feat(plugins): add appendContext to before_prompt_build hook#193
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dgarson merged 1 commit intodgarson/forkfrom Mar 3, 2026
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Cherry-picked from da43291 with conflict resolution. Conflict resolved in src/agents/pi-embedded-runner/run/attempt.ts: - Added appendContext handling after prependContext - Added legacySystemPrompt override support - Changed systemPromptText from const to let to allow reassignment Original commit message: Plugins returning prependContext from before_prompt_build inject text before the user prompt, which is part of the stable system-prompt prefix and benefits from prompt caching. However, transient or per-turn context (e.g. enforcement nudges, dynamic warnings) inserted via prependContext invalidates the cached prefix for the entire block. Add appendContext as the counterpart: text returned in appendContext is appended after the user prompt, leaving the stable prefix intact and preserving prompt cache hits for the prependContext portion. Merge semantics mirror prependContext — multiple hooks' appendContext values are concatenated with "\n\n" separators. Empty results are coerced to undefined to avoid polluting the return type. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
prependContext, which prepends text before the user prompt. This invalidates the stable system-prompt prefix and breaks prompt caching for that block and everything after it.prependContextportion of the system prompt, even though the rest (role instructions, fleet status, etc.) is stable turn-over-turn.appendContextfield toPluginHookBeforePromptBuildResult. Text returned inappendContextis appended after the user prompt, leaving the stableprependContextprefix intact and preserving prompt cache hits. Merge semantics mirrorprependContext-- multiple hooks' values are concatenated with "\n\n". Empty results are coerced toundefined.prependContextbehavior is unchanged.systemPromptbehavior is unchanged. No existing plugin behavior is affected -- this is purely additive.Change Type (select all)
Scope (select all touched areas)
Linked Issue/PR
None -- discovered during agent-orchestrator enforcement work where memory-search nudges were breaking prompt caching.
User-visible / Behavior Changes
None. This is a new opt-in capability for plugin authors. No existing behavior changes.
Security Impact (required)
Repro + Verification
Steps
{ appendContext: "some transient text" }from abefore_prompt_buildhookprependContextfrom other hooks is unaffectedExpected
appendContexttext appears after the user promptprependContextprefix remains cache-stable across turnsActual
Evidence
attempt.test.tswhich validatesresolvePromptBuildHookResultappendContextfor nudge injection (in consuming branch)Human Verification (required)
appendContextonly,prependContextonly, both, neither. Multiple hooks returningappendContext(merge concatenation).appendContextcoerced toundefined(no empty string pollution). Legacybefore_agent_starthookappendContextmerged correctly.Compatibility / Migration
Failure Recovery (if this breaks)
src/plugins/types.ts,src/plugins/hooks.ts,src/agents/pi-embedded-runner/run/attempt.tsappendContexttext missing from agent prompt (would indicate merge regression)Risks and Mitigations
None -- purely additive type field with no behavioral impact on existing code paths.