serve: honour conv_route_batch_after from the config, not only the flag

serve/conv_router.py

@@ -24,6 +24,27 @@
 CHAT = "chat"        # cache on
 BATCH = "batch"      # cache off
 
+DEFAULT_BATCH_AFTER = 4
+
+
+def resolve_batch_after(cli, cfg):
+    """The run length the router uses: the CLI flag wins, then the config's key, then 4.
+
+    The launcher writes `conv_route_batch_after` into the run config and starts the server without the flag
+    (`strata-b70-serve.sh`), so a server that only read the flag ran 4 for every deployment - measured 2026-11-14:
+    a config of 1 and the default produced byte-identical mixed-traffic results until the flag was passed by hand.
+    """
+    for v in (cli, (cfg or {}).get("conv_route_batch_after") if isinstance(cfg, dict) else None):
+        if v is None or isinstance(v, bool):
+            continue
+        try:
+            n = int(v)
+        except (TypeError, ValueError):
+            continue
+        if n >= 1:
+            return n
+    return DEFAULT_BATCH_AFTER
+
 
 @dataclass
 class Decision:

serve/server.py

@@ -65,6 +65,7 @@
                             tool_choice_of, unmark_think_literals)
 from serve.mcp import McpCancelled, hub_from_config  # noqa: E402
 from serve import conv_router as conv_route_mod  # noqa: E402
+from serve.conv_router import resolve_batch_after  # noqa: E402
 from serve import runconfig  # noqa: E402
 from serve.webfiles import subtree_file as web_subtree_file  # noqa: E402
 from strata_control import engine_component  # noqa: E402
@@ -10386,8 +10387,9 @@ def notify_lifecycle(kind: str, payload: dict) -> None:
                     help="switch the engine's prompt/conversation cache per traffic shape: single-shot batches off "
                          "(prefill +33 %%), chat/agent on (a repeated context reuses, 5.3 s -> 0.3 s). 'shadow' only "
                          "logs what it would do (serve/conv_router.py)")
-    ap.add_argument("--conv-route-batch-after", type=int, default=4,
-                    help="consecutive single-shot requests before the cache is switched off (default 4)")
+    ap.add_argument("--conv-route-batch-after", type=int, default=None,
+                    help="consecutive single-shot requests before the cache is switched off (default: the config's "
+                         "conv_route_batch_after, else 4)")
     ap.add_argument("--thinking-policy", choices=["off", "shadow", "on"], default=None,
                     help="do not pay for a thinking block on a request that asks for a short direct answer "
                          "(extraction, formatting, translation, a one-word reply; serve/prompt_kind.py "
@@ -10731,7 +10733,7 @@ def request_stop(reason: str):
             _pc = 6
     _cr = cfg.get("conv_route")
     svc.set_conv_route(a.conv_route or (_cr if isinstance(_cr, str) else "") or "off",
-                       cache_on=_pc, batch_after=a.conv_route_batch_after)
+                       cache_on=_pc, batch_after=resolve_batch_after(a.conv_route_batch_after, cfg))
     if (cfg.get("arch") or "") == "deepseek41" and vision is None and a.engine == "strata":
         # DeepSeek-V4.1 reads pictures with the engine's own tower (--vision on, the default): no separate reader process.
         # `--vision off` in the config's args, or a pack without a tower, leaves svc.vision None: pictures are refused with the 400 as before.

serve/test_conv_router.py

@@ -8,7 +8,7 @@
 from pathlib import Path
 
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
-from serve.conv_router import BATCH, CHAT, Router  # noqa: E402
+from serve.conv_router import BATCH, CHAT, Router, resolve_batch_after  # noqa: E402
 
 
 def one(text="Summarise this document."):
@@ -106,6 +106,17 @@ def test_report_counts_every_request():
     assert rep["chat"] + rep["batch"] == 3
 
 
+def test_the_config_key_is_honoured_when_the_flag_is_absent():
+    """The launcher writes conv_route_batch_after into the config and passes no flag, so the config must win over 4."""
+    assert resolve_batch_after(None, {"conv_route_batch_after": 2}) == 2
+    assert resolve_batch_after(None, {"conv_route_batch_after": "3"}) == 3
+    assert resolve_batch_after(1, {"conv_route_batch_after": 2}) == 1, "an explicit flag wins over the config"
+    assert resolve_batch_after(None, {}) == 4, "no flag and no key keeps the old default"
+    assert resolve_batch_after(None, None) == 4
+    for bad in (0, -1, True, "off", [1], {}):
+        assert resolve_batch_after(None, {"conv_route_batch_after": bad}) == 4, f"malformed {bad!r} must not crash"
+
+
 def test_the_prefix_history_is_bounded():
     r = Router(repeat_window=8)
     for i in range(50):