diff --git a/doc/architecture/SPEC/kb_service.md b/doc/architecture/SPEC/kb_service.md
index 2df97413..1c66b065 100644
--- a/doc/architecture/SPEC/kb_service.md
+++ b/doc/architecture/SPEC/kb_service.md
@@ -181,3 +181,9 @@ issue（同一报告只开一个），标签不存在时不带标签重试，绝
 否则运行服务实际生效的 `run_root`（环境变量或其支持的 `.env` 文件）。切换激活快照时记录旧快照的停用时间；
 停用不足 1 小时的快照不删除——在切换前刚解析到它的预留会在此窗口内写下钉住记录（运行服务在其他进程中，无法共享
 激活锁，因此以宽限期协调）。测试通过自动 fixture 把 `KB_RUN_ROOTS` 指向空目录，从不读取真实运行目录。
+
+
+## 2026-09-29 激活前的格式检查
+`verify_snapshot` 在其他检查之前核对快照声明的知识格式：未声明或不在本 Copilot 支持的 `SUPPORTED_FORMATS` 中 →
+`ActivationError`，`active` 保持不变（先升级 Copilot 再激活新格式的知识）。快照随顶层 Markdown 一起携带
+`_format.yaml`。
diff --git a/doc/architecture/SPEC/knowledge_view.md b/doc/architecture/SPEC/knowledge_view.md
index df4d98e1..e94ffb38 100644
--- a/doc/architecture/SPEC/knowledge_view.md
+++ b/doc/architecture/SPEC/knowledge_view.md
@@ -1,6 +1,6 @@
 # knowledge_view.py —— 规范
 
-<!-- verified-against: 2026-09-28 -->
+<!-- verified-against: 2026-09-29 -->
 
 `LOC ~170 · 每请求的知识树视图（打包知识或激活快照） · refactor-status: stable`
 
@@ -35,3 +35,8 @@ stdlib + `.sdk._resources`。叶子模块。
 ## 2026-09-28 可记录的快照标识
 `public_snapshot`：激活快照 ID、`packaged` 或 `unverified`（开发树的绝对路径不外泄）；`tree_sha256`：已验证快照
 按清单重算的树哈希。`direct_routing` 的 `diagnostics.knowledge_snapshot` 与 SDK 计划都使用它们。
+
+## 2026-09-29 知识格式版本
+知识树在顶层 `knowledge/_format.yaml` 声明其格式（`format_version`；当前为 2：规则生命周期尾注、类型化操作、
+`_routes.yaml`、`_tombstones.yaml`）。`knowledge_format(root)` 读取它，`build_manifest` 把它记入 `MANIFEST.json` 的
+`knowledge_format`。该文件位于自动合并白名单之外，改动它需要人工。
diff --git a/knowledge/_format.yaml b/knowledge/_format.yaml
new file mode 100644
index 00000000..942774a3
--- /dev/null
+++ b/knowledge/_format.yaml
@@ -0,0 +1 @@
+format_version: 2
diff --git a/src/infermatrix_copilot/kb_service/activate.py b/src/infermatrix_copilot/kb_service/activate.py
index 48b1540a..a36d0002 100644
--- a/src/infermatrix_copilot/kb_service/activate.py
+++ b/src/infermatrix_copilot/kb_service/activate.py
@@ -98,6 +98,13 @@ def verify_snapshot(snapshot: Path) -> KnowledgeView:
             view.path(rel)
         except (KnowledgeViewError, FileNotFoundError, ValueError) as exc:
             raise ActivationError(f"snapshot content does not match its manifest: {exc}") from exc
+    from ..knowledge_view import SUPPORTED_FORMATS, knowledge_format
+
+    declared = knowledge_format(view.root)
+    if declared not in SUPPORTED_FORMATS:
+        raise ActivationError(
+            f"knowledge format {declared if declared is not None else '(undeclared)'} is not one this Copilot "
+            f"reads ({sorted(SUPPORTED_FORMATS)}); upgrade the Copilot before activating this knowledge")
     files = {p.relative_to(view.root).as_posix(): p.read_text(encoding="utf-8")
              for p in view.root.rglob("*") if p.is_file() and p.suffix in (".md", ".yaml")}
     issues = check_tree({k: v for k, v in files.items() if k.startswith(("repos/", "general/"))})
diff --git a/src/infermatrix_copilot/kb_service/sources.py b/src/infermatrix_copilot/kb_service/sources.py
index 0155e61e..92f14173 100644
--- a/src/infermatrix_copilot/kb_service/sources.py
+++ b/src/infermatrix_copilot/kb_service/sources.py
@@ -159,9 +159,10 @@ def knowledge_files(self, rev: str) -> dict[str, str]:
         return {name.removeprefix("knowledge/"): text for name, text in files.items()}
 
     def top_level_knowledge(self, rev: str) -> dict[str, str]:
-        """knowledge/*.md at the top of the tree (AGENTS.md, README.md, ...):
-        not governed by the gate, but served with every snapshot."""
-        files = self._texts(rev, ("knowledge/",), (".md",))
+        """knowledge/*.md at the top of the tree (AGENTS.md, README.md, ...) and
+        the format declaration: not governed by the gate, but served with every
+        snapshot."""
+        files = self._texts(rev, ("knowledge/",), (".md", "_format.yaml"))
         return {name.removeprefix("knowledge/"): text for name, text in files.items()
                 if "/" not in name.removeprefix("knowledge/")}
 
diff --git a/src/infermatrix_copilot/knowledge_view.py b/src/infermatrix_copilot/knowledge_view.py
index bec8f462..d7e75594 100644
--- a/src/infermatrix_copilot/knowledge_view.py
+++ b/src/infermatrix_copilot/knowledge_view.py
@@ -33,6 +33,25 @@
 KNOWLEDGE_ROOT_ENV = "KNOWLEDGE_ROOT"
 MANIFEST_NAME = "MANIFEST.json"
 MANIFEST_SCHEMA_VERSION = 1
+# The knowledge tree declares the format it is written in (rule lifecycle
+# footers, typed operations, _routes.yaml, _tombstones.yaml). A Copilot only
+# activates snapshots in a format it reads. The file sits at the top of
+# knowledge/, outside the auto-merge whitelist: changing it takes a person.
+FORMAT_FILE = "_format.yaml"
+SUPPORTED_FORMATS = frozenset({2})
+
+
+def knowledge_format(root: Path) -> int | None:
+    """The declared format of the knowledge tree at ``root`` (None: undeclared or unreadable)."""
+    import yaml
+
+    try:
+        data = yaml.safe_load((Path(root) / FORMAT_FILE).read_text(encoding="utf-8"))
+        value = data["format_version"]
+    except (OSError, TypeError, KeyError, yaml.YAMLError):
+        return None
+    # exactly an integer: 2.9, "2", true or .inf are malformed, never coerced
+    return value if type(value) is int else None
 
 
 class KnowledgeViewError(RuntimeError):
@@ -191,6 +210,7 @@ def build_manifest(root: Path, snapshot: str) -> dict:
     return {
         "schema_version": MANIFEST_SCHEMA_VERSION,
         "snapshot": snapshot,
+        "knowledge_format": knowledge_format(root),
         "files": files,
         "tree_sha256": tree.hexdigest(),
     }
diff --git a/test/test_kb_flow.py b/test/test_kb_flow.py
index 695adcf6..9607d50a 100644
--- a/test/test_kb_flow.py
+++ b/test/test_kb_flow.py
@@ -781,3 +781,41 @@ def test_pin_discovery_reads_each_run_status_once(tmp_path, monkeypatch):
     answers = iter([{"state": rs.INTERRUPTED, "child_pid": None}, {"state": rs.QUEUED}])
     monkeypatch.setattr(rs, "read_status", lambda _d: next(answers))   # a reclaim lands between reads
     assert pinned_snapshots([tmp_path / "runs"]) == {(tmp_path / "s").resolve()}
+
+
+def test_activation_refuses_a_knowledge_format_this_copilot_does_not_read(tmp_path):
+    import json
+
+    rt, lifecycle = _flow_runtime(tmp_path)
+    _add_agents(rt)
+    good = rt.knowledge.fetch()
+    snapshot = activate(rt, good)
+    assert json.loads((snapshot / "MANIFEST.json").read_text())["knowledge_format"] == 2
+    newer = _commit_change(rt, "_format.yaml", "format_version: 3\n")
+    with pytest.raises(ActivationError, match="knowledge format 3"):
+        activate(rt, newer)
+    origin = Path(rt.knowledge.path).parent / "origin"
+    (origin / "knowledge" / "_format.yaml").unlink()
+    _git(origin, "add", "-A")
+    _git(origin, "-c", "user.name=t", "-c", "user.email=t@e", "commit", "-q", "-m", "undeclared")
+    with pytest.raises(ActivationError, match="undeclared"):
+        activate(rt, rt.knowledge.fetch())
+    assert Path(active_link(rt.state_dir)).resolve().name == good     # still the good one
+
+
+def test_the_packaged_knowledge_declares_a_supported_format():
+    from infermatrix_copilot.knowledge_view import SUPPORTED_FORMATS, KnowledgeView, knowledge_format
+
+    assert knowledge_format(KnowledgeView.current().root) in SUPPORTED_FORMATS
+
+
+@pytest.mark.parametrize("declared", ["2.9", "'2'", "true", ".inf", "[2]", "{}"])
+def test_a_malformed_format_declaration_is_refused_not_coerced(tmp_path, declared):
+    rt, lifecycle = _flow_runtime(tmp_path)
+    _add_agents(rt)
+    good = rt.knowledge.fetch()
+    activate(rt, good)
+    bad = _commit_change(rt, "_format.yaml", f"format_version: {declared}\n")
+    with pytest.raises(ActivationError, match="undeclared"):
+        activate(rt, bad)
+    assert Path(active_link(rt.state_dir)).resolve().name == good
diff --git a/test/test_kb_intake_gate.py b/test/test_kb_intake_gate.py
index 24add7bb..76942c59 100644
--- a/test/test_kb_intake_gate.py
+++ b/test/test_kb_intake_gate.py
@@ -195,6 +195,7 @@ def _knowledge_remote(tmp_path: Path) -> Path:
         target = origin / "knowledge" / rel
         target.parent.mkdir(parents=True, exist_ok=True)
         target.write_text(text, encoding="utf-8")
+    (origin / "knowledge" / "_format.yaml").write_text("format_version: 2\n", encoding="utf-8")
     (origin / "skills").mkdir()
     (origin / "skills" / "x.md").write_text("see DEMO-1a\n", encoding="utf-8")
     subprocess.run(["git", "init", "-q", "-b", "main", str(origin)], check=True)