fix: Return None for malformed feature flag payloads

[marandaneto]

💡 Motivation and Context

Malformed JSON payloads currently return the raw string through evaluate_flags(...).get_flag_payload(...), while the deprecated get_feature_flag_payload(...) can raise JSONDecodeError.

Both APIs now use a shared parser that returns None for malformed or empty JSON strings, for local and server-provided payloads. Valid JSON and pre-decoded values are preserved.

💚 How did you test it?

• Reproduced the inconsistent behavior with failing regression tests before the fix.
• All 327 focused payload, evaluation, feature flag, and type tests pass.
• Ruff formatting and lint checks, the public API snapshot check, and import with warnings treated as errors pass.
• Autoreview of commit 3781203 against origin/main reported no actionable findings.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with Pi using file editing, shell tests, GitHub CLI, and the isolated autoreview helper. The requested behavior is to treat malformed JSON payloads as absent rather than return invalid strings or raise decoding errors. The change shares parsing between the newer and deprecated APIs and includes local and remote regression coverage. Human review is required.

[github-actions]

posthog-python Compliance Report

Date: 2026-09-22T08:06:37.755802+00:00
Duration: 259354ms

✅ All Tests Passed!

121/121 tests passed

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Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	514ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	508ms
Required Headers.Has Authorization Bearer Header	✅	509ms
Required Headers.Has Content Type Json	✅	509ms
Required Headers.Has Posthog Sdk Info Format	✅	509ms
Required Headers.Has Posthog Attempt Header	✅	508ms
Required Headers.Has Posthog Request Id	✅	509ms
Required Headers.Has Posthog Request Timestamp	✅	509ms
Required Headers.Has User Agent	✅	508ms
Body Format.Body Has Created At And Batch	✅	508ms
Body Format.No Api Key In Body	✅	509ms
Body Format.No Sent At In Body	✅	508ms
Event Format.Event Has Required Root Fields	✅	509ms
Event Format.Event Uuid Is Valid	✅	509ms
Event Format.Event Timestamp Is Rfc3339	✅	508ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	514ms
Event Format.Distinct Id Is String	✅	508ms
Event Format.Distinct Id At Root Not Properties	✅	508ms
Event Format.Custom Properties Preserved	✅	509ms
Event Format.Set Properties Preserved	✅	509ms
Event Format.Set Once Properties Preserved	✅	508ms
Event Format.Groups Properties Preserved	✅	509ms
Event Format.Sdk Generates Uuid If Not Provided	✅	508ms
Event Format.Event Has Required Root Fields Batch	✅	510ms
Event Format.Event Uuid Is Valid Batch	✅	512ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	512ms
Event Format.Distinct Id Is String Batch	✅	524ms
Event Format.Distinct Id At Root Not Properties Batch	✅	511ms
Event Format.Custom Properties Preserved Batch	✅	510ms
Event Format.Set Properties Preserved Batch	✅	510ms
Event Format.Set Once Properties Preserved Batch	✅	510ms
Event Format.Groups Properties Preserved Batch	✅	510ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	511ms
Batch Behavior.Multiple Events In Single Batch	✅	514ms
Batch Behavior.Batch Envelope Smoke	✅	512ms
Batch Behavior.Flush With No Events Sends Nothing	✅	505ms
Batch Behavior.Flush At Triggers Batch	✅	1008ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	509ms
Deduplication.Generates Unique Uuids	✅	513ms
Deduplication.Different Events Same Content Different Uuids	✅	510ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6513ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6517ms
Deduplication.No Duplicate Events In Batch	✅	514ms
Header Behavior On Retry.Attempt Header Starts At One	✅	507ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13525ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6513ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3018ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6517ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	509ms
Response Format Validation.Success Response Has Ok For Each Event	✅	511ms
Response Format Validation.Success No Retry After When All Ok	✅	510ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1513ms
Response Format Validation.Success No Retry After When Drop Only	✅	509ms
Response Format Validation.Response Echoes Request Id	✅	508ms
Retry Behavior.Retries On 408	✅	6518ms
Retry Behavior.Retries On 500	✅	6517ms
Retry Behavior.Retries On 503	✅	8519ms
Retry Behavior.Retries On 504	✅	6514ms
Retry Behavior.Retryable Errors Have Retry After	✅	3513ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11521ms
Retry Behavior.Does Not Retry On 400	✅	2510ms
Retry Behavior.Does Not Retry On 401	✅	2511ms
Retry Behavior.Does Not Retry On 402	✅	2511ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2527ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2510ms
Retry Behavior.Implements Backoff	✅	22533ms
Retry Behavior.Max Retries Respected	✅	22532ms
Partial Batch Handling.Handles 200 Full Success	✅	2511ms
Partial Batch Handling.Handles 200 With All Ok	✅	3515ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3510ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3513ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6514ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6515ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6519ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6518ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6514ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6519ms
Partial Batch Handling.Respects Retry After On Partial	✅	8518ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3510ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3517ms
Compression.Sends Gzip Content Encoding	✅	509ms
Compression.No Content Encoding When Disabled	✅	508ms
Compression.Compressed Body Is Decompressible	✅	509ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2511ms
Event Options.Cookieless Mode Override	✅	508ms
Event Options.Disable Skew Correction Override	✅	508ms
Event Options.Process Person Profile Override	✅	509ms
Event Options.Product Tour Id Override	✅	508ms
Event Options.Unset Options Omitted	✅	508ms
Event Options.Options Override In Batch	✅	511ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	508ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	508ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	508ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	509ms
Routing.Capture Does Not Reroute Ai Named Events	✅	508ms
Identity.Capture Ai Event Has Uuid	✅	508ms
Identity.Capture Ai Keeps Supplied Uuid	✅	508ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	508ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	10ms
Request Payload.Flags Request Uses V2 Query Param	✅	8ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	7ms
Request Payload.Flags Request Omits Authorization Header	✅	9ms
Request Payload.Token In Flags Body Matches Init	✅	8ms
Request Payload.Groups Round Trip	✅	8ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	8ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	8ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	8ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	508ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	11ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	7ms
Retry Behavior.Retries Flags On 502	✅	311ms
Retry Behavior.Retries Flags On 504	✅	311ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	510ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	49ms
Versioned Boolean Matching.Matching Version 1	✅	46ms
Versioned Boolean Matching.Matching Version 2	✅	46ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	29ms

[marandaneto]

idk if this one should be fixed or if the other sdks should be fixed
a malformed json should return as is or none
cc @PostHog/team-feature-flags ?

[JamesPatrickGill]

Just a couple of small blocking points below. Looks good otherwise.

To answer your question I think None is the correct direction in this regard. We already have the API rejecting malformed payloads and the raw string is just a bit ambiguous.

[greptile-apps]

The PR appears safe to merge, with a non-blocking observability concern around repeated payload parse warnings.

_{Reviews (1) · Last reviewed commit: "fix: merge main and restore JSON import ..."}

[greptile-apps]

Payload warnings can flood logs

Every empty, whitespace-only, or persistently malformed payload emits this warning on every evaluation. Because empty payloads are supported inputs and warnings are not deduplicated, routine evaluations or one bad flag can flood application logs. Return None silently for empty payloads, and rate-limit, deduplicate, or lower the log level for repeated malformed payloads.

Prompt To Fix With AI

This is a comment left during a code review.
Path: posthog/types.py
Line: 17-20

Comment:
**Payload warnings can flood logs**

Every empty, whitespace-only, or persistently malformed payload emits this warning on every evaluation. Because empty payloads are supported inputs and warnings are not deduplicated, routine evaluations or one bad flag can flood application logs. Return `None` silently for empty payloads, and rate-limit, deduplicate, or lower the log level for repeated malformed payloads.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!