diff --git a/docs/CONTRIBUTING.md b/docs/CONTRIBUTING.md
index 5bf4bed..145c238 100644
--- a/docs/CONTRIBUTING.md
+++ b/docs/CONTRIBUTING.md
@@ -60,8 +60,11 @@ These functions are provided for documentation examples and return mock data:
 - `process_tenant_request(tenant_id, request)` - Returns tenant result
 
 #### Configuration
-- `secret_key` - Test encryption key (value: `"a" * 64`)
-- `CACHEKIT_MASTER_KEY` - Environment variable set to `secret_key` (enables `@cache.secure` examples)
+- `secret_key` - Test encryption key (value: `"a" * 64`). `@cache.secure` examples must
+  pass it explicitly (`master_key=secret_key`); the conftest does **not** set
+  `CACHEKIT_MASTER_KEY` in the environment because an ambient key turns encryption on
+  globally and breaks plain `@cache` fences that use `serializer="auto"` or custom
+  serializers.
 
 ## Skipping Examples with `notest`
 
diff --git a/docs/conftest.py b/docs/conftest.py
index e47447e..4318807 100644
--- a/docs/conftest.py
+++ b/docs/conftest.py
@@ -6,7 +6,6 @@
 """
 
 import logging
-import os
 import time
 
 try:
@@ -91,10 +90,16 @@ def expensive_computation():
     logger = logging.getLogger("cachekit.examples")
     logger.setLevel(logging.INFO)
 
-    # Secret key for encryption examples (test value only)
+    # Secret key for encryption examples (test value only). Exposed as a global
+    # so @cache.secure fences can pass it explicitly (master_key=secret_key).
+    #
+    # We deliberately DO NOT set CACHEKIT_MASTER_KEY in the environment here.
+    # Since the PR #127 auto-detect + PR #200 settings re-read, an ambient
+    # master key turns encryption on globally, and the v0.6.0 cross-SDK rule
+    # then rejects any plain `@cache` fence using a non-cross-SDK serializer
+    # (serializer="auto", custom serializer instances) at decoration time.
+    # tests/conftest.py hit the same trap and fixed it the same way. See #205.
     secret_key = "a" * 64  # 32 bytes in hex
-    # Set env var so @cache.secure validation passes
-    os.environ["CACHEKIT_MASTER_KEY"] = secret_key
 
     globals_dict = {
         "cache": cache,
diff --git a/tests/unit/test_docs_conftest_no_key_leak.py b/tests/unit/test_docs_conftest_no_key_leak.py
new file mode 100644
index 0000000..f514333
--- /dev/null
+++ b/tests/unit/test_docs_conftest_no_key_leak.py
@@ -0,0 +1,48 @@
+"""Regression guard for issue #205.
+
+The markdown-docs suite runs only in the post-merge CI job (push to main), not
+on PRs, so a regression here would turn `main` red instead of failing review.
+This fast unit test runs on every PR and fails loudly if `docs/conftest.py`
+starts setting CACHEKIT_MASTER_KEY in the process environment again.
+
+Why it matters: an ambient master key turns encryption on globally (PR #127
+auto-detect + PR #200 settings re-read), and the v0.6.0 cross-SDK rule then
+rejects every plain `@cache` doc fence that uses a non-cross-SDK serializer
+(serializer="auto", custom serializer instances) at decoration time.
+"""
+
+from __future__ import annotations
+
+import importlib.util
+import os
+from pathlib import Path
+
+import pytest
+
+DOCS_CONFTEST = Path(__file__).resolve().parents[2] / "docs" / "conftest.py"
+
+
+def _load_docs_conftest():
+    spec = importlib.util.spec_from_file_location("docs_conftest_under_test", DOCS_CONFTEST)
+    assert spec and spec.loader, f"could not load {DOCS_CONFTEST}"
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+@pytest.mark.unit
+def test_docs_globals_hook_does_not_set_master_key_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Invoking the markdown-docs globals hook must not leak CACHEKIT_MASTER_KEY."""
+    monkeypatch.delenv("CACHEKIT_MASTER_KEY", raising=False)
+
+    module = _load_docs_conftest()
+    globals_dict = module.pytest_markdown_docs_globals()
+
+    assert "CACHEKIT_MASTER_KEY" not in os.environ, (
+        "docs/conftest.py set CACHEKIT_MASTER_KEY in the environment. This turns "
+        "encryption on globally and breaks plain @cache fences using serializer='auto' "
+        "or custom serializers. Pass the key explicitly (master_key=secret_key) in the "
+        "@cache.secure fences instead. See issue #205."
+    )
+    # The key must still be available to fences that opt in explicitly.
+    assert globals_dict.get("secret_key") == "a" * 64