Claude's API now swaps agent tools mid-conversation, cache intact
Anthropic's platform shipped three agent-builder upgrades this week: dynamic toolsets, default refusal fallbacks, and Managed Agent sessions you can pre-seed.
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Swap tools mid-turn, keep your cache
Add or remove tools between conversation turns without invalidating the prompt cache. The beta works on Opus 5, Opus 4.8, Fable 5 and Mythos 5 behind the `mid-conversation-tool-changes-2026-07-01` header. Dynamic tool retrieval (loading only the tools a turn needs) no longer forces you to re-prime and re-pay for your whole context each turn.
Refusal fallbacks get a "default" mode
The `fallbacks` parameter now accepts `"default"`, auto-applying Anthropic's recommended fallback models by refusal category (header `server-side-fallback-2026-07-01`). A refusal can reroute to a backup model server-side, instead of you catching the error and retrying client-side.
Managed Agents: seed a session, tune its effort
Boot a session with up to 50 `initial_events` (user messages or defined outcomes) on `POST /v1/sessions`, set an effort level per agent, and get webhooks covering environment and memory-store lifecycle. You can now start an agent already holding prior context and dial its reasoning spend up or down.
Session threads now stream event deltas
Thread-level streams (`GET /v1/sessions/{id}/threads/{tid}/stream`) accept the same `event_deltas[]` query param as the session-level stream. Per-thread agent UIs get lean incremental payloads instead of full event snapshots on every poll.