fix(upstream): break OpenRouter bare-tail ties on combined token cost

The bare-tail tie-break ranked candidates by prompt price alone, so two
entries sharing a tail where one is cheaper on prompt but far dearer on
completion could resolve to the entry that undercharges output-heavy
traffic — contradicting the "highest-priced wins for money safety" promise.

Rank by the combined prompt + completion per-token cost instead, so the
choice stays deterministic and money-safe whichever way traffic leans. As
before there are zero bare-tail collisions in the live feed, so this changes
no resolved price today; it only governs the latent case.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

routstr/upstream/pricing_resolver.py

@@ -123,10 +123,12 @@ def _match_openrouter(model_id: str, feed: list[dict]) -> dict | None:
     Bare-tail matching (``deepseek-chat`` ↔ ``deepseek/deepseek-chat``) is a
     looser, lower-trust match — OpenRouter fans a model out across resellers —
     so an exact id match always wins first. When several entries share the bare
-    tail, the highest-priced one wins: the choice must be deterministic (not
-    feed-order-dependent) and money-safe, since undercharging is the hazard.
-    The live feed has no such collisions today; this only governs the latent
-    case.
+    tail, the one with the highest *combined* (prompt + completion) per-token
+    cost wins: the choice must be deterministic (not feed-order-dependent) and
+    money-safe whichever way traffic leans, since undercharging is the hazard.
+    Ranking on prompt alone could pick an entry that is cheap on input but dear
+    on output. The live feed has no such collisions today; this only governs
+    the latent case.
     """
     bare = model_id.split("/", 1)[-1]
     exact = next((m for m in feed if m.get("id") == model_id), None)
@@ -135,10 +137,14 @@ def _match_openrouter(model_id: str, feed: list[dict]) -> dict | None:
     matches = [m for m in feed if m.get("id", "").split("/", 1)[-1] == bare]
     if not matches:
         return None
-    return max(
-        matches,
-        key=lambda m: _as_float(m.get("pricing", {}).get("prompt")) or 0.0,
-    )
+
+    def _combined_cost(m: dict) -> float:
+        pricing = m.get("pricing", {})
+        return (_as_float(pricing.get("prompt")) or 0.0) + (
+            _as_float(pricing.get("completion")) or 0.0
+        )
+
+    return max(matches, key=_combined_cost)
 
 
 def _from_openrouter(model_id: str, feed: list[dict]) -> ResolvedPricing | None:

tests/unit/test_upstream_generic.py

@@ -404,6 +404,44 @@ async def test_openrouter_bare_tail_collision_picks_highest_price() -> None:
     assert model.pricing.completion == pytest.approx(1e-05)
 
 
+@pytest.mark.asyncio
+async def test_openrouter_bare_tail_tie_breaks_on_combined_cost() -> None:
+    """The bare-tail tie-break must weigh *both* rates, not prompt alone.
+    Given two colliding entries where one is cheaper on prompt but far dearer
+    on completion, ranking by prompt would pick the entry that undercharges
+    output-heavy traffic. Pick the highest *combined* per-token cost so the
+    money-safe choice holds whichever way the traffic leans."""
+    payload = {
+        "data": [
+            {"id": "yyy-phantom-model", "object": "model", "owned_by": "mystery"},
+        ]
+    }
+    # dear-overall is listed first with the *lower* prompt, so a prompt-only max
+    # would wrongly pick the second (cheaper-overall) entry.
+    or_feed = AsyncMock(
+        return_value=[
+            {
+                "id": "dearco/yyy-phantom-model",
+                "context_length": 8192,
+                "pricing": {"prompt": "0.000001", "completion": "0.000100"},
+            },
+            {
+                "id": "cheapco/yyy-phantom-model",
+                "context_length": 8192,
+                "pricing": {"prompt": "0.000009", "completion": "0.000002"},
+            },
+        ]
+    )
+
+    with _patch_models_endpoint(payload):
+        with patch("routstr.payment.models.async_fetch_openrouter_models", or_feed):
+            models = await GenericUpstreamProvider(base_url="http://x").fetch_models()
+
+    model = _model_by_id(models, "yyy-phantom-model")
+    assert model.pricing.prompt == pytest.approx(1e-06)
+    assert model.pricing.completion == pytest.approx(1e-04)
+
+
 @pytest.mark.asyncio
 async def test_openrouter_feed_fetched_once_per_discovery() -> None:
     """Two models both missing litellm must share a single OpenRouter fetch —