docs(protocol): reject unadvertised prompt content before insertion (#2601)

Author: benbrandt

docs/protocol/v2/content.mdx

@@ -19,7 +19,7 @@ ACP's `ContentBlock` is compatible with, and a superset of, the [Model Context P
 
 A content type from a later MCP revision that ACP doesn't define isn't forwarded as is. Agents convert it to a supported type, such as an embedded resource, or omit it.
 
-Content block `type` values can be custom or future variants when the receiver can preserve or display them generically. Custom content block types **MUST** begin with `_`. Unknown non-underscore content block types are reserved for future ACP variants.
+Content block `type` values can be custom or future variants when the receiver can preserve or display them generically. Custom content block types **MUST** begin with `_`. Unknown non-underscore content block types are reserved for future ACP variants. In prompts, Clients send only content types the Agent advertised, including custom types advertised through a [custom capability](/protocol/v2/extensibility#advertising-custom-capabilities), and Agents reject others with `-32602`.
 
 ### Text Content
 

docs/protocol/v2/draft/content.mdx

@@ -19,7 +19,7 @@ ACP's `ContentBlock` is compatible with, and a superset of, the [Model Context P
 
 A content type from a later MCP revision that ACP doesn't define isn't forwarded as is. Agents convert it to a supported type, such as an embedded resource, or omit it.
 
-Content block `type` values can be custom or future variants when the receiver can preserve or display them generically. Custom content block types **MUST** begin with `_`. Unknown non-underscore content block types are reserved for future ACP variants.
+Content block `type` values can be custom or future variants when the receiver can preserve or display them generically. Custom content block types **MUST** begin with `_`. Unknown non-underscore content block types are reserved for future ACP variants. In prompts, Clients send only content types the Agent advertised, including custom types advertised through a [custom capability](/protocol/v2/draft/extensibility#advertising-custom-capabilities), and Agents reject others with `-32602`.
 
 ### Text Content
 

docs/protocol/v2/draft/prompt-lifecycle.mdx

@@ -261,7 +261,7 @@ The Client sends a user message with `session/prompt`:
 <ParamField path="prompt" type="ContentBlock[]" required>
     The contents of the user message, e.g. text, images, files, etc.
 
-    Clients **MUST** restrict types of content according to the [Prompt Capabilities](/protocol/v2/draft/initialization#session-prompt-capabilities) established during [initialization](/protocol/v2/draft/initialization). Agents **MUST** reject content types they don't support with `-32602`.
+    Clients **MUST** restrict types of content according to the [Prompt Capabilities](/protocol/v2/draft/initialization#session-prompt-capabilities) established during [initialization](/protocol/v2/draft/initialization). Agents **MUST** reject a prompt containing a content type they did not advertise or don't understand with `-32602`, without inserting it. Custom content types need a [custom capability](/protocol/v2/draft/extensibility#advertising-custom-capabilities), such as an entry in `capabilities.session.prompt._meta`.
 
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docs/protocol/v2/prompt-lifecycle.mdx

@@ -190,7 +190,7 @@ The Client sends a user message with `session/prompt`:
 <ParamField path="prompt" type="ContentBlock[]" required>
     The contents of the user message, e.g. text, images, files, etc.
 
-    Clients **MUST** restrict types of content according to the [Prompt Capabilities](/protocol/v2/initialization#session-prompt-capabilities) established during [initialization](/protocol/v2/initialization). Agents **MUST** reject content types they don't support with `-32602`.
+    Clients **MUST** restrict types of content according to the [Prompt Capabilities](/protocol/v2/initialization#session-prompt-capabilities) established during [initialization](/protocol/v2/initialization). Agents **MUST** reject a prompt containing a content type they did not advertise or don't understand with `-32602`, without inserting it. Custom content types need a [custom capability](/protocol/v2/extensibility#advertising-custom-capabilities), such as an entry in `capabilities.session.prompt._meta`.
 
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