docs: document best practice for handling argument errors in MCP tools#891
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…ations Resolves modelcontextprotocol#356 Clarify the two-tier error model: - Recoverable tool errors: use CallToolResult with isError(true) - Protocol-level errors: throw McpError / let exceptions propagate as JSON-RPC errors
Kehrlann
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Apr 1, 2026
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| ```java | ||
| // Example: missing required argument | ||
| if (lastName == null || lastName.isBlank()) { | ||
| return CallToolResult.builder() | ||
| .content(List.of(new McpSchema.TextContent("Missing required argument: last_name"))) | ||
| .isError(true) | ||
| .build(); | ||
| } | ||
| ``` |
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This example is a bit contrived, because MCP already has semantics for input parameter "required".
Please change it to something more realistic, e.g. validating an address, or an email?
Kehrlann
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Adds "DB timeout" as an explicit example of an infrastructure error to distinguish from input validation. Co-authored-by: Daniel Garnier-Moiroux <daniel.garnier-moiroux@broadcom.com>
Folds duplicate domain validation rows in the summary table to improve readability. Co-authored-by: Daniel Garnier-Moiroux <daniel.garnier-moiroux@broadcom.com>
Replaces the contrived missing argument check with a realistic email validation example per maintainer review. This clarifies that CallToolResult(isError=true) should be used for business logic/domain validation, since MCP natively handles missing required arguments via JSON Schema.
Author
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@Kehrlann Thanks for the review! I've updated the example to use email format validation. I also committed your suggestion for the DB timeout clarification and folded the two validation rows in the table into one. Let me know if everything looks good to go! |
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Resolves #356
Clarify the two-tier error model:
Motivation and Context
Issue #356 raised uncertainty about whether tool argument errors should throw
exceptions or be returned as CallToolResult. The maintainer clarified the correct
approach in the thread but it was not yet documented. This PR formalizes that guidance.
How Has This Been Tested?
Documentation-only change. The full test suite passes locally (747 tests, 0 failures).
Breaking Changes
None.
Types of changes
Checklist
Additional context
The new section was added under the existing
## Error Handlingsection indocs/server.mdto keep all error-related documentation in one place.