[bartlomieju]

Closes #35612

Problem

When the server closes the connection mid-response (or any other transport-level failure occurs), Deno rejected fetch with the raw reqwest/hyper detail directly in .message and left .cause undefined:

msg="error sending request from 127.0.0.1:.. for http://.. (..): client error (SendRequest): connection closed before message completed" | cause=undefined

Node/undici instead reject with TypeError: "fetch failed" and put the detail in .cause:

msg="fetch failed" | cause=other side closed

Node-compatible error handling branches on this shape (err.message === "fetch failed" / inspecting err.cause), so the divergence breaks libraries — surfaced while running the fastify suite under deno test (fastify/fastify#6804).

Change

op_fetch_send now remaps ClientSend transport errors (connection refused, DNS failure, connection reset, connection closed mid-response, request body stream errors, …) to Node's shape: a TypeError whose message is "fetch failed" and whose .cause carries the low-level detail. Permission errors and other typed errors keep their own type. The detail is still emitted to the inspector Network domain (Network.loadingFailed.errorText) and to OpenTelemetry (exception.message).

After the fix, the issue's reproduction matches Node:

msg="fetch failed" | cause=error sending request from .. (..): client error (SendRequest): connection closed before message completed

Tests

• New regression test fetchConnectionClosedBeforeMessageComplete mirroring the issue reproduction.
• Updated existing fetch unit tests and spec fixtures (otel_basic, fetch_async_error_stack) to assert the Node-compatible shape (detail moved from .message to .cause).

Note on scope

This covers the request/response phase ("fetch failed"). A failure while reading the response body after headers are received is a separate Node shape (TypeError: "terminated") and is left for a follow-up.

[bartlomieju]

CI failures addressed in a9d1559 (merged latest main in 2891071 along the way):

• specs::cert::untrusted_cert_hint (all platforms): the DENO_TLS_CA_STORE hint matcher in runtime/fmt_errors.rs only inspected the top-level error message, and the TLS detail now lives in the cause of TypeError: fetch failed. get_message_suggestions now falls back to walking the cause chain when the top-level message yields no suggestions — this keeps the hint working for any error that wraps its actionable detail in .cause.
• unit::serve_test "Throw if disturbed" / "Throw if locked" (all platforms): these asserted the raw transport message (message.includes("client error")) on the client-side fetch rejection; updated to the new shape (message === "fetch failed", detail in .cause), matching the assertions in fetch_test.ts.