diff --git a/nanobot/config/schema.py b/nanobot/config/schema.py
index 06c64ce9..ac8022db 100644
--- a/nanobot/config/schema.py
+++ b/nanobot/config/schema.py
@@ -56,7 +56,10 @@ class DreamConfig(Base):
 
     enabled: bool = True  # Register the periodic Dream consolidation job on startup
     interval_h: int = Field(default=2, ge=1)  # Every 2 hours by default
-    cron: str | None = Field(default=None, exclude=True)  # Legacy cron expression override
+    cron: str | None = Field(
+        default=None,
+        exclude_if=lambda value: value is None,
+    )  # Legacy cron expression override
     model_override: str | None = Field(
         default=None,
         validation_alias=AliasChoices("modelOverride", "model", "model_override"),
diff --git a/tests/config/test_dream_config.py b/tests/config/test_dream_config.py
index 9266792b..feff587a 100644
--- a/tests/config/test_dream_config.py
+++ b/tests/config/test_dream_config.py
@@ -29,12 +29,18 @@ def test_dream_config_honors_legacy_cron_override() -> None:
     assert cfg.describe_schedule() == "cron 0 */4 * * * (legacy)"
 
 
-def test_dream_config_dump_uses_interval_h_and_hides_legacy_cron() -> None:
+def test_dream_config_dump_preserves_legacy_cron_override() -> None:
     cfg = DreamConfig.model_validate({"intervalH": 5, "cron": "0 */4 * * *"})
 
     dumped = cfg.model_dump(by_alias=True)
 
     assert dumped["intervalH"] == 5
+    assert dumped["cron"] == "0 */4 * * *"
+
+
+def test_dream_config_dump_omits_empty_legacy_cron() -> None:
+    dumped = DreamConfig().model_dump(by_alias=True)
+
     assert "cron" not in dumped
 
 
diff --git a/tests/config/test_env_interpolation.py b/tests/config/test_env_interpolation.py
index a0e47d20..dad44382 100644
--- a/tests/config/test_env_interpolation.py
+++ b/tests/config/test_env_interpolation.py
@@ -81,6 +81,52 @@ class TestResolveConfig:
         saved = json.loads(config_path.read_text(encoding="utf-8"))
         assert saved["channels"]["telegram"]["token"] == "${MY_TOKEN}"
 
+    def test_save_preserves_dream_legacy_cron(self, tmp_path):
+        config_path = tmp_path / "config.json"
+        config_path.write_text(
+            json.dumps(
+                {"agents": {"defaults": {"dream": {"cron": "0 */4 * * *"}}}}
+            ),
+            encoding="utf-8",
+        )
+
+        config = load_config(config_path)
+        config.agents.defaults.max_tokens = 1234
+        save_config(config, config_path)
+
+        saved = json.loads(config_path.read_text(encoding="utf-8"))
+        assert saved["agents"]["defaults"]["dream"]["cron"] == "0 */4 * * *"
+
+        reloaded = load_config(config_path)
+        schedule = reloaded.agents.defaults.dream.build_schedule("UTC")
+        assert schedule.kind == "cron"
+        assert schedule.expr == "0 */4 * * *"
+
+    def test_save_keeps_oauth_provider_configs_excluded(self, tmp_path):
+        config_path = tmp_path / "config.json"
+        config_path.write_text(
+            json.dumps(
+                {
+                    "agents": {"defaults": {"dream": {"cron": "0 */4 * * *"}}},
+                    "providers": {
+                        "openaiCodex": {"apiKey": "codex-secret"},
+                        "githubCopilot": {"apiKey": "copilot-secret"},
+                        "groq": {"apiKey": "groq-secret"},
+                    },
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        config = load_config(config_path)
+        save_config(config, config_path)
+
+        saved = json.loads(config_path.read_text(encoding="utf-8"))
+        assert saved["agents"]["defaults"]["dream"]["cron"] == "0 */4 * * *"
+        assert "openaiCodex" not in saved["providers"]
+        assert "githubCopilot" not in saved["providers"]
+        assert saved["providers"]["groq"]["apiKey"] == "groq-secret"
+
     def test_preserves_excluded_fields_when_no_env_refs(self, tmp_path):
         """Regression: fields with ``exclude=True`` (e.g. ProviderConfig.openai_codex)
         must survive ``resolve_config_env_vars`` when the config has no