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Python: [Bug]: Workflow build rejects a list[T] output sent to a Sequence[T] handler #9449
Description
Observed Behavior
An executor that sends list[str] can't be wired to a handler that takes Sequence[str]. WorkflowBuilder.build() raises TypeCompatibilityError.
The same check breaks Workflow.as_agent(): if the start executor's handler takes Sequence[Message], it raises ValueError: Workflow's start executor cannot handle list[Message].
At runtime the message would be delivered fine. is_instance_of(["a"], Sequence[str]) is True. Only the build-time check says no.
It's also inconsistent with the bare case. list[str] -> Sequence passes since #8731, but list[str] -> Sequence[str] doesn't.
Expected Behavior
list[str] is a Sequence[str], so the edge builds. Same for dict[str, int] -> Mapping[str, int], list[str] -> Iterable[str], set[int] -> collections.abc.Set[int]. Element types still have to fit, so list[int] -> Sequence[str] stays rejected.
Steps to Reproduce
- Executor A has
WorkflowContext[list[str]]and sends a list. - Executor B handles
words: Sequence[str]. WorkflowBuilder(start_executor=a).add_edge(a, b).build()
Minimal Reproduction
from collections.abc import Sequence
from agent_framework import Executor, Message, WorkflowBuilder, WorkflowContext, handler
class Splitter(Executor):
@handler
async def split(self, text: str, ctx: WorkflowContext[list[str]]) -> None:
await ctx.send_message(text.split())
class Counter(Executor):
@handler
async def count(self, words: Sequence[str], ctx: WorkflowContext[None, int]) -> None:
await ctx.yield_output(len(words))
splitter, counter = Splitter(id="splitter"), Counter(id="counter")
workflow = WorkflowBuilder(start_executor=splitter).add_edge(splitter, counter).build() # raises
class Chat(Executor):
@handler
async def chat(self, messages: Sequence[Message], ctx: WorkflowContext[None, str]) -> None:
await ctx.yield_output(messages[-1].text)
WorkflowBuilder(start_executor=Chat(id="chat")).build().as_agent() # raisesError Messages and Stack Traces
is_type_compatible(list[str], Sequence[str]): False
is_type_compatible(list[str], Sequence): True
1. build: TypeCompatibilityError - [TYPE_COMPATIBILITY] Type incompatibility between executors 'splitter' -> 'counter'. Source executor outputs types ['list[str]'] but target executor can only handle types ['collections.abc.Sequence[str]'].
2. as_agent(): ValueError - Workflow's start executor cannot handle list[Message]
Package Versions
agent-framework-core: 1.22.0 (main at bfc4fdb)
Python Version
Python 3.12
Operating System
macOS
Regression
No
Additional Context
Cause: is_type_compatible in python/packages/core/agent_framework/_workflows/_typing_utils.py. Origins are list and collections.abc.Sequence, so Case 5 ("different container types are not compatible") returns False before any argument check.
#8731 handled the bare target (list[str] -> Sequence) and its description mentions list[Message] -> Sequence[Message] as left out. I didn't find an issue for it.
Fix I have: when the source is a builtin container, the target is a collections.abc class it implements, and both have the same number of type arguments, compare the arguments pairwise. 13 lines plus a unit test. Happy to open the PR.
Acknowledgements
- I searched existing issues and did not find a duplicate.
- I personally verified this behavior and the reproduction details are authentic.
- I will wait for explicit maintainer agreement before starting implementation of a non-trivial change.
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github-actions commentedon Oct 16, 2026
🤖 Automated triage reproduction notes (agent-authored — trust but verify)
Agent analysis
The bug is in
python/packages/core/agent_framework/_workflows/_typing_utils.py::is_type_compatibleat lines 551-553, where differing parameterized container origins are rejected before generic arguments are checked. It triggers for assignable pairs such aslist[str]andSequence[str], affecting edge validation and workflow-agent construction. Minimal repro: assertis_type_compatible(list[str], Sequence[str])and build a workflow edge with those output/input annotations.python/packages/core/tests/workflow/test_typing_utils.py1.22.01.22.0Python: fix(core): accept a builtin container as an edge target for a…
Somay-kousis commentedon Oct 24, 2026
Eduard van Valkenburg (@eavanvalkenburg) is a PR welcome for this one? The fix is on my fork, rebased on main today: main...Somay-kousis:agent-framework:fix/edge-abc-container-types (879d77d).
is_type_compatiblegets one case: a builtin container whosecollections.abctarget it implements, with the same number of type arguments, compares the arguments pairwise. Solist[str] -> Sequence[str]anddict[str, int] -> Mapping[str, int]build, andlist[int] -> Sequence[str]is still rejected. 3 of the 4 new tests fail on main (the type check, a workflow that builds and runs with aSequence[str]handler, andas_agent()with aSequence[Message]start executor); the mismatch test passes on both.tests/workflow: 1441 passed.I'll open the PR as soon as you're OK with the direction.