ADR 0005: keep Harn's A2A adapter; do not adopt a2a-lf

Status

Accepted on 2026-08-06 for #6089. Harn keeps the bespoke A2A server and client under crates/harn-serve/src/adapters/a2a/ and crates/harn-vm/src/a2a/. The pinned schema at conformance/protocols/schemas/a2a-0.3.0.schema.json remains the contract of record.

Context

#6072 cut MCP over to the official rmcp SDK. The same question was open for A2A. Harn currently carries about 4,900 non-test lines of A2A adapter and client code (about 8,700 with tests).

The official Rust SDK lives at a2aproject/a2a-rs and publishes under -lf crate names. Verified on crates.io on 2026-08-06:

Crate	Version	Role
a2a-lf	0.3.0	Core types (a2a::VERSION = "1.0")
a2a-server-lf	0.4.1	Async server framework
a2a-client-lf	0.2.1	Async client

The crates.io name a2a-rs is a different, community crate (emillindfors/a2a-rs). Any future adoption must pin the -lf package names and say why in the manifest.

Harn's public A2A surface is A2A protocol 0.3.0, matching the upstream JSON schema pinned with provenance to https://a2a-protocol.org/v0.3.0/specification.

Decision

Do not cut the A2A adapter over to a2a-lf, a2a-server-lf, or a2a-client-lf on current evidence.

Do not adopt a2a-lf for types only. A types-only import still fails the falsifier below: SDK types cannot round-trip Harn's pinned 0.3.0 fixtures without field renaming or a Harn-owned shim.

Keep ownership as it is today:

• Harn owns transport, auth, task lifecycle, streaming, push notifications, workflow extensions, and audit seams.
• The pinned 0.3.0 schema and conformance fixtures under conformance/protocols/ own the wire contract.

Revisit when either of these is true:

1. a2a-lf (or a successor under the same official project) exposes A2A 0.3.0 wire shapes that deserialize Harn's pinned fixtures without a shim, or
2. Harn deliberately moves its public A2A pin to the SDK's protocol generation and regenerates conformance against that pin.

Until then, do not add the -lf crates as workspace dependencies.

Falsifier

Cutover, including types-only adoption, is justified only if a conformance run against the pinned upstream A2A 0.3.0 schema passes using unmodified SDK types with no Harn-side field renaming or shim. If SDK types need a fork or adapter layer to speak Harn's wire, ADR 0003's earlier MCP lesson applies and the bespoke adapter stays.

Evidence

A throwaway probe depended on a2a = { package = "a2a-lf", version = "0.3.0" } and tried to deserialize Harn's committed A2A fixtures. Observed on 2026-08-06:

Probe	Result
a2a::VERSION	"1.0", not Harn's pin "0.3.0"
SDK Message serialize	role: "ROLE_USER", field messageId
Harn fixture agent_card.valid.json → SDK AgentCard	missing field supportedInterfaces
Harn fixture task_and_stream.valid.json message → SDK Message	Part must have one of: text, raw, url, data (0.3.0 file parts use { "type": "file", "file": ... })
Harn status "working" → SDK TaskState	unknown variant; SDK expects TASK_STATE_WORKING

Material shape gaps between Harn's 0.3.0 pin and a2a-lf 0.3.0:

• Protocol generation: 0.3.0 vs SDK 1.0.
• Roles: user / agent vs ROLE_USER / ROLE_AGENT.
• Task states: working vs TASK_STATE_WORKING.
• Message identity: id vs messageId.
• Parts: typed 0.3.0 parts (type + file / data) vs SDK field-presence parts (text / raw / url / data).
• Agent card: preferredTransport + additionalInterfaces[].transport vs SDK supportedInterfaces[].protocolBinding.

Server and client crates were not evaluated further. Types already fail the falsifier, and those crates remain pre-1.0 with lower adoption than the MCP SDK path that justified ADR 0003.

Consequences

• No dependency on a2a-lf, a2a-server-lf, or a2a-client-lf.
• Continue refreshing the pinned schema from the upstream 0.3.0 source named in x-harn-provenance when the contract changes.
• Treat crates.io name a2a-rs as a supply-chain hazard in reviews: the official package names end in -lf.
• The companion ACP evaluation (#6088) stays independent; this ADR does not decide ACP ownership.

See MCP, ACP, and A2A integration for the public A2A server contract.