fix(image_routing): check stripped custom:<name> provider key for vision override

When model.provider is set to custom:<name>, _supports_vision_override()
previously tried only the runtime provider key ('custom') and the raw
config value ('custom:my-proxy'). It did not try the stripped name
('my-proxy'), which is the actual key under providers: in config.yaml.

This caused native image routing to fall back to text mode even when the
user explicitly declared supports_vision: true on the named provider's
model entry.

Fixes #39963

agent/image_routing.py

@@ -185,7 +185,8 @@ def _supports_vision_override(
       2. ``providers.<provider>.models.<model>.supports_vision``
          (named custom providers — ``provider`` may be the runtime-resolved
          value ``"custom"`` and/or the user-declared name under
-         ``model.provider``; both are tried)
+         ``model.provider``; both are tried. For ``custom:<name>`` syntax,
+         the stripped ``<name>`` is also tried as a provider key.)
 
     Returns None when no override is set, so the caller falls through to
     models.dev. Returns False explicitly only when the user wrote a
@@ -205,11 +206,16 @@ def _supports_vision_override(
     # get rewritten to provider="custom" at runtime
     # (hermes_cli/runtime_provider.py:_resolve_named_custom_runtime), so the
     # config still holds the user-declared name under model.provider. Try
-    # both as candidate provider keys.
+    # both as candidate provider keys, plus the stripped suffix from
+    # "custom:<name>" (where <name> is the key under providers:).
     config_provider = str(model_cfg.get("provider") or "").strip()
+    # Extract the stripped name from "custom:<name>" if present
+    stripped_suffix = ""
+    if config_provider.startswith("custom:"):
+        stripped_suffix = config_provider[len("custom:"):]
     providers_raw = cfg.get("providers")
     providers_cfg: Dict[str, Any] = providers_raw if isinstance(providers_raw, dict) else {}
-    for p in dict.fromkeys(filter(None, (provider, config_provider))):
+    for p in dict.fromkeys(filter(None, (provider, config_provider, stripped_suffix))):
         entry_raw = providers_cfg.get(p)
         entry: Dict[str, Any] = entry_raw if isinstance(entry_raw, dict) else {}
         models_raw = entry.get("models")

tests/agent/test_image_routing.py

@@ -224,6 +224,37 @@ class TestSupportsVisionOverride:
         cfg = {"model": "some-string", "providers": ["not-a-dict"]}
         assert _supports_vision_override(cfg, "custom", "my-llava") is None
 
+    def test_custom_colon_name_stripped_suffix_lookup(self):
+        # model.provider: custom:my-proxy → should resolve stripped key "my-proxy"
+        cfg = {
+            "model": {"provider": "custom:my-proxy"},
+            "providers": {
+                "my-proxy": {"models": {"gpt-5.5": {"supports_vision": True}}},
+            },
+        }
+        assert _supports_vision_override(cfg, "custom", "gpt-5.5") is True
+
+    def test_custom_colon_name_stripped_suffix_false(self):
+        # Explicitly disabled vision on the stripped key.
+        cfg = {
+            "model": {"provider": "custom:my-proxy"},
+            "providers": {
+                "my-proxy": {"models": {"gpt-5.5": {"supports_vision": False}}},
+            },
+        }
+        assert _supports_vision_override(cfg, "custom", "gpt-5.5") is False
+
+    def test_custom_colon_name_no_stripped_key_falls_through(self):
+        # custom:my-proxy but providers only has "custom" — stripped key
+        # doesn't match, but "custom" does via runtime provider.
+        cfg = {
+            "model": {"provider": "custom:my-proxy"},
+            "providers": {
+                "custom": {"models": {"gpt-5.5": {"supports_vision": True}}},
+            },
+        }
+        assert _supports_vision_override(cfg, "custom", "gpt-5.5") is True
+
 
 # ─── _lookup_supports_vision (override-aware) ────────────────────────────────