diff --git a/frontend/src/lib/rectification-agentic/v9/agent-run-messages.ts b/frontend/src/lib/rectification-agentic/v9/agent-run-messages.ts
index 7f51ad49..af17c781 100644
--- a/frontend/src/lib/rectification-agentic/v9/agent-run-messages.ts
+++ b/frontend/src/lib/rectification-agentic/v9/agent-run-messages.ts
@@ -10,6 +10,7 @@ import type { V9CaseDossier } from "./tool-service";
 import { cachedSystemMessage } from "../../agent-generation-settings.ts";
 import { OPENING_COLLECT_DOMAINS } from "../user-copy";
 import { retryConstraintForAttempt } from "./host-fallback";
+import { sliceRectificationSkill } from "./skill-slice";
 import type { V9AgentRunOptions } from "./agent-run";
 
 function clockWindow(range: { start_time?: string | null; end_time?: string | null } | null | undefined): string | null {
@@ -54,9 +55,12 @@ export function buildAgentMessages(
   const timeContext = options.timeContext
     ?? `服务端当前时间（权威）：${new Date().toISOString()}。涉及“现在、今天、今年、未来几个月”等相对时间时，以此为准。`;
   const caseContext = `【服务端 Case ID】${options.caseId}。所有 rectification 工具调用的 caseId 必须原样使用此值。`;
+  // T6 (TASK-rectification-grounding-20260927): an evidence turn gets only
+  // the Skill sections it uses; other actions keep the whole bound body.
+  const boundSkill = sliceRectificationSkill(skillInstructions, options.action).text;
   const bootstrapContent = [
     "【服务器已绑定当前 Case 的精确 Skill】运行器已在本 attempt 内加载并核验下列指令；不要重复调用 skill。第一步必须调用 rectification-read-case。",
-    skillInstructions,
+    boundSkill,
     ...(attempt > 1 ? [retryConstraintForAttempt(previousErrorCode)] : []),
   ].join("\n\n");
   const bootstrap = cachedSystemMessage(bootstrapContent, options.generationModel)
diff --git a/frontend/src/lib/rectification-agentic/v9/skill-slice.ts b/frontend/src/lib/rectification-agentic/v9/skill-slice.ts
new file mode 100644
index 00000000..ba9b0188
--- /dev/null
+++ b/frontend/src/lib/rectification-agentic/v9/skill-slice.ts
@@ -0,0 +1,71 @@
+/**
+ * Per-turn slice of the bound rectification Skill (TASK-rectification-
+ * grounding-20260927 T6).
+ *
+ * The runner used to send the whole bound SKILL.md body (about 10.7K
+ * characters for 10.0.31) on every model call. An evidence turn only uses
+ * §5 (ConversationFocus and intent) and §7 (batch evidence and date
+ * truthfulness); the rest describes the opening, case status table, fields
+ * that only exist in `full_diagnostics` (method_followup_plan,
+ * CaseConversationSummary, guided_collect_windows …), the candidate language
+ * of offer turns, and upstream sync. Other actions keep the whole body.
+ *
+ * The slice is keyed on `## N. <title>` heading titles, not on numbers:
+ * the 9.0.0 snapshot numbers its sections differently (its §5 is the
+ * candidate language). If any wanted title is missing, the whole body is
+ * sent unchanged, so a historical Case still runs with its exact bound Skill
+ * (BUG-621). All 10.0.x snapshots share the 10.0.31 headings.
+ */
+import type { RectificationAgentAction } from "./step-budget";
+
+/** Heading titles per action; evidence = §5 and §7 of the 10.0.x layout. */
+export const RECTIFICATION_SKILL_SECTIONS_BY_ACTION: Readonly<
+  Record<RectificationAgentAction, readonly string[] | "all">
+> = {
+  opening: "all",
+  read_only: "all",
+  evidence: ["ConversationFocus 与意图承接", "批量证据与日期真实性"],
+  rescore: "all",
+  accept: "all",
+  confirm: "all",
+};
+
+export type RectificationSkillSlice = Readonly<{
+  text: string;
+  /** Heading lines sent, or "all" when the whole body went out. */
+  sections: readonly string[] | "all";
+}>;
+
+function headingTitle(line: string): string | null {
+  const match = /^##\s+(?:\d+\.\s*)?(.+?)\s*$/.exec(line);
+  return match ? match[1] : null;
+}
+
+export function sliceRectificationSkill(
+  instructions: string,
+  action: RectificationAgentAction,
+): RectificationSkillSlice {
+  const wanted = RECTIFICATION_SKILL_SECTIONS_BY_ACTION[action];
+  if (wanted === "all") return { text: instructions, sections: "all" };
+  const preamble: string[] = [];
+  const sections: Array<{ title: string; lines: string[] }> = [];
+  for (const line of instructions.split("\n")) {
+    const title = line.startsWith("## ") ? headingTitle(line) : null;
+    if (title !== null) {
+      sections.push({ title, lines: [line] });
+      continue;
+    }
+    if (sections.length > 0) sections.at(-1)!.lines.push(line);
+    else preamble.push(line);
+  }
+  const picked = wanted.map((title) => sections.find((section) => section.title === title));
+  if (picked.some((section) => !section)) return { text: instructions, sections: "all" };
+  const title = preamble.find((line) => line.startsWith("# ")) ?? "";
+  const headings = picked.map((section) => section!.lines[0]!.trim());
+  const note = `（本轮是证据轮：以下只附本轮适用的 Skill 章节「${picked.map((section) => section!.title).join("」「")}」；其余章节不适用于本轮，按服务器投影与工具说明执行。）`;
+  const body = picked.map((section) => section!.lines.join("\n").trim()).join("\n\n");
+  return {
+    text: [title, note, body].filter(Boolean).join("\n\n"),
+    sections: headings,
+  };
+}
diff --git a/frontend/src/mastra/agentic-rectification.ts b/frontend/src/mastra/agentic-rectification.ts
index 13634b97..2ea87710 100644
--- a/frontend/src/mastra/agentic-rectification.ts
+++ b/frontend/src/mastra/agentic-rectification.ts
@@ -1,4 +1,5 @@
 import { Agent } from "@mastra/core/agent";
+import type { Processor } from "@mastra/core/processors";
 import type { ResolvedLanguageModel } from "./model";
 import {
   resolveActiveSkillPackage,
@@ -56,8 +57,30 @@ export function getRectificationV9Agent(
     model: model.model,
     instructions: agenticRectificationInstructions,
     skills: [resolveSkillPackageRuntimePath(skillPackage)],
+    inputProcessors: [rectificationSkillBoundProcessor],
     tools: createRectificationV9AgentTools(ctx),
   });
 }
 
+/**
+ * R4 (TASK-rectification-grounding-20260927): Mastra's default skills
+ * processor added an `<available_skills>` block (with a temporary package
+ * path) and "call the `skill` tool" to every step, contradicting the runner's
+ * "do not call skill" and costing about 1K characters per call.
+ * `providesSkillDiscovery: "on-demand"` declares that the caller owns skill
+ * loading, which is true: the runner loads the exact bound Skill through
+ * `agent.getSkill` and quotes it (sliced per turn) in the bootstrap message.
+ * With this marker Mastra adds neither the system block nor the `skill` /
+ * `skill_search` tools (same mechanism as the consultation agents'
+ * `jyotishSkillBoundProcessor`).
+ */
+export const rectificationSkillBoundProcessor: Processor & { processInputStep: NonNullable<Processor["processInputStep"]> } = {
+  id: "rectification-skill-bound",
+  name: "Rectification Skill Bound",
+  providesSkillDiscovery: "on-demand",
+  processInputStep() {
+    // Nothing to inject: the bound Skill text rides in the runner's bootstrap message.
+  },
+};
+
 export { RECTIFICATION_V9_SKILL_NAME };
diff --git a/frontend/tests/rectification-grounding-prompt-20260927.test.ts b/frontend/tests/rectification-grounding-prompt-20260927.test.ts
new file mode 100644
index 00000000..fee56607
--- /dev/null
+++ b/frontend/tests/rectification-grounding-prompt-20260927.test.ts
@@ -0,0 +1,202 @@
+/**
+ * TASK-rectification-grounding-20260927 T6 (R4 + Skill slicing): what a real
+ * rectification Agent sends the model on an evidence turn. Real
+ * `runV9AgentTurn` + real `getRectificationV9Agent` (real bound Skill, real
+ * tools) + a prompt-recording scripted model; case data is a real local
+ * engine response for a public AA chart (9 candidates).
+ *
+ * Token estimate (for the size table): one token per CJK character plus one
+ * per four other characters (`estimateTokens`). "Fixed overhead" is what every
+ * call of the turn re-sends regardless of tool results: the system messages
+ * plus the tool schemas.
+ */
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import test from "node:test";
+import {
+  CASE_ID,
+  SESSION_ID,
+  TURN_ID,
+  USER_ID,
+  dossierFixture,
+  fakeAccounting,
+  receiptHandlers,
+} from "./rectification-v9-test-support.ts";
+import {
+  AA_EVIDENCE_ROWS,
+  AA_RANGE,
+  AA_TURNS,
+  buildGoldenLatest,
+  contentText,
+  estimateTokens,
+  scriptedModel,
+  setFocusHandler,
+  stubGoldenFetch,
+  type RecordedCall,
+} from "./rectification-grounding-support.ts";
+import { getRectificationV9Agent } from "../src/mastra/agentic-rectification.ts";
+import { runV9AgentTurn } from "../src/lib/rectification-agentic/v9/agent-run.ts";
+import {
+  RECTIFICATION_SKILL_SECTIONS_BY_ACTION,
+  sliceRectificationSkill,
+} from "../src/lib/rectification-agentic/v9/skill-slice.ts";
+import {
+  resolveActiveSkillPackage,
+  resolveExactSkillPackage,
+} from "../src/lib/skill-package-registry.ts";
+import { resolve } from "node:path";
+
+export type CallSize = Readonly<{
+  call: number;
+  systemChars: number;
+  systemTokens: number;
+  toolsBytes: number;
+  toolsTokens: number;
+  toolNames: string[];
+  fixedTokens: number;
+  totalTokens: number;
+}>;
+
+export function callSizes(calls: readonly RecordedCall[]): CallSize[] {
+  return calls.map((call, index) => {
+    const system = call.prompt.filter((message) => message.role === "system").map((message) => contentText(message.content)).join("\n");
+    const tools = JSON.stringify(call.tools ?? []);
+    const all = call.prompt.map((message) => contentText(message.content)).join("\n");
+    return {
+      call: index + 1,
+      systemChars: system.length,
+      systemTokens: estimateTokens(system),
+      toolsBytes: Buffer.byteLength(tools, "utf8"),
+      toolsTokens: estimateTokens(tools),
+      toolNames: Array.isArray(call.tools) ? (call.tools as Array<{ name: string }>).map((tool) => tool.name) : [],
+      fixedTokens: estimateTokens(system) + estimateTokens(tools),
+      totalTokens: estimateTokens(all) + estimateTokens(tools),
+    };
+  });
+}
+
+/** One evidence turn: read-case, then one short body (no write claimed). */
+export async function recordEvidenceTurn() {
+  const { latest, compute } = await buildGoldenLatest();
+  const { model, calls } = scriptedModel([
+    { parts: [{ tool: { name: "rectification-read-case", input: { caseId: CASE_ID } } }], finish: "tool-calls" },
+    { parts: [{ text: "2014 年 8 月结婚这件事拿去和星盘对照了。" }], finish: "stop" },
+  ]);
+  const active = resolveActiveSkillPackage("jyotish-birth-time-rectification");
+  const accounting = fakeAccounting({
+    ...receiptHandlers,
+    // The case is bound to the active Skill (10.0.x layout), as a new case is.
+    get_agentic_rectification_skill_identity: () => ({
+      skill_name: active.name,
+      skill_version: active.version,
+      skill_sha256: active.sha256,
+      skill_source_commit: active.sourceCommit,
+    }),
+    insert_agentic_rectification_skill_run_receipt: () => ({
+      receipt_id: "88888888-8888-4888-8888-888888888888",
+      skill_name: active.name,
+      skill_version: active.version,
+      skill_sha256: active.sha256,
+      source_commit: active.sourceCommit,
+    }),
+    get_agentic_rectification_case_dossier: () => {
+      const dossier = dossierFixture({
+        evidence: AA_EVIDENCE_ROWS,
+        latestResult: latest,
+        turns: AA_TURNS as never,
+        candidateRange: AA_RANGE as never,
+      }) as { case: Record<string, unknown> };
+      return { ...dossier, case: { ...dossier.case, skill_version: active.version } };
+    },
+    get_agentic_rectification_case_compute: () => compute,
+    set_agentic_rectification_conversation_focus: setFocusHandler,
+    append_agentic_rectification_turn: () => ({ turn_id: TURN_ID, should_execute: true, status: "pending" }),
+    finalize_agentic_rectification_turn: () => ({ turn_id: TURN_ID, status: "completed", idempotent: false }),
+  }, { fallback: () => null });
+  const restore = stubGoldenFetch();
+  try {
+    await runV9AgentTurn({
+      userId: USER_ID,
+      caseId: CASE_ID,
+      sessionId: SESSION_ID,
+      requestId: "req-grounding-prompt",
+      action: "evidence",
+      message: "2014 年 8 月结婚。",
+      modelName: "fake",
+      accounting: accounting.client as never,
+      billing: { reserve: async () => ({ success: true, status: 200 }), complete: async () => true, release: async () => true },
+      emit: () => {},
+      expectedWrite: "none",
+      buildAgent: async (turnId, skillPackage, attemptId) => getRectificationV9Agent(
+        { id: "fake", label: "fake", description: "", creditCost: 1, isDefault: true, mode: "compatible", model: model as never } as never,
+        { userId: USER_ID, caseId: CASE_ID, turnId, attemptId, userMessage: "2014 年 8 月结婚。", accounting: accounting.client as never },
+        skillPackage,
+      ),
+    });
+  } finally {
+    restore();
+  }
+  return calls;
+}
+
+function activeSkillBody(): string {
+  const skill = resolveActiveSkillPackage("jyotish-birth-time-rectification");
+  const raw = readFileSync(resolve(skill.resolvedPath, "SKILL.md"), "utf8");
+  return raw.replace(/^---\r?\n[\s\S]*?\r?\n---\r?\n/, "").trim();
+}
+
+test("R4: an evidence-turn call carries no <available_skills> block and no skill tools", async () => {
+  const calls = await recordEvidenceTurn();
+  assert.ok(calls.length >= 2);
+  for (const call of calls) {
+    const system = call.prompt.filter((message) => message.role === "system").map((message) => contentText(message.content)).join("\n");
+    assert.doesNotMatch(system, /<available_skills>/);
+    assert.doesNotMatch(system, /Skills are NOT tools|call the `skill` tool/);
+    const names = Array.isArray(call.tools) ? (call.tools as Array<{ name: string }>).map((tool) => tool.name) : [];
+    assert.equal(names.includes("skill"), false);
+    assert.equal(names.includes("skill_search"), false);
+  }
+});
+
+test("T6: the evidence-turn bootstrap carries Skill §5 and §7 only", async () => {
+  const calls = await recordEvidenceTurn();
+  const system = calls[0]!.prompt.filter((message) => message.role === "system").map((message) => contentText(message.content)).join("\n");
+  assert.match(system, /## 5\. ConversationFocus 与意图承接/);
+  assert.match(system, /## 7\. 批量证据与日期真实性/);
+  for (const heading of ["## 1. ", "## 2. ", "## 3. ", "## 4. ", "## 6. ", "## 8. ", "## 9. ", "## 10. ", "## 11. "]) {
+    assert.equal(system.includes(heading), false, `${heading} in ${system.slice(system.indexOf(heading) - 200, system.indexOf(heading) + 200)}`);
+  }
+  // full_diagnostics-only fields described in §6 are not in the evidence turn.
+  assert.doesNotMatch(system, /guided_collect_windows|method_followup_plan/);
+  assert.match(system, /本轮是证据轮/);
+  assert.match(system, /# Jyotish 生时校正（V10）/);
+});
+
+test("T6: fixed overhead per evidence-turn call is under 8K estimated tokens", async () => {
+  const sizes = callSizes(await recordEvidenceTurn());
+  console.log(JSON.stringify({ scope: "T6 evidence turn sizes", sizes }));
+  for (const size of sizes) assert.ok(size.fixedTokens < 8_000, JSON.stringify(size));
+});
+
+test("T6: the slice rule is a code constant; other actions and unknown layouts get the whole Skill", () => {
+  assert.deepEqual(RECTIFICATION_SKILL_SECTIONS_BY_ACTION.evidence, ["ConversationFocus 与意图承接", "批量证据与日期真实性"]);
+  assert.equal(RECTIFICATION_SKILL_SECTIONS_BY_ACTION.opening, "all");
+  assert.equal(RECTIFICATION_SKILL_SECTIONS_BY_ACTION.read_only, "all");
+  const body = activeSkillBody();
+  assert.equal(sliceRectificationSkill(body, "opening").text, body);
+  const evidence = sliceRectificationSkill(body, "evidence");
+  assert.deepEqual(evidence.sections, ["## 5. ConversationFocus 与意图承接", "## 7. 批量证据与日期真实性"]);
+  assert.ok(evidence.text.length < body.length / 2);
+  const oldLayout = "# 旧版\n\n## 概述\n\n没有编号的章节。";
+  assert.equal(sliceRectificationSkill(oldLayout, "evidence").text, oldLayout);
+  // BUG-621: the oldest deprecated snapshot still resolves and still slices safely.
+  const oldest = resolveExactSkillPackage(
+    "jyotish-birth-time-rectification",
+    "9.0.0",
+    "5acb3103e80993ea611b93d2c1746e70b74aff8f8636bcffc80a837b954b470d",
+  );
+  const oldestBody = readFileSync(resolve(oldest.resolvedPath, "SKILL.md"), "utf8");
+  // 9.0.0 numbers its sections differently (its §5 is the candidate language):
+  // no title match, so the whole bound body goes out unchanged.
+  assert.equal(sliceRectificationSkill(oldestBody, "evidence").text, oldestBody);
+});