[turnipdabeets]

💡 Motivation and Context

Part of the "Migrate to Swift 6 (and strong concurrency?)" item in #557. This PR only covers the part that needs a major version: the callbacks apps pass to the SDK. Compiling the SDK itself in Swift 6 language mode is internal work with no API impact, so it can follow in a 4.x minor.

Several public callbacks run off the main thread but aren't typed @Sendable. That causes two problems for apps:

• Swift 6 apps crash. A reloadFeatureFlags { _ in ... } closure written in main-actor code (a @MainActor class or a SwiftUI view) is main-actor isolated, and the SDK calls it from its network completion queue. The Swift 6 runtime check traps (_swift_task_checkIsolatedSwift → SIGTRAP). The closure doesn't need to touch any state; { _ in print("x") } is enough. Swift 5 apps don't crash.
• Silent data races. For closures passed variadically (setBeforeSend) or assigned to a property (getAnonymousId, logSanitizer, pushIdentityProvider), Swift doesn't insert that runtime check. A main-actor closure there compiles cleanly and then mutates main-actor state off the main thread.

API	Runs on
reloadFeatureFlags(_:) callback	network completion queue, or the caller's thread when the reload is skipped
setBeforeSend(_:) (events and logs)	the thread that called capture / captureLog
getAnonymousId	whichever thread generates the ID
logSanitizer	the console pipe's background reader
pushIdentityProvider	the SDK's push mint queue; its completion from any thread

Changes:

• Mark those callbacks @Sendable. The compiler now catches main-actor access inside them, in Swift 5 and Swift 6 apps alike, and apps hop explicitly (Task { @MainActor in ... }). Objective-C is unaffected; the generated header doesn't change.
• Make PostHogFeatureFlagsLoaded Sendable (@unchecked: all lets, and variants only holds Bool/String). Swift 6 apps can then pass it across tasks.
• onFeatureFlags keeps its signature. It already delivers on the main thread, so main-actor closures pass the runtime check. Typing it @MainActor adds a Swift 6 error for apps that use plain classes and fixes nothing, so I left it out.
• Document threading on getAnonymousId and logSanitizer, and tell callers of reloadFeatureFlags(_:) how to hop to the main actor. Fix PostHogExample, which set a @Published property from the callback on a background thread.

Who breaks:

App code inside these callbacks	Swift 5 app	Swift 6 app
Plain class capturing self	nothing	error
SwiftUI view setting @State	warning	warning
@MainActor class touching its own state	error	error

Every error lands on code that today either crashes (Swift 6) or races (Swift 5).

One upgrade hazard: a Swift 5 app that passes a pre-typed array such as let hooks: [(PostHogEvent) -> PostHogEvent?] to setBeforeSend(_:) gets only a warning, then aborts at runtime ("Could not cast value of type '(…) -> …' to '@sendable (…) -> …'"). Swift can't convert arrays of function types that differ only in @Sendable. Swift 6 apps get a compile error instead. Array literals, single closures, variadic calls and arrays built with map are fine. The SDK hit the same thing internally, which is why BeforeSendChain now stores @Sendable blocks. The changeset tells apps to type those arrays as [BeforeSendBlock] / [PostHogBeforeSendLogBlock].

Open question for reviewers: we could prevent that crash instead of documenting it. A Swift-only @available(swift, obsoleted: 6.0) @_disfavoredOverload setBeforeSend(_: [(PostHogEvent) -> PostHogEvent?]) on each config would take typed arrays in Swift 5 and box them, without changing overload resolution for literals or [BeforeSendBlock]. I left it out because it adds public API using an underscored attribute for a narrow pattern.

The Swift-only setBeforeSend(_:) overloads get new mangled symbol names, so prebuilt binaries need rebuilding. That's expected for a major version.

Spec: reload-feature-flags lists the iOS signature as reloadFeatureFlags(_ callback: @escaping () -> Void). With #913 and this PR it becomes @escaping @Sendable (PostHogFeatureFlagsLoaded) -> Void. Neither it nor on-feature-flags says anything about threads, so there's no behavior divergence.

Stacked on #913, which passes a PostHogFeatureFlagsLoaded to the reload callback. This PR adds @Sendable to that new signature, so the API breaks only once. #913 sits on #873, whose tools bump to 6.2 the per-target Swift 6 test setting needs. #912 renames PostHogLogEntry to PostHogConsoleLogEntry, so the logSanitizer line will conflict trivially.

💚 How did you test it?

• New PostHogSwift6Tests target compiled in Swift 6 language mode. It calls the API from main-actor code the way an app does:
  • reloadFeatureFlags with a closure written in main-actor code, which passes its result into a main-actor Task. Without @Sendable the test process traps (signal 5); with this PR it passes.
  • onFeatureFlags delivers on main and its payload can be sent to another task. Without the Sendable conformance the target doesn't compile.
• No runtime test covers the silent-race callbacks: nothing traps at runtime, and Swift Testing can't assert that code fails to compile. I checked them with throwaway Swift 6 and Swift 5 app packages (the table above).
• make test: XCTest 169/169 passes. Swift Testing has 13 issues, all in PostHogStorageTest's app-group container tests; the base branch fails the same way on this machine, and nothing else fails.
• make buildSdk: all 6 platforms build, with no concurrency warnings.
• make buildExamplesPlatforms: all 8 example builds succeed.
• iOS simulator: PostHogConsoleLogInterceptorTest and PostHogSessionReplayPluginTests pass (logSanitizer only runs on iOS).
• make lint: no serious violations.
• Regenerated api/posthog-ios.public-api.txt with make apiUpdate (xcpretty isn't installed here, so I ran it with a pass-through shim).
• The change adds 29 compiler warnings to existing Swift 5 mode tests, all closures mutating captured vars inside these callbacks. Tests still pass. They'll be fixed with the SDK's Swift 6 language-mode migration, or I can do it here if reviewers prefer.
• Reviewed by two separate review agents (correctness and compatibility). Their verified findings are applied: threading docs, a fuller changeset, and a test assertion that doesn't depend on the /flags request failing.
• Not run: the full iOS simulator suite. The new Swift 6 target is SPM-only, so CI runs it in the macOS make test job, not the simulator job.

📝 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 changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Claude Code (Opus 5.5).
• Considered and dropped: typing onFeatureFlags as @MainActor, and switching the SDK to Swift 6 language mode in this PR. A first build in Swift 6 mode shows at least 35 errors, mostly mutable globals, and that's internal work.
• PostHogSDK: @unchecked Sendable (so Swift 6 apps can pass SDK instances into tasks and actors) is additive and goes in a separate PR.

🤖 Generated with Claude Code

[github-actions]

posthog-ios Compliance Report

Date: 2026-10-07 02:18:33 UTC
Duration: 215454ms

⚠️ Some Tests Failed

44/45 tests passed, 1 failed

---

Capture Tests

⚠️ 28/29 tests passed, 1 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	2895ms
Format Validation.Event Has Uuid	✅	2678ms
Format Validation.Event Has Lib Properties	✅	325ms
Format Validation.Distinct Id Is String	✅	2819ms
Format Validation.Token Is Present	✅	573ms
Format Validation.Custom Properties Preserved	✅	2949ms
Format Validation.Event Has Timestamp	✅	2834ms
Retry Behavior.Retries On 503	✅	11799ms
Retry Behavior.Does Not Retry On 400	✅	4734ms
Retry Behavior.Does Not Retry On 401	✅	4760ms
Retry Behavior.Respects Retry After Header	✅	7757ms
Retry Behavior.Implements Backoff	✅	21802ms
Retry Behavior.Retries On 500	✅	5279ms
Retry Behavior.Retries On 502	✅	5443ms
Retry Behavior.Retries On 504	✅	5457ms
Retry Behavior.Max Retries Respected	❌	21845ms
Deduplication.Generates Unique Uuids	✅	3152ms
Deduplication.Preserves Uuid On Retry	✅	9322ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10431ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7992ms
Deduplication.No Duplicate Events In Batch	✅	2894ms
Deduplication.Different Events Have Different Uuids	✅	2821ms
Compression.Sends Gzip When Enabled	✅	2781ms
Batch Format.Uses Proper Batch Structure	✅	2701ms
Batch Format.Flush With No Events Sends Nothing	✅	312ms
Batch Format.Multiple Events Batched Together	✅	2985ms
Error Handling.Does Not Retry On 403	✅	4810ms
Error Handling.Does Not Retry On 413	✅	4809ms
Error Handling.Retries On 408	✅	9325ms

Failures

retry_behavior.max_retries_respected

Expected 4 requests, got 6

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	2888ms
Request Payload.Flags Request Uses V2 Query Param	✅	3075ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	2934ms
Request Payload.Flags Request Omits Authorization Header	✅	2795ms
Request Payload.Token In Flags Body Matches Init	✅	2736ms
Request Payload.Groups Round Trip	✅	2854ms
Request Payload.Groups Default To Empty Object	✅	2788ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	2808ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	2842ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	2830ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	2749ms
Request Lifecycle.No Flags Request On Init Alone	✅	57ms
Request Lifecycle.No Flags Request On Normal Capture	✅	2816ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	5504ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	2761ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	3068ms

[dustinbyrne]

Thanks—the callback annotations and major-version migration notes match the execution paths. One important issue remains in the loaded-flags payload: accepted mutable Foundation bootstrap variants can survive the filter, undermining its new unchecked Sendable guarantee. Could we store the bridged Bool/String values and add the snapshot regression described inline?

Agent-led review, human-reviewed before posting. Validation was static inspection plus existing CI evidence.