Consolidate sampling examples and fix tool_choice bug (#2618)

* Fix tool_choice to always require tools when result_type is set

* Consolidate sampling examples with rich output

* Replace eval() with explicit add/multiply tools

examples/sampling/structured_output.py

@@ -0,0 +1,110 @@
+# /// script
+# dependencies = ["anthropic", "fastmcp", "rich"]
+# ///
+"""
+Structured Output Sampling
+
+Demonstrates using `result_type` to get validated Pydantic models from an LLM.
+The server exposes a sentiment analysis tool that returns structured data.
+
+Run:
+    uv run examples/sampling/structured_output.py
+"""
+
+import asyncio
+
+from pydantic import BaseModel
+from rich.console import Console
+from rich.panel import Panel
+from rich.table import Table
+
+from fastmcp import Client, Context, FastMCP
+from fastmcp.client.sampling import SamplingMessage, SamplingParams
+from fastmcp.client.sampling.handlers.anthropic import AnthropicSamplingHandler
+
+console = Console()
+
+
+class LoggingAnthropicHandler(AnthropicSamplingHandler):
+    async def __call__(
+        self, messages: list[SamplingMessage], params: SamplingParams, context
+    ):  # type: ignore[override]
+        console.print("      [bold blue]SAMPLING[/] Calling Claude API...")
+        result = await super().__call__(messages, params, context)
+        console.print("      [bold blue]SAMPLING[/] Response received")
+        return result
+
+
+# Define a structured output model
+class SentimentAnalysis(BaseModel):
+    sentiment: str  # "positive", "negative", or "neutral"
+    confidence: float  # 0.0 to 1.0
+    keywords: list[str]  # Key words that influenced the analysis
+    explanation: str  # Brief explanation of the analysis
+
+
+# Create the MCP server
+mcp = FastMCP("Sentiment Analyzer")
+
+
+@mcp.tool
+async def analyze_sentiment(text: str, ctx: Context) -> dict:
+    """Analyze the sentiment of the given text."""
+    console.print("    [bold cyan]SERVER[/] Analyzing sentiment...")
+
+    result = await ctx.sample(
+        messages=f"Analyze the sentiment of this text:\n\n{text}",
+        system_prompt="You are a sentiment analysis expert. Analyze text carefully.",
+        result_type=SentimentAnalysis,
+    )
+
+    console.print("    [bold cyan]SERVER[/] Analysis complete")
+    return result.result.model_dump()  # type: ignore[attr-defined]
+
+
+async def main():
+    console.print(
+        Panel.fit("[bold]MCP Sampling Flow Demo[/]", subtitle="structured_output.py")
+    )
+    console.print()
+
+    handler = LoggingAnthropicHandler(default_model="claude-sonnet-4-5")
+
+    async with Client(mcp, sampling_handler=handler) as client:
+        texts = [
+            "I absolutely love this product! It exceeded all my expectations.",
+            "The service was okay, nothing special but got the job done.",
+            "This is the worst experience I've ever had. Never again.",
+        ]
+
+        for text in texts:
+            console.print(f"[bold green]CLIENT[/] Analyzing: [italic]{text[:50]}...[/]")
+            console.print()
+
+            result = await client.call_tool("analyze_sentiment", {"text": text})
+            data = result.data
+
+            # Display results in a table
+            table = Table(show_header=False, box=None, padding=(0, 2))
+            table.add_column(style="bold")
+            table.add_column()
+
+            sentiment_color = {
+                "positive": "green",
+                "negative": "red",
+                "neutral": "yellow",
+            }.get(
+                data["sentiment"],
+                "white",  # type: ignore[union-attr]
+            )
+            table.add_row("Sentiment", f"[{sentiment_color}]{data['sentiment']}[/]")  # type: ignore[index]
+            table.add_row("Confidence", f"{data['confidence']:.0%}")  # type: ignore[index]
+            table.add_row("Keywords", ", ".join(data["keywords"]))  # type: ignore[index]
+            table.add_row("Explanation", data["explanation"])  # type: ignore[index]
+
+            console.print(Panel(table, border_style=sentiment_color))
+            console.print()
+
+
+if __name__ == "__main__":
+    asyncio.run(main())