Cloud Agent: pasted images fail with 'attachment <uuid>.png could not be retrieved (undefined is not a function)'

[maphew]

Summary

Pasting an image into a Kilo Cloud Agent thread (kilo.ai web app) shows a preview in the composer, but after submitting, the message instead contains:

attachment a7393f97-c6bf-4c1f-9fec-2bbb11987962.png could not be retrieved (undefined is not a function)

The agent then "carries on" without the image. Reproduced with different UUIDs (e.g. 4024b67d-4294-4493-8884-ce0f9f40b836.png) in separate sessions — the image is silently dropped and the model never sees it.

Diagnosis

The string is generated by services/cloud-agent-next/wrapper/src/session-bootstrap.ts (downloadAndMaterializeAttachment): when the wrapper in the sandbox fetches the presigned R2 attachment URL, any fetch/read error is interpolated verbatim into a synthetic text part:

text: `attachment ${attachment.filename} could not be retrieved (${message})`

So (undefined is not a function) is a raw JS TypeError message leaking into the user-visible transcript. It matches a known class of Bun Web Streams bugs where reading a fetch() response body whose bytes arrive after the response head throws exactly TypeError: undefined is not a function (see oven-sh/bun#34302, #28952, #28953). I reproduced the exact message 100/100 times on Bun 1.3.12 by iterating a delayed-body Response.body; the failure is intermittent in production and depends on network latency + runtime version.

Impact

• Pasted images are lost without the user being told anything actionable.
• The raw internal error message (undefined is not a function) is meaningless to users and agents.
• No retry exists, so a transient network/stream race permanently drops the attachment.

Proposed fix

PR (once open): https://github.com/Kilo-Org/cloud/pull/XXXX

• Retry transient attachment-download failures (3 attempts, short backoff).
• On retry, re-fetch and read the body with response.arrayBuffer() (the workaround the Bun maintainers verified never hits the stream race) when content-length is bounded.
• On exhaustion, surface attachment <name> could not be retrieved (download failed after N attempts) and log the raw reason instead of leaking raw JS errors.

[maphew]

Fix PR: #5898 — retries transient attachment-download failures (with a buffered read on retry to bypass the Bun Web Streams race) and no longer leaks raw JS error text like undefined is not a function into the transcript.