diff --git a/adapters/codex/AGENTS.md.snippet b/adapters/codex/AGENTS.md.snippet
new file mode 100644
index 00000000..593f97df
--- /dev/null
+++ b/adapters/codex/AGENTS.md.snippet
@@ -0,0 +1,31 @@
+# Vouch — knowledge base
+
+This repo uses **vouch** for durable agent knowledge. The KB lives in
+`.vouch/` and is reviewed in PRs like any other code.
+
+## How to remember things
+
+To preserve a fact, decision, or workflow across sessions:
+
+1. Register evidence: `kb_register_source` (or
+   `kb_register_source_from_path` for a file).
+2. Propose a claim that cites it: `kb_propose_claim`. Every claim
+   MUST cite at least one source or evidence id.
+3. For richer write-ups, propose pages: `kb_propose_page` with a
+   markdown body that references claims.
+
+You **cannot** write durable knowledge directly. Proposals land in
+`.vouch/proposed/` and require human approval via `vouch approve`.
+This is intentional.
+
+## How to read
+
+- `kb_search` for keyword search.
+- `kb_context` to fill a working set for a task ("what does this KB know
+  about X?").
+- `kb_read_*` for specific ids.
+
+## Identity
+
+Set `VOUCH_AGENT=codex` in your env (or the MCP entry's `env:` block) so the
+audit log can attribute writes to this host.
diff --git a/adapters/codex/install.yaml b/adapters/codex/install.yaml
index 7ba68b2f..40c6a5a2 100644
--- a/adapters/codex/install.yaml
+++ b/adapters/codex/install.yaml
@@ -1,10 +1,17 @@
 # OpenAI Codex CLI adapter manifest.
 #
+# T1 = project-local MCP config at `<project>/.codex/config.toml`.
 # Codex reads `<project>/.codex/config.toml` (also `~/.codex/config.toml` for
 # user-global). We ship the project-local form so `vouch install-mcp codex`
 # doesn't touch home-directory state -- see issue #179 scope decision.
+# T2 = AGENTS.md fenced snippet (Codex reads AGENTS.md the way Cursor does).
 host: codex
 pretty: OpenAI Codex CLI
+fence:
+  begin: "<!-- BEGIN vouch -->"
+  end: "<!-- END vouch -->"
 tiers:
   T1:
     - { src: config.toml, dst: .codex/config.toml }
+  T2:
+    - { src: AGENTS.md.snippet, dst: AGENTS.md, fenced_append: true }
diff --git a/tests/test_install_adapter.py b/tests/test_install_adapter.py
index 83036e8c..1714520e 100644
--- a/tests/test_install_adapter.py
+++ b/tests/test_install_adapter.py
@@ -480,3 +480,46 @@ def test_installed_wheel_resolves_adapters(tmp_path: Path) -> None:
     assert "claude-code" in result["hosts"], (
         f"installed copy can't resolve adapters: {result}"
     )
+
+
+# --- codex: T2 AGENTS.md fenced snippet (vouchdev/vouch#385) ---------------
+
+
+def test_install_codex_t1_writes_config_toml(tmp_path: Path) -> None:
+    """T1 produces the project-local .codex/config.toml."""
+    result = install("codex", target=tmp_path, tier="T1")
+    assert (tmp_path / ".codex" / "config.toml").is_file()
+    assert not (tmp_path / "AGENTS.md").exists()
+    assert result.written and not result.appended
+
+
+def test_install_codex_t2_creates_agents_md(tmp_path: Path) -> None:
+    """T2 appends the fenced AGENTS.md snippet; creates file when absent."""
+    result = install("codex", target=tmp_path, tier="T2")
+    assert (tmp_path / ".codex" / "config.toml").is_file()
+    assert (tmp_path / "AGENTS.md").is_file()
+    assert "AGENTS.md" in result.written
+    content = (tmp_path / "AGENTS.md").read_text()
+    assert "<!-- BEGIN vouch -->" in content
+    assert "<!-- END vouch -->" in content
+    assert "VOUCH_AGENT=codex" in content
+
+
+def test_install_codex_t2_idempotent(tmp_path: Path) -> None:
+    """Re-running T2 leaves AGENTS.md unchanged — fence dedup."""
+    install("codex", target=tmp_path, tier="T2")
+    content_first = (tmp_path / "AGENTS.md").read_text()
+    again = install("codex", target=tmp_path, tier="T2")
+    content_again = (tmp_path / "AGENTS.md").read_text()
+    assert content_first == content_again
+    assert "AGENTS.md" not in again.appended
+
+
+def test_install_codex_t2_appends_to_existing_agents_md(tmp_path: Path) -> None:
+    """When AGENTS.md already exists without a fence, the snippet is appended."""
+    (tmp_path / "AGENTS.md").write_text("# Existing project notes\n")
+    result = install("codex", target=tmp_path, tier="T2")
+    assert "AGENTS.md" in result.appended
+    content = (tmp_path / "AGENTS.md").read_text()
+    assert content.startswith("# Existing project notes\n")
+    assert "<!-- BEGIN vouch -->" in content