   document.head.appendChild(s);
 }
 
+// ── JSON/YAML structured code-block default-view configuration (#484) ──
+// Read the user's configured default-view mode for valid JSON/YAML fenced
+// blocks. Falls back to 'auto' for any missing/invalid value so the renderer
+// stays safe before settings load and in non-browser test contexts.
+function _structuredCodeMode(){
+  const m=(typeof window!=='undefined')?window._structuredCodeDefaultView:undefined;
+  return (m==='on'||m==='off'||m==='auto')?m:'auto';
+}
+// Read the configured 'auto'-mode line threshold, clamped to a sane integer
+// range. Invalid/missing values fall back to 10 (the original hardcoded value).
+function _structuredCodeThreshold(){
+  const raw=(typeof window!=='undefined')?window._structuredCodeAutoTreeLines:undefined;
+  const n=parseInt(raw,10);
+  return (Number.isFinite(n)&&n>=1&&n<=1000)?n:10;
+}
+// Pure decision helper: should a structured block default to Tree view?
+// Factored out so the (mode, threshold, lineCount) contract is unit-testable.
+//   mode 'on'   => always Tree
+//   mode 'off'  => always Raw
+//   mode 'auto' => Tree only when lineCount >= threshold (threshold sanitized,
+//                  fallback 10)
+function _structuredCodeShowTree(mode,threshold,lineCount){
+  if(mode==='on') return true;
+  if(mode==='off') return false;
+  const th=(Number.isFinite(threshold)&&threshold>=1&&threshold<=1000)?threshold:10;
+  return lineCount>=th;
+}
+
 function initTreeViews(container){
   const root=container||document;
   root.querySelectorAll('.code-tree-wrap:not([data-tree-init])').forEach(wrap=>{
@@ -13474,8 +13502,11 @@ function initTreeViews(container){
       return; // leave as raw view
     }
     const lineCount=rawText.split('\n').length;
-    // Default to raw for short blocks (<10 lines), tree for longer
-    const showTree=lineCount>=10;
+    // Default view is user-configurable (#484 follow-up). 'on' => always Tree,
+    // 'off' => always Raw, 'auto' => Tree only when the block is >= the
+    // configured line threshold (default 10, preserving the original behavior).
+    // The per-block Raw/Tree toggle below always remains available regardless.
+    const showTree=_structuredCodeShowTree(_structuredCodeMode(),_structuredCodeThreshold(),lineCount);
     // Build tree DOM
     const treeDiv=document.createElement('div');
     treeDiv.className='tree-view'+(showTree?'':' tree-hidden');
diff --git a/tests/test_issue484_json_tree_viewer.py b/tests/test_issue484_json_tree_viewer.py
index 789c8d17..1b0978e3 100644
--- a/tests/test_issue484_json_tree_viewer.py
+++ b/tests/test_issue484_json_tree_viewer.py
@@ -1,7 +1,32 @@
-"""Tests for issue #484 — collapsible JSON/YAML tree viewer."""
+"""Tests for issue #484 — collapsible JSON/YAML tree viewer.
+
+The original feature hardcoded the default view: valid JSON/YAML fenced blocks
+opened in Tree view at 10+ lines (`const showTree=lineCount>=10;`). That default
+is now user-configurable (#484 follow-up) via two settings:
+
+  - structured_code_default_view: "auto" | "on" | "off"
+        on   => always default to Tree
+        off  => always default to Raw
+        auto => Tree only when the block line count >= the configured threshold
+  - structured_code_auto_tree_lines: integer 1..1000 (default 10)
+
+`auto` + threshold 10 reproduces the original behavior, so the default is
+preserved. These tests pin the new configurable shape while keeping the existing
+Tree/Raw renderer invariants (wrapper class, helpers, value types, toggle, YAML
+support) intact.
+"""
+import shutil
+import subprocess
+from pathlib import Path
+
 import pytest
 
 
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+UI_JS_PATH = REPO_ROOT / "static" / "ui.js"
+NODE = shutil.which("node")
+
+
 class TestTreeRenderer:
     """Fenced JSON/YAML blocks should get a tree view toggle."""
 
@@ -68,11 +93,125 @@ class TestTreeRenderer:
             content = f.read()
         assert "jsyaml" in content
 
-    def test_short_json_defaults_to_raw(self):
-        """Blocks under 10 lines should default to raw view."""
+
+class TestConfigurableDefaultView:
+    """The default Tree/Raw decision is configurable, not hardcoded (#484 follow-up)."""
+
+    def test_hardcoded_threshold_is_gone(self):
+        """The original `lineCount>=10` hardcode must no longer drive the default."""
         with open("static/ui.js", "r", encoding="utf-8") as f:
             content = f.read()
-        assert "lineCount>=10" in content
+        assert "const showTree=lineCount>=10;" not in content, (
+            "The hardcoded `const showTree=lineCount>=10;` must be replaced by the "
+            "configurable decision helper."
+        )
+        assert "lineCount>=10" not in content, (
+            "No hardcoded `lineCount>=10` comparison should remain; the threshold "
+            "is configurable and the default is read from a sanitized helper."
+        )
+
+    def test_decision_helper_exists(self):
+        with open("static/ui.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        assert "function _structuredCodeShowTree(mode,threshold,lineCount)" in content
+        assert "function _structuredCodeMode(" in content
+        assert "function _structuredCodeThreshold(" in content
+
+    def test_all_three_modes_represented(self):
+        """auto / on / off must all be handled in the renderer."""
+        with open("static/ui.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        start = content.find("function _structuredCodeShowTree")
+        body = content[start:start + 400]
+        assert "'on'" in body, "mode 'on' (always Tree) must be handled"
+        assert "'off'" in body, "mode 'off' (always Raw) must be handled"
+        # 'auto' is the fallthrough; the mode reader sanitizes to it.
+        mode_start = content.find("function _structuredCodeMode")
+        mode_body = content[mode_start:mode_start + 300]
+        assert "'auto'" in mode_body, "mode 'auto' must be the sanitized fallback"
+
+    def test_default_threshold_fallback_is_10(self):
+        """An invalid/missing threshold must fall back to 10 (original behavior)."""
+        with open("static/ui.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        thr_start = content.find("function _structuredCodeThreshold")
+        thr_body = content[thr_start:thr_start + 300]
+        assert ":10" in thr_body or "?10" in thr_body or " 10" in thr_body, (
+            "_structuredCodeThreshold must fall back to 10 for invalid/missing values"
+        )
+        # Sanitization bounds (1..1000) keep the value safe.
+        assert "1000" in thr_body, "threshold should be clamped to an upper bound"
+
+    def test_renderer_uses_helper_for_decision(self):
+        with open("static/ui.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        assert "_structuredCodeShowTree(_structuredCodeMode(),_structuredCodeThreshold(),lineCount)" in content
+
+    @pytest.mark.skipif(NODE is None, reason="node not on PATH")
+    def test_decision_helper_behaviour_via_node(self, tmp_path):
+        """Evaluate the real _structuredCodeShowTree() against the prompt's cases.
+
+          - off, lineCount 999            => False
+          - on, lineCount 1               => True
+          - auto, threshold 10, count 9   => False
+          - auto, threshold 10, count 10  => True
+          - auto, invalid threshold       => fallback 10 (count 10 => True, 9 => False)
+        """
+        src = UI_JS_PATH.read_text(encoding="utf-8")
+        # Extract the brace-balanced body of _structuredCodeShowTree.
+        marker = "function _structuredCodeShowTree("
+        start = src.find(marker)
+        assert start >= 0, "_structuredCodeShowTree not found"
+        i = src.index("{", start)
+        depth = 1
+        i += 1
+        while depth > 0 and i < len(src):
+            if src[i] == "{":
+                depth += 1
+            elif src[i] == "}":
+                depth -= 1
+            i += 1
+        fn_src = src[start:i]
+
+        driver = tmp_path / "driver.js"
+        driver.write_text(
+            fn_src
+            + "\n"
+            + r"""
+const cases = [
+  ['off', 5, 999, false],
+  ['on', 5, 1, true],
+  ['auto', 10, 9, false],
+  ['auto', 10, 10, true],
+  ['auto', 11, 10, false],
+  // invalid threshold falls back to 10
+  ['auto', NaN, 10, true],
+  ['auto', NaN, 9, false],
+  // out-of-range thresholds fall back to 10 inside the pure helper
+  ['auto', 0, 10, true],
+  ['auto', 0, 9, false],
+  ['auto', 99999, 10, true],
+  ['auto', 99999, 9, false],
+  // unknown mode is treated as auto by the pure helper's fallthrough
+  ['bogus', 10, 10, true],
+  ['bogus', 10, 9, false],
+];
+let failures = [];
+for (const [mode, thr, count, want] of cases) {
+  const got = _structuredCodeShowTree(mode, thr, count);
+  if (got !== want) failures.push(`${mode},${thr},${count} => ${got} (want ${want})`);
+}
+if (failures.length) { console.error('FAIL: ' + failures.join(' | ')); process.exit(1); }
+console.log('OK');
+""",
+            encoding="utf-8",
+        )
+        result = subprocess.run(
+            [NODE, str(driver)], capture_output=True, text=True, timeout=30
+        )
+        assert result.returncode == 0, (
+            f"_structuredCodeShowTree behaviour mismatch: {result.stdout}{result.stderr}"
+        )
 
 
 class TestTreeCSS:
@@ -103,3 +242,50 @@ class TestTreeI18n:
         for key in ("tree_view", "raw_view"):
             count = content.count(key)
             assert count >= 7, f"{key} found {count} times, expected >= 7"
+
+    def test_structured_code_setting_i18n_keys_present(self):
+        """The new settings labels/options/help text must exist in i18n."""
+        with open("static/i18n.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        for key in (
+            "settings_label_structured_code",
+            "settings_option_structured_code_auto",
+            "settings_option_structured_code_on",
+            "settings_option_structured_code_off",
+            "settings_label_structured_code_auto_lines",
+            "settings_desc_structured_code",
+        ):
+            assert f"{key}:" in content, f"Missing i18n key: {key}"
+
+
+class TestStructuredCodeSettingsWiring:
+    """The setting must be a real, persisted WebUI setting — not a localStorage hack."""
+
+    def test_server_defaults_and_validation(self):
+        with open("api/config.py", "r", encoding="utf-8") as f:
+            content = f.read()
+        # Defaults preserve current behavior: auto + threshold 10.
+        assert '"structured_code_default_view": "auto"' in content
+        assert '"structured_code_auto_tree_lines": 10' in content
+        # Enum + range validation are registered.
+        assert '"structured_code_default_view": {"auto", "on", "off"}' in content
+        assert '"structured_code_auto_tree_lines": (1, 1000)' in content
+
+    def test_settings_ui_controls_present(self):
+        with open("static/index.html", "r", encoding="utf-8") as f:
+            content = f.read()
+        assert 'id="settingsStructuredCodeMode"' in content
+        assert 'id="settingsStructuredCodeAutoLines"' in content
+
+    def test_boot_initializes_runtime_globals(self):
+        with open("static/boot.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        assert "window._structuredCodeDefaultView" in content
+        assert "window._structuredCodeAutoTreeLines" in content
+
+    def test_panel_persists_setting(self):
+        with open("static/panels.js", "r", encoding="utf-8") as f:
+            content = f.read()
+        assert "_structuredCodeViewFromUi" in content
+        assert "structured_code_default_view" in content
+        assert "structured_code_auto_tree_lines" in content