jason810496 commented on code in PR #69757:
URL: https://github.com/apache/airflow/pull/69757#discussion_r3654479476


##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:
+    """
+    Build the per-map-index arg spec for a mapped (``.expand()``) stub task.
+
+    A mapped stub never instantiates at parse time, so no spec is captured in 
the
+    serialized Dag; it is derived here from the serialized expand input 
instead, with
+    the map-index decomposition delegated to
+    ``SchedulerDictOfListsExpandInput.resolve_expansion_sub_indexes``.
+    Value schemas come from the stub function's annotations, which are not 
available
+    server-side, so mapped bindings omit them (runtimes fall back to 
decode-only checks).
+    """
+    expand_input = task._get_specified_expand_input()
+    if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
+        _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")

Review Comment:
   ```suggestion
       # TODO: Support the `expand_kwargs` path once #69757 merged and all the 
Lang-SDKs adapt it.
       if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
           _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")
   ```



##########
airflow-core/src/airflow/api_fastapi/execution_api/datamodels/task_arg_binding.py:
##########
@@ -0,0 +1,102 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""
+Positional-argument binding spec for stub (foreign-runtime) tasks.
+
+Captured at parse time from the ``@task.stub`` TaskFlow call, stored in the 
serialized
+Dag, and delivered to the lang-SDK runtime via ``TIRunContext.arg_bindings``.
+"""
+
+from __future__ import annotations
+
+from functools import cache
+from typing import Annotated, Literal
+
+from pydantic import Field, JsonValue, TypeAdapter
+from typing_extensions import TypeAliasType
+
+from airflow.api_fastapi.core_api.base import BaseModel
+
+# A named, titled alias (like TaskArgBinding below) kept as free-form JSON 
rather than a
+# typed model, so unknown JSON-schema keywords survive re-serialization along 
the way.
+ArgValueSchema = TypeAliasType(
+    "ArgValueSchema", Annotated[dict[str, JsonValue], 
Field(title="ArgValueSchema")]
+)
+"""JSON-schema fragment constraining the value a stub-task argument binds to; 
generated
+by pydantic from the stub annotation, carried verbatim, unknown keywords 
ignored."""
+
+
+class XComArgBinding(BaseModel):
+    """One positional stub-task argument pulled from an upstream task's 
XCom."""
+
+    # No default: it would drop ``kind`` from ``required``, and the generated 
task-sdk
+    # client then types it ``Literal | None``, invalid as a tagged-union 
discriminator.
+    kind: Literal["xcom"]
+
+    name: str
+    """The stub function's parameter name this binding fills, in declaration 
order."""
+
+    value_schema: ArgValueSchema | None = None
+    """Schema fragment from the stub function's annotation; omitted when 
unconstrained."""
+
+    task_id: str
+    """Upstream task id whose ``return_value`` XCom is pulled."""
+
+    map_index: int = -1
+    """Map index of the upstream XCom row to pull; -1 is the unmapped row."""
+
+    element_index: int | None = None
+    """When set, the pulled value is a sequence and this binding takes the 
element at
+    this index (the stub was expanded over an unmapped upstream's output)."""

Review Comment:
   ```suggestion
       this index (the stub was expanded over an unmapped upstream's output). 
The Lang-SDK side will GET the single row (task_id, map_index=-1), decode it, 
take value[i]."""
   ```



##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:
+    """
+    Build the per-map-index arg spec for a mapped (``.expand()``) stub task.
+
+    A mapped stub never instantiates at parse time, so no spec is captured in 
the
+    serialized Dag; it is derived here from the serialized expand input 
instead, with
+    the map-index decomposition delegated to
+    ``SchedulerDictOfListsExpandInput.resolve_expansion_sub_indexes``.
+    Value schemas come from the stub function's annotations, which are not 
available
+    server-side, so mapped bindings omit them (runtimes fall back to 
decode-only checks).
+    """
+    expand_input = task._get_specified_expand_input()
+    if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
+        _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")
+    if ti.map_index < 0:
+        _unsupported_arg_bindings("the task instance has not been expanded to 
a map index")
+
+    expand_value = expand_input.value
+    try:
+        sub_indexes = expand_input.resolve_expansion_sub_indexes(ti.map_index, 
ti.run_id, session=session)
+    except NotFullyPopulated as e:
+        _unsupported_arg_bindings(f"upstream map lengths are not yet known for 
{sorted(e.missing)}")

Review Comment:
   ```suggestion
       except NotFullyPopulated as e:
           # In most of the happy path, this shouldn't happed at all unless 
someone clear upstream TIs or XCom themself during the DagRun.
           _unsupported_arg_bindings(f"upstream map lengths are not yet known 
for {sorted(e.missing)}")
   ```



##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:
+    """
+    Build the per-map-index arg spec for a mapped (``.expand()``) stub task.
+
+    A mapped stub never instantiates at parse time, so no spec is captured in 
the
+    serialized Dag; it is derived here from the serialized expand input 
instead, with
+    the map-index decomposition delegated to
+    ``SchedulerDictOfListsExpandInput.resolve_expansion_sub_indexes``.
+    Value schemas come from the stub function's annotations, which are not 
available
+    server-side, so mapped bindings omit them (runtimes fall back to 
decode-only checks).
+    """
+    expand_input = task._get_specified_expand_input()
+    if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
+        _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")
+    if ti.map_index < 0:
+        _unsupported_arg_bindings("the task instance has not been expanded to 
a map index")
+
+    expand_value = expand_input.value
+    try:
+        sub_indexes = expand_input.resolve_expansion_sub_indexes(ti.map_index, 
ti.run_id, session=session)
+    except NotFullyPopulated as e:
+        _unsupported_arg_bindings(f"upstream map lengths are not yet known for 
{sorted(e.missing)}")
+
+    spec = [
+        _bind_mapped_stub_arg(name, value, sub_index=None)
+        for name, value in (task.partial_kwargs.get("op_kwargs") or {}).items()
+    ]
+    spec += [
+        _bind_mapped_stub_arg(name, value, sub_index=sub_indexes[name])
+        for name, value in expand_value.items()
+    ]
+    return spec
+
+
+def _bind_mapped_stub_arg(name: str, value: Any, *, sub_index: int | None) -> 
dict[str, Any]:
+    """Build one arg-binding dict; ``sub_index`` is set for expanded kwargs, 
None for partial ones."""
+    if isinstance(value, SchedulerPlainXComArg):
+        if value.key != "return_value":
+            _unsupported_arg_bindings(f"parameter {name!r} references the XCom 
key {value.key!r}")
+        entry: dict[str, Any] = {"name": name, "kind": "xcom", "task_id": 
value.operator.task_id}
+        if sub_index is not None:
+            if value.operator.is_mapped:
+                entry["map_index"] = sub_index
+            else:
+                entry["element_index"] = sub_index
+        return entry
+    if isinstance(value, SchedulerXComArg):
+        _unsupported_arg_bindings(
+            f"parameter {name!r} received a {type(value).__name__}; only 
direct upstream"
+            " task outputs and literals are supported"
+        )
+    if sub_index is not None:
+        items = list(value.items()) if isinstance(value, dict) else value

Review Comment:
   ```suggestion
       if sub_index is not None:
           #  this kwarg was expanded over a literal collection written in the 
Dag file
           items = list(value.items()) if isinstance(value, dict) else value
   ```



##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:
+    """
+    Build the per-map-index arg spec for a mapped (``.expand()``) stub task.
+
+    A mapped stub never instantiates at parse time, so no spec is captured in 
the
+    serialized Dag; it is derived here from the serialized expand input 
instead, with
+    the map-index decomposition delegated to
+    ``SchedulerDictOfListsExpandInput.resolve_expansion_sub_indexes``.
+    Value schemas come from the stub function's annotations, which are not 
available
+    server-side, so mapped bindings omit them (runtimes fall back to 
decode-only checks).
+    """
+    expand_input = task._get_specified_expand_input()
+    if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
+        _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")
+    if ti.map_index < 0:
+        _unsupported_arg_bindings("the task instance has not been expanded to 
a map index")
+
+    expand_value = expand_input.value
+    try:
+        sub_indexes = expand_input.resolve_expansion_sub_indexes(ti.map_index, 
ti.run_id, session=session)
+    except NotFullyPopulated as e:
+        _unsupported_arg_bindings(f"upstream map lengths are not yet known for 
{sorted(e.missing)}")
+
+    spec = [
+        _bind_mapped_stub_arg(name, value, sub_index=None)
+        for name, value in (task.partial_kwargs.get("op_kwargs") or {}).items()
+    ]
+    spec += [
+        _bind_mapped_stub_arg(name, value, sub_index=sub_indexes[name])
+        for name, value in expand_value.items()
+    ]
+    return spec
+
+
+def _bind_mapped_stub_arg(name: str, value: Any, *, sub_index: int | None) -> 
dict[str, Any]:

Review Comment:
   Would it be possible to set the `value_schema` (typing) as well?



##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:
+    """
+    Build the per-map-index arg spec for a mapped (``.expand()``) stub task.
+
+    A mapped stub never instantiates at parse time, so no spec is captured in 
the
+    serialized Dag; it is derived here from the serialized expand input 
instead, with
+    the map-index decomposition delegated to
+    ``SchedulerDictOfListsExpandInput.resolve_expansion_sub_indexes``.
+    Value schemas come from the stub function's annotations, which are not 
available
+    server-side, so mapped bindings omit them (runtimes fall back to 
decode-only checks).
+    """
+    expand_input = task._get_specified_expand_input()
+    if not isinstance(expand_input, SchedulerDictOfListsExpandInput):
+        _unsupported_arg_bindings("expand_kwargs() is not supported on stub 
tasks")
+    if ti.map_index < 0:
+        _unsupported_arg_bindings("the task instance has not been expanded to 
a map index")
+
+    expand_value = expand_input.value
+    try:
+        sub_indexes = expand_input.resolve_expansion_sub_indexes(ti.map_index, 
ti.run_id, session=session)
+    except NotFullyPopulated as e:
+        _unsupported_arg_bindings(f"upstream map lengths are not yet known for 
{sorted(e.missing)}")
+
+    spec = [
+        _bind_mapped_stub_arg(name, value, sub_index=None)
+        for name, value in (task.partial_kwargs.get("op_kwargs") or {}).items()
+    ]
+    spec += [
+        _bind_mapped_stub_arg(name, value, sub_index=sub_indexes[name])
+        for name, value in expand_value.items()
+    ]

Review Comment:
   ```suggestion
   
       # partial() kwargs
       spec = [
           _bind_mapped_stub_arg(name, value, sub_index=None)
           for name, value in (task.partial_kwargs.get("op_kwargs") or 
{}).items()
       ]
       # expand() kwargs
       spec += [
           _bind_mapped_stub_arg(name, value, sub_index=sub_indexes[name])
           for name, value in expand_value.items()
       ]
   ```



##########
airflow-core/src/airflow/api_fastapi/execution_api/services/task_instances.py:
##########
@@ -0,0 +1,128 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Business logic backing the task-instance execution routes."""
+
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING, Any, NoReturn
+
+from fastapi import HTTPException, status
+
+from airflow.models.expandinput import NotFullyPopulated, 
SchedulerDictOfListsExpandInput
+from airflow.serialization.definitions.xcom_arg import SchedulerPlainXComArg, 
SchedulerXComArg
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+    from airflow.models.dagbag import DBDagBag
+
+# Task type recorded on the TI row (``TaskInstance.operator``) for
+# ``airflow.providers.standard.decorators.stub._StubOperator``. Used to gate 
the
+# serialized-Dag lookup for ``arg_bindings`` so regular tasks never pay for it.
+# The gate matches the exact class name; a subclass would need its own entry 
here.
+STUB_TASK_TYPE = "_StubOperator"
+
+
+def get_arg_bindings(dag_bag: DBDagBag, ti: Any, *, session: Session) -> list 
| None:
+    """Extract or derive the stub task's TaskFlow arg spec from its Dag 
version."""
+    if ti.dag_version_id is None:
+        return None
+    if (dag := dag_bag.get_dag(ti.dag_version_id, session=session)) is None:
+        return None
+    if (task := dag.task_dict.get(ti.task_id)) is None:
+        return None
+    if task.is_mapped:
+        return _resolve_mapped_stub_arg_bindings(task, ti, session=session)
+    return getattr(task, "_arg_bindings", None)
+
+
+def _unsupported_arg_bindings(detail: str) -> NoReturn:
+    raise HTTPException(
+        status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+        detail={
+            "reason": "invalid_arg_bindings",
+            "message": f"The stub task's TaskFlow arguments cannot be 
delivered: {detail}.",
+        },
+    )
+
+
+def _resolve_mapped_stub_arg_bindings(task: Any, ti: Any, *, session: Session) 
-> list[dict[str, Any]]:

Review Comment:
   The task should be annotated with `Operator` where `Operator` is
   
   ```
   Operator: TypeAlias = BaseOperator | MappedOperator
   ```
   
   at `TYPE_CHECKING` level.



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