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workflow extra's anyio<4.7 pin blocks the mcp extra (anyio>=4.9) #139
Description
osw[workflow] and osw[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 lock fails when both extras are resolvable together.
Why the anyio cap exists
prefect 2.20.25's GatherTaskGroup subclasses anyio's TaskGroup but does not
implement 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 the
workflowextra and thedevdependency-group.mcpis excluded from theallextra and from thedevgroup, so the default
resolution keeps the workflow-compatible anyio.- The
devgroup listsprefectandanyiodirectly instead of inheriting them
fromosw[workflow]. Being in a conflict set, a self-referentialosw[workflow]
entry would only activate under--extra workflow, leaving a bareuv syncon
anyio 4.14 (the version that breaks prefect). These pins now have to be kept in
sync with the workflow extra by hand. - The pytest stack moved into its own
testgroup, sincedevconflicts with the
mcpextra and there would otherwise be no environment containing both pytest
and mcp. src/osw/mcpis excluded fromty, since its imports are unresolvable in the
default dev environment.- The MCP unit tests use
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 run
explicitly:
uv sync --extra mcp --group test --no-dev
uv run --extra mcp --group test --no-dev --no-sync python -m pytest tests/test_mcp_*.pyCI 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_task path, or because the workflow extra can move to
prefect 3.x. If the cap can be lifted, revert all six workarounds above and put
mcp back into all and dev.
Activity
feat(mcp): port server to mcp 2.x and isolate the extra
build: type-check src/osw/mcp instead of excluding it
LukasGold commentedon Aug 28, 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()):GatherTaskGroup.__abstractmethods__[]['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:No 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 2.20.25test_notify_teams,test_tags_str_to_list,test_deploy_serve);test_deploy_runneris skipped by a pre-existing unconditional markerFollow-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)testgroup, which existed only becausedevconflicted with themcpextraimportorskipguards in the MCP testsThe sixth (the
tyexclude forsrc/osw/mcp) is already undone on that branch.feat: an osw CLI and an MCP server for live OSL instances (#133)