Task capability not configured at the server level when expected

[LucaButBoring]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

The enable_tasks() method doesn't properly set the tasks.requests.tools.call capability on the server, causing compatibility issues with clients that assert against server capabilities before attempting task support on a tool invocation.

Identified during strands-agents/harness-sdk#1475.

Example Code

Python & MCP Python SDK

Python 3.12.9
MCP SDK 1.25.0

[adityuhkapoor]

Have a potential fix, putting up pr

[BabyChrist666]

This was fixed in #1854 (merged Jan 15) — the TasksToolsCapability() was missing the call=TasksCallCapability() argument in update_capabilities().

Before the fix:

capabilities.tasks.requests = ServerTasksRequestsCapability(
    tools=TasksToolsCapability()  # call was None
)

After #1854:

capabilities.tasks.requests = ServerTasksRequestsCapability(
    tools=TasksToolsCapability(call=TasksCallCapability())
)

This should be available in SDK versions > 1.25.0. Can this issue be closed?

[LucaButBoring]

This is still a problem, was that reverted? Relevant code is here.

[BabyChrist666]

@LucaButBoring The fix is present on main and was included in v1.26.0 (released Jan 24). The line you linked (experimental.py:L81) shows:

capabilities.tasks.requests = ServerTasksRequestsCapability(
    tools=TasksToolsCapability(call=TasksCallCapability())
)

That call=TasksCallCapability() argument is the fix from #1854. Before the fix it was just TasksToolsCapability() with no call.

Are you maybe running an older SDK version? Check with pip show mcp — you need >= 1.26.0.

[LucaButBoring]

It looks fixed on main, but I am on 1.26.0 - this is the line from the commit at which that was released from:

python-sdk/src/mcp/server/lowlevel/experimental.py

Line 82 in 3d9d345

tools=TasksToolsCapability()

[BabyChrist666]

You're absolutely right, apologies for the incorrect info! I double-checked:

• PR Add missing TasksCallCapability to enable proper MCP task support #1854 was merged to main on Jan 15
• v1.26.0 was tagged on Jan 24 from the v1.x branch

The fix commit (b26e5b9) is on main but was never backported to v1.x, so it's not in any release yet.

# v1.26.0 (v1.x branch) — still has the bug:
tools=TasksToolsCapability()

# main — has the fix:
tools=TasksToolsCapability(call=TasksCallCapability())

This needs a backport to v1.x to ship in the next release. I can open a cherry-pick PR if the maintainers would like.

[BabyChrist666]

Opened #2137 to backport the fix to v1.x so it ships in the next release.

[maxisbey]

Fixed in #1854 (main) and #2137 (v1.x backport).