Evolution of agentic surfaces — Gagan Bhat & Isabella Kai He, Anthropic
Anthropic’s harness fixes for Sonnet 4.5 became harmful under Opus 4.5. Treat model-specific scaffolding as replaceable, and keep sessions durable enough to recover context and tool failures.
**Sonnet 4.5** sometimes ended work early near its context limit, so Anthropic added context resets. With **Opus 4.5**, that behavior disappeared; the resets instead added latency and could discard cache state incorrectly.
Keep harness components independently replaceable and retest every workaround when changing models. Persist session events outside the active context window so agents can restore discarded context, resume runs, and recover from tool failures.
**Sonnet 4.5** sometimes ended work early near its context limit, so Anthropic added context resets. With **Opus 4.5**, that behavior disappeared; the resets instead added latency and could discard cache state incorrectly. Keep harness components independently replaceable and retest every workaround when changing models. Persist session events outside the active context window so agents can restore discarded context, resume runs, and recover from tool failures. Claude managed agents shift loops, memory, and observability toward Anthropic while leaving context management and domain tools customizable. That reduces maintenance, but also places more behavior inside a managed surface that will continue changing with model releases.
This makes model upgrades an explicit harness-maintenance event: a context workaround can become pure latency or corrupt recovery assumptions when model behavior changes. It strengthens the candidates’ durable-state and deliberate-compaction patterns, but narrows them by showing that resets are not universally beneficial. The stable investment is replaceable controls plus external session history, not any fixed context policy.