feat: split system prompt into static/dynamic blocks with per-block cache_control#1330
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Split buildSystemPrompt to return SystemPromptBlock[] instead of string,
separating static content (base prompt, instructions, tool policy, tone)
from dynamic content (permission mode, language, env info, auto memory,
MEMORY.md). For Claude models, callAgent maps cacheable blocks with
cache_control: {type: 'ephemeral'} and non-cacheable blocks without it,
so dynamic content changes don't invalidate the static cache.
transformMessagesForExplicitCache idempotency check prevents double-marking.
- Add SystemPromptBlock interface to prompts/index.ts
- Widen CallAgentOptions.systemPrompt to string | SystemPromptBlock[]
- Update 9 existing tests across 3 files for array-based assertions
- Add 4 block structure tests in prompts.test.ts
- Add 3 SystemPromptBlock[] input tests in aiService.cacheControl.test.ts
- Update spec 019 with user story 5, FR-008, edge cases 7-8
- All 235 test files pass (3150 tests)
Follow the agent.noParams.test.ts pattern to avoid CI environment issues: - Add workdir: /tmp/test-system-prompt to isolate from CI filesystem - Add apiKey: test-key for proper agent initialization - Re-create AIManager after Agent.create() to pick up mock ToolManager - Register mock ConfigurationService in container - Clear mockCallAgent call history before sendMessage to ensure mock.calls[0] captures the sendMessage call, not any init call The CI test process was producing zero output (crash/hang), likely due to real MemoryService reading actual files from process.cwd() in CI.
Align with Claude Code: permission mode is handled purely at the tool execution layer (PermissionManager), not in the system prompt. This prevents dynamic block changes when mode switches, preserving prompt cache.
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Summary
Split the system prompt into static/dynamic blocks with per-block
cache_control, mirroring Claude Code's approach where static prompt content getscache_controland survives cache invalidation, while dynamic content does not.Changes
Source
packages/agent-sdk/src/prompts/index.ts: AddedSystemPromptBlockinterface{ text: string; cacheable: boolean }. ChangedbuildSystemPromptto returnSystemPromptBlock[]instead ofstring, splitting into:cacheable: true): base prompt + DOING_TASKS + EXECUTING_ACTIONS + TOOL_POLICY + OUTPUT_EFFICIENCY + TONE_AND_STYLEcacheable: false): permission mode + language + env info + auto memory + MEMORY.mdpackages/agent-sdk/src/services/aiService.ts: WidenedCallAgentOptions.systemPrompttostring | SystemPromptBlock[].callAgentmaps blocks to content parts withcache_controlon cacheable blocks only for Claude models; joins to string for non-Claude models. ExistingtransformMessagesForExplicitCacheidempotency check prevents double-marking.Tests
prompts.test.tsSystemPromptBlock[]input tests inaiService.cacheControl.test.tsagent.systemPrompt.test.ts,aiManager.plan.test.ts,aiManager.test.tsfor array-based assertionsSpec
specs/019-prompt-cache-control.mdwith user story 5, FR-008, edge cases 7-8, andSystemPromptBlockkey entityWhy
Previously, the entire system prompt was a single string. Any change to dynamic content (date, MEMORY.md, permission mode) invalidated the entire cache. By separating static and dynamic content into independent blocks, the static block's cache survives dynamic content changes.