swift-acp

A test-driven, fully-conformant Swift implementation of the Agent Client Protocol (ACP) — the JSON-RPC standard that connects any editor/host to any agent.

Conformance target: schema crate 0.13.6 / protocol v1 (the latest stable). Every type is proven against the authoritative wire schema, the reference crate's serialization vectors, and the method registry — not assumed.

Why

ACP's core (session/prompt → streamed session/update of plans, thoughts, tool calls, and permission requests → session/cancel) is a domain-agnostic contract for a host observing and steering a single agent's working session. The file-system and terminal capabilities are optional client-side bolt-ons, so this is just as usable as the progress/control plane for a non-coding agent as for a coding one. The transport is a swappable boundary: in-process the messages cross as Swift values with no serialization; over stdio they are JSON-RPC.

Targets

Target	Role
`ACPJSONRPC`	JSON-RPC 2.0 envelope (`RequestId`, `RPCError`, `ErrorCode`, `JSONValue`), decoupled from ACP
`ACPCore`	The 135 ACP v1 `$defs` as Codable value types — sum-typed unions, open string enums, `unknown` cases for forward-compat
`ACPAgent`	The agent-role contract (`protocol ACPAgent`)
`ACPClient`	The client/host-role contract (`protocol ACPClient`)
`ACPTransport`	`InProcessConnection` (typed, no serialization) + `StdioTransport`/`AgentConnection` (JSON-RPC) + `StreamingSessionClient`

In-process (no serialization)

import ACPTransport

let connection = InProcessConnection { client in
    MyResearchAgent(client: client)   // the agent reports progress via `client`
}

Task {
    for await update in connection.updates {   // the progress channel a UI renders
        render(update)
    }
}

let response = try await connection.agent.prompt(promptRequest)
connection.finish()

Over stdio (interop)

let connection = AgentConnection(transport: StdioTransport())
await connection.start { client in MyAgent(client: client) }
try await connection.run()   // serves the agent to any ACP client over stdin/stdout

Conformance

The ACPConformanceTests suite enforces three independent guarantees against the version-pinned spec vendored under Tests/ACPConformanceTests/Spec/v1:

Schema coverage — every one of the 135 $defs maps to exactly one modelled type.
Golden round-trip — 30 wire samples harvested verbatim from the reference crate's #[test] assertions decode and re-encode losslessly (field-level conformance).
Method parity — the Swift method-name tables equal meta.json exactly.

swift test

Apache-2.0, matching the reference protocol.