feat(chat): stream model thinking over SSE and show it in a collapsible block

tests/unit/test_config.py

@@ -36,6 +36,8 @@ def test_defaults_match_locked_decisions(monkeypatch: pytest.MonkeyPatch) -> Non
     assert s.top_n_docs >= 1
     # Owner instruction 2026-08-22: answers may run up to 32 768 tokens.
     assert s.max_output_tokens == 32_768
+    # Phase 17: the model's thinking streams by default (kill-switch off).
+    assert s.stream_thinking is True
     assert len(s.suggestions) >= 3
     # A9 (revised): the import scope covers the seven A9 formats.
     assert s.import_extension_set == {
@@ -57,6 +59,19 @@ def test_max_output_tokens_env_override(monkeypatch) -> None:
     assert s.max_output_tokens == 1234
 
 
+def test_stream_thinking_default_true_and_env_parse(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Phase 17 kill-switch (``BOR_STREAM_THINKING``): on by default,
+    ``0``/``false`` turn the ``thinking`` SSE frames off."""
+    assert _settings().stream_thinking is True
+    assert _settings(stream_thinking=False).stream_thinking is False
+    monkeypatch.setenv("BOR_STREAM_THINKING", "0")
+    assert _settings().stream_thinking is False
+    monkeypatch.setenv("BOR_STREAM_THINKING", "false")
+    assert _settings().stream_thinking is False
+    monkeypatch.setenv("BOR_STREAM_THINKING", "1")
+    assert _settings().stream_thinking is True
+
+
 def test_import_extensions_env_override_is_a_csv_list(monkeypatch) -> None:
     monkeypatch.setenv("BOR_IMPORT_EXTENSIONS", "md,yml")
     s = _settings()