test: lock in CallToolResult(is_error=True) with non-text content (#348)

A FastMCP tool can already report failure while returning rich (non-text)
content by returning a CallToolResult with is_error=True directly -- MCPServer
passes it through unchanged. This was untested and undocumented, which is what
issue #348 reports ("no way to set isError=True for arbitrary tool result
content").

Add a regression test proving an image content block plus is_error=True
round-trips to the caller over all transports, and a matching requirements
manifest entry (source issue:#348). Test-only; no library change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cognis-digital

tests/interaction/_requirements.py

@@ -528,6 +528,14 @@ def __post_init__(self) -> None:
             "in content, not as a JSON-RPC error."
         ),
     ),
+    "tools:call:is-error-with-content": Requirement(
+        source="issue:#348",
+        behavior=(
+            "A tool can return a hand-built CallToolResult with isError true that carries arbitrary "
+            "content (e.g. an image), not just text; the content blocks and the isError flag reach the "
+            "caller intact."
+        ),
+    ),
     "tools:call:logging-mid-execution": Requirement(
         source=f"{SPEC_BASE_URL}/server/utilities/logging#log-message-notifications",
         behavior=(

tests/interaction/mcpserver/test_tools.py

@@ -16,6 +16,7 @@
     CallToolResult,
     ElicitRequestURLParams,
     ErrorData,
+    ImageContent,
     LoggingMessageNotification,
     LoggingMessageNotificationParams,
     TextContent,
@@ -133,6 +134,30 @@ def add() -> None:
     assert result == snapshot(CallToolResult(content=[TextContent(text="Unknown tool: nope")], is_error=True))
 
 
+@requirement("tools:call:is-error-with-content")
+async def test_tool_returning_call_tool_result_can_flag_is_error_on_non_text_content(connect: Connect) -> None:
+    """A tool may hand back a CallToolResult with is_error=True carrying non-text content.
+
+    Raising an exception is the usual way to produce an is_error result, but that only yields a
+    text message. A tool that wants to report failure while returning richer content (here, an
+    image) can return a CallToolResult directly; MCPServer passes it through unchanged, so both the
+    image block and the is_error flag reach the caller. Regression lock-in for issue #348.
+    """
+    mcp = MCPServer("imager")
+
+    @mcp.tool()
+    def render() -> CallToolResult:
+        return CallToolResult(
+            content=[ImageContent(data="aW1n", mime_type="image/png")],
+            is_error=True,
+        )
+
+    async with connect(mcp) as client:
+        result = await client.call_tool("render", {})
+
+    assert result == snapshot(CallToolResult(content=[ImageContent(data="aW1n", mime_type="image/png")], is_error=True))
+
+
 @requirement("mcpserver:tool:output-schema:model")
 @requirement("tools:call:structured-content:text-mirror")
 async def test_call_tool_model_return_becomes_structured_content(connect: Connect) -> None: