diff --git a/src/cmdforge/cli/__init__.py b/src/cmdforge/cli/__init__.py
index 74864a0..8c01db8 100644
--- a/src/cmdforge/cli/__init__.py
+++ b/src/cmdforge/cli/__init__.py
@@ -103,6 +103,12 @@ def main():
     )
     p_inspect.set_defaults(func=cmd_inspect)
 
+    # 'optimize' command
+    p_optimize = subparsers.add_parser("optimize", help="Generate and test prompt variations for a tool")
+    p_optimize.add_argument("name", help="Tool name")
+    p_optimize.add_argument("-n", "--count", type=int, default=5, help="Number of variations")
+    p_optimize.set_defaults(func=cmd_optimize)
+
     # 'check' command
     p_check = subparsers.add_parser("check", help="Check dependencies for a tool (meta-tools)")
     p_check.add_argument("name", help="Tool name")
@@ -674,5 +680,38 @@ def cmd_inspect(args):
     return 0 if not report.errors else 1
 
 
+def cmd_optimize(args):
+    """Generate and test prompt variations for a tool."""
+    from ..prompt_optimizer import optimize_tool
+    from ..tool import load_tool
+
+    tool = load_tool(args.name)
+    if not tool:
+        print(f"Error: Tool '{args.name}' not found.", file=sys.stderr)
+        return 1
+
+    print(f"Optimizing '{tool.name}' — generating {args.count} prompt variations...")
+    print()
+
+    result = optimize_tool(tool, count=args.count)
+
+    print(f"Baseline score: {result.baseline_score}")
+    print(f"Variations tested: {len(result.variations)}")
+    print()
+
+    if result.best:
+        print(f"Best variation (index {result.best_index + 1}, strategy: {result.best.strategy}):")
+        print(f"  Score: {result.scores.get(result.best_index, 0)}")
+        print(f"  Improvement: {result.improvement:.0f}%")
+        print(f"  Original: {result.best.original[:100]}{'...' if len(result.best.original) > 100 else ''}")
+        print(f"  Variation: {result.best.variation[:100]}{'...' if len(result.best.variation) > 100 else ''}")
+        print()
+        print("To apply this variation, edit the tool's config.yaml and update the prompt step.")
+    else:
+        print("No improvements found. The original prompt may already be optimal.")
+
+    return 0
+
+
 if __name__ == "__main__":
     sys.exit(main())
diff --git a/src/cmdforge/prompt_optimizer.py b/src/cmdforge/prompt_optimizer.py
new file mode 100644
index 0000000..a3f0b18
--- /dev/null
+++ b/src/cmdforge/prompt_optimizer.py
@@ -0,0 +1,216 @@
+"""Prompt variation generation and optimization (M9.1).
+
+Generates N variations of a tool's prompt steps, runs each through M8.V1
+contract conformance tests, and surfaces the highest-scoring variation.
+"""
+
+from dataclasses import dataclass, field
+from typing import Any, Dict, List, Optional
+
+from .tool import Tool, PromptStep
+
+
+@dataclass
+class PromptVariation:
+    """One generated variation of a tool's prompt."""
+
+    step_index: int
+    original: str
+    variation: str
+    strategy: str  # how this variation was created
+
+    def to_dict(self) -> dict:
+        return {
+            "step_index": self.step_index,
+            "original": self.original,
+            "variation": self.variation,
+            "strategy": self.strategy,
+        }
+
+
+@dataclass
+class OptimizationResult:
+    """Results of running all variations through contract tests."""
+
+    tool_name: str
+    variations: List[PromptVariation] = field(default_factory=list)
+    scores: Dict[int, int] = field(default_factory=dict)  # variation_index -> score
+    best_index: Optional[int] = None
+    baseline_score: int = 0
+
+    @property
+    def best(self) -> Optional[PromptVariation]:
+        if self.best_index is not None and self.best_index < len(self.variations):
+            return self.variations[self.best_index]
+        return None
+
+    @property
+    def improvement(self) -> float:
+        if self.best is None or self.baseline_score == 0:
+            return 0.0
+        best = self.scores.get(self.best_index, 0)
+        return (best - self.baseline_score) / max(self.baseline_score, 1) * 100
+
+    def to_dict(self) -> dict:
+        return {
+            "tool": self.tool_name,
+            "baseline_score": self.baseline_score,
+            "best_index": self.best_index,
+            "improvement_pct": round(self.improvement, 1),
+            "best": self.best.to_dict() if self.best else None,
+            "variations": [v.to_dict() for v in self.variations],
+            "scores": {str(k): v for k, v in self.scores.items()},
+        }
+
+
+def generate_variations(
+    tool: Tool,
+    count: int = 5,
+    provider: str = "mock",
+) -> List[PromptVariation]:
+    """Generate prompt variations for each prompt step in the tool.
+
+    Strategies:
+      - rephrase: ask the AI to rephrase the prompt
+      - structure: convert to bullet points
+      - concise: make it shorter
+      - detailed: add more context
+      - focused: emphasize the key task
+
+    Args:
+        tool: The tool to optimize.
+        count: Number of variations per prompt step.
+        provider: Provider to use for variation generation.
+
+    Returns:
+        List of PromptVariation objects.
+    """
+    variations: List[PromptVariation] = []
+
+    for i, step in enumerate(tool.steps):
+        if not isinstance(step, PromptStep):
+            continue
+
+        original = step.prompt
+
+        strategies = _get_strategies(original, count)
+
+        for strategy, variation_text in strategies:
+            if variation_text and variation_text != original:
+                variations.append(PromptVariation(
+                    step_index=i,
+                    original=original,
+                    variation=variation_text,
+                    strategy=strategy,
+                ))
+
+    return variations
+
+
+def _get_strategies(original: str, count: int) -> List[tuple]:
+    """Generate variations using different strategies."""
+    strategies: List[tuple] = []
+
+    # Attempt AI-driven rephrase if a real provider is available
+    rephrased = _call_provider_for_variation(
+        f"Rephrase this instruction while keeping the same meaning. Output only the rephrased text:\n\n{original}"
+    )
+    if rephrased and rephrased != original:
+        strategies.append(("rephrase", rephrased))
+
+    # Deterministic mock variations
+    mock_strategies = [
+        ("focused", f"Complete this specific task — {original}"),
+        ("concise", _truncate(original, 120)),
+        ("detailed", f"Carefully perform the following: {original}. Ensure correctness and completeness."),
+        ("rephrase", f"I need you to do this: {original}"),
+        ("structure", f"Step through this task:\n\n{original}\n\nRespond step by step."),
+    ]
+    for strat, text in mock_strategies:
+        if len(strategies) >= count:
+            break
+        if text and text != original:
+            strategies.append((strat, text))
+
+    return strategies[:count]
+
+
+def _truncate(text: str, max_len: int) -> str:
+    if len(text) <= max_len:
+        return text
+    return text[:max_len - 3].rsplit(" ", 1)[0] + "..."
+
+
+def _call_provider_for_variation(prompt: str) -> Optional[str]:
+    """Try to call a provider for variation generation."""
+    try:
+        from .providers import call_provider
+        result = call_provider("opencode-pickle", prompt, timeout=15)
+        if result.success and result.text:
+            text = result.text.strip()
+            if text and len(text) >= 10:
+                return text
+    except Exception:
+        pass
+    return None
+
+
+def optimize_tool(
+    tool: Tool,
+    count: int = 5,
+    provider: str = "mock",
+) -> OptimizationResult:
+    """Run prompt optimization for a tool.
+
+    Generates variations, runs each through contract tests, and returns
+    the best-performing variation.
+
+    Args:
+        tool: The tool to optimize.
+        count: Number of variations per prompt step.
+        provider: Provider for variation generation.
+
+    Returns:
+        OptimizationResult with scores and best variation.
+    """
+    from .contract_testing import run_contract_tests
+
+    result = OptimizationResult(tool_name=tool.name)
+
+    # Baseline score
+    baseline = run_contract_tests(tool)
+    result.baseline_score = _count_passed(baseline)
+
+    # Generate variations
+    result.variations = generate_variations(tool, count=count, provider=provider)
+
+    if not result.variations:
+        return result
+
+    # Test each variation
+    for idx, variation in enumerate(result.variations):
+        # Create a copy with the modified prompt
+        import copy
+        tool_copy = copy.deepcopy(tool)
+        if 0 <= variation.step_index < len(tool_copy.steps):
+            step = tool_copy.steps[variation.step_index]
+            if isinstance(step, PromptStep):
+                step.prompt = variation.variation
+
+        try:
+            test_result = run_contract_tests(tool_copy)
+            result.scores[idx] = _count_passed(test_result)
+        except Exception:
+            result.scores[idx] = 0
+
+    # Find best
+    if result.scores:
+        result.best_index = max(result.scores, key=lambda k: result.scores[k])
+
+    return result
+
+
+def _count_passed(test_result) -> int:
+    """Count passed tests in a ConformanceReport."""
+    return sum(1 for r in getattr(test_result, "results", [])
+               if getattr(r, "state", "") == "passed")
diff --git a/tests/test_prompt_optimizer.py b/tests/test_prompt_optimizer.py
new file mode 100644
index 0000000..0972925
--- /dev/null
+++ b/tests/test_prompt_optimizer.py
@@ -0,0 +1,148 @@
+"""Tests for prompt optimization (M9.1)."""
+
+from unittest.mock import patch
+
+from cmdforge.prompt_optimizer import (
+    PromptVariation,
+    OptimizationResult,
+    generate_variations,
+    optimize_tool,
+    _get_strategies,
+    _truncate,
+)
+
+
+class TestTruncate:
+    def test_short_text_unchanged(self):
+        assert _truncate("hello", 10) == "hello"
+
+    def test_long_text_truncated(self):
+        result = _truncate("this is a very long sentence that needs truncation", 20)
+        assert len(result) <= 20
+        assert result.endswith("...")
+
+
+class TestGenerateVariations:
+    def test_generates_for_prompt_steps(self, tmp_path):
+        from cmdforge.tool import Tool, PromptStep
+
+        with patch("cmdforge.tool.TOOLS_DIR", tmp_path / ".cmdforge"):
+            tool = Tool(
+                name="test-tool",
+                steps=[
+                    PromptStep(
+                        prompt="Analyze the input and produce a summary",
+                        provider="mock",
+                        output_var="summary",
+                    ),
+                ],
+                output="{summary}",
+            )
+            variations = generate_variations(tool, count=3)
+            assert len(variations) >= 1
+            for v in variations:
+                assert v.step_index == 0
+                assert v.original == "Analyze the input and produce a summary"
+                assert v.variation != v.original
+
+    def test_skips_non_prompt_steps(self, tmp_path):
+        from cmdforge.tool import Tool, CodeStep
+
+        with patch("cmdforge.tool.TOOLS_DIR", tmp_path / ".cmdforge"):
+            tool = Tool(
+                name="test-tool",
+                steps=[CodeStep(code="print('hello')", output_var="out")],
+                output="{out}",
+            )
+            variations = generate_variations(tool)
+            assert len(variations) == 0
+
+
+class TestGetStrategies:
+    def test_produces_variations(self):
+        strategies = _get_strategies("Summarize the text", 3)
+        assert len(strategies) == 3
+        for strat, text in strategies:
+            assert len(text) > 0
+            assert strat in ("focused", "concise", "detailed", "rephrase", "structure")
+
+    def test_truncates_concise(self):
+        original = "A" * 200
+        strategies = _get_strategies(original, 3)
+        concise = [t for s, t in strategies if s == "concise"]
+        if concise:
+            assert len(concise[0]) <= 125  # 120 + "..."
+
+
+class TestOptimizeTool:
+    def test_baseline_score_recorded(self, tmp_path):
+        from cmdforge.tool import Tool, PromptStep
+
+        with patch("cmdforge.tool.TOOLS_DIR", tmp_path / ".cmdforge"):
+            tool = Tool(
+                name="opt-test",
+                steps=[
+                    PromptStep(
+                        prompt="Summarize: {input}",
+                        provider="mock",
+                        output_var="result",
+                    ),
+                ],
+                output="{result}",
+            )
+            result = optimize_tool(tool, count=2)
+            assert result.tool_name == "opt-test"
+            assert result.baseline_score >= 0
+            assert len(result.variations) >= 1
+            assert len(result.scores) == len(result.variations)
+
+    def test_no_prompt_steps_returns_empty(self, tmp_path):
+        from cmdforge.tool import Tool, CodeStep
+
+        with patch("cmdforge.tool.TOOLS_DIR", tmp_path / ".cmdforge"):
+            tool = Tool(
+                name="code-only",
+                steps=[CodeStep(code="pass", output_var="x")],
+                output="{x}",
+            )
+            result = optimize_tool(tool)
+            assert len(result.variations) == 0
+            assert result.best is None
+
+
+class TestPromptVariation:
+    def test_to_dict(self):
+        v = PromptVariation(
+            step_index=0,
+            original="hello",
+            variation="hi there",
+            strategy="rephrase",
+        )
+        d = v.to_dict()
+        assert d["original"] == "hello"
+        assert d["strategy"] == "rephrase"
+
+
+class TestOptimizationResult:
+    def test_improvement_calculation(self):
+        result = OptimizationResult(
+            tool_name="test",
+            variations=[
+                PromptVariation(0, "a", "b", "rephrase"),
+            ],
+            scores={0: 5},
+            best_index=0,
+            baseline_score=3,
+        )
+        assert result.best is not None
+        assert result.improvement > 0
+
+    def test_no_improvement_when_no_baseline(self):
+        result = OptimizationResult(
+            tool_name="test",
+            variations=[],
+            scores={},
+            baseline_score=0,
+        )
+        assert result.best is None
+        assert result.improvement == 0.0