apache/answer · error

unknown tool: %s

Error message

unknown tool: %s

What it means

Fires in callMCPTool's default switch branch when an LLM tool call names a toolName not in the whitelist of supported MCP tools (get_questions, get_answers_by_question_id, get_comments, get_tags, get_tag_detail, get_user, semantic_search). It is a plain guard rejecting the unknown tool name sent by the model.

Source

Thrown at internal/controller/ai_controller.go:776

	log.Debugf("Calling MCP tool: %s with arguments: %v", toolName, arguments)

	switch toolName {
	case "get_questions":
		result, err = c.mcpController.MCPQuestionsHandler()(ctx, request)
	case "get_answers_by_question_id":
		result, err = c.mcpController.MCPAnswersHandler()(ctx, request)
	case "get_comments":
		result, err = c.mcpController.MCPCommentsHandler()(ctx, request)
	case "get_tags":
		result, err = c.mcpController.MCPTagsHandler()(ctx, request)
	case "get_tag_detail":
		result, err = c.mcpController.MCPTagDetailsHandler()(ctx, request)
	case "get_user":
		result, err = c.mcpController.MCPUserDetailsHandler()(ctx, request)
	case "semantic_search":
		result, err = c.mcpController.MCPSemanticSearchHandler()(ctx, request)
	default:
		return "", fmt.Errorf("unknown tool: %s", toolName)
	}

	if err != nil {
		return "", err
	}

	data, _ := json.Marshal(result)
	log.Debugf("MCP tool %s called successfully, result: %v", toolName, string(data))

	if result != nil && len(result.Content) > 0 {
		if textContent, ok := result.Content[0].(mcp.TextContent); ok {
			return textContent.Text, nil
		}
	}

	return "No result found", nil
}

View on GitHub (pinned to 3b9f137061)

Solutions

1. Use one of the registered tool names: get_questions, get_answers_by_question_id, get_comments, get_tags, get_tag_detail, get_user, etc.
2. Check the tool name passed by the LLM for typos or hallucinated names; constrain tool choices in the prompt/schema
3. Verify the LLM client advertises only the tools registered by callMCPTool so the model cannot select unknown names
4. Add the missing tool case to the switch if a new MCP tool was implemented but not registered here

When it happens

Trigger: Thrown at internal/controller/ai_controller.go:776 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/answer@3b9f137061 (2026-09-05). Data as JSON: /api/errors/dfc0315af0385bb0. Report an issue: GitHub.