feat(backend): every generation call carries a session name

The release audit swept every agent.run and raw chat call: the two
streaming orchestrate paths never carried a binding (they predate the
v2.6.0 pinning), and the biographer and housekeeper ran bare. Through
the wrapper a session-less call takes idle slots only and 503s once
four sessions are resident — so the streaming paths join
tatlock-orchestrate, and the experts share tatlock-experts at rank 35:
between the librarian and the phases, the two least-used consumers
trade the spare slot by rank instead of anyone hitting an empty pool.
Rank mutation shown to fail its test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/agents/biographer/agent.py

@@ -191,9 +191,12 @@ async def run_biographer(
     )
 
     try:
+        from src.anthropic.model_selector import with_phase_binding
+
         result = await agent.run(
             prompt,
             message_history=message_history,
+            model_settings=with_phase_binding(None, "experts"),
         )
 
         logger.info(

src/agents/housekeeper/agent.py

@@ -206,12 +206,12 @@ async def run_housekeeper(
 
     try:
         # Temperature 0.1 for slight exploration (skipped on Claude backend)
-        from src.anthropic.model_selector import get_sampling_settings
+        from src.anthropic.model_selector import get_sampling_settings, with_phase_binding
 
         result = await agent.run(
             prompt,
             message_history=message_history,
-            model_settings=get_sampling_settings(0.1),
+            model_settings=with_phase_binding(get_sampling_settings(0.1), "experts"),
         )
 
         logger.info(

src/agents/tatlock.py

@@ -404,10 +404,13 @@ class TatlockAgent(AgentInterface):
             # with async context managers inside generators
             # The StreamingCoordinator will handle word-by-word streaming
             # Pass message_history to maintain conversation context and tracker for tool logging
+            from src.anthropic.model_selector import with_phase_binding
+
             result = await self.agent.run(
                 user_message,
                 message_history=message_history if message_history else None,
                 deps=tracker,
+                model_settings=with_phase_binding(None, "orchestrate"),
             )
             final_text = result.output
 
@@ -625,10 +628,13 @@ class TatlockAgent(AgentInterface):
         # Use run() instead of run_stream() to avoid Ollama 400 bug
         # with streaming + tool calls (PydanticAI issues #1292, #2256)
         # We yield the final response in chunks to maintain streaming interface
+        from src.anthropic.model_selector import with_phase_binding
+
         result = await scoped_agent.run(
             enriched_message,
             message_history=pydantic_history if pydantic_history else None,
             deps=tool_tracker,
+            model_settings=with_phase_binding(None, "orchestrate"),
         )
 
         # Stream the final response in chunks to maintain UX

src/anthropic/model_selector.py

@@ -313,6 +313,12 @@ _PHASE_BINDINGS: dict[str, tuple[int | None, str, int]] = {
     "steward": (0, "tatlock-steward", 40),
     "orchestrate": (1, "tatlock-orchestrate", 40),
     "synthesize": (2, "tatlock-synthesize", 40),
+    # Experts (biographer, housekeeper) share one session between the
+    # librarian and the phases: the two least-used consumers trade the
+    # spare slot by rank instead of anyone hitting an empty pool — a
+    # session-less call 503s once four sessions are resident (D-4's
+    # idle-only rule), so every generation call here must carry a name.
+    "experts": (None, "tatlock-experts", 35),
     "librarian": (None, "librarian", 30),
 }
 

tests/anthropic/test_model_selector.py

@@ -121,11 +121,12 @@ class TestPhaseBinding:
         monkeypatch.setattr(model_selector, "_local_flavor", "boilerroom")
         ranks = {
             phase: model_selector.phase_extra_body(phase)["eviction_order"]
-            for phase in ("steward", "orchestrate", "synthesize", "librarian")
+            for phase in ("steward", "orchestrate", "synthesize", "experts", "librarian")
         }
         assert ranks["steward"] == ranks["orchestrate"] == ranks["synthesize"] == 40
+        assert ranks["experts"] == 35
         assert ranks["librarian"] == 30
-        assert ranks["librarian"] < ranks["steward"]
+        assert ranks["librarian"] < ranks["experts"] < ranks["steward"]
         assert ranks["librarian"] > 20  # webber's rank stays below
 
     def test_wrapper_session_names_are_stable(self, local_first, monkeypatch):
@@ -134,12 +135,13 @@ class TestPhaseBinding:
         monkeypatch.setattr(model_selector, "_local_flavor", "boilerroom")
         names = {
             phase: model_selector.phase_extra_body(phase)["session"]
-            for phase in ("steward", "orchestrate", "synthesize", "librarian")
+            for phase in ("steward", "orchestrate", "synthesize", "experts", "librarian")
         }
         assert names == {
             "steward": "tatlock-steward",
             "orchestrate": "tatlock-orchestrate",
             "synthesize": "tatlock-synthesize",
+            "experts": "tatlock-experts",
             "librarian": "librarian",
         }