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Durable A2A task results: implementation amendment ​

Audited on 23 October 2026 against the approved LangGraph compatibility plan. All six independent audit-plan dimensions passed after the commitments below were added. This is an additive persistence amendment; it adds no task database, scheduler, deployment policy or Agent Protocol Run wire field.

Gate before adding surface ​

First build an official-client A2A adapter against existing authorized execution/resource records. Send a task, start another turn in the same context, prune history with keep_latest, and retrieve the first task, including after SQLite reopening. Compare its original artifact parts. If that passes, omit the result primitive and keep the projection-only adapter. Do not introduce an adapter cache, Store.items mirror or after-settlement output capture to make the baseline pass.

The verified premise is that Run rows store status/error and execution kwargs, while final values, interrupts and declared reply exist only in the immediate engine outcome. Delivery payloads are cleared after delivery; event replay expires; pruning removes earlier checkpoints. These records cannot be assumed to retain the first task’s output.

Narrow persistence contract if the gate fails ​

  • Export RunResult through core’s existing root surface. Reuse DefaultValues, Interrupt and Checkpoint: { values, interrupts?, reply?, checkpoint? }. Share it with engine outcome and settlement payloads. Status/error remain on Run.
  • Add optional RunRepo.recordResult(runId, result): Promise<void> and getResult(runId): Promise<RunResult | null>. Writes to unknown/deleted runs are no-ops; missing results read as null. Writes are atomic first-write-wins and reads/writes use defensive, wire-normalized JSON copies. Serialization failure fails capture before terminal success.
  • Capture only server-marked A2A runs before the terminal status write. The marker, task lineage and body-to-input transfer stay internal. JSON extras cannot request capture. Rendering follows the effective terminal status so a cancellation winner never appears completed.
  • Store results beside existing runs in memory, SQLite and Postgres. Keep schema upgrades additive; regenerate the compiled Postgres migration asset through Nx. Ordinary run serialization excludes results.
  • Optional snapshot result entries tolerate old snapshots, ignore orphan results, attach only to existing/imported runs and never replace an existing first-written result. Run/thread erasure cascades results. Thread copy reattaches results to new run IDs and remaps lineage/checkpoint thread coordinates.
  • Preserve optional-method absence through wrappers. A2A admission requires both methods before creating any thread/run. Result access requires the same scoped run read/write as other run operations; private lineage grants no authority.
  • Keep the first run ID as task ID. Continuations use existing run admission, queue and cancellation. Resume validates the exact persisted checkpoint and current interrupted run under existing withThreadExecution, using existing saver lookup and checkpoint projection. Reject stale or pruned checkpoints; result values cannot resurrect them.

Evidence and validation ​

The ownership/reuse audits inspected RunRepo, RunOutcome, checkpoint history/projection, @langchain/langgraph-api public storage declarations, A2A SDK TaskStore/DatabaseTaskStore and the existing delivery and StoreRepo contracts. SDK task storage introduces another task authority and uses overwrite semantics; existing kwargs describe execution input. Neither substitutes for this narrow result contract.

The delivery tranche must include shared conformance on all three stores; SQLite real-file reopen and subprocess crash; Postgres migrations/container tests; snapshot/hydrate/restore/dev import; run/thread/rollback erasure and thread copy; scoped result reads/writes; unsupported custom drivers; settlement reentry and cancellation races. Keep the original official-client failing baseline in this report before introducing the primitive.

Baseline gate result ​

On 23 October 2026 the official @a2a-js/sdk 1.3.0 client reproducer failed on both memory and SQLite after reopening. GetTask(firstTaskId) returned {value: "echo: second"} where the original artifact contained {value: "echo: first"}. The test used existing run/thread/checkpoint records only, with no adapter cache or output mirror. Nx command: nx test agent-protocol --args='run src/interop/a2a.test.ts'. Both artifact-equality assertions failed. The result primitive is therefore retained in the implementation.