feat!: remove capture v0 and legacy transport options

[eli-r-ph]

💡 Motivation and Context

Second PR in the capture v1 series, stacked on #1017. Capture v1 becomes the only capture path, and the v0 transport is removed.

Breaking removals:

• CaptureMode, the capture_mode kwarg, and the POSTHOG_CAPTURE_MODE env var. This follows posthog-go's v1 branch, which removed its capture mode switch too.
• The gzip kwarg on Client/AsyncPosthog/Consumer. Use capture_compression instead (default none, env var POSTHOG_CAPTURE_COMPRESSION still honored).
• request.batch_post, EVENTS_ENDPOINT, AI_EVENTS_ENDPOINT, and the async async_batch_post.
• The gzip parameter of request.post and request.flags. No caller passed it, and flags requests were never gzipped by the client, so flags behavior does not change.
• The backoff dependency.
• Client and AsyncPosthog constructors are keyword-only after host.

Other changes:

• Consumers always send through the v1 submitter to their lane's endpoint. The AI lane stays on /i/v1/ai/events, uncompressed.
• AsyncPosthog capture no longer needs httpx. The httpx client is now only built for flags and remote config.
• The server payload snapshots now record the v1 request (URL, headers, body). legacy_event_family.json is renamed to event_family.json.
• The compliance workflow is one job again (harness 1.1.1, capture_v1). The adapter drops its v0 batch_post patching.

Things to review:

• v1 emits options: {} on every event. This shows in the regenerated snapshots. The event options PR later in the series revisits it.
• The old async v0 path followed same-origin 307/308 redirects itself. The v1 async path sends through requests in a thread, which matches the v1 behavior users already had when opted in.

No new changeset: the major changeset on v1 already covers v0 removal. The migration guide PR lists each breaking change.

💚 How did you test it?

• Ran the full pytest suite, ruff format --check, ruff check, mypy with the baseline filter, python -W error -c "import posthog", the public API snapshot check, the adapter tests, and uv lock --check, all locally.
• v0-only tests are deleted. Their v1 counterparts already live in test_capture_v1.py (retries, Retry-After, 408, terminal errors).
• test_ai_capture_lane.py now asserts that the AI lane keeps its own path and no compression while analytics uses capture_compression="gzip", in both async and sync_mode.
• The async http client reuse/close test moved from capture to the flags path.

📝 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)

Written with Cursor (Claude Opus) under the direction of the assignee. Removing CaptureMode outright (rather than keeping a v1-only enum), collapsing the compliance job, and dropping gzip from request.post/request.flags were agent calls made to match posthog-go's v1 branch; flag them if you want them kept.

[github-actions]

posthog-python Compliance Report

Date: 2026-10-06T19:45:58.833293+00:00
Duration: 257225ms

✅ All Tests Passed!

116/116 tests passed

---

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	516ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	510ms
Required Headers.Has Authorization Bearer Header	✅	509ms
Required Headers.Has Content Type Json	✅	510ms
Required Headers.Has Posthog Sdk Info Format	✅	509ms
Required Headers.Has Posthog Attempt Header	✅	510ms
Required Headers.Has Posthog Request Id	✅	509ms
Required Headers.Has Posthog Request Timestamp	✅	508ms
Required Headers.Has User Agent	✅	510ms
Body Format.Body Has Created At And Batch	✅	509ms
Body Format.No Api Key In Body	✅	509ms
Body Format.No Sent At In Body	✅	509ms
Event Format.Event Has Required Root Fields	✅	509ms
Event Format.Event Uuid Is Valid	✅	509ms
Event Format.Event Timestamp Is Rfc3339	✅	510ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	515ms
Event Format.Distinct Id Is String	✅	510ms
Event Format.Distinct Id At Root Not Properties	✅	509ms
Event Format.Custom Properties Preserved	✅	509ms
Event Format.Set Properties Preserved	✅	509ms
Event Format.Set Once Properties Preserved	✅	510ms
Event Format.Groups Properties Preserved	✅	509ms
Event Format.Sdk Generates Uuid If Not Provided	✅	509ms
Event Format.Event Has Required Root Fields Batch	✅	513ms
Event Format.Event Uuid Is Valid Batch	✅	512ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	513ms
Event Format.Distinct Id Is String Batch	✅	512ms
Event Format.Distinct Id At Root Not Properties Batch	✅	512ms
Event Format.Custom Properties Preserved Batch	✅	512ms
Event Format.Set Properties Preserved Batch	✅	513ms
Event Format.Set Once Properties Preserved Batch	✅	512ms
Event Format.Groups Properties Preserved Batch	✅	513ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	513ms
Batch Behavior.Multiple Events In Single Batch	✅	517ms
Batch Behavior.Batch Envelope Smoke	✅	514ms
Batch Behavior.Flush With No Events Sends Nothing	✅	506ms
Batch Behavior.Flush At Triggers Batch	✅	1011ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	510ms
Deduplication.Generates Unique Uuids	✅	515ms
Deduplication.Different Events Same Content Different Uuids	✅	512ms
Deduplication.Preserves Uuid On Retry	✅	6514ms
Deduplication.Preserves Timestamp On Retry	✅	6519ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6519ms
Deduplication.No Duplicate Events In Batch	✅	517ms
Header Behavior On Retry.Attempt Header Starts At One	✅	510ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13526ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6519ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3018ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6519ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	510ms
Response Format Validation.Success Response Has Ok For Each Event	✅	514ms
Response Format Validation.Success No Retry After When All Ok	✅	511ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1514ms
Response Format Validation.Success No Retry After When Drop Only	✅	511ms
Response Format Validation.Response Echoes Request Id	✅	509ms
Retry Behavior.Retries On 408	✅	6519ms
Retry Behavior.Retries On 500	✅	6519ms
Retry Behavior.Retries On 503	✅	8518ms
Retry Behavior.Retries On 504	✅	6519ms
Retry Behavior.Retryable Errors Have Retry After	✅	3516ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11522ms
Retry Behavior.Does Not Retry On 400	✅	2513ms
Retry Behavior.Does Not Retry On 401	✅	2513ms
Retry Behavior.Does Not Retry On 402	✅	2513ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2512ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2510ms
Retry Behavior.Implements Backoff	✅	22529ms
Retry Behavior.Max Retries Respected	✅	22536ms
Partial Batch Handling.Handles 200 Full Success	✅	2513ms
Partial Batch Handling.Handles 200 With All Ok	✅	3515ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3516ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3516ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6521ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6517ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6523ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6521ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6522ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6520ms
Partial Batch Handling.Respects Retry After On Partial	✅	8519ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3513ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3519ms
Compression.Sends Gzip Content Encoding	✅	510ms
Compression.No Content Encoding When Disabled	✅	509ms
Compression.Compressed Body Is Decompressible	✅	510ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2511ms
Event Options.Cookieless Mode Override	✅	511ms
Event Options.Disable Skew Correction Override	✅	509ms
Event Options.Process Person Profile Override	✅	509ms
Event Options.Product Tour Id Override	✅	509ms
Event Options.Unset Options Omitted	✅	509ms
Event Options.Options Override In Batch	✅	512ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	509ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	509ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	509ms

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	✅	9ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	8ms
Request Payload.Flags Request Omits Authorization Header	✅	9ms
Request Payload.Token In Flags Body Matches Init	✅	9ms
Request Payload.Groups Round Trip	✅	9ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	9ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	509ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	13ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Retry Behavior.Retries Flags On 502	✅	311ms
Retry Behavior.Retries Flags On 504	✅	312ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	511ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	67ms
Versioned Boolean Matching.Matching Version 1	✅	67ms
Versioned Boolean Matching.Matching Version 2	✅	66ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	33ms

[eli-r-ph]

@greptileai review

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 1 · PR risk: 0/10

[dustinbyrne]

AI-assisted review.

[dustinbyrne]

are these repopulated at all during ingestion? we rely on these for analytics and support. /flags also considers it (though it'll fall back to the user-agent header in absence, so i suspect this still works)