Optimize prompt cache hit rate by freezing deferred tool list in initial context

[bhavyaus]

Optimizes Anthropic prompt cache hit rate by moving the dynamic deferred-tool list out of the cached system prompt and into the global agent context (first user message), which is frozen across turns via GlobalContextMessageMetadata. The instructional/guidance prose stays in the system prompt; only the dynamic tool inventory moves.

Also wires up tool-search support for HiddenModelB.

cc: @kevin-m-kent

[Copilot]

Pull request overview

This PR improves Anthropic prompt-cache hit rates by relocating the dynamic deferred-tool inventory from the system prompt into the cached “global agent context” (initial user message), while keeping tool-search guidance in the system prompt. It also enables tool-search prompt support for HiddenModelB.

Changes:

• Split tool-search prompt responsibilities: keep guidance in ToolSearchToolPromptOptimized, and emit the deferred-tool inventory once via DeferredToolListReminder.
• Render the deferred-tool inventory inside GlobalAgentContext so it’s frozen via GlobalContextMessageMetadata across turns.
• Update Anthropic and HiddenModelB prompts + snapshots to reflect the new placement of <availableDeferredTools>.

Show a summary per file

File	Description
extensions/copilot/src/extension/prompts/node/agent/toolSearchInstructions.tsx	Adds DeferredToolListReminder and removes the inline deferred-tool list from the optimized tool-search system prompt.
extensions/copilot/src/extension/prompts/node/agent/agentPrompt.tsx	Emits <availableDeferredTools> from GlobalAgentContext to keep it in the cached initial context.
extensions/copilot/src/extension/prompts/node/agent/anthropicPrompts.tsx	Updates legacy Anthropic tool-search prompt to no longer inline the deferred-tool list and to reference initial context.
extensions/copilot/src/extension/prompts/node/agent/openai/hiddenModelBPrompt.tsx	Adds tool-search guidance to HiddenModelB system prompt + a per-turn reminder gated on supportsToolSearch.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-sonnet-4.6/all_tools.spec.snap	Snapshot updates: <availableDeferredTools> moved from tool-search instructions to global context.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-sonnet-4.6/all_non_edit_tools.spec.snap	Snapshot updates for the same relocation.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-sonnet-4.5/all_tools.spec.snap	Snapshot updates for the same relocation + prose tweaks.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-sonnet-4.5/all_non_edit_tools.spec.snap	Snapshot updates for the same relocation + prose tweaks.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-opus-4.6/all_tools.spec.snap	Snapshot updates: <availableDeferredTools> moved to global context.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-opus-4.6/all_non_edit_tools.spec.snap	Snapshot updates for the same relocation.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-opus-4.5/all_tools.spec.snap	Snapshot updates for the same relocation + prose tweaks.
extensions/copilot/src/extension/prompts/node/agent/test/snapshots/agentPrompts-claude-opus-4.5/all_non_edit_tools.spec.snap	Snapshot updates for the same relocation + prose tweaks.

Copilot's findings

• Files reviewed: 12/12 changed files
• Comments generated: 1