Surface the router rationale in the shown message, and log it

The subagent-routing hook fetched the gateway's rationale but only put it in
permissionDecisionReason, so the line the harness surfaces (the systemMessage,
e.g. "Using Smart Routing. Routing to gpt-5.6-sol.") omitted the "why". Append
the rationale to the systemMessage too, so users see the reason alongside the
model.

Also persist the rationale in the decisions.jsonl record, so an empty value
cleanly distinguishes "the gateway returned no rationale" from a display-side
placement bug.

Co-authored-by: Isaac

Author: masonc08

src/ucode/smart_routing/routing.py

@@ -200,14 +200,17 @@ def route_spawn_tool(
     if record_decision is not None:
         record_decision(payload, task, decision, routed_model)
     routing_message = f"Using Smart Routing. Routing to {routed_model}."
+    # Append the router's rationale to BOTH the systemMessage (the line the
+    # harness surfaces to the user) and permissionDecisionReason, so the "why"
+    # is visible, not just the "what". Empty when the gateway returns none.
+    if decision.rationale:
+        routing_message += f" {decision.rationale}"
     output: dict[str, Any] = {
         "hookEventName": "PreToolUse",
         "permissionDecision": "allow",
         "updatedInput": {**tool_input, "model": routed_model},
         "permissionDecisionReason": routing_message,
     }
-    if decision.rationale:
-        output["permissionDecisionReason"] += f" {decision.rationale}"
     return {"systemMessage": routing_message, "hookSpecificOutput": output}
 
 
@@ -271,6 +274,9 @@ def write_decision_record(
             "task_name": task_name,
             "router_model": decision.raw_model,
             "requested_model": requested_model,
+            # Persisted for diagnosis: an empty value means the gateway returned
+            # no rationale (vs. a placement bug in how we surface it).
+            "rationale": decision.rationale,
             "at": time.time(),
         },
     )

tests/test_codex_routing.py

@@ -143,7 +143,11 @@ def test_spawn_rewrite_preserves_original_input(monkeypatch):
     )
 
     hook = output["hookSpecificOutput"]
-    assert output["systemMessage"] == "Using Smart Routing. Routing to gpt-5.5."
+    # The rationale is surfaced in BOTH the systemMessage (shown to the user) and
+    # permissionDecisionReason, so the "why" is visible, not just the "what".
+    assert output["systemMessage"] == (
+        "Using Smart Routing. Routing to gpt-5.5. Review needs deeper reasoning."
+    )
     assert hook["permissionDecision"] == "allow"
     assert hook["updatedInput"] == {
         "task_name": "reviewer",
@@ -276,3 +280,37 @@ def test_decision_is_reconciled_with_actual_subagent_model(tmp_path, monkeypatch
     assert record["router_model"] == "gpt-5-6-luna"
     assert record["requested_model"] == "gpt-5.6-luna"
     assert record["matches_router_decision"] is True
+
+
+def test_decision_record_persists_rationale(tmp_path, monkeypatch):
+    decisions = tmp_path / "decisions.jsonl"
+    monkeypatch.setattr(codex_routing, "DECISIONS_PATH", decisions)
+    monkeypatch.setattr(
+        codex_routing,
+        "request_routing_decision",
+        lambda *args, **kwargs: (
+            codex_routing.RoutingDecision(
+                model="system.ai.gpt-5-6-sol",
+                raw_model="gpt-5-6-sol",
+                rationale="Cross-cutting refactor needs the strongest model.",
+            ),
+            None,
+        ),
+    )
+
+    codex_routing.route_pre_tool_use(
+        {
+            "session_id": "s1",
+            "tool_name": "collaborationspawn_agent",
+            "tool_input": {"task_name": "refactor", "message": "encrypted"},
+        },
+        workspace=WS,
+        token="token",
+        available_models=["system.ai.gpt-5-6-sol", "system.ai.gpt-5-6-luna"],
+        audit_decision=True,
+    )
+
+    # The rationale is persisted so an empty value distinguishes "gateway
+    # returned none" from a display-placement bug.
+    record = json.loads(decisions.read_text().strip())
+    assert record["rationale"] == "Cross-cutting refactor needs the strongest model."