fix: preserve typed error as cause in SSE errors

[birdayz]

What

Wrap the typed error from mapToError() as the cause when throwing SSE stream errors.

Why

When an SSE stream contains a JSON-RPC error, we throw an Error with a formatted message string. The structured error data (code, message, data) is serialized into that string and lost. Consumers who need the structured data have to parse it back out with regex.

Implementation details

Use the standard JS error cause option to preserve the typed error:

throw new Error(
  `SSE event contained an error: ${err.message} (Code: ${err.code}) Data: ${JSON.stringify(err.data || {})}`,
  { cause: JsonRpcTransport.mapToError(a2aStreamResponse as JSONRPCErrorResponse) }
);

Backward compatible - the message string is unchanged. Consumers can access structured data via:

const cause = error.cause as JSONRPCTransportError;
const { code, message, data } = cause.errorResponse.error;

[gemini-code-assist]

Summary of Changes

Hello @birdayz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves error handling within the JsonRpcTransport by ensuring that when an SSE stream encounters a JSON-RPC error, the original structured error object is not merely converted into a string but is instead preserved as the cause property of the thrown Error. This allows for programmatic access to the detailed error information, making error handling more robust and developer-friendly without breaking existing integrations.

Highlights

• Preservation of Typed Errors: The pull request modifies the JsonRpcTransport to preserve the original typed error from mapToError() as the cause when throwing SSE stream errors. This prevents the loss of structured error data (code, message, data) that previously occurred when it was only serialized into a message string.
• Enhanced Error Accessibility: Consumers of the API can now access the structured error data directly via the error.cause property, which will be an instance of JSONRPCTransportError, eliminating the need for regex parsing of the error message string.
• Backward Compatibility: The change is backward compatible as the original error message string remains unchanged, ensuring existing error handling logic that relies on the message string will continue to function as before.

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[github-actions]

🧪 Code Coverage

⬇️ Download Full Report

	Main	PR	Delta
src/client/transports/json_rpc_transport.ts	60.79%	60.62%	🔴 -0.17%
Total	80.06%	80.04%	🔴 -0.02%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

The pull request modifies src/client/transports/json_rpc_transport.ts to enhance error handling. Specifically, when an SSE event contains an error, the thrown Error object now includes a cause property. This cause is populated by mapping the JSONRPCErrorResponse using JsonRpcTransport.mapToError, providing more detailed context for the error.

[ishymko]

Thank you for the PR!

Error handling definitely requires improvements here (i.e. using predefined transport-agnostic errors across all transports in a consistent way and documenting them at the client level). It'll hopefully improve for 1.0 release as we'll be able to drop some backward compatible leftovers from #169 required for #198.