docs(rfd): move client system prompt to preview

agent-client-protocol-schema/src/v1/agent.rs

@@ -3859,17 +3859,18 @@ impl HttpHeader {
 pub struct SystemPrompt {
     /// How the Agent combines `content` with its own instructions.
     ///
-    /// Required on the wire, so every request states its mode. In Rust,
-    /// Unknown
-    /// values are preserved. Agents MUST return an error
-    /// for modes they do not support.
+    /// Required. Agents MUST return an error for a mode they do not support rather than
+    /// treating it as `append` or `override`.
     pub mode: SystemPromptMode,
     /// The system prompt content.
     ///
     /// Agents advertising the capability MUST accept `text` content blocks.
     /// Clients MUST NOT send other content block types until a future
     /// capability allows them, and Agents MUST return an error for content
     /// blocks they do not support.
+    ///
+    /// Clients MUST NOT send an empty array. Agents MAY reject one with
+    /// `-32602` without creating, loading, resuming, or forking the session.
     pub content: Vec<ContentBlock>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Implementations MUST NOT make assumptions about values at

agent-client-protocol-schema/src/v2/agent.rs

@@ -4062,6 +4062,9 @@ pub struct SystemPrompt {
     /// Clients MUST NOT send other content block types until a future
     /// capability allows them, and Agents MUST return an error for content
     /// blocks they do not support, including unknown types.
+    ///
+    /// Clients MUST NOT send an empty array. Agents MAY reject one with
+    /// `-32602` without creating, resuming, or forking the session.
     pub content: Vec<ContentBlock>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Omitted and `null` are equivalent and mean no metadata.

docs/docs.json

@@ -211,7 +211,6 @@
             "pages": [
               "rfds/streamable-http-websocket-transport",
               "rfds/subagents",
-              "rfds/client-system-prompt",
               "rfds/mcp-over-acp",
               {
                 "group": "v2",
@@ -237,7 +236,7 @@
           },
           {
             "group": "Preview",
-            "pages": []
+            "pages": ["rfds/client-system-prompt"]
           },
           {
             "group": "Completed",

docs/protocol/v1/draft/schema.mdx

@@ -8592,14 +8592,15 @@ Clients MUST NOT send other content block types until a future
 capability allows them, and Agents MUST return an error for content
 blocks they do not support.
 
+Clients MUST NOT send an empty array. Agents MAY reject one with
+`-32602` without creating, loading, resuming, or forking the session.
+
 </ResponseField>
 <ResponseField name="mode" type={<a href="#systempromptmode">SystemPromptMode</a>} required>
   How the Agent combines `content` with its own instructions.
 
-Required on the wire, so every request states its mode. In Rust,
-Unknown
-values are preserved. Agents MUST return an error
-for modes they do not support.
+Required. Agents MUST return an error for a mode they do not support rather than
+treating it as `append` or `override`.
 
 </ResponseField>
 

docs/protocol/v1/draft/session-setup.mdx

@@ -416,6 +416,7 @@ and, when the Agent supports it, `session/fork`.
 - `append` adds `content` to the Agent's own instructions rather than replacing them. The Agent **SHOULD** give it system-level precedence, for example in the model's system prompt or developer instructions rather than in user-turn content. Where it is placed and how it is formatted is up to the Agent
 - `override` replaces the Agent's built-in default instructions. It does not change tool availability, MCP servers, permissions, or session configuration. Context the Agent supplies from the workspace or environment, such as `AGENTS.md` files or skill listings, **MAY** still be included, and Agents **SHOULD** document what they keep. Clients that use `override` take responsibility for any tool guidance or safety instructions the Agent's default instructions provided
 - only `text` content blocks are supported for now: Agents **MUST** accept them, and Clients **MUST NOT** send other content block types until a future capability allows them
+- Clients **MUST NOT** send an empty `content` array. Agents **MAY** reject one with `-32602` without creating, loading, resuming, or forking the session
 - omitting the field or setting it to `null` on `session/new` means the Agent uses its default instructions
 - on `session/load`, `session/resume`, and `session/fork`, Clients **SHOULD** send the same `systemPrompt` they used when the session was created. When it is omitted, the Agent may keep a prompt it stored with the session or use its default. When it differs, the Agent **SHOULD** apply it, and **MAY** defer the change until it next rebuilds its prompt (for example, after compaction) when applying it immediately would discard prompt caches or preserved reasoning
 - if the Agent cannot apply a supplied `systemPrompt`, for example because of an unadvertised mode, an unsupported content block type, or content that exceeds its limits, it **MUST** return an error and **MUST NOT** create, load, resume, or fork the session. JSON-RPC `-32602` (Invalid params) is the **RECOMMENDED** error for malformed or unsupported values

docs/protocol/v2/draft/schema.mdx

@@ -9637,6 +9637,9 @@ Clients MUST NOT send other content block types until a future
 capability allows them, and Agents MUST return an error for content
 blocks they do not support, including unknown types.
 
+Clients MUST NOT send an empty array. Agents MAY reject one with
+`-32602` without creating, resuming, or forking the session.
+
 </ResponseField>
 <ResponseField name="mode" type={<a href="#systempromptmode">SystemPromptMode</a>} required>
   How the Agent combines `content` with its own instructions.

docs/protocol/v2/draft/session-setup.mdx

@@ -473,6 +473,7 @@ Agent supports it, `session/fork`.
 - `append` adds `content` to the Agent's own instructions rather than replacing them. The Agent **SHOULD** give it system-level precedence, for example in the model's system prompt or developer instructions rather than in user-turn content. Where it is placed and how it is formatted is up to the Agent
 - `override` replaces the Agent's built-in default instructions. It does not change tool availability, MCP servers, permissions, or session configuration. Context the Agent supplies from the workspace or environment, such as `AGENTS.md` files or skill listings, **MAY** still be included, and Agents **SHOULD** document what they keep. Clients that use `override` take responsibility for any tool guidance or safety instructions the Agent's default instructions provided
 - only `text` content blocks are supported for now: Agents **MUST** accept them, and Clients **MUST NOT** send other content block types until a future capability allows them. Agents **MUST** return an error for content blocks they do not support, including unknown types
+- Clients **MUST NOT** send an empty `content` array. Agents **MAY** reject one with `-32602` without creating, resuming, or forking the session
 - omitting the field or setting it to `null` on `session/new` means the Agent uses its default instructions
 - on `session/resume` and `session/fork`, Clients **SHOULD** send the same `systemPrompt` they used when the session was created. When it is omitted, the Agent may keep a prompt it stored with the session or use its default. When it differs, the Agent **SHOULD** apply it, and **MAY** defer the change until it next rebuilds its prompt (for example, after compaction) when applying it immediately would discard prompt caches or preserved reasoning
 - if the Agent cannot apply a supplied `systemPrompt`, for example because of an unadvertised mode, an unsupported content block type, or content that exceeds its limits, it **MUST** return an error and **MUST NOT** create, resume, or fork the session. JSON-RPC `-32602` (Invalid params) is the **RECOMMENDED** error for malformed or unsupported values