fix: correct MCPServer call_tool result type#2816
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Kludex merged 5 commits intoJun 20, 2026
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Move the conversion of the internal tool-result shapes (bare content sequence and (content, structured_content) tuple) out of the low-level handler and into call_tool, so the public API has a single CallToolResult return type. Type the conversion chain with a ToolResult alias and convert_result overloads keyed on the convert_result flag, removing the cast and type: ignore the previous approach needed.
ToolManager.call_tool forwards convert_result as a bool, never a literal, so the Literal-keyed overloads never resolved. The overloads that matter are on ToolManager.call_tool, which MCPServer.call_tool calls with convert_result=True.
Have FuncMetadata.convert_result return a CallToolResult instead of the intermediate (content, structured_content) tuple / bare content sequence shapes. MCPServer.call_tool becomes a passthrough, and the ToolResult alias plus the convert_result overloads on ToolManager.call_tool are no longer needed.
maxisbey
approved these changes
Jun 20, 2026
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Summary
Fixes #2695.
MCPServer.call_tool()currently advertisesSequence[ContentBlock] | dict[str, Any], but theconvert_result=Truepath never returns a raw dict. It can return only:CallToolResult,Sequence[ContentBlock]for unstructured tools, or(content, structured_content)tuple for structured tools.This PR removes the unreachable raw-dict branch in
_handle_call_tool(), drops the now-unusedjsonimport, and introduces aToolResulttype alias so the public annotation and handler implementation share the same result shape.Tests
uv run --frozen pytest tests/server/mcpserver/test_server.py -quv run --frozen pytest tests/server/mcpserver/test_server.py::TestServerTools::test_call_tool_returns_declared_result_shapes -quv run --frozen coverage erase && uv run --frozen coverage run -m pytest tests/server/mcpserver/test_server.py && uv run --frozen coverage combine && uv run --frozen coverage report --include='src/mcp/server/mcpserver/server.py' --fail-under=0 && UV_FROZEN=1 uv run --frozen strict-no-coveruv run --frozen ruff format --check .uv run --frozen ruff check .uv run --frozen pyrightuvx --from pre-commit pre-commit run markdownlint --files docs/migration.mduvx --from pre-commit pre-commit run end-of-file-fixer --files docs/migration.md src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.pyAI assistance disclosure
I used an AI coding assistant to help trace the result shapes and draft the patch. I reviewed the final diff and ran the validation commands above locally.