diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
index 689d01010ad6..0c346cad56f7 100644
--- a/agent/anthropic_adapter.py
+++ b/agent/anthropic_adapter.py
@@ -544,6 +544,7 @@ def _requires_bearer_auth(base_url: str | None) -> bool:
     return (
         normalized.startswith(("https://api.minimax.io/anthropic", "https://api.minimaxi.com/anthropic"))
         or "azure.com" in normalized
+        or "palantirfoundry" in normalized
     )
 
 
diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index bacd3e7749ff..3b1af7ea6f2f 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -4384,10 +4384,26 @@ def _resolve_auto(
         resolved_provider = main_provider
         explicit_base_url = runtime_base_url or None
         explicit_api_key = None
-        if runtime_base_url and (main_provider == "custom" or main_provider.startswith("custom:")):
+        if runtime_base_url and main_provider == "custom":
+            # Anonymous custom endpoint (OPENAI_BASE_URL / config.model.base_url)
+            # — pass through with explicit base_url + api_key.
             resolved_provider = "custom"
             explicit_base_url = runtime_base_url
             explicit_api_key = runtime_api_key or None
+        elif main_provider.startswith("custom:"):
+            # Named custom provider (custom_providers / providers dict entry).
+            # KEEP the full ``custom:<name>`` so resolve_provider_client lands in
+            # the named-custom-provider arm — that arm honours the entry's
+            # api_mode (e.g. anthropic_messages → AnthropicAuxiliaryClient,
+            # avoiding the /anthropic→/v1 rewrite that 404s against proxies
+            # like Palantir Foundry's Anthropic surface).  Do NOT collapse to
+            # plain "custom"; that path strips /anthropic and routes through
+            # OpenAI chat.completions.
+            resolved_provider = main_provider
+            # base_url / api_key come from the named entry itself — leave the
+            # explicit_* overrides unset so the named arm reads them from config.
+            if runtime_api_key:
+                explicit_api_key = runtime_api_key
         elif runtime_api_key:
             # Pin auxiliary to the same api_key as the active main chat session
             # so that a working key is reused instead of re-selecting from the pool