ADR 0003: Stream via a delta callback, not an AsyncSequence wrapper

Date: 2026-09-17. Status: accepted.

Context

The first draft exposed streaming as AsyncThrowingStream<String, Error> built by spawning a Task that iterated session.streamResponse(to:). Under Swift 6 strict concurrency the compiler rejected it: the closure captured the non-Sendable LanguageModelSession, risking a data race.

Decision

Agent.stream(_:onDelta:) is an async throws method that iterates the framework’s stream on the caller’s task and invokes a synchronous callback per delta. It returns the final text.

Consequences

• No @unchecked Sendable or nonisolated(unsafe) anywhere, keeping the strict-concurrency guarantee intact.
• Callers that want an AsyncSequence can build one on their own actor; the library does not force a concurrency model on them.
• The framework’s snapshots are cumulative, so Agent computes deltas by prefix comparison.