DurableAgent: single tool exception breaks entire agent stream

[iNishant]

Problem

In DurableAgent.stream(), tool calls are executed via Promise.all. Inside executeTool, only FatalError is caught and converted to an error-text result — any other exception re-throws, causing Promise.all to reject and killing the entire agent stream.

This differs from the AI SDK's streamText, where individual tool errors are returned as error results to the model, allowing recovery.

Expected behaviour

Non-fatal tool exceptions should be converted to per-tool error results and sent back to the model so the agent can continue. Only FatalError / AbortError should break the stream.

Versions

Package	Version
@workflow/ai	4.0.1-beta.51
workflow	4.0.1-beta.50
ai (Vercel AI SDK)	6.0.49

[TooTallNate]

Thanks for the well-written report — you're right, this is a bug.

The catch handler in executeTool (durable-agent.ts:1186–1202) currently only converts FatalError instances into error-text tool results and re-throws everything else. The assumption was that errors would always originate from within a step execution boundary (where retries are exhausted and the error is surfaced as a FatalError). That assumption doesn't hold — tool implementations don't need to be step functions, and there are other scenarios where a non-FatalError exception can be thrown outside a step boundary.

The fix is straightforward: any error caught in that handler should be converted to an error-text result and sent back to the model, rather than re-throwing and killing the stream. This aligns with the AI SDK's streamText behavior where individual tool failures are returned to the model for recovery.

Happy to review a PR if you'd like to take a crack at it!

[iNishant]

Just added a minor comment on the PR. Thanks folks.