Align with category documentaton

Author: benbrandt

docs/protocol/draft/schema.mdx

@@ -4322,6 +4322,11 @@ This capability is not part of the spec yet, and may be removed or changed at an
 
 Well-known API protocol identifiers for LLM providers.
 
+Agents and clients MUST handle unknown protocol identifiers gracefully.
+
+Protocol names beginning with `_` are free for custom use, like other ACP extension methods.
+Protocol names that do not begin with `_` are reserved for the ACP spec.
+
 **Type:** Union
 
 <ResponseField name="anthropic" type="string">

schema/schema.unstable.json

@@ -3175,7 +3175,7 @@
           "type": "string"
         }
       ],
-      "description": "**UNSTABLE**\n\nThis capability is not part of the spec yet, and may be removed or changed at any point.\n\nWell-known API protocol identifiers for LLM providers."
+      "description": "**UNSTABLE**\n\nThis capability is not part of the spec yet, and may be removed or changed at any point.\n\nWell-known API protocol identifiers for LLM providers.\n\nAgents and clients MUST handle unknown protocol identifiers gracefully.\n\nProtocol names beginning with `_` are free for custom use, like other ACP extension methods.\nProtocol names that do not begin with `_` are reserved for the ACP spec."
     },
     "LoadSessionRequest": {
       "description": "Request parameters for loading an existing session.\n\nOnly available if the Agent supports the `loadSession` capability.\n\nSee protocol docs: [Loading Sessions](https://agentclientprotocol.com/protocol/session-setup#loading-sessions)",

src/agent.rs

@@ -3389,6 +3389,11 @@ impl SetSessionModelResponse {
 /// This capability is not part of the spec yet, and may be removed or changed at any point.
 ///
 /// Well-known API protocol identifiers for LLM providers.
+///
+/// Agents and clients MUST handle unknown protocol identifiers gracefully.
+///
+/// Protocol names beginning with `_` are free for custom use, like other ACP extension methods.
+/// Protocol names that do not begin with `_` are reserved for the ACP spec.
 #[cfg(feature = "unstable_llm_providers")]
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 #[serde(rename_all = "snake_case")]
@@ -6046,7 +6051,7 @@ mod test_serialization {
             SetProvidersRequest::new("main", LlmProtocol::Anthropic, "https://api.anthropic.com")
                 .headers(headers);
 
-        let debug = format!("{:?}", request);
+        let debug = format!("{request:?}");
         assert!(debug.contains("[REDACTED]"));
         assert!(!debug.contains("sk-secret-key"));
     }