diff --git a/changelog.d/1642-folded-thinking-preview.fixed.md b/changelog.d/1642-folded-thinking-preview.fixed.md
new file mode 100644
index 000000000..af5609109
--- /dev/null
+++ b/changelog.d/1642-folded-thinking-preview.fixed.md
@@ -0,0 +1 @@
+The folded thinking row's one-line preview now skips fences and horizontal rules, cuts on a character boundary instead of halving an emoji, and ends a clipped line with an ellipsis; reasoning that is nothing but markup says how many lines it has instead of showing a fence fragment ([#1642](https://github.com/ThinkFlowLab/FlyBot/issues/1642)).
diff --git a/core/client.js b/core/client.js
index 3414e1f00..e3295b9ac 100644
--- a/core/client.js
+++ b/core/client.js
@@ -838,6 +838,8 @@ var ZH = {
   "Message actions": "消息操作",
   Download: "下载",
   "Thought it through": "思考过程",
+  "{n} line of reasoning": "{n} 行思考内容",
+  "{n} lines of reasoning": "{n} 行思考内容",
   "Seen — I’ll get to this right after the current step.": "看到了，这步做完就接着处理你这条。",
   "Earlier messages": "更早的消息",
   "Loading earlier messages…": "正在加载更早的消息…",
@@ -6171,9 +6173,24 @@ function MessageTools({ text, name, sessionId, react, actions }) {
   );
 }
 var PEEK_MAX = 160;
+var CUT = "…";
+var MARKUP = [/^\s{0,3}(?:`{3,}|~{3,})/, /^\s{0,3}([-*_])(?:\s*\1){2,}\s*$/];
+var isMarkup = (line2) => MARKUP.some((pattern) => pattern.test(line2));
+var graphemes = (line2) => typeof Intl !== "undefined" && typeof Intl.Segmenter === "function" ? [...new Intl.Segmenter().segment(line2)].map((part) => part.segment) : [...line2];
+var clip = (line2) => {
+  if (line2.length <= PEEK_MAX) return line2;
+  let kept = "";
+  for (const part of graphemes(line2)) {
+    if (kept.length + part.length > PEEK_MAX - CUT.length) break;
+    kept += part;
+  }
+  return kept + CUT;
+};
+var onlyMarkup = (lines) => lines.length === 1 ? t("{n} line of reasoning", { n: lines.length }) : t("{n} lines of reasoning", { n: lines.length });
 var peekLine = (text) => {
-  const line2 = text.split("\n").map((part) => part.replace(/\s+/g, " ").trim()).find((part) => part !== "");
-  return line2 === void 0 ? "" : line2.slice(0, PEEK_MAX);
+  const lines = text.split("\n").map((part) => part.replace(/\s+/g, " ").trim()).filter((part) => part !== "");
+  const prose = lines.find((line2) => !isMarkup(line2));
+  return prose === void 0 ? onlyMarkup(lines) : clip(prose);
 };
 function ThinkingRow({ text }) {
   if (typeof text !== "string" || text.trim() === "") return null;
diff --git a/core/src/client/cells.js b/core/src/client/cells.js
index d8b5c316c..cdf9a73bc 100644
--- a/core/src/client/cells.js
+++ b/core/src/client/cells.js
@@ -130,9 +130,33 @@ function MessageTools({ text, name, sessionId, react, actions }) {
 // The first line with something on it, inner runs of whitespace squeezed: a folded row shows it, so
 // the row never reads as an empty frame while the text it names sits behind the caret (#1624).
 const PEEK_MAX = 160
+const CUT = '…'
+// A fence (```js, ~~~, a closing fence) and a horizontal rule are markup, not something thought, so
+// the preview skips them and takes the first line that has prose (#1642).
+const MARKUP = [/^\s{0,3}(?:`{3,}|~{3,})/, /^\s{0,3}([-*_])(?:\s*\1){2,}\s*$/]
+const isMarkup = (line) => MARKUP.some(pattern => pattern.test(line))
+// Graphemes where the platform segments them, code points otherwise: either way the cut never lands
+// inside a surrogate pair, so a clipped emoji cannot show up as U+FFFD (#1642).
+const graphemes = (line) => typeof Intl !== 'undefined' && typeof Intl.Segmenter === 'function'
+  ? [...new Intl.Segmenter().segment(line)].map(part => part.segment)
+  : [...line]
+const clip = (line) => {
+  if (line.length <= PEEK_MAX) return line
+  let kept = ''
+  for (const part of graphemes(line)) {
+    if (kept.length + part.length > PEEK_MAX - CUT.length) break
+    kept += part
+  }
+  return kept + CUT
+}
+// Nothing but markup: a line count still says something, where a fence fragment says nothing (#1642).
+const onlyMarkup = (lines) => (lines.length === 1
+  ? t('{n} line of reasoning', { n: lines.length })
+  : t('{n} lines of reasoning', { n: lines.length }))
 const peekLine = (text) => {
-  const line = text.split('\n').map((part) => part.replace(/\s+/g, ' ').trim()).find((part) => part !== '')
-  return line === undefined ? '' : line.slice(0, PEEK_MAX)
+  const lines = text.split('\n').map((part) => part.replace(/\s+/g, ' ').trim()).filter((part) => part !== '')
+  const prose = lines.find((line) => !isMarkup(line))
+  return prose === undefined ? onlyMarkup(lines) : clip(prose)
 }
 
 function ThinkingRow({ text }) {
diff --git a/core/src/client/i18n.js b/core/src/client/i18n.js
index 72d01720e..91e0a1644 100644
--- a/core/src/client/i18n.js
+++ b/core/src/client/i18n.js
@@ -785,6 +785,8 @@ const ZH = {
   'Message actions': '消息操作',
   Download: '下载',
   'Thought it through': '思考过程',
+  '{n} line of reasoning': '{n} 行思考内容',
+  '{n} lines of reasoning': '{n} 行思考内容',
   'Seen — I’ll get to this right after the current step.': '看到了，这步做完就接着处理你这条。',
   'Earlier messages': '更早的消息',
   'Loading earlier messages…': '正在加载更早的消息…',
diff --git a/core/test/thinking-row.test.js b/core/test/thinking-row.test.js
index ef3971cc4..c8bdec65d 100644
--- a/core/test/thinking-row.test.js
+++ b/core/test/thinking-row.test.js
@@ -136,3 +136,43 @@ test('#1636: an opened row hides its preview, so its first line is not printed t
     /\.bt-think\[open\] \.bt-think-peek\{display:none\}/,
   )
 })
+
+test('#1642: a fence or a rule is not the peek line; the first prose line is', () => {
+  const fenced = [think('```js\nconst x = 1\n```')]
+  const peek = classed(thinkingRow(render(step(1, fenced), [step(1, fenced)])), 'bt-think-peek')
+  assert.ok(peek, 'the closed row carries a line of the reasoning')
+  assert.equal(text(peek), 'const x = 1', 'markup is not what the Bot was thinking')
+  assert.doesNotMatch(text(peek), /```/)
+
+  const ruled = [think('---\n***\nLet me read it\n___')]
+  const second = classed(thinkingRow(render(step(1, ruled), [step(1, ruled)])), 'bt-think-peek')
+  assert.ok(second)
+  assert.equal(text(second), 'Let me read it', 'a horizontal rule is markup too')
+})
+
+test('#1642: a cut preview keeps whole characters and says it was cut', () => {
+  const long = [think(`${'a'.repeat(150)}😀${'b'.repeat(100)}`)]
+  const peek = classed(thinkingRow(render(step(1, long), [step(1, long)])), 'bt-think-peek')
+  assert.ok(peek)
+  const shown = text(peek)
+  assert.equal(shown.length, 160, 'the cap still holds')
+  assert.ok(shown.includes('😀'), 'an emoji inside the cap survives whole')
+  assert.ok(shown.endsWith('…'), 'a clipped line says it was clipped')
+  assert.doesNotMatch(shown, /[\uD800-\uDFFF]/u, 'no half a surrogate pair')
+
+  // The reported input: a pair cut exactly at the cap used to render as U+FFFD.
+  const edge = [think(`${'a'.repeat(159)}😀 tail`)]
+  const cut = text(classed(thinkingRow(render(step(1, edge), [step(1, edge)])), 'bt-think-peek'))
+  assert.equal(cut.length, 160)
+  assert.ok(cut.endsWith('…'))
+  assert.doesNotMatch(cut, /[\uD800-\uDFFF]/u, 'no half a surrogate pair at the cap either')
+})
+
+test('#1642: reasoning that is only markup gets a neutral summary, never a bare fence or an empty row', () => {
+  const onlyMarkup = [think('```js\n```')]
+  const details = thinkingRow(render(step(1, onlyMarkup), [step(1, onlyMarkup)]))
+  assert.ok(details, 'the row stays')
+  const peek = classed(details, 'bt-think-peek')
+  assert.ok(peek, 'the folded row never reads as an empty frame')
+  assert.equal(text(peek), '2 lines of reasoning')
+})
diff --git a/patches/ds-bot/0001-core-fixes.patch b/patches/ds-bot/0001-core-fixes.patch
index 3bf37caac..8f10c751e 100644
--- a/patches/ds-bot/0001-core-fixes.patch
+++ b/patches/ds-bot/0001-core-fixes.patch
@@ -1152,7 +1152,7 @@ diff --git a/src/client/i18n.js b/src/client/i18n.js
    // Command palette
    Messages: '消息',
    Commands: '命令',
-@@ -390,10 +390,12 @@ const ZH = {
+@@ -390,10 +390,14 @@ const ZH = {
    Reply: '回复',
    'More message actions': '更多操作',
    More: '更多',
@@ -1161,6 +1161,8 @@ diff --git a/src/client/i18n.js b/src/client/i18n.js
    'Message actions': '消息操作',
    Download: '下载',
    'Thought it through': '思考过程',
++  '{n} line of reasoning': '{n} 行思考内容',
++  '{n} lines of reasoning': '{n} 行思考内容',
 +  'Seen — I’ll get to this right after the current step.': '看到了，这步做完就接着处理你这条。',
    'Earlier messages': '更早的消息',
    'Loading earlier messages…': '正在加载更早的消息…',
diff --git a/patches/ds-bot/0003-flybot-api-client.patch b/patches/ds-bot/0003-flybot-api-client.patch
index 3f1a67494..a83f748bf 100644
--- a/patches/ds-bot/0003-flybot-api-client.patch
+++ b/patches/ds-bot/0003-flybot-api-client.patch
@@ -529,16 +529,40 @@ diff --git a/src/client/cells.js b/src/client/cells.js
  // Closes a popover on a press outside the given elements or on Escape.
  function useDismiss(open, close, refs) {
    useEffect(() => {
-@@ -100,9 +100,21 @@ function MessageTools({ text, name, sessionId, react, actions }) {
+@@ -100,9 +100,45 @@ function MessageTools({ text, name, sessionId, react, actions }) {
        item(t('Copy'), CopyIcon, copy), item(t('Download'), DownloadIcon, download)), document.body) : null)
  }
  
 +// The first line with something on it, inner runs of whitespace squeezed: a folded row shows it, so
 +// the row never reads as an empty frame while the text it names sits behind the caret (#1624).
 +const PEEK_MAX = 160
++const CUT = '…'
++// A fence (```js, ~~~, a closing fence) and a horizontal rule are markup, not something thought, so
++// the preview skips them and takes the first line that has prose (#1642).
++const MARKUP = [/^\s{0,3}(?:`{3,}|~{3,})/, /^\s{0,3}([-*_])(?:\s*\1){2,}\s*$/]
++const isMarkup = (line) => MARKUP.some(pattern => pattern.test(line))
++// Graphemes where the platform segments them, code points otherwise: either way the cut never lands
++// inside a surrogate pair, so a clipped emoji cannot show up as U+FFFD (#1642).
++const graphemes = (line) => typeof Intl !== 'undefined' && typeof Intl.Segmenter === 'function'
++  ? [...new Intl.Segmenter().segment(line)].map(part => part.segment)
++  : [...line]
++const clip = (line) => {
++  if (line.length <= PEEK_MAX) return line
++  let kept = ''
++  for (const part of graphemes(line)) {
++    if (kept.length + part.length > PEEK_MAX - CUT.length) break
++    kept += part
++  }
++  return kept + CUT
++}
++// Nothing but markup: a line count still says something, where a fence fragment says nothing (#1642).
++const onlyMarkup = (lines) => (lines.length === 1
++  ? t('{n} line of reasoning', { n: lines.length })
++  : t('{n} lines of reasoning', { n: lines.length }))
 +const peekLine = (text) => {
-+  const line = text.split('\n').map((part) => part.replace(/\s+/g, ' ').trim()).find((part) => part !== '')
-+  return line === undefined ? '' : line.slice(0, PEEK_MAX)
++  const lines = text.split('\n').map((part) => part.replace(/\s+/g, ' ').trim()).filter((part) => part !== '')
++  const prose = lines.find((line) => !isMarkup(line))
++  return prose === undefined ? onlyMarkup(lines) : clip(prose)
 +}
 +
  function ThinkingRow({ text }) {
diff --git a/patches/ds-bot/0008-client-bundle.patch b/patches/ds-bot/0008-client-bundle.patch
index ba448a312..6ef71528a 100644
--- a/patches/ds-bot/0008-client-bundle.patch
+++ b/patches/ds-bot/0008-client-bundle.patch
@@ -572,7 +572,7 @@ diff --git a/client.js b/client.js
    // Command palette
    Messages: "消息",
    Commands: "命令",
-@@ -446,10 +446,12 @@ var ZH = {
+@@ -446,10 +446,14 @@ var ZH = {
    Reply: "回复",
    "More message actions": "更多操作",
    More: "更多",
@@ -581,6 +581,8 @@ diff --git a/client.js b/client.js
    "Message actions": "消息操作",
    Download: "下载",
    "Thought it through": "思考过程",
++  "{n} line of reasoning": "{n} 行思考内容",
++  "{n} lines of reasoning": "{n} 行思考内容",
 +  "Seen — I’ll get to this right after the current step.": "看到了，这步做完就接着处理你这条。",
    "Earlier messages": "更早的消息",
    "Loading earlier messages…": "正在加载更早的消息…",
@@ -3668,7 +3670,7 @@ diff --git a/client.js b/client.js
  .bt-ububble{max-width:min(560px,78%);background:var(--bt-user);color:var(--bt-user-ink);border-radius:18px;padding:7px 12px;font-size:14px;line-height:20px;box-sizing:border-box;overflow-wrap:anywhere}
  .bt-ububble [class*="_markdown"],.bt-ububble a{color:inherit!important}
  .bt-ububble p{margin:0;line-height:22px}
-@@ -2805,1975 +2805,4080 @@ function sheetClassNames(sheet) {
+@@ -2805,1975 +2805,4095 @@ function sheetClassNames(sheet) {
      visit(sheet.cssRules);
    } catch {
    }
@@ -5172,19 +5174,12 @@ diff --git a/client.js b/client.js
 -    ) : null,
 -    foot
 +    error ? (0, import_react8.createElement)("div", { className: "bt-flybot-approval-error", role: "alert" }, error) : null
-   );
++  );
 +  return (0, import_react8.createElement)(
 +    "div",
 +    { className: "bt-held-actions", "data-flybot": "held-reply-actions" },
 +    custom ? (0, import_react8.createElement)(boundary(), { id: custom.id, fallback: plain }, (0, import_react8.createElement)(custom.component, { held, decide, call: call3, actions })) : plain
-+  );
-+}
-+
-+// src/client/turn-reasoning.js
-+function turnReasoning(steps, current) {
-+  const members = steps.length ? steps : [current];
-+  if (members.find((step) => stepReasoning(step) !== "")?.step !== current.step) return "";
-+  return members.filter((step) => !thinkingLive(step)).map(stepReasoning).filter(Boolean).join("\n\n");
+   );
  }
 -function SettledQuestion({ question, answer }) {
 -  const options = optionsOf(question);
@@ -5203,6 +5198,13 @@ diff --git a/client.js b/client.js
 -    );
 -  }
 -  return (0, import_react4.createElement)(
++
++// src/client/turn-reasoning.js
++function turnReasoning(steps, current) {
++  const members = steps.length ? steps : [current];
++  if (members.find((step) => stepReasoning(step) !== "")?.step !== current.step) return "";
++  return members.filter((step) => !thinkingLive(step)).map(stepReasoning).filter(Boolean).join("\n\n");
++}
 +var texts = (step) => step.blocks.filter((block) => block.kind === "text" && block.text.trim() !== "");
 +var thinkingLive = (step) => step.status === "running" && texts(step).length === 0 && step.blocks.some((block) => block.kind === "reasoning");
 +var stepReasoning = (step) => step.blocks.filter((block) => block.kind === "reasoning").map((block) => block.text).join("\n").trim();
@@ -5388,49 +5390,31 @@ diff --git a/client.js b/client.js
 -  const loading = useSession?.(historyLoading) === true;
 -  if (question === void 0 || question.callId && (drawn || loading)) return null;
 -  return (0, import_react4.createElement)(PendingQuestion, { sessionId, question, actions, inline: false });
-+var PEEK_MAX = 160;
-+var peekLine = (text) => {
-+  const line2 = text.split("\n").map((part) => part.replace(/\s+/g, " ").trim()).find((part) => part !== "");
-+  return line2 === void 0 ? "" : line2.slice(0, PEEK_MAX);
-+};
-+function ThinkingRow({ text }) {
-+  if (typeof text !== "string" || text.trim() === "") return null;
-+  const peek = peekLine(text);
-+  return (0, import_react9.createElement)(
-+    "details",
-+    { className: "bt-think" },
-+    (0, import_react9.createElement)(
-+      "summary",
-+      null,
-+      (0, import_react9.createElement)("span", { className: "bt-think-caret", "aria-hidden": true }, "›"),
-+      t("Thought it through"),
-+      peek ? (0, import_react9.createElement)("span", { className: "bt-think-peek" }, peek) : null
-+    ),
-+    (0, import_react9.createElement)("div", { className: "bt-think-text" }, text)
-+  );
- }
+-}
 -function AnsweredCell({ node }) {
 -  const data = node.data ?? {};
 -  const questions = Array.isArray(data.questions) ? data.questions : [];
 -  if (questions.length === 0) {
 -    return data.text ? (0, import_react4.createElement)("div", { className: "bt-urow" }, (0, import_react4.createElement)("div", { className: "bt-ububble" }, data.text)) : null;
--  }
++var PEEK_MAX = 160;
++var CUT = "…";
++var MARKUP = [/^\s{0,3}(?:`{3,}|~{3,})/, /^\s{0,3}([-*_])(?:\s*\1){2,}\s*$/];
++var isMarkup = (line2) => MARKUP.some((pattern) => pattern.test(line2));
++var graphemes = (line2) => typeof Intl !== "undefined" && typeof Intl.Segmenter === "function" ? [...new Intl.Segmenter().segment(line2)].map((part) => part.segment) : [...line2];
++var clip = (line2) => {
++  if (line2.length <= PEEK_MAX) return line2;
++  let kept = "";
++  for (const part of graphemes(line2)) {
++    if (kept.length + part.length > PEEK_MAX - CUT.length) break;
++    kept += part;
+   }
 -  const answers = Array.isArray(data.answers) ? data.answers : [];
 -  return (0, import_react4.createElement)("div", { className: "bt-q-inline" }, questions.map((question) => (0, import_react4.createElement)(SettledQuestion, {
 -    key: question.id,
 -    question,
 -    answer: answers.find((answer) => answer.id === question.id) ?? { selected: [] }
 -  })));
-+function useFileMentions({ node, openFile, fileMentions, useTurnData }) {
-+  const tail = useTurnData?.("turn-tail");
-+  const location2 = node.location;
-+  const turn = location2?.kind === "turn" || location2?.kind === "step" ? location2.turn : void 0;
-+  const seq = node.data.finalNode?.seq;
-+  return (0, import_react9.useMemo)(() => {
-+    if (typeof fileMentions !== "function" || turn?.status !== "closed" || seq === void 0) return void 0;
-+    return tail?.closing?.finalNode.seq === seq ? fileMentions({ turn, seq, openFile }) : void 0;
-+  }, [fileMentions, openFile, seq, tail, turn]);
- }
+-}
 -
 -// src/client/secret-card.js
 -var import_react5 = require("react");
@@ -5460,7 +5444,8 @@ diff --git a/client.js b/client.js
 -  if (scope === "all") return t("All Bots");
 -  const names = (scope ?? []).map((id) => roster.byId[id]?.name).filter(Boolean);
 -  return names.length === 0 ? t("No Bot") : names.join(t(" and "));
--};
++  return kept + CUT;
+ };
 -var when = (time) => Number.isFinite(time) ? new Date(time).toLocaleString(dateLocale(), { month: "short", day: "numeric", hour: "2-digit", minute: "2-digit" }) : "";
 -function MaskedInput({ value, onChange, onEnter, label, autoFocus = false }) {
 -  return (0, import_react5.createElement)("input", {
@@ -5485,6 +5470,38 @@ diff --git a/client.js b/client.js
 -      event.preventDefault();
 -      onEnter?.();
 -    }
++var onlyMarkup = (lines) => lines.length === 1 ? t("{n} line of reasoning", { n: lines.length }) : t("{n} lines of reasoning", { n: lines.length });
++var peekLine = (text) => {
++  const lines = text.split("\n").map((part) => part.replace(/\s+/g, " ").trim()).filter((part) => part !== "");
++  const prose = lines.find((line2) => !isMarkup(line2));
++  return prose === void 0 ? onlyMarkup(lines) : clip(prose);
++};
++function ThinkingRow({ text }) {
++  if (typeof text !== "string" || text.trim() === "") return null;
++  const peek = peekLine(text);
++  return (0, import_react9.createElement)(
++    "details",
++    { className: "bt-think" },
++    (0, import_react9.createElement)(
++      "summary",
++      null,
++      (0, import_react9.createElement)("span", { className: "bt-think-caret", "aria-hidden": true }, "›"),
++      t("Thought it through"),
++      peek ? (0, import_react9.createElement)("span", { className: "bt-think-peek" }, peek) : null
++    ),
++    (0, import_react9.createElement)("div", { className: "bt-think-text" }, text)
++  );
++}
++function useFileMentions({ node, openFile, fileMentions, useTurnData }) {
++  const tail = useTurnData?.("turn-tail");
++  const location2 = node.location;
++  const turn = location2?.kind === "turn" || location2?.kind === "step" ? location2.turn : void 0;
++  const seq = node.data.finalNode?.seq;
++  return (0, import_react9.useMemo)(() => {
++    if (typeof fileMentions !== "function" || turn?.status !== "closed" || seq === void 0) return void 0;
++    return tail?.closing?.finalNode.seq === seq ? fileMentions({ turn, seq, openFile }) : void 0;
++  }, [fileMentions, openFile, seq, tail, turn]);
++}
 +function useClosingText(node, useTurnData) {
 +  const tail = useTurnData?.("turn-tail");
 +  const seq = node.data.finalNode?.seq;
diff --git a/patches/ds-bot/files/test/thinking-row.test.js b/patches/ds-bot/files/test/thinking-row.test.js
index ef3971cc4..c8bdec65d 100644
--- a/patches/ds-bot/files/test/thinking-row.test.js
+++ b/patches/ds-bot/files/test/thinking-row.test.js
@@ -136,3 +136,43 @@ test('#1636: an opened row hides its preview, so its first line is not printed t
     /\.bt-think\[open\] \.bt-think-peek\{display:none\}/,
   )
 })
+
+test('#1642: a fence or a rule is not the peek line; the first prose line is', () => {
+  const fenced = [think('```js\nconst x = 1\n```')]
+  const peek = classed(thinkingRow(render(step(1, fenced), [step(1, fenced)])), 'bt-think-peek')
+  assert.ok(peek, 'the closed row carries a line of the reasoning')
+  assert.equal(text(peek), 'const x = 1', 'markup is not what the Bot was thinking')
+  assert.doesNotMatch(text(peek), /```/)
+
+  const ruled = [think('---\n***\nLet me read it\n___')]
+  const second = classed(thinkingRow(render(step(1, ruled), [step(1, ruled)])), 'bt-think-peek')
+  assert.ok(second)
+  assert.equal(text(second), 'Let me read it', 'a horizontal rule is markup too')
+})
+
+test('#1642: a cut preview keeps whole characters and says it was cut', () => {
+  const long = [think(`${'a'.repeat(150)}😀${'b'.repeat(100)}`)]
+  const peek = classed(thinkingRow(render(step(1, long), [step(1, long)])), 'bt-think-peek')
+  assert.ok(peek)
+  const shown = text(peek)
+  assert.equal(shown.length, 160, 'the cap still holds')
+  assert.ok(shown.includes('😀'), 'an emoji inside the cap survives whole')
+  assert.ok(shown.endsWith('…'), 'a clipped line says it was clipped')
+  assert.doesNotMatch(shown, /[\uD800-\uDFFF]/u, 'no half a surrogate pair')
+
+  // The reported input: a pair cut exactly at the cap used to render as U+FFFD.
+  const edge = [think(`${'a'.repeat(159)}😀 tail`)]
+  const cut = text(classed(thinkingRow(render(step(1, edge), [step(1, edge)])), 'bt-think-peek'))
+  assert.equal(cut.length, 160)
+  assert.ok(cut.endsWith('…'))
+  assert.doesNotMatch(cut, /[\uD800-\uDFFF]/u, 'no half a surrogate pair at the cap either')
+})
+
+test('#1642: reasoning that is only markup gets a neutral summary, never a bare fence or an empty row', () => {
+  const onlyMarkup = [think('```js\n```')]
+  const details = thinkingRow(render(step(1, onlyMarkup), [step(1, onlyMarkup)]))
+  assert.ok(details, 'the row stays')
+  const peek = classed(details, 'bt-think-peek')
+  assert.ok(peek, 'the folded row never reads as an empty frame')
+  assert.equal(text(peek), '2 lines of reasoning')
+})