diff --git a/api/streaming.py b/api/streaming.py
index bb208a41..648df143 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -1817,7 +1817,7 @@ def _run_agent_streaming(
             # the key is absent or invalid, pass None → agent uses its default.
             try:
                 from api.config import parse_reasoning_effort as _parse_reff
-                _effort_cfg = _cfg.cfg.get('agent', {}) if isinstance(_cfg.cfg, dict) else {}
+                _effort_cfg = _cfg.get('agent', {}) if isinstance(_cfg, dict) else {}
                 _effort_raw = _effort_cfg.get('reasoning_effort') if isinstance(_effort_cfg, dict) else None
                 _reasoning_config = _parse_reff(_effort_raw)
             except Exception:
@@ -1885,6 +1885,7 @@ def _run_agent_streaming(
                     _max_tokens_cfg or '',
                     _fallback_resolved or {},
                     sorted(_toolsets) if _toolsets else [],
+                    _reasoning_config or {},
                 ], sort_keys=True)
                 _agent_sig = _hashlib.sha256(_sig_blob.encode()).hexdigest()[:16]
 
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index 906799b4..126a1e6a 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -608,6 +608,38 @@ def test_streaming_bridge_accepts_current_tool_progress_callback_signature(clean
         "streaming.py must emit live tool completion SSE events"
 
 
+def test_streaming_reads_reasoning_effort_from_config_dict(cleanup_test_sessions):
+    """R17b: WebUI must read agent.reasoning_effort from the dict returned by get_config().
+
+    `get_config()` returns a plain dict (not a wrapper exposing `.cfg`).  The
+    pre-fix line `_cfg.cfg.get('agent', {})` raised AttributeError that the
+    surrounding try/except swallowed, so `_reasoning_config` was always None
+    regardless of what `/reasoning <level>` had been set to.  This static
+    source assertion pins the fix because the runtime symptom is silent.
+    """
+    src = (REPO_ROOT / "api/streaming.py").read_text()
+    assert "_cfg.cfg" not in src, \
+        "get_config() returns a dict; accessing _cfg.cfg drops reasoning_config to None"
+    assert "_cfg.get('agent', {})" in src or '_cfg.get("agent", {})' in src, \
+        "streaming.py must read agent.reasoning_effort via the config dict"
+
+
+def test_streaming_agent_cache_signature_includes_reasoning_config(cleanup_test_sessions):
+    """R17c: changing reasoning effort mid-session must rebuild the cached per-session agent.
+
+    Without `_reasoning_config` participating in `_sig_blob`, the cache key
+    matches the old entry and the operator's `/reasoning xhigh` change has
+    no effect on the live session.
+    """
+    src = (REPO_ROOT / "api/streaming.py").read_text()
+    start = src.find("_sig_blob = _json.dumps")
+    end = src.find("_agent_sig", start)
+    assert start >= 0 and end > start, "agent cache signature block not found"
+    sig_block = src[start:end]
+    assert "_reasoning_config" in sig_block, \
+        "agent cache signature must include reasoning_config so xhigh/medium changes take effect"
+
+
 def test_messages_js_supports_live_reasoning_and_tool_completion(cleanup_test_sessions):
     """R18: messages.js must render live reasoning and react to tool completion events.
     Without these handlers, the operator only sees generic Thinking… or nothing