fix: custom prompts return a list of Message so FastMCP can render them

src/teradata_mcp_server/app.py

@@ -865,7 +865,7 @@ def make_custom_prompt(prompt_name: str, prompt: str, desc: str, parameters: dic
         if parameters is None or len(parameters) == 0:
 
             async def _dynamic_prompt():
-                return Message(role="user", content=TextContent(type="text", text=prompt))
+                return [Message(role="user", content=TextContent(type="text", text=prompt))]
 
             _dynamic_prompt.__name__ = prompt_name
             return mcp.prompt(description=desc)(_dynamic_prompt)
@@ -905,7 +905,7 @@ async def _dynamic_prompt_with_params(**kwargs: Any):  # type: ignore[no-untyped
                 if missing:
                     raise ValueError(f"Missing parameters: {missing}")
                 formatted_prompt = prompt.format(**kwargs)
-                return Message(role="user", content=TextContent(type="text", text=formatted_prompt))
+                return [Message(role="user", content=TextContent(type="text", text=formatted_prompt))]
 
             _dynamic_prompt_with_params.__signature__ = sig  # type: ignore[attr-defined]
             _dynamic_prompt_with_params.__annotations__ = annotations

tests/phase0/test_custom_prompt_return_shape.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python
+"""Unit tests for the return shape of YAML-defined (custom) prompts.
+
+FastMCP (>= the 4.x line this project requires) renders a prompt only when the
+function returns ``str``, ``list[Message]`` or ``PromptResult``. A bare ``Message``
+is rejected at render time with::
+
+    Prompt must return str, list[Message], or PromptResult, got Message
+
+so every prompt registered through ``make_custom_prompt`` (the ``*_objects.yml``
+prompts, with or without parameters) failed on ``prompts/get``. Tools were
+unaffected; only prompt rendering broke.
+"""
+
+import ast
+import asyncio
+from pathlib import Path
+
+import pytest
+from fastmcp import Client, FastMCP
+from fastmcp.prompts import Message
+from mcp.types import TextContent
+
+APP = Path(__file__).resolve().parents[2] / "src" / "teradata_mcp_server" / "app.py"
+
+
+def _render(returns_list: bool):
+    mcp = FastMCP("prompt-shape")
+
+    async def _p():
+        m = Message(role="user", content=TextContent(type="text", text="hello"))
+        return [m] if returns_list else m
+
+    _p.__name__ = "shape_probe"
+    mcp.prompt(description="probe")(_p)
+
+    async def go():
+        async with Client(mcp) as c:
+            return await c.get_prompt("shape_probe")
+
+    return asyncio.run(go())
+
+
+def test_fastmcp_rejects_a_bare_message():
+    """The contract this fix follows: a bare Message does not render."""
+    with pytest.raises(Exception, match="got Message"):
+        _render(returns_list=False)
+
+
+def test_fastmcp_renders_a_list_of_messages():
+    result = _render(returns_list=True)
+    assert [m.content.text for m in result.messages] == ["hello"]
+
+
+def test_make_custom_prompt_returns_a_list_on_every_branch():
+    """Both branches of make_custom_prompt (no parameters / with parameters)."""
+    tree = ast.parse(APP.read_text())
+    fn = next(n for n in ast.walk(tree)
+              if isinstance(n, ast.FunctionDef) and n.name == "make_custom_prompt")
+    prompt_fns = [n for n in ast.walk(fn)
+                  if isinstance(n, ast.AsyncFunctionDef) and n.name.startswith("_dynamic_prompt")]
+    assert len(prompt_fns) == 2, "expected one prompt function per branch"
+    for pf in prompt_fns:
+        returns = [n for n in ast.walk(pf) if isinstance(n, ast.Return) and n.value is not None]
+        assert returns, "the prompt function must return its message"
+        for r in returns:
+            assert isinstance(r.value, ast.List), (
+                f"{pf.name} (line {r.lineno}) must return a list of Message, "
+                f"not a bare Message")