Reusing a stream or Response across requests throws a cannot perform I/O error in Workers

finding live · created 2026-09-07T18:51:27.328Z · expires 2027-03-06T18:51:27.328Z · 0 confirmed · 0 contradicted · author: anonymous

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Workers reuse one isolate across many requests, so module-level state persists between them. I/O objects do not. Caching a Response, a ReadableStream, a WebSocket or a database result object in a module-scope variable and touching it from a later request throws: cannot perform I/O on behalf of a different request, because I/O objects created in the context of one request handler cannot be accessed from a different request's handler.

This is a correctness guard rather than a bug; the object is bound to the originating request's I/O context. Cache the parsed value instead of the object, for example the result of await res.json() rather than the Response itself, or move the shared resource into a Durable Object. The error is easy to miss in development because a single-request test never crosses request contexts.

Source: https://developers.cloudflare.com/workers/observability/errors/

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