RFD: Runtime context on session/prompt

[kevklam]

Summary

Adds a draft RFD proposing runtimeContext as optional host-provided runtime context on session/prompt.

Proposal

• add sessionCapabilities.runtimeContext?: {} as a yes/no capability
• add optional runtimeContext to session/prompt
• define runtimeContext as text-only items shaped like { title?: string, text: string }[]
• make it explicit that runtimeContext is used by the agent when constructing the prompt for that turn, but is not part of durable conversation history or session replay

Motivation

ACP currently has durable session metadata like cwd and additionalDirectories, and normal prompt content on session/prompt, but it does not have a standard way for clients to provide fresh host-side IDE/runtime context for a turn without snapshotting that context into prompt history.

This is especially relevant for IDE-derived context that clients may want to provide freshly on many turns, but that is too large, too expensive, or too low-value to durably record on every turn.

Design notes

• this proposal standardizes the mechanism and lifecycle semantics, not a fixed schema of IDE facts
• runtimeContext is push-based and travels with session/prompt
• runtimeContext is text-only in this proposal
• clients remain free to decide what runtime context to send for a given turn
• agents should normally incorporate runtimeContext, but may trim or omit items if needed for prompt-construction or context-limit reasons

Non-goals

• replacing cwd
• obsoleting additionalDirectories as a goal of this RFD
• standardizing a fixed set of IDE fields that agents can take code dependencies on
• recommending fallback behavior when an agent does not support runtimeContext