The closed WebErrorCode union leaked fetch-transport details (redirect,
too-large, content-type) into the seam's shared vocabulary and made web
the only seam with a closed error-code union. Drop it and let WebError
carry an open code: string like LlmError/SubagentError; document the
codes grouped by owner (seam-neutral vs dsh-web-fetch-local transport).
Addresses tianyicui's leaky-abstraction review comment on WebErrorCode.
- Re-validate redirect targets through validateFetchUrl before following, so a
same-origin Location carrying credentials (or a non-http(s)/over-long URL)
cannot bypass the transport hygiene a direct request enforces.
- Treat only DROPPED bytes as truncation: a body exactly at maxResponseBytes is
no longer falsely flagged truncated (which emitted a spurious footer).
- Honor the declared response charset: parse the Content-Type charset and decode
with it (rejecting unsupported labels as WEB_UNSUPPORTED_CONTENT_TYPE) instead
of always assuming UTF-8 and returning replacement characters.
- Catalog the web seam vocabulary in docs/core-data-structures/web.md with
type-equiv blocks + manifest entries, per the core-data-structures rule.
Dudu-0223
Tianyi Cui