docs(rfd): Draft: Meta Field Propagation Conventions (#276)

* RFD Draft: Meta Field Propagation Conventions

Document `params._meta` as the convention for propagating metadata from clients to agents, such as trace identifiers or correlation IDs.

This aligns with MCP, enabling shared instrumentation since both protocols use stdio JSON-RPC transports.

* Use SHOULD language for W3C trace context keys

Reserves traceparent, tracestate, and baggage using SHOULD rather
than MUST. Allows ecosystem adoption without a protocol version change
while protecting keys existing tooling depends on.

* Add ben as champion, and add to website

* Add ben as champion, and add to website

---------

Co-authored-by: Ben Brandt <[email protected]>

Author: codefromthecrypt

docs/docs.json

@@ -96,7 +96,8 @@
               "rfds/session-config-options",
               "rfds/session-fork",
               "rfds/request-cancellation",
-              "rfds/session-resume"
+              "rfds/session-resume",
+              "rfds/meta-propagation"
             ]
           },
           { "group": "Preview", "pages": [] },

docs/rfds/meta-propagation.mdx

@@ -0,0 +1,65 @@
+---
+title: "Meta Field Propagation Conventions"
+---
+
+- Author(s): [Adrian Cole](https://github.com/codefromthecrypt)
+- Champion: [@benbrandt](https://github.com/benbrandt)
+
+## Elevator pitch
+
+Document `params._meta` as the convention for propagating metadata from clients to agents, such as trace identifiers or correlation IDs. This aligns with [MCP](https://modelcontextprotocol.io/), enabling shared instrumentation since both protocols use stdio JSON-RPC transports.
+
+## Status quo
+
+ACP clients already propagate context to agents via `_meta`. For example, `requestId` is used for request correlation in [AionUi](https://github.com/iOfficeAI/AionUi/blob/main/src/common/codex/types/eventData.ts#L12-L16).
+
+However, the [extensibility](../protocol/extensibility) documentation does not specify the `_meta` type or document its use for propagation. Without documentation, parties must coordinate ad hoc, which can lead to portability accidents (such as one side using `_meta.traceparent` and the other `_meta.otel.traceparent`). Documenting that propagated fields are root keys in `_meta` prevents this.
+
+## What we propose to do about it
+
+Update the [extensibility](../protocol/extensibility#the-_meta-field) documentation with two changes:
+
+1. Add the type `{ [key: string]: unknown }` to the existing summary sentence. This type is compatible with MCP SDKs.
+2. Add a new paragraph after the JSON example about propagation conventions.
+
+## Shiny future
+
+- Same instrumentation (OpenInference, etc.) works for both ACP and MCP.
+- Observability tools can correlate traces across protocols.
+
+## Implementation details
+
+**Change 1**: Update the existing summary sentence in [extensibility](../protocol/extensibility#the-_meta-field):
+
+```diff
+-All types in the protocol include a `_meta` field that implementations can use to attach custom information.
++All types in the protocol include a `_meta` field with type `{ [key: string]: unknown }` that implementations can use to attach custom information.
+```
+
+**Change 2**: After the JSON example, before "Implementations **MUST NOT**", add:
+
+> Clients may propagate fields to the agent for correlation purposes, such as `requestId`. The following root-level keys in `_meta` **SHOULD** be reserved for [W3C trace context](https://www.w3.org/TR/trace-context/) to guarantee interop with existing MCP implementations and OpenTelemetry tooling:
+>
+> - `traceparent`
+> - `tracestate`
+> - `baggage`
+
+## FAQ
+
+### Why document this now?
+
+Clients already propagate context via `_meta`. Documenting prevents incompatible drift and enables shared tooling with MCP.
+
+### Why reference MCP?
+
+ACP and MCP are the two core agentic protocols, both using stdio JSON-RPC. Where `_meta` types are compatible, instrumentation code can be abstracted and reused for both:
+
+Here are several MCP SDKs that propagate W3C trace-context in `_meta`:
+
+- [MCP C# SDK](https://github.com/modelcontextprotocol/csharp-sdk) - native W3C trace-context propagation
+- [OpenInference](https://github.com/Arize-ai/openinference) - Python and TypeScript MCP instrumentation (collaboration between Arize and Elastic)
+- [curioswitch/mcp-go-sdk-otel](https://github.com/curioswitch/mcp-go-sdk-otel) - Go MCP instrumentation
+
+## Revision history
+
+- 2025-11-28: Initial draft

docs/updates.mdx

@@ -4,6 +4,13 @@ description: Updates and announcements about the Agent Client Protocol
 rss: true
 ---
 
+<Update label="December 3, 2025" tags={["RFD"]}>
+## _meta Propagation RFD moves to Draft stage
+
+The RFD for providing more guidance on how the `_meta` parameter should be used within the protocol has been moved to Draft stage. Please review the [RFD](./rfds/meta-propagation) for more information on the current proposal and provide feedback as work on the implementation begins.
+
+</Update>
+
 <Update label="November 26, 2025" tags={["RFD"]}>
 ## session/resume RFD moves to Draft stage