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workflow extra's anyio<4.7 pin blocks the mcp extra (anyio>=4.9) #139
Description
Activity
- added 2 commits that reference this issue
on Aug 22, 2026 The cap is still needed, but
<4.7is seven minors too tightBisected
prefect==2.20.25against every anyio minor (py3.12, exercising prefect's owngather()):anyio GatherTaskGroup.__abstractmethods__result 4.6.2, 4.7.0, 4.9.0, 4.10.0, 4.11.0, 4.12.0, 4.13.0 []works 4.14.0, 4.14.2 (latest) ['create_task']TypeError: Can't instantiate abstract class GatherTaskGroupThe break is exactly at 4.14.0, which added
create_task()to theTaskGroupABC and changedstart_soon()to return aTaskHandle(anyio changelog, agronholm/anyio#1098). That promotedcreate_taskinto the abstract set, so prefect's external subclass can no longer be instantiated. Not reverted in 4.14.1 or 4.14.2.This resolves the conflict
mcp 2.1.1requiresanyio>=4.9. Its stricteranyio>=4.10clause is gated onpython_version >= '3.14', and osw isrequires-python = ">=3.10,<3.14", so that clause never applies here.That leaves a non-empty window:
anyio>=4.9,<4.14. Verified by co-resolving all three:prefect 2.20.25 + mcp 2.1.1 + anyio 4.13.0 -> both import, GatherTaskGroup concreteNo move to prefect 3.x is required, and no osw-python code change is required.
Verified
Branch
fix/widen-anyio-pinchanges theworkflowextra toanyio>=4.9,<4.14:uv lockresolves,anyio 4.6.2.post1 -> 4.13.0, prefect stays 2.20.25- full default suite: 36 passed, 1 skipped
- workflow tests pass (
test_notify_teams,test_tags_str_to_list,test_deploy_serve);test_deploy_runneris skipped by a pre-existing unconditional marker
Follow-up on feat/mcp-server
Five of the six workarounds from #133 become unnecessary once this lands:
[tool.uv] conflicts(mcp vs workflow, mcp vs dev)mcpexcluded from theallextradevlistingprefect/anyiodirectly instead ofosw[workflow](removes the hand-sync hazard)- the separate
testgroup, which existed only becausedevconflicted with themcpextra - the
importorskipguards in the MCP tests
The sixth (the
tyexclude forsrc/osw/mcp) is already undone on that branch.- added a commit that references this issue
on Sep 21, 2026
osw[workflow]andosw[mcp]cannot be installed into the same environment.osw[workflow]pinsanyio>=4.4.0,<4.7mcp>=2requiresanyio>=4.9(anyio>=4.10on Python 3.14+)No anyio version satisfies both, so
uv lockfails when both extras are resolvable together.Why the anyio cap exists
prefect 2.20.25's
GatherTaskGroupsubclasses anyio'sTaskGroupbut does notimplement
create_task, which newer anyio made abstract. Broken on anyio 4.14,verified good on 4.6.2. prefect 2.20.25 is the final 2.x release, so no backport
is coming.
Current workaround
Applied in #133:
[tool.uv] conflictsdeclares themcpextra mutually exclusive with both theworkflowextra and thedevdependency-group.mcpis excluded from theallextra and from thedevgroup, so the defaultresolution keeps the workflow-compatible anyio.
devgroup listsprefectandanyiodirectly instead of inheriting themfrom
osw[workflow]. Being in a conflict set, a self-referentialosw[workflow]entry would only activate under
--extra workflow, leaving a bareuv synconanyio 4.14 (the version that breaks prefect). These pins now have to be kept in
sync with the workflow extra by hand.
testgroup, sincedevconflicts with themcpextra and there would otherwise be no environment containing both pytestand mcp.
src/osw/mcpis excluded fromty, since its imports are unresolvable in thedefault dev environment.
importorskip, so they skip in the default suite.Cost
The MCP server module is neither type-checked nor unit-tested in the default
environment. Its tests pass (32 passed against
mcp==2.0.0) but only when runexplicitly:
CI does not currently run that second environment.
What to check
Whether the anyio cap is still needed, either because a newer anyio restored the
non-abstract
create_taskpath, or because the workflow extra can move toprefect 3.x. If the cap can be lifted, revert all six workarounds above and put
mcpback intoallanddev.