fix: serialize tool outputs as JSON strings for MCP protocol compliance#140
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gsttg wants to merge 1 commit intoClickHouse:mainfrom
Open
fix: serialize tool outputs as JSON strings for MCP protocol compliance#140gsttg wants to merge 1 commit intoClickHouse:mainfrom
gsttg wants to merge 1 commit intoClickHouse:mainfrom
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The MCP protocol requires tool results to be returned as strings (wrapped
in TextContent), but `run_query`, `list_tables`, and `run_chdb_select_query`
were returning Python dicts/lists directly, causing output validation errors
like: `Output validation error: {...} is not of type 'string'`.
`list_databases` already correctly used `json.dumps()` — this commit makes
all tool functions consistent by wrapping their return values with
`json.dumps(..., default=str)`.
Also removes dead error-check code in `run_query` that could never trigger
since `execute_query` raises `ToolError` on failure rather than returning
an error dict.
Co-Authored-By: Claude Opus 4.6 <[email protected]>
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Summary
run_queryandlist_tablesreturn Python dicts instead of JSON strings, causing MCP output validation errors on every successful call:Output validation error: {...} is not of type 'string'json.dumps(..., default=str)to match the pattern already used bylist_databasesexecute_query,list_tables(2 return paths), andrun_chdb_select_query(all return paths)run_query(errors already raiseToolError)list_tablesreturn type annotation fromDict[str, Any]tostrSee linked issue for full details and reproduction steps.
Test plan
json.loads(result.content[0].text)assertions pass (confirms output is valid JSON string)test_run_select_query_syntax_errorpasses (error handling unaffected)🤖 Generated with Claude Code