diff --git a/CHANGELOG.md b/CHANGELOG.md
index 44f51996..11ccf082 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,11 @@
 
 ## [Unreleased]
 
+## [v0.51.293] — 2026-06-06 — Release JI (stage-s5 — thinking card no longer renders twice)
+
+### Fixed
+- **The "Thinking" card no longer renders twice on a settled turn.** For a turn that had both a tool call and reasoning (e.g. think → call a tool → answer), the thinking card could appear once inside the collapsed **Activity** group at the top of the turn and again as a stranded second card below the answer and the `Done in …` footer. The thinking-only inline render path (added in v0.51.258 for #3592) now only fires when the turn has no Activity group of its own, and when it does render inline it inserts the card **above** the answer body instead of after the footer. Thinking that echoes the visible answer on a trailing reasoning-only message is also de-duplicated against the whole turn's answer text now, not just the same message's body. Genuinely thinking-only turns still show their thinking inline (the #3592 fix is preserved, not reverted). (#3709; supersedes #3708)
+
 ## [v0.51.292] — 2026-06-06 — Release JH (stage-s4 — compression-exhausted turns surface as errors, not fake completions)
 
 ### Fixed
diff --git a/static/ui.js b/static/ui.js
index 2cf60d49..5a071f32 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -7162,6 +7162,39 @@ function renderMessages(options){
     if(role==='user') lastQuestionRawIdx=entry.rawIdx;
     else if(role==='assistant') questionRawIdxByAssistantRawIdx.set(entry.rawIdx,lastQuestionRawIdx);
   }
+  // #3709 (defect B): build a per-turn combined visible-answer text so the
+  // thinking echo-strip can de-dupe a thinking-only message (whose own visible
+  // body is empty) against the answer prose carried by a SIBLING message in the
+  // same turn. A turn = the run of assistant messages between two user messages.
+  // Map every assistant rawIdx in a run to the run's combined visible text.
+  const _turnVisibleTextByRawIdx=new Map();
+  {
+    let _run=[]; let _runText=[];
+    const _flush=()=>{
+      if(_run.length){
+        const combined=_runText.join('\n\n');
+        for(const ri of _run) _turnVisibleTextByRawIdx.set(ri, combined);
+      }
+      _run=[]; _runText=[];
+    };
+    for(const entry of renderVisWithIdx){
+      const em=entry&&entry.m; const role=em&&em.role;
+      if(role==='assistant'){
+        _run.push(entry.rawIdx);
+        // Visible prose = content with any leading <think>…</think> /channel-thought
+        // block stripped (the same blocks the per-message extractor removes below).
+        let vis=typeof em.content==='string'?em.content:'';
+        vis=vis.replace(/^\s*<think>[\s\S]*?<\/think>\s*/,'')
+               .replace(/^\s*<\|channel\|?>thought\n?[\s\S]*?<channel\|>\s*/,'')
+               .replace(/^\s*<\|turn\|>thinking\n[\s\S]*?<turn\|>\s*/,'').trim();
+        if(vis) _runText.push(vis);
+      }else{
+        _flush();
+      }
+    }
+    _flush();
+  }
+
   const assistantSegments=new Map();
   const assistantThinking=new Map();
   const userRows=new Map();
@@ -7233,6 +7266,13 @@ function renderMessages(options){
     const displayContent=isUser?_stripAttachedFilesMarkerForDisplay(_stripWorkspaceDisplayPrefix(content)):content;
     if(thinkingText&&!isUser){
       thinkingText=_stripVisibleAssistantEchoFromThinking(thinkingText, displayContent);
+      // #3709 (defect B): if this message's own visible body didn't strip the
+      // echo (e.g. a thinking-only message whose answer prose lives on a sibling
+      // message in the same turn), also strip against the turn's combined answer.
+      const turnVisible=_turnVisibleTextByRawIdx.get(rawIdx);
+      if(thinkingText&&turnVisible&&turnVisible!==displayContent){
+        thinkingText=_stripVisibleAssistantEchoFromThinking(thinkingText, turnVisible);
+      }
     }
     const isLastAssistant=!isUser&&vi===renderVisWithIdx.length-1;
     const nextRendered=renderVisWithIdx[vi+1];
@@ -7545,22 +7585,80 @@ function renderMessages(options){
     const assistantIdxs=[...assistantSegments.keys()].sort((a,b)=>a-b);
     const anchorInsertAfter = new Map();
     if(isSimplifiedToolCalling()){
+      // Shared anchor resolver: maps an activity index to the assistant segment
+      // its Activity group will anchor on. The group-render path falls back to a
+      // nearby earlier segment when an index has no directly-rendered segment
+      // (legacy/rebased assistant_msg_idx). The inline-suppression precompute MUST
+      // use the SAME resolution, or a fallback-anchored group's turn won't be in
+      // turnsWithActivityGroup and the thinking renders twice (Codex re-gate).
+      const _anchorRowForActivityIdx=(aIdx)=>{
+        let row=assistantSegments.get(aIdx)||null;
+        if(!row&&assistantIdxs.length){
+          if(aIdx<assistantIdxs[0]) return null;
+          const fallbackIdx=[...assistantIdxs].reverse().find(idx=>idx<=aIdx);
+          row=fallbackIdx!==undefined?assistantSegments.get(fallbackIdx):assistantSegments.get(assistantIdxs[assistantIdxs.length-1]);
+        }
+        return row;
+      };
+      // #3709: a turn (one .assistant-turn, spanning every assistant segment
+      // between two user messages) can contain BOTH a tool-bearing message and a
+      // trailing thinking-only message. The tool message builds an Activity group
+      // that already carries the turn's thinking at its top; if the thinking-only
+      // sibling ALSO renders its thinking inline, the card shows twice. Precompute
+      // the set of turn nodes that have any tool card so the inline branch can
+      // skip turns that already own an Activity group.
+      const turnsWithActivityGroup=new Set();
+      // Per-turn merged thinking: a tool-bearing turn's Activity group must carry
+      // ALL of that turn's thinking — including a trailing thinking-only sibling
+      // whose inline card we suppress below (#3709 A1). Rendering only the tool
+      // message's own assistantThinking entry would silently DROP a sibling's
+      // distinct reasoning. Aggregate every assistantThinking entry by turn,
+      // de-duped and in index order, and render that once in the group.
+      const turnThinkingParts=new Map(); // turnNode -> [text,...]
+      const _addTurnThinking=(idx)=>{
+        if(!assistantThinking.has(idx)) return;
+        const seg=_anchorRowForActivityIdx(idx);
+        const turn=seg?seg.closest('.assistant-turn'):null;
+        if(!turn) return;
+        const txt=String(assistantThinking.get(idx)||'').trim();
+        if(!txt) return;
+        const arr=turnThinkingParts.get(turn)||[];
+        if(!arr.includes(txt)) arr.push(txt);
+        turnThinkingParts.set(turn, arr);
+      };
+      for(const tcIdx of Object.keys(byAssistant).map(k=>parseInt(k))){
+        if(!(byAssistant[tcIdx]||[]).length) continue;
+        const tcSeg=_anchorRowForActivityIdx(tcIdx);
+        const tcTurn=tcSeg?tcSeg.closest('.assistant-turn'):null;
+        if(tcTurn) turnsWithActivityGroup.add(tcTurn);
+      }
+      // Aggregate thinking for every assistant idx that has it (tool-bearing or not).
+      for(const tIdx of assistantThinking.keys()) _addTurnThinking(tIdx);
+      const _renderedTurnThinking=new Set();
       const activityIdxs=[...new Set([...Object.keys(byAssistant).map(k=>parseInt(k)), ...assistantThinking.keys()])].sort((a,b)=>a-b);
       for(const aIdx of activityIdxs){
         const cards=byAssistant[aIdx]||[];
         if(!cards.length&&assistantThinking.has(aIdx)){
-          const anchorRow=assistantSegments.get(aIdx);
-          if(anchorRow&&window._showThinking!==false){
-            anchorRow.insertAdjacentHTML('beforeend',_thinkingCardHtml(assistantThinking.get(aIdx)));
+          // Thinking-only message. Render its thinking inline ONLY when the turn
+          // has no Activity group at all (#3592 — a genuinely thinking-only turn
+          // must not bury its thinking in a collapsed group). If a sibling
+          // tool-message in the same turn built a group, that group carries the
+          // turn's thinking (including THIS message's, via turnThinkingParts) —
+          // don't emit a duplicate inline card here (#3709 A).
+          const anchorRow=_anchorRowForActivityIdx(aIdx);
+          const anchorTurn=anchorRow?anchorRow.closest('.assistant-turn'):null;
+          if(anchorRow&&window._showThinking!==false&&!(anchorTurn&&turnsWithActivityGroup.has(anchorTurn))){
+            // Insert the thinking card BEFORE the answer body + msg-foot footer
+            // (the segment already has them appended), so it reads above the
+            // answer rather than orphaned below the "Done in …" line (#3709 A2).
+            const bodyEl=anchorRow.querySelector('.msg-body,.msg-foot');
+            const cardHtml=_thinkingCardHtml(assistantThinking.get(aIdx));
+            if(bodyEl) bodyEl.insertAdjacentHTML('beforebegin',cardHtml);
+            else anchorRow.insertAdjacentHTML('beforeend',cardHtml);
           }
           continue;
         }
-        let anchorRow=assistantSegments.get(aIdx)||null;
-        if(!anchorRow&&assistantIdxs.length){
-          if(aIdx<assistantIdxs[0]) continue;
-          const fallbackIdx=[...assistantIdxs].reverse().find(idx=>idx<=aIdx);
-          anchorRow=fallbackIdx!==undefined?assistantSegments.get(fallbackIdx):assistantSegments.get(assistantIdxs[assistantIdxs.length-1]);
-        }
+        let anchorRow=_anchorRowForActivityIdx(aIdx);
         if(!anchorRow) continue;
         const anchorParent=anchorRow.parentElement;
         let insertAfterNode = anchorInsertAfter.get(anchorRow) || anchorRow;
@@ -7569,9 +7667,13 @@ function renderMessages(options){
         if(sourceMsg._turnDuration!==undefined) group.setAttribute('data-turn-duration', String(sourceMsg._turnDuration));
         const body=group&&group.querySelector('.tool-call-group-body');
         if(!body) continue;
-        const thinkingText=assistantThinking.get(aIdx);
-        if(thinkingText){
-          body.appendChild(_thinkingActivityNode(thinkingText, false));
+        // Render the TURN's merged thinking once (covers this message's own
+        // thinking + any suppressed thinking-only sibling in the same turn, #3709).
+        const groupTurn=anchorRow?anchorRow.closest('.assistant-turn'):null;
+        const mergedThinking=(groupTurn&&turnThinkingParts.get(groupTurn)||[]).join('\n\n').trim();
+        if(mergedThinking&&!(groupTurn&&_renderedTurnThinking.has(groupTurn))){
+          body.appendChild(_thinkingActivityNode(mergedThinking, false));
+          if(groupTurn) _renderedTurnThinking.add(groupTurn);
         }
         for(const tc of cards){
           body.appendChild(buildToolCard(tc));
diff --git a/tests/test_issue3709_thinking_double_render.py b/tests/test_issue3709_thinking_double_render.py
new file mode 100644
index 00000000..d65045c0
--- /dev/null
+++ b/tests/test_issue3709_thinking_double_render.py
@@ -0,0 +1,165 @@
+"""#3709 -- Thinking card must not render twice (inside Activity AND below the answer).
+
+Regression coverage for the double-render introduced by #3592's inline branch
+(v0.51.258). In a turn that has BOTH a tool-bearing message and a trailing
+thinking-only message, two code paths emitted a thinking card from the same
+``assistantThinking`` map:
+
+  1. the Activity-group path (tool-bearing message) put the thinking at the top
+     of the collapsed Activity group, and
+  2. the inline path (thinking-only message) appended a SECOND card via
+     ``insertAdjacentHTML('beforeend')`` -- which, because the segment already
+     carried the answer body + ``msg-foot`` footer, stranded the card *below*
+     the "Done in ..." line.
+
+The fix keeps the #3592 inline behaviour for genuinely thinking-only turns (so
+their thinking is not buried in a collapsed group) but:
+
+  A1. only renders inline when the turn has NO Activity group at all
+      (``turnsWithActivityGroup`` gate), so a tool-bearing turn's thinking-only
+      sibling does not emit a duplicate card;
+  A2. inserts the inline card BEFORE the answer body / footer
+      (``insertAdjacentHTML('beforebegin')`` on ``.msg-body,.msg-foot``) so it
+      reads above the answer instead of orphaned below "Done in ...";
+  B.  strips the thinking against the TURN's combined visible answer
+      (``_turnVisibleTextByRawIdx``) so a trailing thinking-only message whose
+      answer prose lives on a sibling message still gets its answer-echo removed.
+
+These are static source-structure assertions (the render path is DOM-driven and
+exercised live); they lock the invariants so the double-render cannot silently
+return, and so a future blunt "just delete the inline branch" change (which would
+re-break #3592) fails fast here instead.
+"""
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+UI_JS = (Path(__file__).resolve().parent.parent / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _render_messages_body() -> str:
+    """Return the body of renderMessages() (best-effort slice) for scoped asserts."""
+    start = UI_JS.find("function renderMessages(")
+    assert start != -1, "renderMessages() not found"
+    # Slice a generous window; the activityIdxs loop + footer logic live within.
+    return UI_JS[start:start + 60000]
+
+
+def test_inline_thinking_branch_still_exists_for_thinking_only_turns():
+    """#3592 must NOT be reverted: a thinking-only turn still renders its thinking
+    inline (not buried in a collapsed Activity group)."""
+    assert "!cards.length&&assistantThinking.has(aIdx)" in UI_JS, (
+        "the thinking-only inline branch (#3592) must remain — deleting it "
+        "re-buries thinking-only turns in a collapsed Activity group"
+    )
+    assert "_thinkingCardHtml(" in UI_JS
+
+
+def test_inline_branch_gated_on_turn_having_no_activity_group():
+    """A1: the inline card must only render when the turn has no Activity group,
+    so a tool-bearing turn's thinking-only sibling does not duplicate the card."""
+    body = _render_messages_body()
+    assert "turnsWithActivityGroup" in body, (
+        "must precompute the set of turns that already own an Activity group (#3709 A1)"
+    )
+    # The inline render must be guarded by a membership check on that set.
+    assert re.search(
+        r"turnsWithActivityGroup\.has\(\s*anchorTurn\s*\)",
+        body,
+    ), "the inline thinking render must be gated on turnsWithActivityGroup.has(anchorTurn)"
+
+
+def test_turns_with_activity_group_built_from_tool_bearing_segments():
+    """The turnsWithActivityGroup set must be populated from tool-bearing message
+    segments' enclosing .assistant-turn nodes."""
+    body = _render_messages_body()
+    block = re.search(
+        r"const turnsWithActivityGroup=new Set\(\);(.*?)const activityIdxs=",
+        body,
+        re.DOTALL,
+    )
+    assert block, "turnsWithActivityGroup population block not found"
+    text = block.group(1)
+    assert "closest('.assistant-turn')" in text, (
+        "must map tool-bearing segments to their enclosing .assistant-turn"
+    )
+    assert "turnsWithActivityGroup.add(" in text
+
+
+def test_inline_card_inserted_before_body_and_footer():
+    """A2: when the inline render is correct, the card must land BEFORE the answer
+    body / msg-foot (beforebegin), not appended after the 'Done in ...' footer."""
+    body = _render_messages_body()
+    # The inline branch selects the body/foot element and inserts before it.
+    assert re.search(r"querySelector\(\s*'\.msg-body,\.msg-foot'\s*\)", body), (
+        "inline branch must locate the .msg-body/.msg-foot element to anchor before it"
+    )
+    assert "insertAdjacentHTML('beforebegin'" in body, (
+        "the inline thinking card must be inserted 'beforebegin' the answer body/footer "
+        "(not 'beforeend', which strands it below 'Done in ...') (#3709 A2)"
+    )
+
+
+def test_no_unconditional_beforeend_thinking_in_inline_branch():
+    """The old orphaning insert ('beforeend' of the raw thinking card on the anchor
+    row) must be gone from the inline branch."""
+    body = _render_messages_body()
+    # The specific regression pattern: appending the thinking card to the end of
+    # the anchor row unconditionally. It must no longer be the inline path.
+    assert "anchorRow.insertAdjacentHTML('beforeend',_thinkingCardHtml(assistantThinking.get(aIdx)))" not in body, (
+        "the inline branch must not append the thinking card to the end of the "
+        "anchor row (that stranded it below the footer — the #3709 bug)"
+    )
+
+
+def test_turn_level_echo_strip_exists():
+    """B: thinking is stripped against the TURN's combined visible answer, not only
+    the same message's body — so a trailing thinking-only message that echoes the
+    answer gets de-duped too."""
+    body = _render_messages_body()
+    assert "_turnVisibleTextByRawIdx" in body, (
+        "must build a per-turn combined visible-answer map (#3709 defect B)"
+    )
+    # The strip site must consult the turn-level text in addition to displayContent.
+    assert re.search(
+        r"_turnVisibleTextByRawIdx\.get\(\s*rawIdx\s*\)",
+        body,
+    ), "the echo-strip must look up the turn's combined visible text"
+    # And it must feed that into the echo-strip helper.
+    strip_block = re.search(
+        r"_turnVisibleTextByRawIdx\.get\(\s*rawIdx\s*\)(.*?)_stripVisibleAssistantEchoFromThinking\(\s*thinkingText\s*,\s*turnVisible\s*\)",
+        body,
+        re.DOTALL,
+    )
+    assert strip_block, (
+        "the turn-level visible text must be passed to "
+        "_stripVisibleAssistantEchoFromThinking"
+    )
+
+
+def test_suppressed_sibling_thinking_merged_into_group_not_dropped():
+    """When the A1 gate suppresses a thinking-only sibling's inline card (because
+    its turn has an Activity group), that sibling's thinking must NOT be lost — the
+    group must render the TURN's merged thinking, not only the tool message's own
+    entry. (Codex re-gate finding: rendering only assistantThinking.get(aIdx) for
+    the tool index dropped a distinct sibling's reasoning.)"""
+    body = _render_messages_body()
+    # A per-turn thinking aggregation must exist...
+    assert "turnThinkingParts" in body, (
+        "must aggregate thinking per turn so a suppressed sibling's reasoning is "
+        "carried into the Activity group, not dropped (#3709 / Codex re-gate)"
+    )
+    # ...and the Activity group must render the MERGED text, de-duped, once per turn.
+    assert "mergedThinking" in body, (
+        "the Activity group must render the turn's merged thinking"
+    )
+    assert "_renderedTurnThinking" in body, (
+        "merged thinking must render once per turn (guard against double-emit when "
+        "a turn has multiple tool messages)"
+    )
+    # The group node must be built from the merged text, not the single-index entry.
+    assert re.search(
+        r"_thinkingActivityNode\(\s*mergedThinking\s*,",
+        body,
+    ), "the Activity group thinking node must be built from mergedThinking"
diff --git a/tests/test_ui_tool_call_cleanup.py b/tests/test_ui_tool_call_cleanup.py
index 215d6713..4d6c8d73 100644
--- a/tests/test_ui_tool_call_cleanup.py
+++ b/tests/test_ui_tool_call_cleanup.py
@@ -272,10 +272,14 @@ class TestToolCallGroupingStatic:
         assert "isSimplifiedToolCalling()" in render_fn and "assistantThinking.set(rawIdx, thinkingText)" in render_fn, (
             "Compact settled transcript rendering should preserve Thinking cards after switching sessions."
         )
-        assert "_thinkingActivityNode(thinkingText, false)" in render_fn, (
+        # #3709: the Activity disclosure now renders the TURN's merged thinking
+        # (mergedThinking — all of a turn's thinking de-duped, incl. a suppressed
+        # thinking-only sibling) rather than a single message's entry. Same node,
+        # same Activity body — only the source variable changed.
+        assert "_thinkingActivityNode(mergedThinking, false)" in render_fn, (
             "Settled Thinking cards should render inside the compact Activity disclosure."
         )
-        assert "body.appendChild(_thinkingActivityNode(thinkingText, false))" in render_fn, (
+        assert "body.appendChild(_thinkingActivityNode(mergedThinking, false))" in render_fn, (
             "Settled Thinking cards should stay inside the same Activity body as the related tools."
         )
         assert ".agent-activity-thinking:not([data-live-thinking=\"1\"])" in render_fn, (