fix(credits): suppress depleted banner on stealth-preview models

Stealth-preview SKUs (e.g. stealth/ox-alpha) are free-tier but carry no
:free suffix, so is_free_tier_model() returned False for them.  On gateway
sessions (which never run the model picker's pricing fetch), the free-model
suppression of the credits.depleted banner never engaged, and any response
carrying paid_access:false triggered a false "Credit access paused" notice.

Add stealth/ prefix detection to is_free_tier_model() as a zero-network
signal, same design as the existing :free suffix check.  Fail-open to
False (banner still shows) if the prefix changes — recoverable noise,
never a masked depletion on a paid model.

Closes #91843

agent/credits_tracker.py

@@ -226,12 +226,15 @@ def is_free_tier_model(model: str, base_url: str = "") -> bool:
     1. The ``:free`` suffix — the canonical Nous free SKU marker (e.g.
        ``nvidia/nemotron-3-ultra:free``). Free by construction on the API side
        (spend is forced to 0 for ``:free`` ids).
-    2. A peek into the in-process pricing cache in ``hermes_cli.models``
+    2. The ``stealth/`` prefix — Nous stealth-preview SKUs (e.g.
+       ``stealth/ox-alpha``) are free-tier but carry no ``:free`` suffix.  Spend
+       is forced to zero server-side, so these are also free by construction.
+    3. A peek into the in-process pricing cache in ``hermes_cli.models``
        (populated when the model picker fetched ``/v1/models`` pricing for
        *base_url*). PEEK ONLY — a cache miss never triggers a fetch. This is
        CLI/TUI-session best-effort: gateway sessions never run the picker's
        pricing fetch, so suppression there rests entirely on the ``:free``
-       suffix (which all Nous free SKUs carry).
+       suffix and ``stealth/`` prefix.
 
     Fail-open to False (the depleted notice still shows) on any error: wrongly
     showing the warning is recoverable noise; wrongly hiding it on a paid model
@@ -241,6 +244,15 @@ def is_free_tier_model(model: str, base_url: str = "") -> bool:
         return False
     if model.endswith(":free"):
         return True
+    # Stealth-preview SKUs (e.g. stealth/ox-alpha) are free-tier but carry no
+    # ``:free`` suffix.  Spend is forced to zero server-side, so a ``paid_access:
+    # false`` header on these models is a false positive for the depleted banner.
+    # The ``stealth/`` prefix is the Nous naming convention for these SKUs and
+    # is checked here as a zero-network signal, same design as the ``:free``
+    # suffix above.  Fail-open to False (the banner still shows) if the prefix
+    # ever changes — recoverable noise, never a masked depletion on a paid model.
+    if model.startswith("stealth/"):
+        return True
     if not base_url:
         return False
     try:

tests/agent/test_credits_policy.py

@@ -351,6 +351,47 @@ class TestIsFreeTierModel:
         monkeypatch.setattr(models_mod, "_pricing_cache", _Exploding())
         assert is_free_tier_model("some/model", "https://inference-api.nousresearch.com") is False
 
+    def test_stealth_prefix_detected_as_free(self):
+        """Stealth-preview SKUs (stealth/...) are free-tier but carry no
+        :free suffix.  Suppression must engage so the depleted banner doesn't
+        fire on a false paid_access:false from the server's stealth pool."""
+        from agent.credits_tracker import is_free_tier_model
+
+        # No base_url needed — stealth/ is a zero-network signal, same as :free.
+        assert is_free_tier_model("stealth/ox-alpha", "") is True
+        assert is_free_tier_model("stealth/ox-alpha", "https://inference-api.nousresearch.com/v1") is True
+        # Non-stealth model without :free suffix → not free (without pricing cache).
+        assert is_free_tier_model("some/paid-model", "") is False
+
+    def test_depleted_suppressed_for_stealth_model(self):
+        """End-to-end: paid_access:false on a stealth/ model must NOT fire
+        the depleted banner (the exact scenario from issue #91843)."""
+        from agent.credits_tracker import (
+            CreditsState, evaluate_credits_notices, is_free_tier_model,
+        )
+
+        state = CreditsState(
+            version=1,
+            remaining_micros=0,
+            remaining_usd="0.00",
+            subscription_micros=0,
+            subscription_usd="0.00",
+            purchased_micros=0,
+            purchased_usd="0.00",
+            paid_access=False,
+            captured_at=1.0,
+            from_header=True,
+        )
+        model = "stealth/ox-alpha"
+        base_url = "https://inference-api.nousresearch.com/v1"
+        model_is_free = is_free_tier_model(model, base_url)
+        assert model_is_free is True
+
+        latch = fresh_latch()
+        to_show, to_clear = evaluate_credits_notices(state, latch, model_is_free=model_is_free)
+        assert all(n.key != "credits.depleted" for n in to_show)
+        assert "credits.depleted" not in latch["active"]
+
 
 # ── Scenario 6: denominator none (uf is None) ────────────────────────────────