diff --git a/ouroboros/config.py b/ouroboros/config.py
index 658230fec..c2cb0d219 100644
--- a/ouroboros/config.py
+++ b/ouroboros/config.py
@@ -952,6 +952,21 @@ def get_mcp_tool_timeout_sec() -> int:
     return parsed if parsed > 0 else int(SETTINGS_DEFAULTS["MCP_TOOL_TIMEOUT_SEC"])
 
 
+def _serialize_env_setting(key: str, value: object) -> str:
+    """Project the incoming settings shape without leaking Python repr syntax.
+
+    Settings writers accept structured values from the UI before the persistence
+    prologue canonicalizes them for disk.  The process projection must carry the
+    same JSON-shaped value, otherwise ``str(dict)`` produces single-quoted text
+    that strict runtime readers cannot parse.
+    """
+    if key in (MODEL_ACCOUNTS_KEY, MODEL_CONTEXT_WINDOWS_KEY):
+        return normalize_model_role_options(key, value)[1]
+    if isinstance(value, (dict, list)):
+        return json.dumps(value, ensure_ascii=False, separators=(",", ":"))
+    return str(value)
+
+
 def apply_settings_to_env(settings: dict, *, environ=None) -> None:
     """Push settings into environment variables for supervisor modules."""
     with _settings_integrity.SETTINGS_ENV_LOCK:
@@ -974,7 +989,7 @@ def apply_settings_to_env(settings: dict, *, environ=None) -> None:
             if val is None or val == "":
                 environ.pop(k, None)
             else:
-                environ[k] = str(val)
+                environ[k] = _serialize_env_setting(k, val)
         # Reviewer-model floors moved into the structured-slot projection (6.1):
         from ouroboros.reviewer_slot_config import project_reviewer_slots_into_env
         project_reviewer_slots_into_env(environ=environ)
diff --git a/tests/test_owner_settings_write_seam.py b/tests/test_owner_settings_write_seam.py
index 5907f72b9..363ef0311 100644
--- a/tests/test_owner_settings_write_seam.py
+++ b/tests/test_owner_settings_write_seam.py
@@ -348,6 +348,28 @@ def test_generic_settings_save_validates_and_canonicalizes_available_subagents(
     assert '"name"' not in canonical
 
 
+def test_generic_settings_save_projects_model_role_objects_as_json(
+        monkeypatch, isolated_settings,
+):
+    from ouroboros import config as cfg
+    from ouroboros.gateway import settings as settings_mod
+
+    app = _settings_app(monkeypatch, isolated_settings)
+    monkeypatch.setattr(settings_mod, "_apply_settings_to_env", cfg.apply_settings_to_env)
+    response = TestClient(app).post("/api/settings", json={
+        "OUROBOROS_MODEL_ACCOUNTS": {"main": "", "fallback": ["work", ""]},
+        "OUROBOROS_MODEL_CONTEXT_WINDOWS": {"main": 131072, "fallback": [0, 65536]},
+    })
+
+    assert response.status_code == 200, response.text
+    stored = json.loads(isolated_settings.read_text(encoding="utf-8"))
+    assert json.loads(stored["OUROBOROS_MODEL_ACCOUNTS"]) == {
+        "fallback": ["work", ""], "main": "",
+    }
+    assert json.loads(os.environ["OUROBOROS_MODEL_ACCOUNTS"])["main"] == ""
+    assert json.loads(os.environ["OUROBOROS_MODEL_CONTEXT_WINDOWS"])["main"] == 131072
+
+
 def test_generic_settings_save_rejects_malformed_available_subagents_without_write(
     monkeypatch, isolated_settings,
 ):
diff --git a/tests/test_settings_env_on_disk.py b/tests/test_settings_env_on_disk.py
index e328e4714..fe7b0bf0e 100644
--- a/tests/test_settings_env_on_disk.py
+++ b/tests/test_settings_env_on_disk.py
@@ -150,6 +150,28 @@ def test_a_disk_authored_key_is_not_projected_back_out_of_a_silent_file(
     assert os.environ["OUROBOROS_CONTEXT_MODE"] == "max"
 
 
+def test_structured_model_settings_keep_json_shape_in_environment(isolated_settings):
+    """UI-shaped model role objects stay parseable after process projection."""
+    from ouroboros import config as cfg
+
+    projected = {}
+    cfg.apply_settings_to_env({
+        "OUROBOROS_MODEL_ACCOUNTS": {"main": "", "fallback": ["work", ""]},
+        "OUROBOROS_MODEL_CONTEXT_WINDOWS": {"main": 131072, "fallback": [0, 65536]},
+    }, environ=projected)
+
+    assert json.loads(projected["OUROBOROS_MODEL_ACCOUNTS"]) == {
+        "fallback": ["work", ""], "main": "",
+    }
+    assert json.loads(projected["OUROBOROS_MODEL_CONTEXT_WINDOWS"]) == {
+        "fallback": [0, 65536], "main": 131072,
+    }
+    parsed, _ = cfg.normalize_model_role_options(
+        cfg.MODEL_ACCOUNTS_KEY, projected["OUROBOROS_MODEL_ACCOUNTS"])
+    assert parsed["fallback"] == ["work", ""]
+    assert "'main'" not in projected["OUROBOROS_MODEL_ACCOUNTS"]
+
+
 def test_install_time_facts_are_disk_only_in_both_directions(isolated_settings, monkeypatch):
     """`ENDPOINT_AUTHORED_SETTINGS` is stricter than the ratchets: those project
     once the file carries them, these never leave disk at all. An environment