diff --git a/src/prometheus/application/dto.py b/src/prometheus/application/dto.py
index 3292d8d..931a095 100644
--- a/src/prometheus/application/dto.py
+++ b/src/prometheus/application/dto.py
@@ -4,20 +4,48 @@ from __future__ import annotations
 from dataclasses import dataclass, field
 from typing import Any
 
+from pydantic import BaseModel, Field, field_validator, model_validator
 
-@dataclass
-class OptimizationConfig:
-    """Complete configuration for a PROMETHEUS run."""
 
-    # --- Prompt ---
-    seed_prompt: str
-    task_description: str
+# Current config schema version.
+CONFIG_VERSION = 1
+
+_ERROR_STRATEGY_VALUES = {"skip", "retry", "abort"}
+
+
+class OptimizationConfig(BaseModel):
+    """Complete configuration for a PROMETHEUS run.
+
+    Validated with Pydantic so missing or wrong-type fields produce clear,
+    actionable error messages at the CLI boundary instead of cryptic failures
+    deep in the pipeline.
+    """
+
+    model_config = {"extra": "forbid"}
+
+    # --- Schema version (for migration support) ---
+    config_version: int = Field(
+        default=CONFIG_VERSION,
+        description="Config schema version. Set automatically; used for migration.",
+    )
+
+    # --- Prompt (required) ---
+    seed_prompt: str = Field(
+        ...,
+        min_length=1,
+        description="The initial prompt to optimize.",
+    )
+    task_description: str = Field(
+        ...,
+        min_length=1,
+        description="Description of the task the prompt should accomplish.",
+    )
 
     # --- Models ---
-    task_model: str = "openai/gpt-4o-mini"
-    judge_model: str = "openai/gpt-4o"
-    proposer_model: str = "openai/gpt-4o"
-    synth_model: str = "openai/gpt-4o"
+    task_model: str = Field(default="openai/gpt-4o-mini", min_length=1)
+    judge_model: str = Field(default="openai/gpt-4o", min_length=1)
+    proposer_model: str = Field(default="openai/gpt-4o", min_length=1)
+    synth_model: str = Field(default="openai/gpt-4o", min_length=1)
 
     # --- Per-model API overrides (optional, fall back to global api_base/api_key_env) ---
     task_api_base: str | None = None
@@ -29,28 +57,54 @@ class OptimizationConfig:
     synth_api_base: str | None = None
     synth_api_key_env: str | None = None
 
+    # --- Global API settings (optional) ---
+    api_base: str | None = None
+    api_key_env: str | None = None
+
     # --- Evolution parameters ---
-    max_iterations: int = 30
-    n_synthetic_inputs: int = 20
-    minibatch_size: int = 5
-    perfect_score: float = 1.0
+    max_iterations: int = Field(default=30, ge=1, description="Maximum evolution iterations.")
+    n_synthetic_inputs: int = Field(default=20, ge=1, description="Number of synthetic inputs to generate.")
+    minibatch_size: int = Field(default=5, ge=1, description="Inputs per evaluation minibatch.")
+    perfect_score: float = Field(default=1.0, ge=0.0, le=1.0)
 
     # --- Reproducibility ---
-    seed: int = 42
+    seed: int = Field(default=42, ge=0)
 
     # --- Concurrency ---
-    max_concurrency: int = 5
+    max_concurrency: int = Field(default=5, ge=1, description="Max parallel LLM calls.")
 
     # --- Error handling ---
-    max_retries: int = 3
-    retry_delay_base: float = 1.0
-    circuit_breaker_threshold: int = 5
-    error_strategy: str = "retry"  # skip | retry | abort
+    max_retries: int = Field(default=3, ge=0, description="Max retry attempts for transient errors.")
+    retry_delay_base: float = Field(default=1.0, gt=0, description="Base delay in seconds for retry backoff.")
+    circuit_breaker_threshold: int = Field(default=5, ge=1, description="Consecutive failures before circuit opens.")
+    error_strategy: str = Field(default="retry", description="Error handling strategy: skip | retry | abort.")
 
     # --- Output ---
-    output_path: str = "output.yaml"
+    output_path: str = Field(default="output.yaml", min_length=1)
     verbose: bool = False
 
+    @field_validator("error_strategy")
+    @classmethod
+    def _validate_error_strategy(cls, v: str) -> str:
+        if v not in _ERROR_STRATEGY_VALUES:
+            raise ValueError(
+                f"error_strategy must be one of {sorted(_ERROR_STRATEGY_VALUES)}, got '{v}'"
+            )
+        return v
+
+    @model_validator(mode="before")
+    @classmethod
+    def _migrate_config(cls, data: Any) -> Any:
+        """Apply migration transforms for older config versions."""
+        if isinstance(data, dict):
+            version = data.get("config_version", CONFIG_VERSION)
+            # Future migrations go here, e.g.:
+            # if version < 2:
+            #     data = _migrate_v1_to_v2(data)
+            # Always stamp current version.
+            data["config_version"] = CONFIG_VERSION
+        return data
+
 
 @dataclass
 class OptimizationResult:
diff --git a/src/prometheus/cli/app.py b/src/prometheus/cli/app.py
index 6da3017..776f70e 100644
--- a/src/prometheus/cli/app.py
+++ b/src/prometheus/cli/app.py
@@ -12,6 +12,7 @@ from dataclasses import asdict
 
 import dspy
 import typer
+from pydantic import ValidationError
 from rich.console import Console
 from rich.panel import Panel
 from rich.table import Table
@@ -102,75 +103,58 @@ async def _async_optimize(
         )
     )
 
-    # 1. Load config
+    # 1. Load & validate config
     persistence = YamlPersistence()
     raw_config = persistence.read_config(input)
 
+    # CLI flags override config file values
+    raw_config.setdefault("max_retries", max_retries)
+    raw_config.setdefault("error_strategy", error_strategy)
+    raw_config.setdefault("max_concurrency", max_concurrency)
+    raw_config["output_path"] = output
+    raw_config["verbose"] = verbose
+
+    try:
+        config = OptimizationConfig.model_validate(raw_config)
+    except ValidationError as exc:
+        console.print("[bold red]Configuration error:[/bold red]\n")
+        for err in exc.errors():
+            loc = " → ".join(str(l) for l in err["loc"])
+            console.print(f"  [red]• {loc}: {err['msg']}[/red]")
+        raise typer.Exit(code=1) from exc
+    console.print(f"[dim]Task: {config.task_description[:80]}...[/dim]")
+    console.print(f"[dim]Seed prompt: {config.seed_prompt[:80]}...[/dim]")
+
+    # 2. Create per-model DSPy LM instances
     def _model_lm_kwargs(
         model_api_base: str | None,
         model_api_key_env: str | None,
-        global_api_base: str | None,
-        global_api_key_env: str | None,
     ) -> dict:
         """Build kwargs for dspy.LM, using per-model overrides with global fallback."""
         kwargs: dict = {}
-        api_base = model_api_base or global_api_base
-        api_key_env = model_api_key_env or global_api_key_env
+        api_base = model_api_base or config.api_base
+        api_key_env = model_api_key_env or config.api_key_env
         if api_base:
             kwargs["api_base"] = api_base
         if api_key_env:
             kwargs["api_key"] = os.environ.get(api_key_env, "")
         return kwargs
 
-    global_api_base = raw_config.get("api_base")
-    global_api_key_env = raw_config.get("api_key_env")
-
-    config = OptimizationConfig(
-        seed_prompt=raw_config["seed_prompt"],
-        task_description=raw_config["task_description"],
-        task_model=raw_config.get("task_model", "openai/gpt-4o-mini"),
-        judge_model=raw_config.get("judge_model", "openai/gpt-4o"),
-        proposer_model=raw_config.get("proposer_model", "openai/gpt-4o"),
-        synth_model=raw_config.get("synth_model", "openai/gpt-4o"),
-        task_api_base=raw_config.get("task_api_base"),
-        task_api_key_env=raw_config.get("task_api_key_env"),
-        judge_api_base=raw_config.get("judge_api_base"),
-        judge_api_key_env=raw_config.get("judge_api_key_env"),
-        proposer_api_base=raw_config.get("proposer_api_base"),
-        proposer_api_key_env=raw_config.get("proposer_api_key_env"),
-        synth_api_base=raw_config.get("synth_api_base"),
-        synth_api_key_env=raw_config.get("synth_api_key_env"),
-        max_iterations=raw_config.get("max_iterations", 30),
-        n_synthetic_inputs=raw_config.get("n_synthetic_inputs", 20),
-        minibatch_size=raw_config.get("minibatch_size", 5),
-        seed=raw_config.get("seed", 42),
-        max_retries=raw_config.get("max_retries", max_retries),
-        retry_delay_base=raw_config.get("retry_delay_base", 1.0),
-        circuit_breaker_threshold=raw_config.get("circuit_breaker_threshold", 5),
-        error_strategy=raw_config.get("error_strategy", error_strategy),
-        max_concurrency=raw_config.get("max_concurrency", max_concurrency),
-        output_path=output,
-        verbose=verbose,
-    )
-    console.print(f"[dim]Task: {config.task_description[:80]}...[/dim]")
-    console.print(f"[dim]Seed prompt: {config.seed_prompt[:80]}...[/dim]")
-
-    # 2. Create per-model DSPy LM instances
     task_lm = dspy.LM(
         config.task_model,
-        **_model_lm_kwargs(config.task_api_base, config.task_api_key_env, global_api_base, global_api_key_env),
+        **_model_lm_kwargs(config.task_api_base, config.task_api_key_env),
     )
     judge_lm = dspy.LM(
         config.judge_model,
-        **_model_lm_kwargs(config.judge_api_base, config.judge_api_key_env, global_api_base, global_api_key_env),
+        **_model_lm_kwargs(config.judge_api_base, config.judge_api_key_env),
     )
     proposer_lm = dspy.LM(
         config.proposer_model,
-        **_model_lm_kwargs(config.proposer_api_base, config.proposer_api_key_env, global_api_base, global_api_key_env),
+        **_model_lm_kwargs(config.proposer_api_base, config.proposer_api_key_env),
     )
     synth_lm = dspy.LM(
         config.synth_model,
-        **_model_lm_kwargs(config.synth_api_base, config.synth_api_key_env, global_api_base, global_api_key_env),
+        **_model_lm_kwargs(config.synth_api_base, config.synth_api_key_env),
     )
 
     # 3. Build adapters (Dependency Injection — each gets its own LM + retry config)
diff --git a/src/prometheus/config.py b/src/prometheus/config.py
index 744fbcc..783e687 100644
--- a/src/prometheus/config.py
+++ b/src/prometheus/config.py
@@ -1,12 +1,2 @@
 """Application settings."""
 from __future__ import annotations
-
-from dataclasses import dataclass
-
-
-@dataclass
-class AppSettings:
-    """Non-sensitive settings, hardcoded for the MVP."""
-
-    app_name: str = "prometheus"
-    version: str = "0.1.0"
diff --git a/tests/unit/test_config.py b/tests/unit/test_config.py
new file mode 100644
index 0000000..bdf138b
--- /dev/null
+++ b/tests/unit/test_config.py
@@ -0,0 +1,302 @@
+"""Unit tests for config and config loading via CLI.
+
+Tests config validation scenarios: missing fields, wrong types, defaults.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+import yaml
+from pydantic import ValidationError
+
+from prometheus.application.dto import OptimizationConfig
+from prometheus.infrastructure.file_io import YamlPersistence
+
+
+class TestOptimizationConfig:
+    """Tests for OptimizationConfig Pydantic model defaults."""
+
+    def test_default_values(self) -> None:
+        config = OptimizationConfig(
+            seed_prompt="test prompt",
+            task_description="test task",
+        )
+        assert config.task_model == "openai/gpt-4o-mini"
+        assert config.judge_model == "openai/gpt-4o"
+        assert config.proposer_model == "openai/gpt-4o"
+        assert config.synth_model == "openai/gpt-4o"
+        assert config.max_iterations == 30
+        assert config.n_synthetic_inputs == 20
+        assert config.minibatch_size == 5
+        assert config.perfect_score == 1.0
+        assert config.seed == 42
+        assert config.output_path == "output.yaml"
+        assert config.verbose is False
+        assert config.error_strategy == "retry"
+        assert config.max_retries == 3
+        assert config.max_concurrency == 5
+
+    def test_custom_values(self) -> None:
+        config = OptimizationConfig(
+            seed_prompt="custom prompt",
+            task_description="custom task",
+            max_iterations=100,
+            minibatch_size=10,
+            seed=123,
+            verbose=True,
+        )
+        assert config.max_iterations == 100
+        assert config.minibatch_size == 10
+        assert config.seed == 123
+        assert config.verbose is True
+
+    def test_roundtrip_to_dict(self) -> None:
+        config = OptimizationConfig(
+            seed_prompt="test",
+            task_description="task",
+        )
+        d = config.model_dump()
+        assert d["seed_prompt"] == "test"
+        assert d["task_description"] == "task"
+        assert "history" not in d  # OptimizationResult has history, not config
+
+    def test_config_version_defaults(self) -> None:
+        config = OptimizationConfig(seed_prompt="a", task_description="b")
+        assert config.config_version == 1
+
+    def test_config_version_stamps_current(self) -> None:
+        config = OptimizationConfig(
+            seed_prompt="a", task_description="b", config_version=0,
+        )
+        # Migration always stamps current version
+        assert config.config_version == 1
+
+
+class TestConfigLoading:
+    """Tests for loading OptimizationConfig from YAML via YamlPersistence."""
+
+    def test_minimal_config_loads(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        data = {
+            "seed_prompt": "You are helpful.",
+            "task_description": "Answer questions.",
+        }
+        config_file = tmp_path / "config.yaml"
+        with open(config_file, "w") as f:
+            yaml.dump(data, f)
+
+        raw = persistence.read_config(str(config_file))
+        config = OptimizationConfig.model_validate(raw)
+
+        assert config.seed_prompt == "You are helpful."
+        assert config.task_description == "Answer questions."
+        assert config.max_iterations == 30  # default
+
+    def test_full_config_loads(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        data = {
+            "seed_prompt": "You are helpful.",
+            "task_description": "Answer questions.",
+            "task_model": "openai/gpt-4o",
+            "judge_model": "openai/gpt-4o-mini",
+            "max_iterations": 50,
+            "n_synthetic_inputs": 30,
+            "minibatch_size": 8,
+            "seed": 99,
+            "verbose": True,
+        }
+        config_file = tmp_path / "config.yaml"
+        with open(config_file, "w") as f:
+            yaml.dump(data, f)
+
+        raw = persistence.read_config(str(config_file))
+        config = OptimizationConfig.model_validate(raw)
+
+        assert config.task_model == "openai/gpt-4o"
+        assert config.max_iterations == 50
+        assert config.verbose is True
+
+    def test_missing_seed_prompt_raises_validation_error(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        data = {"task_description": "Answer questions."}
+        config_file = tmp_path / "config.yaml"
+        with open(config_file, "w") as f:
+            yaml.dump(data, f)
+
+        raw = persistence.read_config(str(config_file))
+
+        with pytest.raises(ValidationError, match="seed_prompt"):
+            OptimizationConfig.model_validate(raw)
+
+    def test_missing_task_description_raises_validation_error(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        data = {"seed_prompt": "You are helpful."}
+        config_file = tmp_path / "config.yaml"
+        with open(config_file, "w") as f:
+            yaml.dump(data, f)
+
+        raw = persistence.read_config(str(config_file))
+
+        with pytest.raises(ValidationError, match="task_description"):
+            OptimizationConfig.model_validate(raw)
+
+    def test_empty_yaml_raises_on_required_fields(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        config_file = tmp_path / "empty.yaml"
+        config_file.write_text("{}", encoding="utf-8")
+
+        raw = persistence.read_config(str(config_file))
+
+        with pytest.raises(ValidationError):
+            OptimizationConfig.model_validate(raw)
+
+    def test_partial_config_uses_defaults(self, tmp_path: Path) -> None:
+        persistence = YamlPersistence()
+        data = {
+            "seed_prompt": "test",
+            "task_description": "task",
+            "max_iterations": 10,
+        }
+        config_file = tmp_path / "partial.yaml"
+        with open(config_file, "w") as f:
+            yaml.dump(data, f)
+
+        raw = persistence.read_config(str(config_file))
+        config = OptimizationConfig.model_validate(raw)
+
+        assert config.max_iterations == 10
+        assert config.n_synthetic_inputs == 20  # default
+        assert config.minibatch_size == 5  # default
+
+
+class TestConfigValidation:
+    """Tests for Pydantic validation edge cases."""
+
+    def test_wrong_type_max_iterations(self) -> None:
+        with pytest.raises(ValidationError, match="max_iterations"):
+            OptimizationConfig(
+                seed_prompt="a",
+                task_description="b",
+                max_iterations="not_a_number",  # type: ignore[arg-type]
+            )
+
+    def test_wrong_type_seed(self) -> None:
+        with pytest.raises(ValidationError, match="seed"):
+            OptimizationConfig(
+                seed_prompt="a",
+                task_description="b",
+                seed="not_a_number",  # type: ignore[arg-type]
+            )
+
+    def test_negative_max_iterations(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 1"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", max_iterations=0,
+            )
+
+    def test_negative_seed(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 0"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", seed=-1,
+            )
+
+    def test_zero_minibatch_size(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 1"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", minibatch_size=0,
+            )
+
+    def test_zero_max_concurrency(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 1"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", max_concurrency=0,
+            )
+
+    def test_negative_max_retries(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 0"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", max_retries=-1,
+            )
+
+    def test_zero_retry_delay_base(self) -> None:
+        with pytest.raises(ValidationError, match="greater than 0"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", retry_delay_base=0,
+            )
+
+    def test_negative_retry_delay_base(self) -> None:
+        with pytest.raises(ValidationError, match="greater than 0"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", retry_delay_base=-0.5,
+            )
+
+    def test_zero_circuit_breaker_threshold(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 1"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", circuit_breaker_threshold=0,
+            )
+
+    def test_perfect_score_above_one(self) -> None:
+        with pytest.raises(ValidationError, match="less than or equal to 1"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", perfect_score=1.5,
+            )
+
+    def test_perfect_score_negative(self) -> None:
+        with pytest.raises(ValidationError, match="greater than or equal to 0"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", perfect_score=-0.1,
+            )
+
+    def test_invalid_error_strategy(self) -> None:
+        with pytest.raises(ValidationError, match="error_strategy must be one of"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", error_strategy="invalid",
+            )
+
+    def test_valid_error_strategies(self) -> None:
+        for strategy in ("skip", "retry", "abort"):
+            config = OptimizationConfig(
+                seed_prompt="a", task_description="b", error_strategy=strategy,
+            )
+            assert config.error_strategy == strategy
+
+    def test_empty_seed_prompt(self) -> None:
+        with pytest.raises(ValidationError, match="seed_prompt"):
+            OptimizationConfig(seed_prompt="", task_description="b")
+
+    def test_empty_task_description(self) -> None:
+        with pytest.raises(ValidationError, match="task_description"):
+            OptimizationConfig(seed_prompt="a", task_description="")
+
+    def test_empty_model_string(self) -> None:
+        with pytest.raises(ValidationError, match="task_model"):
+            OptimizationConfig(
+                seed_prompt="a", task_description="b", task_model="",
+            )
+
+    def test_extra_fields_rejected(self) -> None:
+        with pytest.raises(ValidationError, match="extra"):
+            OptimizationConfig(
+                seed_prompt="a",
+                task_description="b",
+                nonexistent_field="value",  # type: ignore[call-arg]
+            )
+
+    def test_boundary_values_accepted(self) -> None:
+        config = OptimizationConfig(
+            seed_prompt="a",
+            task_description="b",
+            max_iterations=1,
+            n_synthetic_inputs=1,
+            minibatch_size=1,
+            max_concurrency=1,
+            max_retries=0,
+            retry_delay_base=0.001,
+            circuit_breaker_threshold=1,
+            perfect_score=0.0,
+            seed=0,
+        )
+        assert config.max_iterations == 1
+        assert config.perfect_score == 0.0