ui: prefer structuredContent preview for multi-block tool results

examples/mcp/structured-content-preview/example.py

@@ -0,0 +1,34 @@
+"""Manual demo for structuredContent preview behavior.
+
+Run from this directory:
+    uv run example.py
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+from fast_agent import FastAgent
+
+fast = FastAgent("Structured content preview demo")
+
+
+@fast.agent(
+    instruction=(
+        "Use the passthrough model to invoke the requested tool exactly as provided."
+    ),
+    servers=["structured_preview"],
+)
+async def main() -> None:
+    async with fast.run() as agent:
+        print("\n=== matching text + structuredContent ===")
+        matching = await agent.send("***CALL_TOOL structured_content_match {}")
+        print(matching)
+
+        print("\n=== mismatched text + structuredContent ===")
+        mismatched = await agent.send("***CALL_TOOL structured_content_mismatch {}")
+        print(mismatched)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())

examples/mcp/structured-content-preview/fastagent.config.yaml

@@ -0,0 +1,13 @@
+default_model: "passthrough"
+
+logger:
+  level: "error"
+  type: "console"
+  show_chat: false
+  show_tools: true
+  truncate_tools: false
+
+mcp:
+  targets:
+    - name: structured_preview
+      target: "uv run preview_server.py"

examples/mcp/structured-content-preview/preview_server.py

@@ -0,0 +1,75 @@
+"""MCP server for exercising structured tool result previews.
+
+Run with:
+    uv run preview_server.py
+"""
+
+from __future__ import annotations
+
+import json
+from typing import Any
+
+from mcp.server.fastmcp import FastMCP
+from mcp.types import CallToolResult, TextContent
+
+app = FastMCP(name="Structured Content Preview Demo")
+
+
+def _text_block(payload: Any) -> TextContent:
+    return TextContent(
+        type="text",
+        text=json.dumps(payload, ensure_ascii=False, separators=(",", ":")),
+    )
+
+
+def _tool_result(*, text_payloads: list[Any], structured_payload: dict[str, Any]) -> CallToolResult:
+    result = CallToolResult(
+        content=[_text_block(payload) for payload in text_payloads],
+        isError=False,
+    )
+    setattr(result, "structuredContent", structured_payload)
+    return result
+
+
+@app.tool(
+    name="structured_content_match",
+    description=(
+        "Return multiple text blocks that match the structuredContent payload. "
+        "Useful for checking the new preview path."
+    ),
+)
+def structured_content_match() -> CallToolResult:
+    tickets = [
+        {"ticket_id": "T-100", "status": "open", "owner": "alex"},
+        {"ticket_id": "T-101", "status": "pending", "owner": "sam"},
+    ]
+    return _tool_result(
+        text_payloads=tickets,
+        structured_payload={"tickets": tickets, "match_state": "match"},
+    )
+
+
+@app.tool(
+    name="structured_content_mismatch",
+    description=(
+        "Return multiple text blocks that do not match structuredContent. "
+        "Useful for seeing how the preview behaves when the two disagree."
+    ),
+)
+def structured_content_mismatch() -> CallToolResult:
+    text_tickets = [
+        {"ticket_id": "T-100", "status": "closed", "owner": "alex"},
+        {"ticket_id": "T-101", "status": "pending", "owner": "sam"},
+    ]
+    structured_tickets = [
+        {"ticket_id": "T-100", "status": "open", "owner": "alex"},
+        {"ticket_id": "T-101", "status": "escalated", "owner": "sam"},
+    ]
+    return _tool_result(
+        text_payloads=text_tickets,
+        structured_payload={"tickets": structured_tickets, "match_state": "mismatch"},
+    )
+
+
+if __name__ == "__main__":
+    app.run(transport="stdio")