chore: import upstream snapshot with attribution

examples/sampling/server_fallback.py

@@ -0,0 +1,88 @@
+# /// script
+# dependencies = ["anthropic", "fastmcp", "rich"]
+# ///
+"""
+Server-Side Fallback Handler
+
+Demonstrates configuring a sampling handler on the server. This ensures
+sampling works even when the client doesn't provide a handler.
+
+The server runs as an HTTP server that can be connected to by any MCP client.
+
+Run:
+    uv run examples/sampling/server_fallback.py
+
+Then connect with any MCP client (e.g., Claude Desktop) or test with:
+    curl http://localhost:8000/mcp/
+"""
+
+import asyncio
+
+from rich.console import Console
+from rich.panel import Panel
+
+from fastmcp import FastMCP
+from fastmcp.client.sampling.handlers.anthropic import AnthropicSamplingHandler
+from fastmcp.server.context import Context
+
+console = Console()
+
+
+# Create server with a fallback sampling handler
+# This handler is used when the client doesn't support sampling
+mcp = FastMCP(
+    "Server with Fallback Handler",
+    sampling_handler=AnthropicSamplingHandler(default_model="claude-sonnet-4-5"),
+    sampling_handler_behavior="fallback",  # Use only if client lacks sampling
+)
+
+
+@mcp.tool
+async def summarize(text: str, ctx: Context) -> str:
+    """Summarize the given text."""
+    console.print(f"[bold cyan]SERVER[/] Summarizing text ({len(text)} chars)...")
+
+    result = await ctx.sample(
+        messages=f"Summarize this text in 1-2 sentences:\n\n{text}",
+        system_prompt="You are a concise summarizer.",
+        max_tokens=150,
+    )
+
+    console.print("[bold cyan]SERVER[/] Summary complete")
+    return result.text or ""
+
+
+@mcp.tool
+async def translate(text: str, target_language: str, ctx: Context) -> str:
+    """Translate text to the target language."""
+    console.print(f"[bold cyan]SERVER[/] Translating to {target_language}...")
+
+    result = await ctx.sample(
+        messages=f"Translate to {target_language}:\n\n{text}",
+        system_prompt=f"You are a translator. Output only the {target_language} translation.",
+        max_tokens=500,
+    )
+
+    console.print("[bold cyan]SERVER[/] Translation complete")
+    return result.text or ""
+
+
+async def main():
+    console.print(
+        Panel.fit(
+            "[bold]Server-Side Fallback Handler Demo[/]\n\n"
+            "This server has a built-in Anthropic handler that activates\n"
+            "when clients don't provide their own sampling support.",
+            subtitle="server_fallback.py",
+        )
+    )
+    console.print()
+    console.print("[bold yellow]Starting HTTP server on http://localhost:8000[/]")
+    console.print("Connect with an MCP client or press Ctrl+C to stop")
+    console.print()
+
+    await mcp.run_http_async(host="localhost", port=8000)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())