docs: agree on public API changes before implementing them

[turnipdabeets]

💡 Motivation and Context

Port of PostHog/posthog-js#4906 to posthog-php.

On contributor PRs we sometimes only settle the public API after several rounds of implementation review. By then the contributor, or their agent, has built each suggestion along the way, and a late change of direction wastes their work. This asks contributors to agree on the API shape on the issue first, and tells their agents to stop and ask.

• CONTRIBUTING.md: new "Public API changes" section, same as posthog-js. The last paragraph points at composer api:update / composer api:check and api/public-api.json.
• AGENTS.md: new file, since this repo didn't have one. Same shape as the posthog-js one: it points agents to CONTRIBUTING.md, RELEASING.md, and the new section, and asks to keep shared guidance out of AGENTS.md.

💚 How did you test it?

Docs only. Checked that the commands and file paths it mentions exist in this repo.

📝 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 pnpm change to generate a change intent file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Written with Claude Code, porting the posthog-js section and adapting the API-snapshot paragraph and beforeSend-style hook name to this SDK.

🤖 Generated with Claude Code

https://claude.ai/code/session_016syAmdpp8imNvs7PWkpwvb

[github-actions]

posthog-php Compliance Report

Date: 2026-09-11 15:27:29 UTC
Duration: 95350ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	12ms
Format Validation.Event Has Uuid	✅	6ms
Format Validation.Event Has Lib Properties	✅	7ms
Format Validation.Distinct Id Is String	✅	6ms
Format Validation.Token Is Present	✅	6ms
Format Validation.Custom Properties Preserved	✅	7ms
Format Validation.Event Has Timestamp	✅	6ms
Retry Behavior.Retries On 503	✅	5316ms
Retry Behavior.Does Not Retry On 400	✅	2009ms
Retry Behavior.Does Not Retry On 401	✅	2009ms
Retry Behavior.Respects Retry After Header	✅	8013ms
Retry Behavior.Implements Backoff	✅	15737ms
Retry Behavior.Retries On 500	✅	5114ms
Retry Behavior.Retries On 502	✅	5116ms
Retry Behavior.Retries On 504	✅	5115ms
Retry Behavior.Max Retries Respected	✅	16530ms
Deduplication.Generates Unique Uuids	✅	11ms
Deduplication.Preserves Uuid On Retry	✅	5114ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10316ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5117ms
Deduplication.No Duplicate Events In Batch	✅	11ms
Deduplication.Different Events Have Different Uuids	✅	7ms
Compression.Sends Gzip When Enabled	✅	5ms
Batch Format.Uses Proper Batch Structure	✅	6ms
Batch Format.Flush With No Events Sends Nothing	✅	4ms
Batch Format.Multiple Events Batched Together	✅	9ms
Error Handling.Does Not Retry On 403	✅	2008ms
Error Handling.Does Not Retry On 413	✅	2010ms
Error Handling.Retries On 408	✅	5115ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	7ms
Request Payload.Flags Request Uses V2 Query Param	✅	5ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	4ms
Request Payload.Flags Request Omits Authorization Header	✅	7ms
Request Payload.Token In Flags Body Matches Init	✅	13ms
Request Payload.Groups Round Trip	✅	14ms
Request Payload.Groups Default To Empty Object	✅	10ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	11ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	8ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	7ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	10ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	9ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	5ms
Retry Behavior.Retries Flags On 502	✅	108ms
Retry Behavior.Retries Flags On 504	✅	108ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	8ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "docs: agree on public API changes before..." | Re-trigger Greptile}