fix(anchors): resolve workspace agents from session cwd

bundles/anchors/context/system.md

@@ -14,3 +14,5 @@ Generated with Amplifier
 
 Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>
 ```
+
+@AGENTS.md

docs/lanes/zc6t-lean-head-ship/DONE-NOTE.md

@@ -291,3 +291,13 @@ pin **per artifact**, never to a whole-head absolute · draft PR, ready on green
 Two children (`4qg2`, `va53`) additionally require their **no-op** target
 (`web_search`, `mode`) to be verified and **stated** — an unchanged file in a PR
 is noise, but a silently skipped one is a gap.
+
+## Addendum — 2026-09-08
+
+The current static census for `bundles/anchors/context/system.md` is 1,154
+chars, +12 over its recorded 1,142-char lean budget. The required `@AGENTS.md`
+rule auto-loads session-CWD `AGENTS.md`, so the budget is pinned at 1,154 rather
+than compressing unrelated prompt text. The owned-head total is now 3,392 chars
+against the unchanged 3,980-char stock: a current saving of 588 chars versus the
+original 600. Historical experimental results above are unchanged; no vendor
+cache metrics were rerun.

tests/test_instruction_mentions_lead_context.py

@@ -102,6 +102,11 @@ def _write(path: Path, content: str) -> Path:
     return path
 
 
+_ANCHORS_SYSTEM = (
+    Path(__file__).parent.parent / "bundles" / "anchors" / "context" / "system.md"
+)
+
+
 @pytest.mark.asyncio
 async def test_public_factory_uses_prepared_bundle_and_target_session(
     tmp_path: Path,
@@ -299,3 +304,60 @@ async def test_mentions_resolved_payload_semantics_unchanged(tmp_path: Path) ->
 
     # The instruction's mention now leads the resolutions list as well.
     assert payload["resolutions"][0]["mention"] == "@ns:context/system.md"
+
+
+@pytest.mark.asyncio
+async def test_anchors_system_loads_session_cwd_agents_not_bundle_or_process_cwd(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """The actual anchors system mention resolves nested @AGENTS.md in session_cwd."""
+    anchors_root = _ANCHORS_SYSTEM.parent.parent
+    assert _ANCHORS_SYSTEM.read_text(encoding="utf-8").endswith("```\n\n@AGENTS.md\n")
+
+    session_cwd = tmp_path / "session-cwd"
+    cwd_agents = _write(
+        session_cwd / "AGENTS.md",
+        "SESSION CWD AGENTS\n@AGENTS.md",
+    )
+    bundle_root = tmp_path / "bundle-root"
+    _write(bundle_root / "AGENTS.md", "BUNDLE-LOCAL AGENTS DECOY")
+    process_cwd = tmp_path / "process-cwd"
+    _write(process_cwd / "AGENTS.md", "PROCESS CWD AGENTS DECOY")
+    monkeypatch.chdir(process_cwd)
+
+    bundle = Bundle(
+        name="host",
+        base_path=bundle_root,
+        source_base_paths={"anchors": anchors_root},
+        instruction="@anchors:context/system.md",
+    )
+    prompt = await _make_prepared(bundle)._create_system_prompt_factory(
+        bundle, _make_mock_session(), session_cwd=session_cwd
+    )()
+
+    assert str(cwd_agents.resolve()) in prompt
+    assert prompt.count("SESSION CWD AGENTS") == 1
+    assert "BUNDLE-LOCAL AGENTS DECOY" not in prompt
+    assert "PROCESS CWD AGENTS DECOY" not in prompt
+
+
+@pytest.mark.asyncio
+async def test_anchors_system_missing_cwd_agents_is_nonfatal(tmp_path: Path) -> None:
+    """Absent optional cwd AGENTS.md leaves the anchors system prompt usable."""
+    bundle_root = tmp_path / "bundle-root"
+    _write(bundle_root / "AGENTS.md", "BUNDLE-LOCAL AGENTS DECOY")
+    session_cwd = tmp_path / "session-cwd"
+    session_cwd.mkdir()
+    bundle = Bundle(
+        name="host",
+        base_path=bundle_root,
+        source_base_paths={"anchors": _ANCHORS_SYSTEM.parent.parent},
+        instruction="@anchors:context/system.md",
+    )
+
+    prompt = await _make_prepared(bundle)._create_system_prompt_factory(
+        bundle, _make_mock_session(), session_cwd=session_cwd
+    )()
+
+    assert "You are Amplifier" in prompt
+    assert "BUNDLE-LOCAL AGENTS DECOY" not in prompt

tests/test_lean_head_guardrail.py

@@ -74,7 +74,8 @@
 # above what shipped would let the head regrow up to it silently, which is the
 # whole failure mode.
 OWNED_HEAD_BUDGET_CHARS = {
-    "bundles/anchors/context/system.md": 1142,
+    # Required cwd-AGENTS auto-loading rule costs +12 chars.
+    "bundles/anchors/context/system.md": 1154,
     "bundles/anchors-amp-dev/context/amplifier-ecosystem.md": 1300,
     "delegate:preamble": 938,
 }
@@ -247,10 +248,11 @@ def test_the_owned_head_is_smaller_than_it_was(self) -> None:
         lean = sum(OWNED_HEAD_BUDGET_CHARS.values())
         stock = sum(OWNED_HEAD_STOCK_CHARS.values())
         assert lean < stock, f"lean {lean} is not below stock {stock}"
-        assert stock - lean == 600, (
-            f"The recorded saving moved: {stock - lean} chars, was 600. Update "
-            "both tables and the DONE-NOTE together, or the published census "
-            "and this repo disagree."
+        assert stock - lean == 588, (
+            f"The current saving moved: {stock - lean} chars, expected 588 "
+            "after the original measured 600-char saving absorbed the required "
+            "cwd-AGENTS rule. Update the budget and DONE-NOTE together, or the "
+            "current census and this repo disagree."
         )