Summary
The Anthropic provider already supports per-part cache_control everywhere — including system blocks: toPrompt in providers/anthropic/anthropic.go reads GetCacheControl(part.Options()) for each system TextPart (falling back to msg.ProviderOptions on the last part), so a multi-part system message can pin a cache breakpoint exactly where the stable prefix ends.
But that capability is unreachable through the Agent API: agentSettings.systemPrompt is a plain string (agent.go), so the system prompt always becomes a single text part with no way to attach per-part provider options. The only lever available on an AgentCall/AgentStreamCall is call-level ProviderOptions, which can't express "cache the first N% of the system prompt, keep the tail volatile."
Why it matters
The classic Anthropic prompt-caching layout is a large stable system prefix (instructions, rubric, reference material) plus a small volatile suffix (recent context). Apps with that shape currently have to choose between the Agent conveniences and effective prefix caching — or drop down to the LanguageModel layer and build the prompt by hand.
Suggested shape (whatever fits the API's direction)
agent.WithSystemPromptParts(parts ...fantasy.TextPart) (or accept a pre-built system Message), letting callers attach ProviderOptions/part options carrying anthropic.ProviderCacheControlOptions — the provider plumbing already honors them; or- a lighter
WithSystemPromptCacheBoundary(offset int) style hint the Anthropic provider translates into a two-block system message.
Related precedent: #302 added per-message cache markers for OpenAI-compatible providers — this is the same need aimed at the system prompt through the Agent path.
Happy to take a swing at a PR if one of these directions sounds right to you.
Summary
The Anthropic provider already supports per-part
cache_controleverywhere — including system blocks:toPromptinproviders/anthropic/anthropic.goreadsGetCacheControl(part.Options())for each systemTextPart(falling back tomsg.ProviderOptionson the last part), so a multi-part system message can pin a cache breakpoint exactly where the stable prefix ends.But that capability is unreachable through the Agent API:
agentSettings.systemPromptis a plainstring(agent.go), so the system prompt always becomes a single text part with no way to attach per-part provider options. The only lever available on anAgentCall/AgentStreamCallis call-levelProviderOptions, which can't express "cache the first N% of the system prompt, keep the tail volatile."Why it matters
The classic Anthropic prompt-caching layout is a large stable system prefix (instructions, rubric, reference material) plus a small volatile suffix (recent context). Apps with that shape currently have to choose between the Agent conveniences and effective prefix caching — or drop down to the
LanguageModellayer and build the prompt by hand.Suggested shape (whatever fits the API's direction)
agent.WithSystemPromptParts(parts ...fantasy.TextPart)(or accept a pre-built systemMessage), letting callers attachProviderOptions/part options carryinganthropic.ProviderCacheControlOptions— the provider plumbing already honors them; orWithSystemPromptCacheBoundary(offset int)style hint the Anthropic provider translates into a two-block system message.Related precedent: #302 added per-message cache markers for OpenAI-compatible providers — this is the same need aimed at the system prompt through the Agent path.
Happy to take a swing at a PR if one of these directions sounds right to you.