docs (rfd): propose client-set session titles

docs/rfds/session-set-title.mdx

@@ -0,0 +1,115 @@
+---
+title: "Client-Set Session Titles"
+---
+
+Author(s): [@lifeizhou-ap](https://github.com/lifeizhou-ap)
+
+## Elevator pitch
+
+> What are you proposing to change?
+
+Add an optional `session/set_title` request that lets a Client set the title of an Agent-owned session. The Agent persists the title as authoritative session metadata and publishes changes through the existing `session_info_update` notification.
+
+An Agent may generate session titles. After `session/set_title` succeeds, the Agent must not replace the Client-supplied title with a title it generates.
+
+## Status quo
+
+> How do things work today and what problems does this cause? Why would we change things?
+
+ACP allows an Agent to report a session title, but provides no operation for a Client to set one. As a result, a user-selected title cannot be stored as Agent-owned session metadata through ACP.
+
+[Issue #1978](https://github.com/agentclientprotocol/agent-client-protocol/issues/1978) identifies the same missing operation while also proposing semantics for Agent-generated titles and an optional initial title. This RFD addresses only the Client-to-Agent operation.
+
+## What we propose to do about it
+
+> What are you proposing to improve the situation?
+
+Add a capability-gated `session/set_title` request that lets a Client set the title of an Agent-owned session. The Agent persists the title and publishes the change through `session_info_update`.
+
+## Shiny future
+
+> How will things will play out once this feature exists?
+
+<!--
+    Use this section to describe the "status quo" as it will play out once
+    we have made these changes.
+
+    Note: This section is OPTIONAL when RFDs are first opened.
+-->
+
+## Implementation details and plan
+
+> Tell me more about your implementation. What is your detailed implementation plan?
+
+### Protocol
+
+An Agent advertises support through `sessionCapabilities.setTitle`:
+
+```json
+{
+  "agentCapabilities": {
+    "sessionCapabilities": {
+      "setTitle": {}
+    }
+  }
+}
+```
+
+Clients must not call `session/set_title` unless this capability is present.
+
+The request is:
+
+```json
+{
+  "jsonrpc": "2.0",
+  "id": 42,
+  "method": "session/set_title",
+  "params": {
+    "sessionId": "sess_123",
+    "title": "Investigate OAuth timeout"
+  }
+}
+```
+
+`sessionId` identifies the Agent-owned session and `title` is the new session title.
+
+The response is empty:
+
+```json
+{
+  "jsonrpc": "2.0",
+  "id": 42,
+  "result": {}
+}
+```
+
+- Title validation rules are defined by the Agent. If a title fails validation, the Agent returns `InvalidParams` (`-32602`) with a message explaining why it was rejected. For example:
+
+  ```json
+  {
+    "jsonrpc": "2.0",
+    "id": 42,
+    "error": {
+      "code": -32602,
+      "message": "Title must not exceed 100 characters."
+    }
+  }
+  ```
+
+- The Agent returns success only after persisting the title exactly as supplied. If it cannot do so, it returns an error.
+- After a successful `session/set_title` request, the Agent sends a `session/update` notification containing a `session_info_update` with the persisted title. Future `session/list` calls return the same title.
+- A title set through `session/set_title` must not be overwritten by a title generated by the Agent.
+- If the Agent is generating a title when `session/set_title` succeeds, it must discard the generated title.
+- Each subsequent successful `session/set_title` request replaces the previous title.
+
+## Frequently asked questions
+
+> What questions have arisen over the course of authoring this document or during subsequent discussions?
+
+<!--
+    Keep this section up-to-date as discussion proceeds. The goal is to capture major points that came up on a PR or in a discussion forum -- and if they reoccur, to point people to the FAQ so that we can start the dialog from a more informed place.
+-->
+
+## Revision history
+
+- 2026-08-19: Initial proposal.