[Bugfix] Bracket IPv6 hosts in the DP supervisor probe URL

[stantheman0128]

Purpose

DPSupervisor starts its own server on --data-parallel-supervisor-port (9256 by default) only after every child API server answers /health with 200. It probes each child at _child_base_url(args, port) + "/health", and _child_base_url maps --host :: to ::1 but then returns http://::1:8001 without brackets. aiohttp rejects that URL with InvalidUrlClientError.

_probe_endpoint catches aiohttp.ClientError, logs it at debug level and returns False. Before the children are ready, _probe_all_children just probes again on the next interval. So with --host :: or any IPv6 literal, the supervisor never becomes ready and never starts its server, and nothing above debug level says why.

The fix builds the URL with join_host_port from vllm/utils/network_utils.py. It brackets IPv6 literals and leaves IPv4 addresses and hostnames as they were. launcher.py and snapshot/runtime.py also bracket IPv6 hosts.

How to reproduce

On main with aiohttp 3.14.3 and yarl 1.24.5. No GPU is needed.

1. dp_sup._child_base_url(argparse.Namespace(host="::", ssl_keyfile=None, ssl_certfile=None), 8001) returns http://::1:8001.
2. A GET to http://::1:8001/health through aiohttp.ClientSession raises aiohttp.client_exceptions.InvalidUrlClientError, caused by ValueError("Invalid URL: port can't be converted to integer").
3. Start an aiohttp server on [::1] that answers /health with 200. A direct GET to http://[::1]:<port>/health returns 200, but dp_sup._probe_endpoint(session, args, port, "/health", conn_err_failure_threshold=1) with the same host="::" args returns False.

Why this is not duplicating an existing PR

• Two open PRs edit the same two files. [Bugfix] Keep the multi-port DP supervisor serving through fault-tolerant recovery #54963 keeps the supervisor serving through fault-tolerant recovery. It does not change _child_base_url, but it adds a new caller: _child_state requests _child_base_url(self.args, port) + "/fault_tolerance/status", so that URL has the same IPv6 problem on main and gets the same fix here. [Bugfix][Frontend] Allow selecting Uvicorn HTTP implementation #57332 passes a new --http option into the supervisor's uvicorn config. Neither one changes the lines this PR changes, but whichever merges later may need a rebase. First Attempt at Pod Snapshotting #46089 and [Core][PCP] Support data parallelism with MRV2 PCP #49246 also edit the supervisor, at its old vllm/entrypoints/openai/ path, as part of pod snapshots and PCP with data parallelism.
• [Bugfix] Fix host and port join for ipv6 in bench serve #28679 fixed the same unbracketed join in vllm bench serve, and [Bugfix] Parse sweep server addresses consistently with CLI options #55716 does it for the sweep server. This PR covers the DP supervisor probe, which neither of them touches.
• None of the 11 open PRs that match ipv6 is about the DP supervisor. I read the file lists of the four that change listener or socket code ([Bugfix] Make unspecified --host bind to dual stack #22823, [Bugfix] Fix socket utilities for IPv6 dual-stack support #39619, [Bugfix] Eliminate frontend ZMQ port TOCTOU with inherited listeners #54113, [Frontend] Give API workers independent reuseport listeners #58278), and none of them touches dp_supervisor.py.

GitHub searches run on 2026-09-26:

repo:vllm-project/vllm is:pr is:open supervisor
repo:vllm-project/vllm is:pr is:open ipv6
repo:vllm-project/vllm is:pr is:open dp_supervisor

The dp_supervisor query returns nothing. The PRs named above come from reading the file lists of all 14 results of the broader supervisor query.

Test Plan

test_child_base_url_brackets_ipv6_hosts goes into the existing tests/entrypoints/launchers/test_dp_supervisor.py and reuses _make_unit_args. It covers six hosts: the default, 0.0.0.0, localhost, ::, fd00::1, and :: with SSL. The first three are controls that already pass on main.

python -m pytest tests/entrypoints/launchers/test_dp_supervisor.py

Test Result

With the fix, the new test gives 6 passed.

With only dp_supervisor.py reverted to main and the same test file, it gives 3 failed, 3 passed. The three IPv4 and hostname controls pass. Example failure:

AssertionError: assert 'http://::1:8001' == 'http://[::1]:8001'

Step 3 of the reproduction returns False on main and True with the fix.

The whole file on Linux (WSL, Python 3.12.3, pytest 9.1.1, pytest-asyncio 1.4.0) gives 26 passed with the fix and 20 passed on main, so the existing 20 tests are unaffected. In one earlier whole-file run, test_dp_server_fails got a ConnectionRefusedError on 127.0.0.1 while other jobs were loading the machine. It passed three times on its own and in the next whole-file run.

These hooks from .pre-commit-config.yaml pass with pre-commit run <hook> --files on the two changed files: ruff-check, ruff-format, typos, check-spdx-header, check-root-lazy-imports, check-forbidden-imports, check-torch-cuda-call, check-boolean-context-manager, validate-config and check-test-tethering. My machine runs Windows, where the mypy-3.10 through mypy-3.13 hooks report three errors that are already on main: os.setpgrp in dp_supervisor.py and signal.SIGKILL in the test file do not exist on Windows. So I also ran mypy --platform linux --python-version 3.10 (and 3.11, 3.12, 3.13) on dp_supervisor.py, and the same with --follow-imports skip on the test file. Every run reports no issues, both with the fix and on main.

Model evaluation

Not run. The change only affects the loopback URL the supervisor uses for its health probes and does not touch model code. I did not start a multi GPU data parallel deployment, so the end to end evidence is the loopback probe above.

AI assistance

AI assistance (Claude) was used to prepare this change. The commit carries a Co-authored-by trailer. I reviewed every changed line and ran the tests above locally.

🤖 Generated with Claude Code

[claude]

Claude Code Review

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