[jurajmajerik]

💡 Motivation and Context

An app with many threads, such as a worker with 50 activity slots, sends one request per thread when a prompt is not in the cache or its TTL has expired. That burst repeats every TTL and is enough to hit the per-minute prompt fetch limit.

Now one thread makes the request and the others wait for its result. With capture_errors=True, a shared failure is reported once instead of once per thread.

Same change as PostHog/posthog-js#5266 for the JS client. Part of #106807 in PostHog/posthog.

💚 How did you test it?

Two new tests in test_prompts.py: ten threads on a cold cache make one request, and ten threads on a failing fetch make one request and one capture_exception call. Both fail on the old code with 11 and 10 requests.

📝 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

[posthog]

PostHog Review · Standard · reviewed 4ad9304: 3 Should fix.

[posthog]

P2 · Publish cache state before removing the in-flight entry

When a leader finishes fetching but has not yet updated _cache in _get_internal, _fetch_prompt_once removes its in-flight entry. Another concurrent caller sees both a cache miss and no pending fetch, so it starts a duplicate request. This leaves a gap in the new request sharing on cold caches and expiry; publish the cache or cooldown state before removing the entry.

[jurajmajerik]

fixed in 327fa79

[posthog]

P2 · Release waiters before invoking error-tracking callbacks

With capture_errors=True, a capture_exception callback such as before_send that calls get() for the same prompt enters pending.done.wait() on the leader's own unfinished fetch. The leader cannot reach finally, so the callback and all waiting callers hang indefinitely, even with a fallback. Complete the shared failure before invoking error-tracking callbacks.

[jurajmajerik]

fixed in 327fa79

[posthog]

P2 · Reset prompt flight state after a fork

When a process forks while another thread fetches a prompt, the child inherits _in_flight but loses the leader thread. Its next get() for that key blocks forever in pending.done.wait(), even with a fallback; the client's existing fork hook does not reset this state. Replace the lock and clear inherited flights in the child before accepting requests.

[jurajmajerik]

fixed in 327fa79

[github-actions]

posthog-python Compliance Report

Date: 2026-10-15T07:25:50.360414+00:00
Duration: 259075ms

✅ All Tests Passed!

121/121 tests passed

---

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	513ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	507ms
Required Headers.Has Authorization Bearer Header	✅	507ms
Required Headers.Has Content Type Json	✅	508ms
Required Headers.Has Posthog Sdk Info Format	✅	507ms
Required Headers.Has Posthog Attempt Header	✅	506ms
Required Headers.Has Posthog Request Id	✅	506ms
Required Headers.Has Posthog Request Timestamp	✅	508ms
Required Headers.Has User Agent	✅	507ms
Body Format.Body Has Created At And Batch	✅	507ms
Body Format.No Api Key In Body	✅	506ms
Body Format.No Sent At In Body	✅	507ms
Event Format.Event Has Required Root Fields	✅	506ms
Event Format.Event Uuid Is Valid	✅	507ms
Event Format.Event Timestamp Is Rfc3339	✅	507ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	511ms
Event Format.Distinct Id Is String	✅	506ms
Event Format.Distinct Id At Root Not Properties	✅	506ms
Event Format.Custom Properties Preserved	✅	506ms
Event Format.Set Properties Preserved	✅	506ms
Event Format.Set Once Properties Preserved	✅	506ms
Event Format.Groups Properties Preserved	✅	506ms
Event Format.Sdk Generates Uuid If Not Provided	✅	507ms
Event Format.Event Has Required Root Fields Batch	✅	509ms
Event Format.Event Uuid Is Valid Batch	✅	508ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	508ms
Event Format.Distinct Id Is String Batch	✅	508ms
Event Format.Distinct Id At Root Not Properties Batch	✅	508ms
Event Format.Custom Properties Preserved Batch	✅	508ms
Event Format.Set Properties Preserved Batch	✅	509ms
Event Format.Set Once Properties Preserved Batch	✅	508ms
Event Format.Groups Properties Preserved Batch	✅	508ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	508ms
Batch Behavior.Multiple Events In Single Batch	✅	511ms
Batch Behavior.Batch Envelope Smoke	✅	510ms
Batch Behavior.Flush With No Events Sends Nothing	✅	504ms
Batch Behavior.Flush At Triggers Batch	✅	1007ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	507ms
Deduplication.Generates Unique Uuids	✅	511ms
Deduplication.Different Events Same Content Different Uuids	✅	508ms
Deduplication.Preserves Uuid On Retry	✅	6511ms
Deduplication.Preserves Timestamp On Retry	✅	6515ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6518ms
Deduplication.No Duplicate Events In Batch	✅	514ms
Header Behavior On Retry.Attempt Header Starts At One	✅	508ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13515ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6517ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3016ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6518ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	508ms
Response Format Validation.Success Response Has Ok For Each Event	✅	510ms
Response Format Validation.Success No Retry After When All Ok	✅	509ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1512ms
Response Format Validation.Success No Retry After When Drop Only	✅	509ms
Response Format Validation.Response Echoes Request Id	✅	508ms
Retry Behavior.Retries On 408	✅	6516ms
Retry Behavior.Retries On 500	✅	6517ms
Retry Behavior.Retries On 503	✅	8518ms
Retry Behavior.Retries On 504	✅	6517ms
Retry Behavior.Retryable Errors Have Retry After	✅	3513ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11519ms
Retry Behavior.Does Not Retry On 400	✅	2510ms
Retry Behavior.Does Not Retry On 401	✅	2507ms
Retry Behavior.Does Not Retry On 402	✅	2508ms
Retry Behavior.Does Not Retry On 413	✅	2508ms
Retry Behavior.Does Not Retry On 415	✅	2507ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2508ms
Retry Behavior.Implements Backoff	✅	22527ms
Retry Behavior.Max Retries Respected	✅	22532ms
Partial Batch Handling.Handles 200 Full Success	✅	2510ms
Partial Batch Handling.Handles 200 With All Ok	✅	3513ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3513ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3511ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6515ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6516ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6517ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6516ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6517ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6511ms
Partial Batch Handling.Respects Retry After On Partial	✅	8515ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3513ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3514ms
Compression.Sends Gzip Content Encoding	✅	507ms
Compression.No Content Encoding When Disabled	✅	509ms
Compression.Compressed Body Is Decompressible	✅	507ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2507ms
Event Options.Cookieless Mode Override	✅	506ms
Event Options.Disable Skew Correction Override	✅	505ms
Event Options.Process Person Profile Override	✅	506ms
Event Options.Product Tour Id Override	✅	506ms
Event Options.Unset Options Omitted	✅	505ms
Event Options.Options Override In Batch	✅	508ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	506ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	506ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	506ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	505ms
Routing.Capture Does Not Reroute Ai Named Events	✅	506ms
Identity.Capture Ai Event Has Uuid	✅	505ms
Identity.Capture Ai Keeps Supplied Uuid	✅	506ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	506ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	6ms
Request Payload.Flags Request Uses V2 Query Param	✅	6ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	6ms
Request Payload.Flags Request Omits Authorization Header	✅	6ms
Request Payload.Token In Flags Body Matches Init	✅	6ms
Request Payload.Groups Round Trip	✅	5ms
Request Payload.Groups Default To Empty Object	✅	6ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	6ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	6ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	6ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	506ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	9ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	6ms
Retry Behavior.Retries Flags On 502	✅	307ms
Retry Behavior.Retries Flags On 504	✅	308ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	507ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	44ms
Versioned Boolean Matching.Matching Version 1	✅	44ms
Versioned Boolean Matching.Matching Version 2	✅	44ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	22ms

[posthog]

PostHog Review · Standard · reviewed 327fa79: 3 Should fix.

[posthog]

P2 · Preserve the replacement flight when a timed-out leader finishes

When a fetch exceeds the 30-second waiter timeout, another caller replaces its flight and starts fetching. If the original leader then finishes, this unconditional pop() removes the replacement's entry while its request is still running. Subsequent expired-cache or zero-TTL reads start additional requests; remove the entry only when it still refers to pending.

[jurajmajerik]

fixed in 246c2c0

[posthog]

P2 · Set the callback recursion guard before fetching the nested prompt

With the mocked fetch always failing, the nested prompts.get() invokes capture_exception again before nested_results.append() executes. The guard therefore remains false until recursion exhaustion, and unwinding appends many results, failing the assertion expecting one nested result. Set a separate guard before entering the nested fetch.

[jurajmajerik]

fixed in efa529d

[posthog]

P2 · Register one fork callback for a weak registry of prompt instances

On platforms supporting register_at_fork, every Prompts instance permanently registers a new closure. The weak reference lets the instance disappear, but Python retains the callback and its closure for the process lifetime. Applications repeatedly constructing instances accumulate memory and an ever-growing callback list executed on every fork; register one callback over a weak instance registry.

[jurajmajerik]

fixed in 246c2c0

[posthog]

PostHog Review · Standard · reviewed 246c2c0: 2 Should fix.

[posthog]

P2 · Recheck cache freshness before electing a new fetch leader

When a caller passes _get_internal's cache check and pauses before acquiring this lock, another caller can fetch, populate the cache, and remove its flight. The paused caller then creates a new flight and sends a duplicate request despite the fresh cache. Publishing the cache first leaves this race open; recheck freshness under the election lock while preserving zero-TTL behavior.

[jurajmajerik]

fixed in 7863971

[posthog]

P2 · Avoid replacing live fetches after the fixed waiter timeout

When the server returns 429 with Retry-After: 60, _get_session()'s urllib3 retries sleep beyond the 30-second waiter limit. Waiters delete the live flight and launch another request every 30 seconds, amplifying traffic while the server asks the client to back off. The transport has no 30-second total deadline; align replacement with an actual bounded fetch lifetime.

[jurajmajerik]

fixed in 7863971

[jurajmajerik]

Simplified after the review rounds: the timed wait and replacement-fetch logic are gone, waiters just wait for the leader, a single fork hook resets inherited state in a child, and the cache freshness check also runs under the election lock so no caller starts a second request for a prompt that was just fetched.

[marandaneto]

The code looks good; no qualifying findings. Verdict: correct.