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ref: Add support for custom sampling context to span first #5628

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7 changes: 7 additions & 0 deletions sentry_sdk/scope.py
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Original file line number Diff line number Diff line change
Expand Up @@ -696,6 +696,13 @@ def get_active_propagation_context(self) -> "PropagationContext":
isolation_scope._propagation_context = PropagationContext()
return isolation_scope._propagation_context

def set_custom_sampling_context(
self, custom_sampling_context: "dict[str, Any]"
) -> None:
self.get_active_propagation_context()._set_custom_sampling_context(
custom_sampling_context
)

def clear(self) -> None:
"""Clears the entire scope."""
self._level: "Optional[LogLevelStr]" = None
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11 changes: 11 additions & 0 deletions sentry_sdk/tracing_utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -417,6 +417,7 @@ class PropagationContext:
"parent_span_id",
"parent_sampled",
"baggage",
"custom_sampling_context",
)

def __init__(
Expand Down Expand Up @@ -450,6 +451,8 @@ def __init__(
if baggage is None and dynamic_sampling_context is not None:
self.baggage = Baggage(dynamic_sampling_context)

self.custom_sampling_context: "Optional[dict[str, Any]]" = None

@classmethod
def from_incoming_data(
cls, incoming_data: "Dict[str, Any]"
Expand Down Expand Up @@ -537,6 +540,11 @@ def update(self, other_dict: "Dict[str, Any]") -> None:
except AttributeError:
pass

def _set_custom_sampling_context(
self, custom_sampling_context: "dict[str, Any]"
) -> None:
self.custom_sampling_context = custom_sampling_context
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@ericapisani ericapisani Mar 10, 2026

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Related to the comment Cursor made - should we consider adding validation to the value that's going be set on the custom_sampling_context?


def __repr__(self) -> str:
return "<PropagationContext _trace_id={} _span_id={} parent_span_id={} parent_sampled={} baggage={}>".format(
self._trace_id,
Expand Down Expand Up @@ -1420,6 +1428,9 @@ def _make_sampling_decision(
"attributes": dict(attributes) if attributes else {},
}

if propagation_context.custom_sampling_context:
sampling_context.update(propagation_context.custom_sampling_context)
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@cursor cursor bot Mar 10, 2026

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Custom sampling context can silently override built-in keys

Medium Severity

Using sampling_context.update(propagation_context.custom_sampling_context) allows user-provided custom sampling context to silently override built-in keys like name, trace_id, parent_span_id, parent_sampled, and attributes. Unlike the old start_transaction path where the built-in keys were transaction_context and parent_sampled (uncommon names), keys like name and attributes are very common and likely to be used innocuously by someone calling scope.set_custom_sampling_context({"name": "my_feature"}), resulting in a corrupted sampling context reaching the traces_sampler.

Fix in Cursor Fix in Web


sample_rate = client.options["traces_sampler"](sampling_context)
else:
if propagation_context.parent_sampled is not None:
Expand Down
56 changes: 56 additions & 0 deletions tests/tracing/test_span_streaming.py
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Original file line number Diff line number Diff line change
Expand Up @@ -233,6 +233,62 @@ def traces_sampler(sampling_context):
assert len(spans) == 1


def test_custom_sampling_context(sentry_init):
class MyClass: ...

my_class = MyClass()

def traces_sampler(sampling_context):
assert "class" in sampling_context
assert "string" in sampling_context
assert sampling_context["class"] == my_class
assert sampling_context["string"] == "my string"
return 1.0

sentry_init(
traces_sampler=traces_sampler,
_experiments={"trace_lifecycle": "stream"},
)

sentry_sdk.get_current_scope().set_custom_sampling_context(
{
"class": my_class,
"string": "my string",
}
)

with sentry_sdk.traces.start_span(name="span"):
...


def test_new_custom_sampling_context(sentry_init):
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@ericapisani ericapisani Mar 10, 2026

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I think both of the test cases introduced here are good, but I think the names could be improved slightly to make it easier to understand (in terms of what broke) should they ever fail.

For this particular test, maybe something like test_custom_sampling_context_update_to_context_value_persists could work?

def traces_sampler(sampling_context):
if sampling_context["attributes"]["first"] is True:
assert sampling_context["custom_value"] == 1
else:
assert sampling_context["custom_value"] == 2
return 1.0

sentry_init(
traces_sampler=traces_sampler,
_experiments={"trace_lifecycle": "stream"},
)

sentry_sdk.traces.new_trace()

sentry_sdk.get_current_scope().set_custom_sampling_context({"custom_value": 1})

with sentry_sdk.traces.start_span(name="span", attributes={"first": True}):
...

sentry_sdk.traces.new_trace()

sentry_sdk.get_current_scope().set_custom_sampling_context({"custom_value": 2})

with sentry_sdk.traces.start_span(name="span", attributes={"first": False}):
...


def test_span_attributes(sentry_init, capture_envelopes):
sentry_init(
traces_sample_rate=1.0,
Expand Down
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