diff --git a/tests/unit/test_manifest_validation.py b/tests/unit/test_manifest_validation.py
new file mode 100644
index 0000000..dbc5a90
--- /dev/null
+++ b/tests/unit/test_manifest_validation.py
@@ -0,0 +1,226 @@
+"""Unit: manifest + manifest_agent validation error/edge branches
+(coverage ratchet, ADR 0004).
+
+Drives ManifestBottle / ManifestAgentProvider / ManifestAgent / the
+provider-settings parser and the eager ManifestIndex lookup methods
+through their rejection and edge paths."""
+
+from __future__ import annotations
+
+import unittest
+
+from bot_bottle.manifest import ManifestBottle, ManifestIndex
+from bot_bottle.manifest_agent import (
+    ManifestAgent,
+    ManifestAgentProvider,
+    _parse_provider_settings,
+)
+from bot_bottle.manifest_util import ManifestError
+
+
+def _idx(obj: dict[str, object]) -> ManifestIndex:
+    return ManifestIndex.from_json_obj(obj)
+
+
+# ---------------------------------------------------------------------------
+# ManifestBottle.from_dict
+# ---------------------------------------------------------------------------
+
+
+class TestBottleValidation(unittest.TestCase):
+    def test_unknown_key(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestBottle.from_dict("b", {"bogus": 1})
+
+    def test_env_value_not_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestBottle.from_dict("b", {"env": {"X": 5}})
+
+    def test_supervise_not_bool(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestBottle.from_dict("b", {"supervise": "yes"})
+
+    def test_removed_runtime_field(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestBottle.from_dict("b", {"runtime": "runsc"})
+
+    def test_valid_minimal(self) -> None:
+        b = ManifestBottle.from_dict("b", {"supervise": False, "env": {"X": "1"}})
+        self.assertFalse(b.supervise)
+        self.assertEqual({"X": "1"}, dict(b.env))
+
+
+# ---------------------------------------------------------------------------
+# ManifestAgentProvider.from_dict
+# ---------------------------------------------------------------------------
+
+
+class TestAgentProviderValidation(unittest.TestCase):
+    def test_unknown_key(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict("b", {"bogus": 1})
+
+    def test_empty_template(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict("b", {"template": ""})
+
+    def test_dockerfile_not_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict("b", {"dockerfile": 5})
+
+    def test_auth_token_unknown_template(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict("b", {"auth_token": "x", "template": "weird"})
+
+    def test_auth_token_non_claude_template(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict("b", {"auth_token": "x", "template": "codex"})
+
+    def test_forward_creds_unknown_template(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict(
+                "b", {"forward_host_credentials": True, "template": "weird"}
+            )
+
+    def test_forward_creds_non_codex_template(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict(
+                "b", {"forward_host_credentials": True, "template": "claude"}
+            )
+
+    def test_valid_claude_auth_token(self) -> None:
+        p = ManifestAgentProvider.from_dict("b", {"template": "claude", "auth_token": "T"})
+        self.assertEqual("T", p.auth_token)
+
+
+# ---------------------------------------------------------------------------
+# _parse_provider_settings
+# ---------------------------------------------------------------------------
+
+
+class TestProviderSettings(unittest.TestCase):
+    def test_unknown_template_passes_settings_through(self) -> None:
+        out = _parse_provider_settings("b", "weird", {"anything": 1})
+        self.assertEqual({"anything": 1}, out)
+
+    def test_startup_args_not_list(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "claude", {"startup_args": "x"})
+
+    def test_startup_args_empty_item(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "claude", {"startup_args": [""]})
+
+    def test_pi_string_field_empty(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"provider": ""})
+
+    def test_pi_max_tokens_field_invalid(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"max_tokens_field": "bogus"})
+
+    def test_pi_api_key_and_env_conflict(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"api_key": "k", "api_key_env": "E"})
+
+    def test_pi_models_item_not_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"models": [5]})
+
+    def test_pi_bool_field_not_bool(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"supports_developer_role": "yes"})
+
+    def test_pi_context_window_not_positive(self) -> None:
+        with self.assertRaises(ManifestError):
+            _parse_provider_settings("b", "pi", {"context_window": -1})
+
+    def test_pi_valid_settings(self) -> None:
+        out = _parse_provider_settings(
+            "b", "pi",
+            {"provider": "openai", "models": ["gpt"], "context_window": 8000},
+        )
+        self.assertEqual("openai", out["provider"])
+
+
+# ---------------------------------------------------------------------------
+# ManifestAgent.from_dict
+# ---------------------------------------------------------------------------
+
+
+class TestAgentValidation(unittest.TestCase):
+    def test_bottle_empty_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"bottle": ""}, set())
+
+    def test_bottle_undefined(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"bottle": "x"}, set())
+
+    def test_skills_not_list(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"skills": "x"}, set())
+
+    def test_skill_item_not_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"skills": [5]}, set())
+
+    def test_prompt_not_string(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"prompt": 5}, set())
+
+    def test_git_gate_repos_rejected_at_agent_level(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgent.from_dict("a", {"git-gate": {"repos": {}}}, set())
+
+    def test_git_gate_empty_is_allowed(self) -> None:
+        agent = ManifestAgent.from_dict("a", {"git-gate": {}}, set())
+        self.assertTrue(agent.git_user.is_empty())
+
+
+# ---------------------------------------------------------------------------
+# Eager ManifestIndex lookup methods
+# ---------------------------------------------------------------------------
+
+
+class TestEagerIndexLookups(unittest.TestCase):
+    def _idx(self) -> ManifestIndex:
+        return _idx({
+            "bottles": {"b": {"git-gate": {"user": {"name": "Bot", "email": "b@x"}}}},
+            "agents": {"a": {"bottle": "b"}},
+        })
+
+    def test_unknown_bottle_section_is_empty(self) -> None:
+        # no "bottles" key -> _section_dict(None) path
+        idx = _idx({"agents": {"a": {}}})
+        self.assertEqual(["a"], idx.all_agent_names)
+
+    def test_load_unknown_agent_raises(self) -> None:
+        with self.assertRaises(ManifestError):
+            self._idx().load_for_agent("nope")
+
+    def test_has_agent(self) -> None:
+        idx = self._idx()
+        self.assertTrue(idx.has_agent("a"))
+        self.assertFalse(idx.has_agent("nope"))
+
+    def test_require_agent_known_and_unknown(self) -> None:
+        idx = self._idx()
+        idx.require_agent("a")  # no raise
+        with self.assertRaises(ManifestError):
+            idx.require_agent("nope")
+
+    def test_git_identity_summary(self) -> None:
+        m = self._idx().load_for_agent("a")
+        summary = m.git_identity_summary()
+        assert summary is not None
+        self.assertIn("name=Bot", summary)
+        self.assertIn("email=b@x", summary)
+
+    def test_git_identity_summary_none_when_empty(self) -> None:
+        m = _idx({"bottles": {"b": {}}, "agents": {"a": {"bottle": "b"}}}).load_for_agent("a")
+        self.assertIsNone(m.git_identity_summary())
+
+
+if __name__ == "__main__":
+    unittest.main()