diff --git a/agent/turn_summary.py b/agent/turn_summary.py
index b542daa0ea40..f4440afb50aa 100644
--- a/agent/turn_summary.py
+++ b/agent/turn_summary.py
@@ -151,7 +151,13 @@ def _extract_line_deltas(tool_name: str, result: Any) -> tuple[int, int] | None:
     diff = payload.get("diff")
     if not isinstance(diff, str) or not diff.strip():
         return None
-    return _count_diff_lines(diff)
+    added, removed = _count_diff_lines(diff)
+    # A diff that carries no +/- content lines (e.g. a bare hunk header) tells
+    # us nothing — report it as unknown rather than rendering a misleading
+    # "+0 -0" next to a real edit.
+    if added == 0 and removed == 0:
+        return None
+    return added, removed
 
 
 class TurnSummaryCollector:
diff --git a/tests/agent/test_turn_summary.py b/tests/agent/test_turn_summary.py
index 786a53abaa9c..b3d96f85433c 100644
--- a/tests/agent/test_turn_summary.py
+++ b/tests/agent/test_turn_summary.py
@@ -344,3 +344,28 @@ def test_turn_summary_config_defaults_present():
     display = DEFAULT_CONFIG["display"]
     assert display["turn_summary"] is True
     assert display["spinner_token_flow"] is True
+
+
+def test_content_free_diff_reports_unknown_not_zero_zero():
+    """A diff with no +/- content lines (bare hunk header) must render as an
+    edit with UNKNOWN deltas, never a misleading '+0 -0'.
+
+    Found by E2E-rendering the collector against realistic tool payloads: the
+    unit suite only fed diffs that had real content lines.
+    """
+    from agent.turn_summary import TurnSummaryCollector
+
+    c = TurnSummaryCollector()
+    c.begin()
+    c.record_tool("patch", result={"success": True, "diff": "@@ -1,3 +1,15 @@"})
+    line = c.render(3.0)
+    assert "edited 1 file" in line
+    assert "+0 -0" not in line
+
+    real = TurnSummaryCollector()
+    real.begin()
+    real.record_tool(
+        "patch",
+        result={"success": True, "diff": "--- a/x\n+++ b/x\n@@ -1 +1,2 @@\n-old\n+new\n+extra\n"},
+    )
+    assert "+2 -1" in real.render(1.0)