diff --git a/static/style.css b/static/style.css
index cbf76a6e..a90b2051 100644
--- a/static/style.css
+++ b/static/style.css
@@ -2834,8 +2834,9 @@ body.resizing .sidebar{transition:none!important;}
   margin:4px 0 14px;
 }
 .tool-worklog-summary{
-  width:auto;
   display:inline-flex;
+  max-width:100%;
+  min-width:0;
   align-items:center;
   gap:9px;
   padding:5px 8px;
@@ -2856,6 +2857,8 @@ body.resizing .sidebar{transition:none!important;}
   font-weight:500;
   line-height:var(--message-body-line-height);
   letter-spacing:0;
+  flex:1 1 auto;
+  min-width:0;
   overflow:hidden;
   text-overflow:ellipsis;
   white-space:nowrap;
diff --git a/tests/test_issue4064_worklog_mobile_overflow.py b/tests/test_issue4064_worklog_mobile_overflow.py
new file mode 100644
index 00000000..a26b3c8d
--- /dev/null
+++ b/tests/test_issue4064_worklog_mobile_overflow.py
@@ -0,0 +1,44 @@
+"""Regression guard for worklog summary overflow on narrow mobile viewports."""
+
+from pathlib import Path
+
+
+REPO = Path(__file__).resolve().parent.parent
+STYLE_CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
+
+
+def _rule(selector: str) -> str:
+    start = STYLE_CSS.find(selector)
+    assert start >= 0, f"{selector} selector not found in style.css"
+    end = STYLE_CSS.find("}", start)
+    assert end >= 0, f"{selector} rule did not close"
+    return STYLE_CSS[start:end]
+
+
+def test_worklog_summary_no_longer_uses_auto_width_inline_flex():
+    """The summary button must be width-bounded inside the chat column."""
+    rule = _rule(".tool-worklog-summary{")
+    assert "width:auto" not in rule, (
+        "The worklog summary must not force width:auto; that lets long one-line "
+        "labels widen narrow mobile viewports."
+    )
+    assert "display:inline-flex" in rule, "The summary should keep its inline-flex layout."
+    assert "max-width:100%" in rule, (
+        "The worklog summary must cap itself to the available message width."
+    )
+    assert "min-width:0" in rule, (
+        "The worklog summary must allow its inner label to shrink inside the chat column."
+    )
+
+
+def test_worklog_summary_label_can_shrink_with_ellipsis():
+    """The long summary label must be the shrinkable flex child."""
+    rule = _rule(".tool-worklog-summary .tool-worklog-label{")
+    assert "white-space:nowrap" in rule, "The summary copy should stay single-line."
+    assert "text-overflow:ellipsis" in rule, "The summary copy should still ellipsize."
+    assert "flex:1 1 auto" in rule, (
+        "The summary label must be a shrinkable flex child so it truncates before widening the viewport."
+    )
+    assert "min-width:0" in rule, (
+        "The summary label must opt out of the default flex min-width:auto clamp."
+    )