diff --git a/src/claude_agent_sdk/_internal/sessions.py b/src/claude_agent_sdk/_internal/sessions.py
index cb1cb1342..1cd92db6b 100644
--- a/src/claude_agent_sdk/_internal/sessions.py
+++ b/src/claude_agent_sdk/_internal/sessions.py
@@ -230,6 +230,10 @@ def _extract_last_json_string_field(text: str, key: str) -> str | None:
     """Like _extract_json_string_field but finds the LAST occurrence."""
     patterns = [f'"{key}":"', f'"{key}": "']
     last_value: str | None = None
+    # Each pattern is scanned separately, so compare positions rather than
+    # letting the later pattern's match overwrite an earlier one's: with both
+    # spacings present, the last match by POSITION has to win.
+    last_idx = -1
     for pattern in patterns:
         search_from = 0
         while True:
@@ -244,7 +248,9 @@ def _extract_last_json_string_field(text: str, key: str) -> str | None:
                     i += 2
                     continue
                 if text[i] == '"':
-                    last_value = _unescape_json_string(text[value_start:i])
+                    if idx > last_idx:
+                        last_idx = idx
+                        last_value = _unescape_json_string(text[value_start:i])
                     break
                 i += 1
             search_from = i + 1
diff --git a/tests/test_session_mutations.py b/tests/test_session_mutations.py
index 18ad8aeb5..ff3ca2a9b 100644
--- a/tests/test_session_mutations.py
+++ b/tests/test_session_mutations.py
@@ -215,6 +215,47 @@ def test_last_wins_via_list_sessions(self, claude_config_dir: Path, tmp_path: Pa
         assert sessions[0].custom_title == "Final Title"
         assert sessions[0].summary == "Final Title"
 
+    def test_rename_wins_over_earlier_spaced_title(
+        self, claude_config_dir: Path, tmp_path: Path
+    ):
+        """A rename must win over a pre-existing spaced-form custom-title line.
+
+        rename_session appends compact JSON. A transcript written by a host
+        tool with a bare ``json.dumps`` carries the spaced form instead, so a
+        file can hold both. The rename is physically the last line and has to
+        be the one that list_sessions reports.
+        """
+        project_path = str(tmp_path / "proj")
+        Path(project_path).mkdir(parents=True)
+        project_dir = _make_project_dir(
+            claude_config_dir, os.path.realpath(project_path)
+        )
+        sid, file_path = _make_session_file(project_dir)
+
+        # json.dumps defaults -> '"customTitle": "Imported Title"' (spaced).
+        with file_path.open("a", encoding="utf-8") as f:
+            f.write(
+                json.dumps(
+                    {
+                        "type": "custom-title",
+                        "customTitle": "Imported Title",
+                        "sessionId": sid,
+                    }
+                )
+                + "\n"
+            )
+
+        rename_session(sid, "Renamed Title", directory=project_path)
+
+        assert (
+            json.loads(file_path.read_text().strip().split("\n")[-1])["customTitle"]
+            == "Renamed Title"
+        )
+
+        sessions = list_sessions(directory=project_path, include_worktrees=False)
+        assert sessions[0].custom_title == "Renamed Title"
+        assert sessions[0].summary == "Renamed Title"
+
     def test_search_all_projects(self, claude_config_dir: Path):
         """When no directory given, searches all project directories."""
         project_dir = _make_project_dir(claude_config_dir, "/some/project")
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 1f324f001..cf0a8a914 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -180,6 +180,25 @@ def test_extract_last_json_string_field(self):
         text = '{"summary":"first"}\n{"summary":"second"}\n{"summary":"third"}'
         assert _extract_last_json_string_field(text, "summary") == "third"
 
+    def test_extract_last_json_string_field_mixed_spacing(self):
+        """The last occurrence by POSITION wins, whichever spacing it uses.
+
+        A transcript can mix both serializations: the CLI and this SDK write
+        compact (``json.dumps(..., separators=(",", ":"))``), while a host
+        tool writing the same file with a bare ``json.dumps`` produces the
+        spaced form. Scanning the patterns one after another must not let an
+        earlier spaced match outrank a later compact one.
+        """
+        spaced_then_compact = '{"customTitle": "old"}\n{"customTitle":"new"}'
+        assert _extract_last_json_string_field(spaced_then_compact, "customTitle") == (
+            "new"
+        )
+
+        compact_then_spaced = '{"customTitle":"old"}\n{"customTitle": "new"}'
+        assert _extract_last_json_string_field(compact_then_spaced, "customTitle") == (
+            "new"
+        )
+
     def test_extract_first_prompt_simple(self):
         head = json.dumps({"type": "user", "message": {"content": "Hello!"}}) + "\n"
         assert _extract_first_prompt_from_head(head) == "Hello!"