René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"web_fetch tool fails on all URLs (TypeError: fetch failed) in IPv4-only sandbox environments","articleBody":"### Description\n\nThe built-in `web_fetch` tool fails on **every** URL, including trivial always-up test sites (e.g. `https://example.com`), returning a generic error:\n\n```\nTypeError: fetch failed\n```\n\nThis happens both with `raw: true` and default markdown mode, and regardless of the target site (tested `https://example.com` and `https://www.intel.com/...`).\n\n### Environment\n\n- CLI version: 1.0.68\n- OS: Linux (Copilot CLI cloud/session sandbox)\n- Host network: no global IPv6 address on the primary interface (`eth0` only has a link-local `fe80::/64` address; `ip -6 route show` shows no default IPv6 route)\n\n### Steps to reproduce\n\n1. In a Copilot CLI session, invoke the `web_fetch` tool against `https://example.com`.\n2. Observe `TypeError: fetch failed` with no further detail.\n\n### Diagnosis / evidence that this is not a general network outage\n\nFrom the same session's bash shell:\n- `curl https://example.com` → succeeds (`HTTP 200`)\n- `curl -4 https://example.com` (IPv4 forced) → succeeds consistently\n- `curl -6 https://example.com` (IPv6 forced) → fails consistently: `Failed to connect to example.com port 443 ... Couldn't connect to server`\n- Plain Node `fetch()` (same Node runtime available in the sandbox, v24.13.0) → succeeds, because Node's `fetch`/undici implementation performs Happy-Eyeballs-style fallback from IPv6 to IPv4 automatically\n- DNS resolves both `A` (IPv4) and `AAAA` (IPv6) records for the target domains\n\nThis strongly suggests the `web_fetch` tool's underlying HTTP client attempts the IPv6 (`AAAA`) address first (as returned by DNS) and does not fall back to IPv4 on failure, causing an immediate, opaque `TypeError: fetch failed` in environments where IPv6 egress is unavailable (no global IPv6 address/route on the host, only link-local).\n\nI even tried disabling IPv6 system-wide on the sandbox host via `sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1` (and `default`) — `curl`/Node fetch continued to work fine afterward, but `web_fetch` still failed identically. This indicates `web_fetch` likely executes in a separate process/container from the interactive bash sandbox, so host-level network changes made from within the session don't reach it — the fix needs to happen wherever that tool's HTTP client actually runs.\n\n### Expected behavior\n\n`web_fetch` should successfully fetch pages in environments with IPv4-only egress, ideally by:\n- Implementing Happy Eyeballs / IPv4 fallback when IPv6 connection attempts fail, and/or\n- Surfacing a more actionable error message (e.g., distinguishing DNS failure vs. connection refused vs. TLS failure) instead of a generic `TypeError: fetch failed`.\n\n### Workaround\n\nUsing `curl` via the `bash` tool as a substitute for `web_fetch` works reliably in the same session.\n","author":{"url":"https://github.com/lanrebr","@type":"Person","name":"lanrebr"},"datePublished":"2026-07-07T03:47:35.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/4041/copilot-cli/issues/4041"}