Anthropic Prompt Caching: Fix SYSTEM_ONLY cache strategy to not expli…

[sobychacko]

…citly cache tools

This commit fixes a bug where the SYSTEM_ONLY cache strategy was incorrectly placing cache_control on tool definitions, making it functionally identical to SYSTEM_AND_TOOLS.

Problem:

• SYSTEM_ONLY and SYSTEM_AND_TOOLS both placed cache breakpoints on tools AND system messages, using 2 breakpoints
• This violated the documented behavior and wasted breakpoints
• The strategies were functionally identical

Root Cause:

• CacheEligibilityResolver.resolveToolCacheControl() checked if eligible message types included SYSTEM, which both strategies have
• It should have checked the strategy directly instead

Solution:

• Changed resolveToolCacheControl() to use allowlist approach
• Only SYSTEM_AND_TOOLS and CONVERSATION_HISTORY explicitly cache tools
• SYSTEM_ONLY now places cache_control only on system message
• Tools still get cached implicitly via Anthropic's cache hierarchy (tools → system → messages)

Impact:

• SYSTEM_ONLY: Uses 1 breakpoint (system only) instead of 2
• SYSTEM_AND_TOOLS: Uses 2 breakpoints (last tool + system) as before
• Users can now optimize breakpoint usage more effectively

Changes:

• Fix CacheEligibilityResolver.resolveToolCacheControl() to check strategy directly instead of eligible message types
• Enhance testSystemOnlyCacheStrategy() to include tools and verify they don't have cache_control
• Update toolCacheControlRespectsStrategy() test to verify all strategies behave correctly

Thank you for taking time to contribute this pull request!
You might have already read the contributor guide, but as a reminder, please make sure to:

• Add a Signed-off-by line to each commit (git commit -s) per the DCO
• Rebase your changes on the latest main branch and squash your commits
• Add/Update unit tests as needed
• Run a build and make sure all tests pass prior to submission

For more details, please check the contributor guide.
Thank you upfront!

[sobychacko]

@adase11 I made some slight semantic changes here. Could you take a look and see what you think?

[adase11]

@adase11 I made some slight semantic changes here. Could you take a look and see what you think?

Can do, sorry I am traveling until next week but I'll try to get to it in a timely manner. I also saw your other question and am giving it some thought. Thanks!