fix(celery): capture task exceptions with exception autocapture

[posthog]

💡 Motivation and Context

• Python SDK users who turn on enable_exception_autocapture get no $exception events for Celery task errors. They must find and call PosthogCeleryIntegration.instrument() themselves, and the error tracking docs do not mention it.
• Root cause: autocapture installs only sys.excepthook and threading.excepthook. Celery catches every task exception in its trace code, so those hooks never run.
• Also, PosthogCeleryIntegration._on_task_retry did not capture the exception of a retried attempt.

Changes

• Autocapture connects to Celery signals. When Celery is in sys.modules at client creation, ExceptionCapture connects to task_failure and task_retry. A Celery worker always imports Celery first, so workers are covered. Processes that do not use Celery import nothing new.
• No double capture. When a PosthogCeleryIntegration with capture_exceptions=True is instrumented, autocapture skips the exception. The integration captures it once and applies its task_filter.
• Retries are captured by both paths, from Retry.exc. A retry with no exception captures nothing.
• ExceptionCapture.close() disconnects the signals. Lifecycle events and context propagation stay opt-in through the integration.

Signal	source	level	mechanism.handled
task_failure	celery.task_failure	error	false
task_retry	celery.task_retry	warning	true

Note

Behavior change: projects with autocapture and Celery will start to get $exception events for task errors (and per-attempt retry warnings). The rate limiter of autocapture applies to these events too. Count $exception events with mechanism source celery.task_failure / celery.task_retry to measure adoption.

Follow-up outside this repo: add a Celery section to the Python error tracking install docs.

💚 How did you test it?

• New unit tests for autocapture (failure, retry, defer to integration, close() disconnects, no Celery) and for retry capture in the integration.
• End-to-end with real Celery (eager mode, uv run --with celery), with a task that raises KeyError and a task that retries once:

	main	this branch
$exception events	0	3: KeyError error, ConnectionError retry warning, ConnectionError final error
after close()	—	0

• ruff format --check, ruff check, mypy baseline, and the public API snapshot check pass.
• Full pytest: 4716 passed. 3 AI-wrapper tests (langchain/test_callbacks.py x2, anthropic::test_integration_stop_reason) fail on main too, and are not related.

📝 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: Fully autonomous

• Claude Code (PostHog Desktop), from an inbox report.
• Rejected: auto-instrumenting the full PosthogCeleryIntegration. That adds billable lifecycle events and edits task headers without consent. Rejected: only a warning log. It still leaves users with no events.
• Rejected: importing Celery on client creation. It costs import time in processes that only have Celery installed. A process that imports Celery after the client is created still needs the integration.

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Created with PostHog Desktop from this inbox report.

🤖 Generated with Claude Code

Origin

• Conversations: ticket
• First signal: 2026-10-16
• Inbox report: open
• Likely cause: 52cd20e
• Task started by: auto-start, after the report was rated P2 and ready to fix

[github-actions]

posthog-python Compliance Report

Date: 2026-10-16T19:48:02.724184+00:00
Duration: 259898ms

✅ 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	✅	517ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	511ms
Required Headers.Has Authorization Bearer Header	✅	510ms
Required Headers.Has Content Type Json	✅	511ms
Required Headers.Has Posthog Sdk Info Format	✅	510ms
Required Headers.Has Posthog Attempt Header	✅	510ms
Required Headers.Has Posthog Request Id	✅	511ms
Required Headers.Has Posthog Request Timestamp	✅	510ms
Required Headers.Has User Agent	✅	510ms
Body Format.Body Has Created At And Batch	✅	509ms
Body Format.No Api Key In Body	✅	510ms
Body Format.No Sent At In Body	✅	510ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	510ms
Event Format.Event Timestamp Is Rfc3339	✅	510ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	517ms
Event Format.Distinct Id Is String	✅	510ms
Event Format.Distinct Id At Root Not Properties	✅	510ms
Event Format.Custom Properties Preserved	✅	510ms
Event Format.Set Properties Preserved	✅	510ms
Event Format.Set Once Properties Preserved	✅	511ms
Event Format.Groups Properties Preserved	✅	510ms
Event Format.Sdk Generates Uuid If Not Provided	✅	510ms
Event Format.Event Has Required Root Fields Batch	✅	515ms
Event Format.Event Uuid Is Valid Batch	✅	514ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	514ms
Event Format.Distinct Id Is String Batch	✅	513ms
Event Format.Distinct Id At Root Not Properties Batch	✅	515ms
Event Format.Custom Properties Preserved Batch	✅	514ms
Event Format.Set Properties Preserved Batch	✅	513ms
Event Format.Set Once Properties Preserved Batch	✅	514ms
Event Format.Groups Properties Preserved Batch	✅	513ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	514ms
Batch Behavior.Multiple Events In Single Batch	✅	518ms
Batch Behavior.Batch Envelope Smoke	✅	515ms
Batch Behavior.Flush With No Events Sends Nothing	✅	507ms
Batch Behavior.Flush At Triggers Batch	✅	1011ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	511ms
Deduplication.Generates Unique Uuids	✅	518ms
Deduplication.Different Events Same Content Different Uuids	✅	513ms
Deduplication.Preserves Uuid On Retry	✅	6519ms
Deduplication.Preserves Timestamp On Retry	✅	6520ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6524ms
Deduplication.No Duplicate Events In Batch	✅	518ms
Header Behavior On Retry.Attempt Header Starts At One	✅	510ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13522ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6517ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3021ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6515ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	511ms
Response Format Validation.Success Response Has Ok For Each Event	✅	515ms
Response Format Validation.Success No Retry After When All Ok	✅	513ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1516ms
Response Format Validation.Success No Retry After When Drop Only	✅	513ms
Response Format Validation.Response Echoes Request Id	✅	511ms
Retry Behavior.Retries On 408	✅	6516ms
Retry Behavior.Retries On 500	✅	6520ms
Retry Behavior.Retries On 503	✅	8522ms
Retry Behavior.Retries On 504	✅	6517ms
Retry Behavior.Retryable Errors Have Retry After	✅	3517ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11524ms
Retry Behavior.Does Not Retry On 400	✅	2514ms
Retry Behavior.Does Not Retry On 401	✅	2512ms
Retry Behavior.Does Not Retry On 402	✅	2514ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2513ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2514ms
Retry Behavior.Implements Backoff	✅	22537ms
Retry Behavior.Max Retries Respected	✅	22524ms
Partial Batch Handling.Handles 200 Full Success	✅	2514ms
Partial Batch Handling.Handles 200 With All Ok	✅	3517ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3517ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3515ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6523ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6518ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6524ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6522ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6517ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6522ms
Partial Batch Handling.Respects Retry After On Partial	✅	8522ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3517ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3520ms
Compression.Sends Gzip Content Encoding	✅	511ms
Compression.No Content Encoding When Disabled	✅	511ms
Compression.Compressed Body Is Decompressible	✅	511ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2513ms
Event Options.Cookieless Mode Override	✅	511ms
Event Options.Disable Skew Correction Override	✅	510ms
Event Options.Process Person Profile Override	✅	511ms
Event Options.Product Tour Id Override	✅	510ms
Event Options.Unset Options Omitted	✅	510ms
Event Options.Options Override In Batch	✅	514ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	511ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	510ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	511ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	511ms
Routing.Capture Does Not Reroute Ai Named Events	✅	510ms
Identity.Capture Ai Event Has Uuid	✅	511ms
Identity.Capture Ai Keeps Supplied Uuid	✅	511ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	510ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	12ms
Request Payload.Flags Request Uses V2 Query Param	✅	10ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	9ms
Request Payload.Flags Request Omits Authorization Header	✅	11ms
Request Payload.Token In Flags Body Matches Init	✅	10ms
Request Payload.Groups Round Trip	✅	12ms
Request Payload.Groups Default To Empty Object	✅	11ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	10ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	11ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	510ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10ms
Retry Behavior.Retries Flags On 502	✅	313ms
Retry Behavior.Retries Flags On 504	✅	314ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	512ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	71ms
Versioned Boolean Matching.Matching Version 1	✅	68ms
Versioned Boolean Matching.Matching Version 2	✅	68ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	36ms