fix: recompress request bodies on retry

[marandaneto]

💡 Motivation and Context

Req re-encodes JSON request bodies when retrying, but retains the content-encoding: gzip header from the previous attempt. The retry body is therefore sent as plain JSON while marked as gzip. This caused retry requests to be unreadable and made six SDK compliance tests fail.

This change removes the stale content encoding before the compression step runs again. The retry is sent with a valid gzip body that preserves the original event UUID and timestamp. Compression fallback also removes the gzip header before sending an uncompressed body.

💚 How did you test it?

• mix format --check-formatted
• mix test - 331 passed, 18 excluded
• mix credo --strict
• mix test test/posthog/api/client_test.exs - 17 passed
• docker compose up --build --abort-on-container-exit --exit-code-from test-harness in sdk_compliance_adapter - 46/46 compliance tests passed

📝 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)

Pi was used to reproduce the compliance failures, inspect Req's retry pipeline, implement the fix, and run the project and Docker compliance suites. Autoreview checked the final committed diff and found no actionable issues. GitHub CLI was used to create this pull request.

[marandaneto]

detected by compliance tests

[github-actions]

posthog-elixir Compliance Report

Date: 2026-08-12 07:19:37 UTC
Duration: 117538ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	608ms
Format Validation.Event Has Uuid	✅	609ms
Format Validation.Event Has Lib Properties	✅	609ms
Format Validation.Distinct Id Is String	✅	609ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	609ms
Format Validation.Event Has Timestamp	✅	608ms
Retry Behavior.Retries On 503	✅	5614ms
Retry Behavior.Does Not Retry On 400	✅	2611ms
Retry Behavior.Does Not Retry On 401	✅	2612ms
Retry Behavior.Respects Retry After Header	✅	5614ms
Retry Behavior.Implements Backoff	✅	15625ms
Retry Behavior.Retries On 500	✅	5615ms
Retry Behavior.Retries On 502	✅	5615ms
Retry Behavior.Retries On 504	✅	5614ms
Retry Behavior.Max Retries Respected	✅	15625ms
Deduplication.Generates Unique Uuids	✅	620ms
Deduplication.Preserves Uuid On Retry	✅	5613ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10619ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	613ms
Compression.Sends Gzip When Enabled	✅	609ms
Batch Format.Uses Proper Batch Structure	✅	609ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	613ms
Error Handling.Does Not Retry On 403	✅	2612ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5615ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	610ms
Request Payload.Flags Request Uses V2 Query Param	✅	609ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	609ms
Request Payload.Flags Request Omits Authorization Header	✅	609ms
Request Payload.Token In Flags Body Matches Init	✅	609ms
Request Payload.Groups Round Trip	✅	609ms
Request Payload.Groups Default To Empty Object	✅	609ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	609ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	609ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	609ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	609ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1214ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	609ms
Retry Behavior.Retries Flags On 502	✅	912ms
Retry Behavior.Retries Flags On 504	✅	912ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1213ms

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
test/posthog/api/client_test.exs:68
**Retry test stops too early**

Setting `max_retries` to 1 verifies only the initial request and first retry, leaving compression-state regressions on later retries undetected.

```suggestion
        max_retries: 2,
```

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix: recompress request bodies on retry" | Re-trigger Greptile}