docs(capture): document capture_mode + release changeset (capture v1, 6/6)

[eli-r-ph]

💡 Motivation and Context

Final PR in the stacked Capture V1 series (stacked on #705). Documents the new opt-in capture_mode so the feature ships with release notes and in-repo guidance.

• Sampo changeset (.sampo/changesets/capture-v1-mode.md, minor) — describes the new capture_mode / POSTHOG_CAPTURE_MODE opt-in for the next release. CHANGELOG.md is generated by sampo release, so the changeset is the right place to add this (not a manual CHANGELOG edit).
• AGENTS.md — a "Capture protocol (capture_mode)" section mapping the modes to the modules and routing that implement them (capture_mode.py, capture_v1.py, Consumer._send_analytics / Client._enqueue), plus the v1 invariants to preserve when editing (sentinel lifting + coerce-or-omit, $set/$set_once relocation, partial retry with stable request id, 429 terminal).

User-facing usage examples intentionally stay in the official docs — the README explicitly defers to them ("SDK usage examples and code snippets live in the official documentation so they stay up to date"), so this PR adds developer-facing notes rather than README usage.

💚 How did you test it?

Docs/changeset only — no runtime code. import posthog smoke check passes; from posthog import CaptureMode resolves. The changeset format matches the existing .sampo/changesets/*.md convention.

📝 Checklist

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

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Authored with Cursor (Claude Opus 4.8) per the agreed plan. Final PR of the 6-part stack (#701 -> #706): config scaffolding, transforms, transport, wiring, compliance adapter/CI, and these docs.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "docs(capture): document capture_mode and..." | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-06-28 00:52:02 UTC
Duration: 327809ms

✅ All Tests Passed!

110/110 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	518ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	1010ms
Required Headers.Has Authorization Bearer Header	✅	1008ms
Required Headers.Has Content Type Json	✅	1008ms
Required Headers.Has Posthog Sdk Info Format	✅	1010ms
Required Headers.Has Posthog Attempt Header	✅	1008ms
Required Headers.Has Posthog Request Id	✅	1008ms
Required Headers.Has Posthog Request Timestamp	✅	1008ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1007ms
Body Format.No Api Key In Body	✅	1009ms
Body Format.No Sent At In Body	✅	1008ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1009ms
Event Format.Event Timestamp Is Rfc3339	✅	1008ms
Event Format.Distinct Id Is String	✅	1009ms
Event Format.Distinct Id At Root Not Properties	✅	1008ms
Event Format.Custom Properties Preserved	✅	1008ms
Event Format.Set Properties Preserved	✅	1008ms
Event Format.Set Once Properties Preserved	✅	1008ms
Event Format.Groups Properties Preserved	✅	1009ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1008ms
Event Format.Event Has Required Root Fields Batch	✅	1012ms
Event Format.Event Uuid Is Valid Batch	✅	1012ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1012ms
Event Format.Distinct Id Is String Batch	✅	1012ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1012ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1011ms
Event Format.Set Once Properties Preserved Batch	✅	1012ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1012ms
Batch Behavior.Multiple Events In Single Batch	✅	1506ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1010ms
Deduplication.Generates Unique Uuids	✅	1506ms
Deduplication.Different Events Same Content Different Uuids	✅	1508ms
Deduplication.Preserves Uuid On Retry	✅	7511ms
Deduplication.Preserves Timestamp On Retry	✅	7514ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1503ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1009ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13520ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7506ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3519ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7511ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1008ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1507ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2509ms
Response Format Validation.Success No Retry After When Drop Only	✅	1506ms
Response Format Validation.Response Echoes Request Id	✅	1009ms
Retry Behavior.Retries On 408	✅	7515ms
Retry Behavior.Retries On 500	✅	7512ms
Retry Behavior.Retries On 503	✅	8511ms
Retry Behavior.Retries On 504	✅	7514ms
Retry Behavior.Retryable Errors Have Retry After	✅	4508ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12511ms
Retry Behavior.Does Not Retry On 400	✅	3510ms
Retry Behavior.Does Not Retry On 401	✅	3507ms
Retry Behavior.Does Not Retry On 402	✅	3508ms
Retry Behavior.Does Not Retry On 413	✅	3506ms
Retry Behavior.Does Not Retry On 415	✅	3509ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3506ms
Retry Behavior.Implements Backoff	✅	19520ms
Retry Behavior.Max Retries Respected	✅	19532ms
Partial Batch Handling.Handles 200 Full Success	✅	3000ms
Partial Batch Handling.Handles 200 With All Ok	✅	4506ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4508ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4508ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7515ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7511ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7509ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7515ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7512ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7506ms
Partial Batch Handling.Respects Retry After On Partial	✅	9516ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4505ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4508ms
Compression.Sends Gzip Content Encoding	✅	1007ms
Compression.No Content Encoding When Disabled	✅	1008ms
Compression.Compressed Body Is Decompressible	✅	1008ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1007ms
Event Options.Disable Skew Correction Override	✅	1008ms
Event Options.Process Person Profile Override	✅	1009ms
Event Options.Product Tour Id Override	✅	1008ms
Event Options.Unset Options Omitted	✅	1008ms
Event Options.Options Override In Batch	✅	1011ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1009ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1008ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1009ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1006ms
Request Payload.Flags Request Uses V2 Query Param	✅	1006ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1006ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1006ms
Request Payload.Groups Round Trip	✅	1006ms
Request Payload.Groups Default To Empty Object	✅	1007ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	1006ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1006ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1009ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1003ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1508ms