diff --git a/tests/test_config.py b/tests/test_config.py
new file mode 100644
index 0000000..00b3719
--- /dev/null
+++ b/tests/test_config.py
@@ -0,0 +1,42 @@
+"""Tests for Settings field validators."""
+
+import pytest
+from pydantic import ValidationError
+
+from token0.config import Settings
+
+
+def test_storage_mode_valid_values():
+    assert Settings(storage_mode="lite").storage_mode == "lite"
+    assert Settings(storage_mode="full").storage_mode == "full"
+
+
+def test_storage_mode_invalid_raises():
+    with pytest.raises(ValidationError):
+        Settings(storage_mode="prod")
+
+
+def test_text_density_threshold_valid():
+    assert Settings(text_density_threshold=0.0).text_density_threshold == 0.0
+    assert Settings(text_density_threshold=1.0).text_density_threshold == 1.0
+    assert Settings(text_density_threshold=0.52).text_density_threshold == 0.52
+
+
+def test_text_density_threshold_out_of_range_raises():
+    with pytest.raises(ValidationError):
+        Settings(text_density_threshold=1.1)
+    with pytest.raises(ValidationError):
+        Settings(text_density_threshold=-0.1)
+
+
+def test_port_valid():
+    assert Settings(port=8000).port == 8000
+    assert Settings(port=1).port == 1
+    assert Settings(port=65535).port == 65535
+
+
+def test_port_out_of_range_raises():
+    with pytest.raises(ValidationError):
+        Settings(port=0)
+    with pytest.raises(ValidationError):
+        Settings(port=65536)
diff --git a/token0/config.py b/token0/config.py
index 7413ade..743cc01 100644
--- a/token0/config.py
+++ b/token0/config.py
@@ -1,10 +1,13 @@
+from typing import Literal
+
+from pydantic import field_validator
 from pydantic_settings import BaseSettings
 
 
 class Settings(BaseSettings):
     # Storage mode: "lite" (SQLite + in-memory) or "full" (Postgres + Redis + S3)
     # Use "lite" for local dev/testing, "full" for production
-    storage_mode: str = "lite"
+    storage_mode: Literal["lite", "full"] = "lite"
 
     # Database — only needed in full mode
     database_url: str = "postgresql+asyncpg://token0:token0@localhost:5432/token0"
@@ -40,6 +43,20 @@ class Settings(BaseSettings):
     jpeg_quality: int = 85
     text_density_threshold: float = 0.52  # Above this → OCR route instead of vision
 
+    @field_validator("text_density_threshold")
+    @classmethod
+    def validate_threshold(cls, v: float) -> float:
+        if not 0.0 <= v <= 1.0:
+            raise ValueError(f"text_density_threshold must be between 0.0 and 1.0, got {v}")
+        return v
+
+    @field_validator("port")
+    @classmethod
+    def validate_port(cls, v: int) -> int:
+        if not 1 <= v <= 65535:
+            raise ValueError(f"port must be between 1 and 65535, got {v}")
+        return v
+
     @property
     def is_lite(self) -> bool:
         return self.storage_mode == "lite"