feat(system-prompt): cache dynamic policy context

packages/core/agent-loop/src/agent.ts

@@ -34,6 +34,7 @@ import {
   LlmError,
   assertNever,
   createAssistantMessage,
+  createUserMessage,
   deepFreeze,
   errorChain,
   freezeMessage,
@@ -45,7 +46,7 @@ import {
 import type { GenerateOptions, LlmCallConfig, LlmFailure, Message, PreparedLlmCall, ResolvedRetryPolicy } from '@deepseek-ai/dsh-llm'
 import { canonicalHeader, headerEquals } from '@deepseek-ai/dsh-session'
 import type { AssistantMessage, Session, SessionId, TurnEndReason, TurnTrigger, UserMessage } from '@deepseek-ai/dsh-session'
-import { renderPrompt } from '@deepseek-ai/dsh-system-prompt'
+import { renderContextSnapshot, renderPrompt } from '@deepseek-ai/dsh-system-prompt'
 import type {} from '@deepseek-ai/dsh-tools'
 import { executeToolCalls } from './tool-calls.ts'
 
@@ -54,6 +55,34 @@ type StepOutcome =
   | { kind: 'completed'; continueTurn: boolean; concluded: boolean; maxTokens: boolean }
   | { kind: 'request-failed'; error: RequestError; failure: LlmFailure; retryPolicy: ResolvedRetryPolicy | undefined }
 
+const RUNTIME_CONTEXT_SOURCE = '@deepseek-ai/dsh-system-prompt'
+const CLEARED_RUNTIME_CONTEXT = 'Current runtime context: none. Earlier runtime-context snapshots no longer apply.'
+
+/** Latest retained cache-safe runtime-context snapshot, excluding compacted-away history. */
+function retainedRuntimeContext(session: Session): string | undefined {
+  for (const message of [...session.deriveMessages()].reverse()) {
+    if (message.role !== 'user'
+      || message.source.kind !== 'plugin'
+      || message.source.plugin !== RUNTIME_CONTEXT_SOURCE) continue
+    const [block] = message.content
+    if (message.content.length === 1 && block?.type === 'text') return block.text
+    return undefined
+  }
+  return undefined
+}
+
+/** Append a full current snapshot only when it changed or compaction removed it. */
+function materializeRuntimeContext(session: Session, current: string): void {
+  const previous = retainedRuntimeContext(session)
+  if (previous === undefined && current.length === 0) return
+  const snapshot = current.length === 0 ? CLEARED_RUNTIME_CONTEXT : current
+  if (previous === snapshot) return
+  session.append('user/message', createUserMessage({
+    content: [{ type: 'text', text: snapshot }],
+    source: { kind: 'plugin', plugin: RUNTIME_CONTEXT_SOURCE },
+  }), { surfaceOp: 'append' })
+}
+
 /**
  * The concrete {@link Agent}: each `run()` owns one turn and repeats model
  * steps while tools or steering require another request.
@@ -511,10 +540,13 @@ export class ReactLoopAgent implements Agent {
     // this request together.
     this.drainOutbox(turn)
 
-    // Assemble the system prompt fresh each step (it may depend on log state).
+    // Assemble request-owned prompt inputs fresh each step. Dynamic context is
+    // committed at the tail before deriving history, preserving the stable
+    // system/history cache prefix while keeping every model-visible byte logged.
     const assembly = await this.loopCtx.systemPrompt.assemble(assembleContextFor(this, signal))
     signal.throwIfAborted()
     const system = renderPrompt(assembly)
+    materializeRuntimeContext(session, renderContextSnapshot(assembly))
 
     // Snapshot the exact log prefix: the reconstruction boundary. Appends
     // after this synchronous snapshot join the next request.