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(); | ||
| } | ||
| ``` |
Contributor
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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?
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| **2. Protocol-Level Errors (Unrecoverable)** | ||
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| Uncaught exceptions from a tool handler are mapped to a JSON-RPC error response. Use this only for truly unexpected failures (e.g., infrastructure errors), not for input validation. |
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Suggested change
| Uncaught exceptions from a tool handler are mapped to a JSON-RPC error response. Use this only for truly unexpected failures (e.g., infrastructure errors), not for input validation. | |
| Uncaught exceptions from a tool handler are mapped to a JSON-RPC error response. Use this only for truly unexpected failures (e.g., infrastructure errors such as DB timeout), not for input validation. |
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| | Missing / invalid argument | `CallToolResult` with `isError=true` | | ||
| | Domain validation failure | `CallToolResult` with `isError=true` | |
Contributor
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Both cases are the same, let's fold them together.
Suggested change
| | Missing / invalid argument | `CallToolResult` with `isError=true` | | |
| | Domain validation failure | `CallToolResult` with `isError=true` | | |
| | Domain validation failure | `CallToolResult` with `isError=true` | |
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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.