fix(server): send protocol errors on the wire with a single "MCP error <code>:" prefix (#659) - #662
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…r <code>:" prefix (#659) The SDK's Protocol copies a thrown error's .message verbatim into the JSON-RPC error and the client-side McpError constructor prefixes "MCP error <code>: " again, so every McpError thrown from a request handler reached SDK clients doubled ("MCP error -32602: MCP error -32602: …"). The dev proxy hid it with dedupeMcpErrorPrefix. Convert at the JSON-RPC boundary instead of at every throw site: WireMcpError is an McpError whose .message is the bare detail (class, code, data and stack preserved), and tools/call rethrows through toWireError; the prompt and resource handlers throw it directly. Upstream code keeps throwing McpError/DebugError, whose prefixed .message is right in logs, and the existing instanceof/code/message-contains tests hold. Adds an in-memory Server/Client test pinning the exact client-side message for tools/call argument validation, an unknown tool, a wrapped internal error, an unknown prompt and an unknown resource. Closes #659 Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DP1W22KJbkHZi4ueAtMFPF
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DP1W22KJbkHZi4ueAtMFPF
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Summary
Closes #659.
Every
McpErrorthrown from a request handler reached SDK clients asMCP error -32602: MCP error -32602: <reason>. The SDK'sProtocolcopies the thrown error's.messageverbatim into the JSON-RPC error, and the client-sideMcpErrorconstructor prefixesMCP error <code>:again. The dev proxy has been hiding this withdedupeMcpErrorPrefix, which is why it was easy to miss.Fix
Convert at the JSON-RPC boundary instead of at every throw site:
WireMcpError(src/errors/debug-errors.ts) is anMcpErrorwhose.messageis the bare detail; class,.code,.dataand stack are preserved.mcpErrorDetail()recovers the detail (aDebugError's recordeddetail, otherwise.messagewith exactly one own-code prefix stripped) andtoWireError()converts.tools/call(tool-dispatch.ts) rethrows throughtoWireError; the prompt and resource handlers throwWireMcpErrordirectly.McpError/DebugError, whose prefixed.messageis right in logs, and every existinginstanceof/.code/ message-contains test holds unchanged.Tests
tests/core/unit/server/mcp-wire-errors.test.tsdrives the real handlers through an SDKServer/Clientpair overInMemoryTransportand pins the exact client-side message (one prefix, then the reason) for: tools/call argument validation, an unknown tool, a non-McpErrorhandler rejection wrapped as InternalError, an unknown prompt, and an unknown resource (read + subscribe).mcpErrorDetail/WireMcpError/toWireErrorindebug-errors.test.ts.npm run typecheck:all,npm run lint, and the full server unit directory pass; the test ratchet is unchanged.🤖 Generated with Claude Code
https://claude.ai/code/session_01DP1W22KJbkHZi4ueAtMFPF