diff --git a/lib/engine/prompts.ts b/lib/engine/prompts.ts
index ef4e358..42bbfd9 100644
--- a/lib/engine/prompts.ts
+++ b/lib/engine/prompts.ts
@@ -28,22 +28,55 @@ import type {
 //  the bible looks identical to every agent that consumes it.
 // ──────────────────────────────────────────────────────────────────────
 
+// ── Story bible — split spine / dynamic for prefix-cache friendliness ──
+//
+// SPINE = Architect-set, never updated by Writer's storyStatePatch:
+//   logline / genreTags / protagonist / castNotes
+//   → goes in the STABLE PREFIX of every Writer user message
+//
+// DYNAMIC = patched every scene by the Writer:
+//   synopsis / relationships / openThreads / nextHook
+//   → goes in the DYNAMIC SUFFIX
+//
+// Keep both sections present even when empty (固定 section) so position is
+// stable across calls — a missing section here would shift every byte after
+// it and torch the cache.
+
+export function renderStoryStateSpine(s: StoryState | undefined): string {
+  const lines: string[] = ["【故事档案 · 主轴（不变）】"];
+  lines.push(`主线（中心钩子）：${s?.logline ?? "（未设定）"}`);
+  lines.push(`题材基调：${s?.genreTags ?? "（未设定）"}`);
+  lines.push(`主角「你」：${s?.protagonist ?? "（未设定）"}`);
+  lines.push(`核心配角：${s?.castNotes ?? "（未设定）"}`);
+  return lines.join("\n");
+}
+
+export function renderStoryStateDynamic(s: StoryState | undefined): string {
+  const lines: string[] = ["【故事档案 · 当前状态（每幕更新）】"];
+  lines.push(`已发生（梗概）：${s?.synopsis ?? "（暂无）"}`);
+  lines.push(
+    `当前关系/情绪：${
+      s?.relationships?.length
+        ? "\n" + s.relationships.map((r) => `- ${r}`).join("\n")
+        : "（暂无）"
+    }`,
+  );
+  lines.push(
+    `未收的悬念/伏笔：${
+      s?.openThreads?.length
+        ? "\n" + s.openThreads.map((t) => `- ${t}`).join("\n")
+        : "（暂无）"
+    }`,
+  );
+  lines.push(`接下来要往哪走（下一个钩子方向）：${s?.nextHook ?? "（暂无）"}`);
+  return lines.join("\n");
+}
+
+// Back-compat for the Architect's own user message (it sees the full bible
+// at session start, no caching concern there yet).
 export function renderStoryState(s: StoryState | undefined): string {
   if (!s) return "";
-  const lines: string[] = ["【故事档案 / 主线记忆】"];
-  if (s.logline) lines.push(`主线（中心钩子）：${s.logline}`);
-  if (s.genreTags) lines.push(`题材基调：${s.genreTags}`);
-  if (s.protagonist) lines.push(`主角「你」：${s.protagonist}`);
-  if (s.castNotes) lines.push(`核心配角：\n${s.castNotes}`);
-  if (s.synopsis) lines.push(`已发生（梗概）：${s.synopsis}`);
-  if (s.relationships?.length) {
-    lines.push(`当前关系/情绪：\n${s.relationships.map((r) => `- ${r}`).join("\n")}`);
-  }
-  if (s.openThreads?.length) {
-    lines.push(`未收的悬念/伏笔：\n${s.openThreads.map((t) => `- ${t}`).join("\n")}`);
-  }
-  if (s.nextHook) lines.push(`接下来要往哪走（下一个钩子方向）：${s.nextHook}`);
-  return lines.join("\n");
+  return renderStoryStateSpine(s) + "\n\n" + renderStoryStateDynamic(s);
 }
 
 // ──────────────────────────────────────────────────────────────────────
@@ -272,74 +305,127 @@ sceneKey 设计原则（重要 — 用于跨场景视觉一致性）：
 
 不要输出 JSON 以外的任何文本。`;
 
+// Render one history entry as a stable, position-independent block. Used by
+// the Writer to dump both "completed past" (stable prefix) and "the entry the
+// player just finished" (dynamic suffix) — same format, so the model sees a
+// uniform history surface.
+function renderHistoryEntry(
+  entry: Session["history"][number],
+  index: number,
+): string {
+  const lines: string[] = [`【场景 ${index}】`];
+  if (entry.scene.sceneKey) lines.push(`  sceneKey: ${entry.scene.sceneKey}`);
+
+  const visited = entry.visitedBeatIds.length
+    ? entry.visitedBeatIds
+    : [entry.scene.entryBeatId];
+  const beatById = new Map(entry.scene.beats.map((b) => [b.id, b]));
+  const visitedBeats = visited
+    .map((id) => beatById.get(id))
+    .filter((b): b is NonNullable<typeof b> => Boolean(b));
+
+  for (const b of visitedBeats) {
+    const fragments: string[] = [];
+    if (b.narration) fragments.push(`旁白：${b.narration}`);
+    if (b.line) fragments.push(`${b.speaker ?? "?"}：${b.line}`);
+    if (fragments.length) lines.push("  " + fragments.join(" / "));
+  }
+
+  if (entry.exit) {
+    if (entry.exit.kind === "choice") {
+      lines.push(
+        `  玩家最终选择：${entry.exit.label}（去往：${entry.exit.nextSceneSeed}）`,
+      );
+    } else {
+      lines.push(`  玩家自由动作：${entry.exit.action}`);
+    }
+  }
+  return lines.join("\n");
+}
+
 export function buildWriterUserMessage(session: Session): string {
+  // ─── STABLE PREFIX ────────────────────────────────────────────────────
+  // Everything in this section is invariant across consecutive Writer calls
+  // within the session (or monotonically grows in a way that keeps the
+  // earlier bytes byte-identical). Always emit every section header — even
+  // when empty — so positions don't shift between calls.
+  //
+  // Order optimized for DeepSeek/MiMo prefix caching (64-token chunks):
+  //   1. session-immutable scalars (world / style)
+  //   2. story bible spine (Architect-set, never patched)
+  //   3. monotonically-growing lists (characters, sceneKeys)
+  //   4. history entries 0..N-2 (the last entry is what THIS call must
+  //      react to, so it lives in the dynamic suffix instead)
+  //
+  // ─── DYNAMIC SUFFIX ───────────────────────────────────────────────────
+  // Everything below changes on (almost) every call:
+  //   5. story bible dynamic patch (synopsis/threads/relationships/nextHook)
+  //   6. the just-completed entry (history[-1]) — same render format as the
+  //      stable history blocks, just preceded by a "just completed" header
+  //   7. last-beat snippet (the exact emotional cliffhanger)
+  //   8. lastExit hint
+  //   9. format reminder tail
+
   const parts: string[] = [];
 
-  const bible = renderStoryState(session.storyState);
-  if (bible) {
-    parts.push(bible);
-    parts.push("");
-  }
-
+  // ── 1. session scalars ────────────────────────────────────────────────
   parts.push(`世界观：${session.worldSetting}`);
   parts.push(`画风：${session.styleGuide}`);
+  parts.push("");
 
-  if (session.characters.length > 0) {
-    parts.push("\n已登记角色（speaker 必须用这些名字之一，或本场景新引入）：");
-    for (const c of session.characters) {
-      parts.push(`- ${c.name}`);
-    }
-  }
+  // ── 2. story bible — spine only (stable) ──────────────────────────────
+  parts.push(renderStoryStateSpine(session.storyState));
+  parts.push("");
 
-  const priorKeys = collectPriorSceneKeys(session);
-  if (priorKeys.length > 0) {
-    parts.push("\n已使用的 sceneKey（同一物理空间请沿用，不要新造）：");
-    for (const k of priorKeys) parts.push(`- ${k}`);
-  }
+  // ── 3a. registered characters ─────────────────────────────────────────
+  // SENTINEL pattern: header + a constant "after this line, entries follow"
+  // marker, then the entries themselves. The marker is byte-identical even
+  // when the list is empty, so adding a character only ever APPENDS bytes
+  // — earlier bytes never shift. Crucial for prefix caching: a placeholder
+  // like "（暂无）" that gets replaced by entries breaks the prefix the
+  // moment the first character is registered.
+  parts.push("已登记角色（speaker 必须用这些名字之一，或本场景新引入）：");
+  parts.push("（以下每行一个已登记角色，开场前为空。）");
+  for (const c of session.characters) parts.push(`- ${c.name}`);
+  parts.push("");
 
-  if (session.history.length === 0) {
-    parts.push(
-      "\n这是故事的开场。请按【故事档案】里的 nextHook 把第一幕的冷开场写出来——开场即抓人，别花笔墨铺垫世界观。写完后更新 storyStatePatch。严格以 JSON 格式返回。",
-    );
-    return parts.join("\n");
-  }
+  // ── 3b. prior sceneKeys (sentinel pattern, same rationale) ────────────
+  parts.push("已使用的 sceneKey（同一物理空间请沿用，不要新造）：");
+  parts.push("（以下每行一个已用过的 sceneKey，开场前为空。）");
+  for (const k of collectPriorSceneKeys(session)) parts.push(`- ${k}`);
+  parts.push("");
 
-  parts.push("\n场景历史（按时间顺序）：");
-  session.history.forEach((entry, idx) => {
-    const lines: string[] = [`【场景 ${idx + 1}】`];
-    if (entry.scene.sceneKey) lines.push(`  sceneKey: ${entry.scene.sceneKey}`);
-
-    const visited = entry.visitedBeatIds.length
-      ? entry.visitedBeatIds
-      : [entry.scene.entryBeatId];
-    const beatById = new Map(entry.scene.beats.map((b) => [b.id, b]));
-    const visitedBeats = visited
-      .map((id) => beatById.get(id))
-      .filter((b): b is NonNullable<typeof b> => Boolean(b));
-
-    for (const b of visitedBeats) {
-      const fragments: string[] = [];
-      if (b.narration) fragments.push(`旁白：${b.narration}`);
-      if (b.line) fragments.push(`${b.speaker ?? "?"}：${b.line}`);
-      if (fragments.length) lines.push("  " + fragments.join(" / "));
-    }
-
-    if (entry.exit) {
-      if (entry.exit.kind === "choice") {
-        lines.push(
-          `  玩家最终选择：${entry.exit.label}（去往：${entry.exit.nextSceneSeed}）`,
-        );
-      } else {
-        lines.push(`  玩家自由动作：${entry.exit.action}`);
-      }
-    }
-    parts.push(lines.join("\n"));
+  // ── 4. history[0..N-2] — ARCHIVED entries (sentinel, append-only) ─────
+  // CRITICAL: only the ALREADY-ARCHIVED entries (i.e. everything except
+  // history[-1]) go in the stable prefix. The last entry is still "live":
+  // its visitedBeatIds keeps growing as the player walks more beats in the
+  // current scene, and speculative prefetch triggers Writer calls that
+  // observe different snapshots of history[-1] mid-scene. Putting the live
+  // entry in the stable prefix would corrupt every Writer call's cache.
+  //
+  // Archived entries (history[0..N-2]) are immutable — once a scene is
+  // exited, its visitedBeatIds + exit are frozen. Safe to cache.
+  const archivedHistory = session.history.slice(0, -1);
+  parts.push("场景历史（按时间顺序，已完结）：");
+  parts.push("（以下每段一幕已完结的场景，开场前为空。）");
+  archivedHistory.forEach((entry, idx) => {
+    parts.push(renderHistoryEntry(entry, idx + 1));
   });
+  parts.push("");
 
+  // ════════════════ DYNAMIC SUFFIX 从这里开始 ═══════════════════════════
+  // 上面 ~95% 的 prompt 长度应该已经稳定可缓存。下面每次调用都会变化。
+
+  // ── 5. story bible — dynamic patch ────────────────────────────────────
+  parts.push(renderStoryStateDynamic(session.storyState));
+  parts.push("");
+
+  // ── 6. last-beat snippet (the exact emotional cliffhanger) ──
+  // The full last entry is already in the stable history block above; here
+  // we only re-emit the very last beat to sharply focus the Writer on the
+  // emotional moment to continue from. Skip the duplicate full-entry render
+  // that was here previously — it wasted ~200-500 tokens of dynamic suffix.
   const last = session.history.at(-1);
-
-  // The exact last moment the player stopped on — the new scene must continue
-  // seamlessly from this emotional beat, not reset to a neutral state.
   if (last) {
     const lastBeatId = last.visitedBeatIds.at(-1) ?? last.scene.entryBeatId;
     const lastBeat = last.scene.beats.find((b) => b.id === lastBeatId);
@@ -349,12 +435,20 @@ export function buildWriterUserMessage(session: Session): string {
       if (lastBeat.line) frag.push(`${lastBeat.speaker ?? "?"}：${lastBeat.line}`);
       if (frag.length) {
         parts.push(
-          `\n上一刻（玩家停留的最后一个画面，新场景要从这里的情绪无缝承接）：\n  ${frag.join(" / ")}`,
+          `上一刻（玩家停留的最后一个画面，新场景从这里的情绪无缝承接）：\n  ${frag.join(" / ")}`,
         );
       }
     }
   }
 
+  if (session.history.length === 0) {
+    parts.push(
+      "\n这是故事的开场。请按【故事档案】里的 nextHook 把第一幕的冷开场写出来——开场即抓人，别花笔墨铺垫世界观。写完后更新 storyStatePatch。严格以 JSON 格式返回。",
+    );
+    return parts.join("\n");
+  }
+
+  // ── 8. lastExit hint ──────────────────────────────────────────────────
   const lastExit = last?.exit;
   if (lastExit) {
     if (lastExit.kind === "choice") {
@@ -370,6 +464,7 @@ export function buildWriterUserMessage(session: Session): string {
     parts.push("\n无缝续写下一个场景，延续上一刻的情绪。");
   }
 
+  // ── 9. format reminder tail ───────────────────────────────────────────
   parts.push("写完后别忘了更新 storyStatePatch。严格以 JSON 格式返回。");
   return parts.join("\n");
 }
@@ -518,6 +613,22 @@ export const CINEMATOGRAPHER_SYSTEM = `你是视觉小说的「分镜导演」
 
 不要输出 JSON 以外的任何文本。`;
 
+// Stable hint block — invariant across every Cinematographer call in a
+// session. Front-loading this (with the session-scoped styleGuide) gives the
+// prefix cache something substantial to anchor on; without it, the per-scene
+// `sceneSummary` would land in the first content chunk and force the whole
+// user message to miss. Long enough to land beyond the 64-token chunk
+// boundary that follows the system prompt.
+const CINE_STABLE_HINT = [
+  "",
+  "以下为本次场景的输入。请基于这些信息：",
+  "1. 选择最合适的 shotType（依据 system prompt 的动态镜头策略 + entryBeatSpeaker）。",
+  "2. 写一段**只用英文**的 integratedPrompt——纯环境 + 构图 + 角色姿态/位置；服饰由画师另外通过 referenceImages 锁定，你只描述能看到的样貌与镜头。",
+  "3. 若上一场与本场 sceneKey 相同，**强调连续性**（时段/情绪/构图微调），而不是重新设定空间。",
+  "4. 严格按 system prompt 要求的 JSON schema 输出。",
+  "",
+].join("\n");
+
 export function buildCinematographerUserMessage(
   sceneSummary: string,
   styleGuide: string,
@@ -527,38 +638,53 @@ export function buildCinematographerUserMessage(
   currentSceneKey: string | undefined,
 ): string {
   const parts: string[] = [];
-  parts.push(`全局美术画风：${styleGuide}`);
-  parts.push(`\n当前场景（来自编剧）：${sceneSummary}`);
 
+  // ─── STABLE PREFIX ──────────────────────────────────────────────────
+  // styleGuide is session-immutable; CINE_STABLE_HINT is a true constant.
+  // Together they're long enough to cross at least one 64-token chunk
+  // boundary, so every subsequent Cinematographer call in this session can
+  // cache-hit through this block.
+  parts.push(`全局美术画风：${styleGuide}`);
+  parts.push(CINE_STABLE_HINT);
+
+  // ─── DYNAMIC SUFFIX ─────────────────────────────────────────────────
+  // Always emit every section header — even when empty — so positions don't
+  // shift between calls. (Caching of the dynamic section itself isn't
+  // expected, but stable positioning helps when adjacent calls happen to
+  // share a sceneSummary prefix.)
+  parts.push(`当前场景（来自编剧）：${sceneSummary}`);
+  parts.push("");
+
+  parts.push("开场画面里的角色及其姿态：");
   if (entryBeatActive.length > 0) {
-    parts.push("\n开场画面里的角色及其姿态：");
     for (const c of entryBeatActive) {
       parts.push(`- ${c.name}：${c.pose ?? "（无具体姿态描述）"}`);
     }
   } else {
-    parts.push("\n开场画面里没有角色（纯环境）。");
+    parts.push("（无角色，纯环境）");
   }
+  parts.push("");
 
   // entryBeatSpeaker drives the dynamic camera policy (see CINEMATOGRAPHER_SYSTEM).
   // "你" means the player is speaking; an NPC name means an NPC is speaking;
   // empty means no dialog (pure environment / narration beat).
   if (entryBeatSpeaker === "你") {
     parts.push(
-      '\n开场 beat 是**玩家说话**（speaker = "你"）——按动态镜头策略：medium shot，NPC 居中、做听玩家说话的姿态、看向画面外。**绝不要画出玩家**。',
+      '开场 beat 是**玩家说话**（speaker = "你"）——按动态镜头策略：medium shot，NPC 居中、做听玩家说话的姿态、看向画面外。**绝不要画出玩家**。',
     );
   } else if (entryBeatSpeaker) {
     parts.push(
-      `\n开场 beat 是 **${entryBeatSpeaker} 在对玩家说话**（speaker = "${entryBeatSpeaker}"）——按动态镜头策略：close-up 或 medium close-up，${entryBeatSpeaker} 看向画面外（看玩家），眼神交流。`,
+      `开场 beat 是 **${entryBeatSpeaker} 在对玩家说话**（speaker = "${entryBeatSpeaker}"）——按动态镜头策略：close-up 或 medium close-up，${entryBeatSpeaker} 看向画面外（看玩家），眼神交流。`,
     );
   } else {
     parts.push(
-      "\n开场 beat 没有 speaker（纯旁白/环境）——按动态镜头策略：wide establishing shot 展现环境氛围。",
+      "开场 beat 没有 speaker（纯旁白/环境）——按动态镜头策略：wide establishing shot 展现环境氛围。",
     );
   }
 
   if (priorSceneKey && currentSceneKey && priorSceneKey === currentSceneKey) {
     parts.push(
-      `\n注意：上一场和本场 sceneKey 都是 "${currentSceneKey}"——画师会把上一张场景图作为 referenceImages 之一锚定同一空间。你的 integratedPrompt 应该**强调连续性**，描述时段/情绪/构图的细微变化，而不是完全重新设定空间。`,
+      `\n注意：上一场和本场 sceneKey 都是 "${currentSceneKey}"——画师会把上一张场景图作为 referenceImages 之一锚定同一空间。integratedPrompt 应强调连续性。`,
     );
   }