fix(metrics): harden attribute snapshots, retry backoff, and config validation

[DanielVisca]

💡 Motivation and Context

Follow-ups from the #739 review — approved to unblock, but three considerations went unaddressed before merge:

1. Nested attribute values weren't snapshotted — _SeriesState shallow-copied the attributes dict, so mutating a nested list/dict after count() rewrote an already-keyed series' attributes on the wire. Attributes are now deep-copied at capture (with a shallow fallback for un-copyable exotic values, keeping the no-throw guarantee).

2. Retry policy was fixed-cadence with an off-by-one drop — the window was retried at the base flush interval and dropped on the fourth failure while the release note said three. Failed flushes now back off exponentially (2× per consecutive failure, capped at 64× the flush interval — the same ceiling the shared JS logs implementation uses) and the window is dropped after exactly _MAX_CONSECUTIVE_SEND_FAILURES = 8 consecutive failures (~21 min of outage coverage at the default 10s interval). The series cap still bounds buffered memory throughout. Browser/Node parity for this policy is a follow-up on the JS side.

3. Lazy metrics init escaped the client's no-throw contract — Client("phc_...", metrics={"resource_attributes": "bad"}) made the first client.metrics.count() raise AttributeError into application code. Nested config is now validated field-by-field (non-dict config/resource_attributes, non-numeric flush_interval, non-integer max_series_per_flush, non-callable before_send) and degrades to defaults with a warning; the client.metrics property additionally falls back to a default-config instance if construction fails (raising only in debug mode, matching no_throw).

💚 How did you test it?

TDD — new tests written first and confirmed failing against the merged code, then made green:

• test_nested_attribute_values_snapshot_at_capture — mutates a list attribute after capture, asserts the two series keep distinct wire attributes.
• test_failed_flushes_back_off_exponentially_capped — pins the [2, 4, 8, 16, 32, 64, 64]× delay schedule and the reset-on-success.
• test_window_survives_failures_until_the_drop_limit / updated test_window_dropped_after_consecutive_failures — pin the drop-at-exactly-8 boundary from both sides.
• test_hostile_metrics_config_does_not_raise_and_still_records — parameterized over five hostile configs, asserts the full public call chain records with defaults.

Full local validation: pytest posthog/test (1207 passed; the pre-existing test_integration_stop_reason failure reproduces on a clean checkout), ruff check/format --check, mypy | mypy-baseline filter (clean), python -W error -c "import posthog", make public_api_check (no public surface change).

📝 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 (hand-written changeset, patch bump)

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• PostHog Code (Claude) session directed by @DanielVisca; work scoped to the three unresolved review threads on feat(metrics): add posthog.metrics count/gauge/histogram API (alpha) #739.
• Chose deep-snapshot over restricting attributes to scalars (the reviewer offered both): Python's array-attribute support is already released and tested, so snapshotting fixes the aliasing bug without a behavior break.
• Chose 8 attempts × capped-64× backoff over keeping the 3-attempt budget: with backoff spacing the retries out, a small budget would drop data ~2 minutes into an outage; 8 attempts covers ~21 minutes while the series cap bounds memory.

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Created with PostHog Code

[github-actions]

posthog-python Compliance Report

Date: 2026-07-15 18:15:29 UTC
Duration: 338855ms

✅ All Tests Passed!

111/111 tests passed

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Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	516ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	1008ms
Required Headers.Has Authorization Bearer Header	✅	1009ms
Required Headers.Has Content Type Json	✅	1008ms
Required Headers.Has Posthog Sdk Info Format	✅	1010ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
Required Headers.Has Posthog Request Id	✅	1008ms
Required Headers.Has Posthog Request Timestamp	✅	1009ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1008ms
Body Format.No Api Key In Body	✅	1008ms
Body Format.No Sent At In Body	✅	1008ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1008ms
Event Format.Event Timestamp Is Rfc3339	✅	1009ms
Event Format.Distinct Id Is String	✅	1009ms
Event Format.Distinct Id At Root Not Properties	✅	1009ms
Event Format.Custom Properties Preserved	✅	1008ms
Event Format.Set Properties Preserved	✅	1010ms
Event Format.Set Once Properties Preserved	✅	1008ms
Event Format.Groups Properties Preserved	✅	1009ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1009ms
Event Format.Event Has Required Root Fields Batch	✅	1012ms
Event Format.Event Uuid Is Valid Batch	✅	1013ms
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	✅	1013ms
Event Format.Set Properties Preserved Batch	✅	1012ms
Event Format.Set Once Properties Preserved Batch	✅	1013ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1012ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1004ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1011ms
Deduplication.Generates Unique Uuids	✅	1507ms
Deduplication.Different Events Same Content Different Uuids	✅	1507ms
Deduplication.Preserves Uuid On Retry	✅	7516ms
Deduplication.Preserves Timestamp On Retry	✅	7507ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7516ms
Deduplication.No Duplicate Events In Batch	✅	1504ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1008ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14523ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7509ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3514ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7514ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1006ms
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	✅	2511ms
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	✅	7514ms
Retry Behavior.Retries On 503	✅	9516ms
Retry Behavior.Retries On 504	✅	7511ms
Retry Behavior.Retryable Errors Have Retry After	✅	4506ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12520ms
Retry Behavior.Does Not Retry On 400	✅	3502ms
Retry Behavior.Does Not Retry On 401	✅	3509ms
Retry Behavior.Does Not Retry On 402	✅	3508ms
Retry Behavior.Does Not Retry On 413	✅	3506ms
Retry Behavior.Does Not Retry On 415	✅	3508ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3507ms
Retry Behavior.Implements Backoff	✅	23531ms
Retry Behavior.Max Retries Respected	✅	23511ms
Partial Batch Handling.Handles 200 Full Success	✅	3011ms
Partial Batch Handling.Handles 200 With All Ok	✅	4508ms
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	✅	7509ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7516ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7512ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7512ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7510ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7513ms
Partial Batch Handling.Respects Retry After On Partial	✅	9512ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4506ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4508ms
Compression.Sends Gzip Content Encoding	✅	1007ms
Compression.No Content Encoding When Disabled	✅	1009ms
Compression.Compressed Body Is Decompressible	✅	1009ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1007ms
Event Options.Disable Skew Correction Override	✅	1009ms
Event Options.Process Person Profile Override	✅	1007ms
Event Options.Product Tour Id Override	✅	1010ms
Event Options.Unset Options Omitted	✅	1008ms
Event Options.Options Override In Batch	✅	1013ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1012ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1008ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1009ms

Feature_Flags Tests

✅ 17/17 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	✅	1007ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1007ms
Request Payload.Groups Round Trip	✅	1007ms
Request Payload.Groups Default To Empty Object	✅	1006ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1007ms
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	✅	1010ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1003ms
Retry Behavior.Retries Flags On 502	✅	1006ms
Retry Behavior.Retries Flags On 504	✅	1007ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1508ms