script: Is Synchronous; Async JS Belongs in function:

Status: Accepted Absorbs: DEC-1 (JS engine choice: Boa)

script: uses Boa for synchronous expression evaluation only — predicates, header/body transformations, simple logic. Async JS (await, fetch, npm imports) is explicitly out of scope for script: and belongs in function:.

Allowing async in script: would let user code evade circuit breakers, retry logic, and pipeline-level metrics, since those operate at the Tower layer above the script call. It would also introduce a JS event loop inside the Tokio runtime with no clean integration boundary. The function: step already handles the async use case with proper isolation and flow auditability.

Engine choice: Boa (absorbed from DEC-1)

In an integration framework, scripting languages serve as expression evaluators, not orchestration engines. This matches Apache Camel, where all scripting languages (Groovy, JS/Nashorn, Simple, OGNL) are strictly synchronous. Async I/O from a script would also obscure error-handling visibility and lose flow auditability (the YAML would no longer describe the full route). camel-language-js therefore stays on Boa; the original "Boa vs rquickjs" comparison is closed (rquickjs async support is no longer a motivating factor).