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fix(flags): local eval dependency fail to match #734

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matheus-vb merged 2 commits into from
Jul 10, 2026
+354 −79
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fix(flags): local eval dependency fail to match #734

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b1caf23
implement fix
matheus-vb Jul 10, 2026
9e38083
add changeset
matheus-vb Jul 10, 2026
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5 changes: 5 additions & 0 deletions .sampo/changesets/stalwart-iceseeker-sampsa.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
pypi/posthog: patch
---

Fix local evaluation of flag dependencies with a `flag_evaluates_to: false` condition: such conditions never matched, forcing the dependent flag to `false` for every locally-evaluated user.
157 changes: 78 additions & 79 deletions posthog/feature_flags.py
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,13 @@ def evaluate_flag_dependency(
device_id=None,
):
"""
Evaluate a flag dependency property according to the dependency chain algorithm.
Evaluate a flag dependency condition under local evaluation.

The dependency_chain only establishes the order in which flags are evaluated
and cached (the referenced flag itself is always the last member). The
outcome is decided solely by comparing the referenced flag's evaluated value
against the condition's expected value; ancestors influence it only through
the referenced flag's own recursive evaluation.

Args:
property: Flag property with type="flag" and dependency_chain
Expand All @@ -127,7 +133,12 @@ def evaluate_flag_dependency(
device_id: The device ID for bucketing (optional)

Returns:
bool: True if all dependencies in the chain evaluate to True, False otherwise
bool: Whether the referenced flag's evaluated value matches the
condition's expected value.

Raises:
InconclusiveMatchError: If the condition is malformed or the chain
cannot be conclusively evaluated locally.
"""
if flags_by_key is None or evaluation_cache is None:
# Cannot evaluate flag dependencies without required context
Expand All @@ -151,93 +162,81 @@ def evaluate_flag_dependency(
f"Circular dependency detected for flag '{property.get('key', 'unknown')}'"
)

# Evaluate all dependencies in the chain order
for dep_flag_key in dependency_chain:
if dep_flag_key not in evaluation_cache:
# Need to evaluate this dependency first
dep_flag = flags_by_key.get(dep_flag_key)
if not dep_flag:
# Missing flag dependency - cannot evaluate locally
evaluation_cache[dep_flag_key] = None
raise InconclusiveMatchError(
f"Cannot evaluate flag dependency '{dep_flag_key}' - flag not found in local flags"
)
else:
# Check if the flag is active (same check as in client._compute_flag_locally)
if not dep_flag.get("active"):
evaluation_cache[dep_flag_key] = False
else:
# Recursively evaluate the dependency
try:
dep_flag_filters = dep_flag.get("filters") or {}
dep_aggregation_group_type_index = dep_flag_filters.get(
"aggregation_group_type_index"
)
if dep_aggregation_group_type_index is not None:
# Group flags should continue bucketing by the group key
# from the current evaluation context.
dep_bucketing_value = distinct_id
else:
dep_bucketing_value = resolve_bucketing_value(
dep_flag, distinct_id, device_id
)
dep_result = match_feature_flag_properties(
dep_flag,
distinct_id,
properties,
cohort_properties=cohort_properties,
flags_by_key=flags_by_key,
evaluation_cache=evaluation_cache,
device_id=device_id,
bucketing_value=dep_bucketing_value,
)
evaluation_cache[dep_flag_key] = dep_result
except InconclusiveMatchError as e:
# If we can't evaluate a dependency, store None and propagate the error
evaluation_cache[dep_flag_key] = None
raise InconclusiveMatchError(
f"Cannot evaluate flag dependency '{dep_flag_key}': {e}"
) from e

# Check the cached result
cached_result = evaluation_cache[dep_flag_key]
if cached_result is None:
# Previously inconclusive - raise error again
raise InconclusiveMatchError(
f"Flag dependency '{dep_flag_key}' was previously inconclusive"
)
elif not cached_result:
# Definitive False result - dependency failed
return False

# All dependencies in the chain have been evaluated successfully
# Now check if the final flag value matches the expected value in the property
# Validate the condition shape before walking the chain. Test `is None`, not
# truthiness: `False` is a valid expected value (the case this exists for).
flag_key = property.get("key")
expected_value = property.get("value")
operator = property.get("operator", "exact")

if flag_key and expected_value is not None:
# Get the actual value of the flag we're checking
actual_value = evaluation_cache.get(flag_key)
if operator != "flag_evaluates_to":
raise InconclusiveMatchError(
f"Flag dependency property for '{flag_key or 'unknown'}' has invalid operator '{operator}'"
)
if not flag_key or expected_value is None:
raise InconclusiveMatchError(
f"Flag dependency property for '{flag_key or 'unknown'}' is missing a key or value"
)

# Evaluate and cache each flag in the chain; members already cached are
# skipped. This does not decide the outcome — it only populates the cache.
for dep_flag_key in dependency_chain:
if dep_flag_key in evaluation_cache:
continue

if actual_value is None:
# Flag wasn't evaluated - this shouldn't happen if dependency chain is correct
dep_flag = flags_by_key.get(dep_flag_key)
if not dep_flag:
# Missing flag dependency - cannot evaluate locally
evaluation_cache[dep_flag_key] = None
raise InconclusiveMatchError(
f"Flag '{flag_key}' was not evaluated despite being in dependency chain"
f"Cannot evaluate flag dependency '{dep_flag_key}' - flag not found in local flags"
)

# For flag dependencies, we need to compare the actual flag result with expected value
# using the flag_evaluates_to operator logic
if operator == "flag_evaluates_to":
return matches_dependency_value(expected_value, actual_value)
else:
# This should never happen, but just to be defensive.
raise InconclusiveMatchError(
f"Flag dependency property for '{property.get('key', 'unknown')}' has invalid operator '{operator}'"
# Check if the flag is active (same check as in client._compute_flag_locally)
if not dep_flag.get("active"):
evaluation_cache[dep_flag_key] = False
continue

# Recursively evaluate the dependency
try:
dep_flag_filters = dep_flag.get("filters") or {}
dep_aggregation_group_type_index = dep_flag_filters.get(
"aggregation_group_type_index"
)
if dep_aggregation_group_type_index is not None:
# Group flags should continue bucketing by the group key
# from the current evaluation context.
dep_bucketing_value = distinct_id
else:
dep_bucketing_value = resolve_bucketing_value(
dep_flag, distinct_id, device_id
)
dep_result = match_feature_flag_properties(
dep_flag,
distinct_id,
properties,
cohort_properties=cohort_properties,
flags_by_key=flags_by_key,
evaluation_cache=evaluation_cache,
device_id=device_id,
bucketing_value=dep_bucketing_value,
)
evaluation_cache[dep_flag_key] = dep_result
except InconclusiveMatchError as e:
# If we can't evaluate a dependency, store None and propagate the error
evaluation_cache[dep_flag_key] = None
raise InconclusiveMatchError(
f"Cannot evaluate flag dependency '{dep_flag_key}': {e}"
) from e

# If no value check needed, return True (all dependencies passed)
return True
# The condition matches iff the referenced flag's value matches the expected
# value. None means inconclusive or not evaluated — distinct from a
# definitive False, which must be allowed to match `expected_value=False`.
actual_value = evaluation_cache.get(flag_key)
if actual_value is None:
raise InconclusiveMatchError(
f"Flag dependency '{flag_key}' was inconclusive or not evaluated"
)
return matches_dependency_value(expected_value, actual_value)


def matches_dependency_value(expected_value, actual_value):
Expand Down
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