`claude-agent-acp` (since v0.6.0 / PR #91) accepts `_meta.systemPrompt:
{append: text}` on `session/new` to append to the adapter's native
preset while keeping its tool-use prompt intact — the same non-standard
extension pattern as `_session/steering` was before it was standardised.
## What changes
**Rust (`crates/buzz-acp/`)**
- Adds `SystemPromptTransport` enum to `acp.rs`: `Field(&str)` (ACP
protocol v2, unchanged) vs `ClaudeMeta(&str)` (new `_meta.systemPrompt:
{append: text}`). When both `ClaudeMeta` and `session_title` are present
the two `_meta` members are merged into one object so neither clobbers
the other.
- Gates on exact adapter identity
`@agentclientprotocol/claude-agent-acp` in `pool.rs`:
`session_new_system_prompt()` routes that name to `ClaudeMeta`
regardless of reported `protocolVersion` (CC declares v1).
`has_system_prompt_support()` gains the same name check so user-message
`[Base]`/`[System]` framing is suppressed for CC sessions.
- All other paths — goose post-hoc method, protocol-v2 `Field`, legacy
user-message framing — are byte-identical to before.
**Desktop (`desktop/src/features/agents/ui/`)**
- `agentSessionTranscript.ts`: the `session/new` extractor now checks
`params._meta.systemPrompt.append` as a fallback when bare
`params.systemPrompt` is absent. Bare field takes precedence. Net line
count stays at 1173 (ratchet limit).
- `agentSessionTranscript.test.mjs`: two new tests — one verifying the
`_meta` transport produces the identical standalone card (same five
sections, same `turnId: null`, same placement before the first turn) as
the bare-field transport; one proving bare field wins when both
transports are present.
## Gate claim
`@agentclientprotocol/claude-agent-acp` implies `_meta.systemPrompt`
support because the feature landed in v0.6.0 (Oct 2025, commit
`ea796f3`) before the `@zed-industries/claude-code-acp` →
`@agentclientprotocol/claude-agent-acp` package rename (Mar 2026, commit
`b409782`). The new name is therefore a reliable capability gate; the
old name falls through to the protocol-version gate (status quo, no
regression).
## Tests
- Rust: Claude append serialization; `_meta` coexistence with
`sessionTitle`; protocol-v2 bare field byte-identical; codex/old-zed
omission; claude-name support/suppression gate; old `@zed-industries`
name falls through to protocol-version gate.
- Desktop: `_meta` transport → identical standalone card; bare field
wins over `_meta` when both present.
## Pre-existing failures
`just mobile-check` and `just mobile-test` fail identically on clean
`origin/main` (5 `compose_bar` / `channels_page` tests + 3 Flutter lint
warnings) — not caused by this change. All other `just ci` jobs are
green.
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: npub1mn7jgtj4w2pd0g0zeuhxsa6jy6p0rewxz4kujt98my82ahfmp72sxjexk7 <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>
