rust: add isExperimentalMode to session create/resume wire

[jmoseley]

Summary

Adds an optional is_experimental_mode field to the Rust SDK's SessionConfig and ResumeSessionConfig so a consumer can tell the runtime to disable (false) or force-enable (true) the experimental feature-flag tier for a single session — without persisting anything to the user's shared config.

This pairs with a companion copilot-agent-runtime change that adds an optional isExperimentalMode?: boolean field to the classic --server SessionCreateRequest / SessionResumeRequest. Runtime semantics:

• false ⇒ re-resolve that session's feature flags as if experimental were off
• true ⇒ enable even if the process didn't
• omitted ⇒ inherit process flags unchanged (no-op)

The motivating consumer is the GitHub desktop app, which spawns copilot --server and needs per-session control over the experimental tier.

Changes

• rust/src/wire.rs: Add is_experimental_mode: Option<bool> (with skip_serializing_if = "Option::is_none") to SessionCreateWire and SessionResumeWire. Serializes as isExperimentalMode via the existing rename_all = "camelCase", omitted from the wire when None so older CLIs are unaffected.
• rust/src/types.rs:
  • Add pub is_experimental_mode: Option<bool> to SessionConfig and ResumeSessionConfig with doc comments (resume doc notes it only re-resolves on the cold-load path).
  • Wire it through the manual Debug impls, Default/new constructors, and both into_wire mappings.
  • Add with_is_experimental_mode(bool) builders to both, mirroring with_enable_mcp_apps.

Tests

Added four unit tests (co-located with the existing wire/types tests):

• session_config_is_experimental_mode_serializes_when_set — asserts "isExperimentalMode": false is present.
• session_config_is_experimental_mode_omitted_when_none — asserts the key is absent.
• resume_session_config_is_experimental_mode_serializes_when_set
• resume_session_config_is_experimental_mode_omitted_when_none

Validation

cargo fmt, cargo clippy --all-targets, and cargo build are clean. Full lib test suite passes (151 passed; 0 failed), including the 4 new tests.

No schema codegen needed — this --server path is hand-written.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

Adds per-session control of the Copilot CLI experimental feature-flag tier to the Rust SDK by threading an optional is_experimental_mode: Option<bool> through the user-facing config types into the JSON-RPC session.create / session.resume wire payloads (camelCased as isExperimentalMode and omitted when None for backwards compatibility).

Changes:

• Add is_experimental_mode: Option<bool> to SessionCreateWire and SessionResumeWire with skip_serializing_if = "Option::is_none".
• Add is_experimental_mode: Option<bool> to SessionConfig and ResumeSessionConfig, wire it through defaults/debug/into_wire, and add with_is_experimental_mode(bool) builders.
• Add unit tests asserting isExperimentalMode serializes when set and is omitted when None for both create and resume paths.

Show a summary per file

File	Description
rust/src/wire.rs	Extends create/resume JSON wire structs to include optional isExperimentalMode when provided.
rust/src/types.rs	Exposes is_experimental_mode on public config types, maps it into wire payloads, and adds serialization/omission tests.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[stephentoub]

We will want to add this to all 6 SDKs, not just Rust.