docs: clarify custom provider protocol support

maintainer edit: spell out that arbitrary named custom providers use the OpenAI-compatible request format only, and point Anthropic-compatible proxies to the built-in anthropic provider with apiBase.

docs/provider-cookbook.md

@@ -168,6 +168,36 @@ ANTHROPIC_API_KEY="sk-ant-..." nanobot agent -m "Hello!"
 
 If you copied a model name such as `anthropic/claude-sonnet-4.5`, that is a gateway-style model path and belongs under `provider: "openrouter"`, not `provider: "anthropic"`.
 
+If you use an Anthropic-compatible proxy, keep the preset provider as `anthropic` and set `providers.anthropic.apiBase`:
+
+```json
+{
+  "providers": {
+    "anthropic": {
+      "apiKey": "${ANTHROPIC_API_KEY}",
+      "apiBase": "https://anthropic-proxy.example.com"
+    }
+  },
+  "modelPresets": {
+    "primary": {
+      "label": "Anthropic proxy",
+      "provider": "anthropic",
+      "model": "claude-sonnet-4-5",
+      "maxTokens": 4096,
+      "contextWindowTokens": 200000,
+      "temperature": 0.1
+    }
+  },
+  "agents": {
+    "defaults": {
+      "modelPreset": "primary"
+    }
+  }
+}
+```
+
+Do not configure Anthropic-compatible endpoints as arbitrary custom provider names; named custom providers use the OpenAI-compatible request format.
+
 ## Recipe: Custom OpenAI-Compatible Provider
 
 This recipe applies to an OpenAI-compatible service that is not a named nanobot provider.
@@ -246,7 +276,7 @@ For multiple custom endpoints, do not overload the single `custom` block. Name e
 }
 ```
 
-These custom names behave like direct OpenAI-compatible providers: `apiBase` is required, `apiKey` is optional when the endpoint allows anonymous or placeholder credentials, and `apiType` should be left unset.
+These custom names behave like direct OpenAI-compatible providers: `apiBase` is required, `apiKey` is optional when the endpoint allows anonymous or placeholder credentials, and `apiType` should be left unset. They do not support Anthropic-compatible endpoints; use the `anthropic` provider with `apiBase` for that case.
 
 ## Recipe: Ollama Local Model