observability

Write and review Swift code that makes an app observable: emit the right signal (log, signpost, metric, diagnostic) at the right level, keep user data out of the logs, and get the data back from production in a form you can actually read - symbolicated, aggregated, and attributed to the right release.

Review process:

1. Establish the signal model (three pillars, unified logging architecture, levels and persistence, dev vs production visibility) using references/fundamentals.md.
2. Confirm each emission uses the right signal via the decision tree below; reroute if it does not (print where a Logger belongs, a log where a signpost belongs, a hand-rolled timer where MetricKit already measures).
3. Validate Logger usage (subsystem/category structure, level choice, privacy annotations, interpolation performance, wrapper design) using references/logger-api.md.
4. Validate log retrieval workflows (Console.app, log CLI predicates, logging profiles, sysdiagnose) using references/log-cli-and-console.md.
5. Validate in-app log access and support-log export using references/oslogstore.md.
6. Validate signpost instrumentation (OSSignposter, IDs, intervals, points of interest, Instruments wiring) using references/signposts.md.
7. Validate cross-flow correlation (activities, correlation IDs) using references/activity-tracing.md.
8. Validate MetricKit adoption (subscription timing, payload handling, histograms, diagnostics, the iOS 27 MetricManager API) using references/metrickit.md.
9. Validate crash/hang/OOM observability (what leaves a report and what does not, exit reasons, Organizer) using references/crash-reporting.md.
10. Validate symbolication (dSYM handling, UUID matching, atos, MetricKit call stacks) using references/symbolication.md.
11. Validate the production pipeline (export format, units, aggregation strategy, launch-metric skew, alerting) using references/production-monitoring.md.
12. Validate network-request observability (error taxonomy and noise filtering, non-fatal report design, URLSessionTaskMetrics, aggregate monitoring) using references/network-telemetry.md.
13. Validate third-party SDK integration (crash-handler exclusivity, dSYM upload, bridging, privacy) using references/third-party-sdks.md.
14. Catch common mistakes using references/anti-patterns.md.

If doing partial work, load only the relevant reference files.

Task-based routing

Match the user's goal to the read order. Load only what you need.

"Add (proper) logging to my app / replace print statements"

1. references/logger-api.md - Logger setup, categories, levels, privacy
2. references/fundamentals.md - which levels persist, who can see what

"My interpolated values show as <private> / I need to hide user data in logs"

1. references/logger-api.md - privacy annotations, .private(mask: .hash), equality tracking

"Read logs from a device / filter the noise in Console / capture logs while debugging"

1. references/log-cli-and-console.md - Console.app workflow, log stream/show/collect, predicates, scripted simulator/device capture tiers

"Let users send me their logs / show logs inside the app"

1. references/oslogstore.md - OSLogStore, scope limits, export design
2. references/logger-api.md - persistence rules decide what is even retrievable

"Measure how long an operation takes / find what is slow"

1. references/signposts.md - OSSignposter intervals, Instruments os_signpost
2. references/production-monitoring.md - if the measurement must come from the field

"Trace a user flow across subsystems / correlate related logs"

1. references/activity-tracing.md - activities, correlation IDs, when signposts fit better

"Collect performance/battery/launch metrics from production"

1. references/metrickit.md - MXMetricManager, payloads, histograms, custom mxSignpost metrics
2. references/production-monitoring.md - exporting, aggregating, alerting

"Why is my app crashing / hanging / getting killed in production?"

1. references/crash-reporting.md - crash pipeline, exit reasons, OOM, watchdog, hangs
2. references/metrickit.md - diagnostic payloads carry the stacks
3. references/symbolication.md - make the stacks readable

"Symbolicate this crash log / MetricKit stack"

1. references/symbolication.md - dSYMs, UUID matching, atos recipes

"Set up Sentry / Crashlytics / Datadog, or decide if I need them"

1. references/third-party-sdks.md - what they add over MetricKit, integration rules
2. references/crash-reporting.md - the native baseline they compete with

"Build a metrics dashboard / send telemetry to my backend"

1. references/production-monitoring.md - the ASC Power and Performance API, pipeline design, units, buckets, index metrics
2. references/metrickit.md - the payload shapes you are exporting

"Monitor network errors / diagnose slow requests in production"

1. references/network-telemetry.md - error taxonomy, non-fatal report design, URLSessionTaskMetrics, aggregate monitoring
2. references/third-party-sdks.md - if an off-the-shelf RUM/performance SDK is in play

"I'm hitting bugs / logs missing / payloads never arrive"

1. references/anti-patterns.md - the usual causes
2. references/fundamentals.md - persistence and delivery rules

Decision tree

Which signal should I emit?

Recording a discrete event to reconstruct behavior later?
  -> Logger (unified logging), with the level matching persistence needs

Measuring the duration of an operation for local profiling?
  -> OSSignposter interval + Instruments os_signpost / Points of Interest

Measuring durations or counts across the production fleet?
  -> mxSignpost (MetricKit histograms) or your own telemetry event

Daily launch / hang / battery / disk / memory numbers from production?
  -> MetricKit metric payloads (MXMetricPayload)

Crash, hang, CPU-spin, disk-write, or OOM call stacks from production?
  -> MetricKit diagnostic payloads + Xcode Organizer (+ third-party if you
     need real-time alerting and grouping)

Correlating logs that belong to one user flow across subsystems?
  -> os_activity scopes or an explicit correlation ID in the message

Users need to send you what happened on their device?
  -> OSLogStore export in-app, or a sysdiagnose for deep system issues

Real-time dashboards, alerting, release health, session context?
  -> third-party SDK (Sentry/Crashlytics/Datadog) or your own pipeline
     (OpenTelemetry + OTLP backend is the vendor-neutral build-it route)

Which log level?

Only useful while actively developing, high volume?      -> debug / trace
Helpful context, fine if it evaporates after the fact?   -> info
Essential for troubleshooting, must survive on device?   -> notice (default)
Something failed but the app copes?                      -> error
A bug: broken invariant, data loss risk, must stand out? -> fault / critical

Persistence follows the level: debug is never written to disk, info only during a log collect, notice/error/fault persist to the store (until rotation). If support needs to see it from a user's device, it must be notice or above.

Core Instructions

• Never ship print or NSLog as the app's logging mechanism. print is invisible outside a debugger and never persisted; NSLog is synchronous and slow. Use Logger (iOS 14+) or os_log (iOS 10+).
• Create one Logger per subsystem/category pair as a static let; use the bundle identifier as the subsystem and a functional area per category (networking, persistence, ui). Do not funnel the whole app through one category.
• Never wrap Logger behind a function that takes a plain String. Interpolation in an OSLogMessage is evaluated lazily only if the message is actually emitted; a String parameter forces evaluation on every call. If you must abstract, take @autoclosure () -> String or expose per-category Loggers directly.
• Treat interpolated strings and objects as redacted by default (they log as <private> on a device without a debugger). Mark values privacy: .public only after confirming they contain no user data; use privacy: .private(mask: .hash) when you need to correlate identical values without revealing them.
• Never log PII (names, emails, tokens, URLs with credentials, account numbers) as .public, and never write it into files or third-party SDKs where the OS redaction cannot protect it. Anyone with the device can read your app's persisted log messages in Console.
• Choose levels for their persistence and cost, not their vibe: debug (memory only, near-zero cost when not captured), info (memory, persisted only inside a log collect), notice (persisted default), error (persisted, for failures the app handles), fault/critical (persisted, captures extra state, for actual bugs). Reserve fault for programmer errors, not expected failures.
• Static format strings only: interpolate values, never build the message by string concatenation first - concatenation defeats both privacy annotations and lazy evaluation.
• For durations, use OSSignposter intervals, not log lines with timestamps. Generate an OSSignpostID per operation instance (makeSignpostID()), store the returned OSSignpostIntervalState, and end with the same name. Use .pointsOfInterest category for events you want in the dedicated Instruments track.
• Keep signposts in release builds - they are designed to be cheap and Instruments profiling of release builds depends on them. Gate high-volume ones behind OSSignposter.disabled or an environment check if needed.
• Subscribe to MetricKit (MXMetricManager.shared.add(_:)) once, early, on a launch path that always runs (App init / didFinishLaunching). pastPayloads only returns data if the app was subscribed; a late or conditional subscription silently loses history.
• MetricKit delivers on real devices only - never debug it in the simulator. Use Xcode's Debug > Simulate MetricKit Payloads with a device attached, and expect metric payloads at most once per 24h (diagnostics immediately on iOS 15+, crash diagnostics at next launch).
• MetricKit subscriber callbacks arrive on a background queue: do not touch UI, and persist/forward payloads quickly - the payload objects provide dictionaryRepresentation()/jsonRepresentation() for export.
• When exporting MetricKit histograms, keep Apple's units (Measurement values, buckets in the payload's own unit) and convert exactly once at the edge. Verify the unit of every metric before writing transforms - a silent seconds-vs-milliseconds mismatch corrupts a week of dashboards before anyone notices.
• Never measure cold launch as "process start to didFinishLaunching" without accounting for prewarming (check ActivePrewarm in the environment): since iOS 15 the system may start the process long before the user taps, producing minutes-long fake launches. Prefer MetricKit's histogrammedTimeToFirstDraw.
• Watch means and low percentiles, not just p95/p99: prewarming outliers and clamping hide regressions in the middle of the distribution. For a fleet-wide view, convert metrics to a bounded 0-100 score before averaging.
• OOM terminations leave no crash report. Observe them via MXAppExitMetric memory-related exit reasons (and the memory-exception diagnostic in the new API), and reduce them by lowering background footprint - do not wait for a crash reporter to show them.
• On a memory warning, change caching policy (stop caching, cap sizes, prefer NSCache which evicts automatically) rather than eagerly walking and freeing compressed collections - touching compressed pages forces decompression and spikes footprint exactly when you can least afford it.
• Never install two crash-reporting SDKs (or a crash SDK plus your own Mach/signal handlers) in one app - handlers chain unreliably and corrupt each other's reports. Pick one; MetricKit does not conflict because it collects out-of-process.
• Archive dSYMs for every shipped build and verify UUID match (dwarfdump --uuid) before symbolicating. MetricKit call stacks arrive unsymbolicated as binaryUUID + offsets - symbolicate off-device with atos -i (the -i recovers inlined frames; load address = frame address minus offsetIntoBinaryTextSegment). Submit builds to the App Store with symbols so Organizer reports arrive named with jump-to-source.
• Triage hangs by main-thread CPU: near 100% means the work itself is too slow (Time Profiler territory); near idle means blocked on I/O, a lock, IPC, or a priority inversion (System Trace territory) - profiling CPU harder cannot find a blocked thread. Hangs are reported from 250 ms; treat 500 ms+ as severe.
• Never validate watchdog behavior in the simulator or under the debugger - the launch watchdog is disabled in both. Test launch standalone on the oldest supported device, and enable Organizer regression notifications so a rising hang or termination rate pages you instead of your users.
• Read OSLogStore off the main thread; on iOS it is scoped to your own process (.currentProcessIdentifier) and only returns what persistence rules kept - debug/info messages will not be there in production.
• Use activities (os_activity) or an explicit correlation ID to relate logs across a flow; use signposts to measure it. Activities correlate, signposts time - do not bend one into the other.
• Report network failures selectively: decode and post-decode validation failures are contract breaks worth a non-fatal report from every user (wrapped in a custom error with a readable description - never the raw URLError, whose shared domain collapses distinct problems into one dashboard row); offline/connection-lost/timeout noise is counted in aggregate, and cancellations you issued are dropped. Diagnose slow requests with URLSessionTaskMetrics phases (DNS, connect, TLS, time-to-first-byte, isReusedConnection) - see references/network-telemetry.md.
• Instrument before optimizing: when chasing a production-only problem, first add the metrics/diagnostics that would prove the hypothesis, ship, and read the data - do not guess from a local repro that does not reproduce.
• Log the decision points, not the noise: state transitions, branch outcomes, inputs at boundaries, and every error path. A log that fires every frame or every scroll tick belongs behind a signpost or should not exist.

Output Format

If the user asks for a review, organize findings by file. For each issue:

1. State the file and relevant line(s).
2. Name the anti-pattern being replaced.
3. Show a brief before/after code fix.

Skip files with no issues. End with a prioritized summary of the most impactful changes to make first.

If the user asks you to write or fix observability code, make the changes directly instead of returning a findings report.

Example output:

CheckoutService.swift

Line 31: print for a production failure path - invisible and unpersisted.

// Before
print("Payment failed for \(user.email): \(error)")

// After - persisted, categorized, PII kept private
private static let logger = Logger(
    subsystem: Bundle.main.bundleIdentifier!, category: "checkout")

Self.logger.error("Payment failed for \(user.email, privacy: .private(mask: .hash)): \(error.localizedDescription, privacy: .public)")

Line 58: hand-rolled timing log - use a signpost interval.

// Before
let start = Date()
let receipt = try await submit(order)
logger.info("submit took \(Date().timeIntervalSince(start))s")

// After
private static let signposter = OSSignposter(
    subsystem: Bundle.main.bundleIdentifier!, category: "checkout")

let id = Self.signposter.makeSignpostID()
let state = Self.signposter.beginInterval("SubmitOrder", id: id)
let receipt = try await submit(order)
Self.signposter.endInterval("SubmitOrder", state)

Summary

1. Privacy (high): hash-mask the email; the raw value is currently public in a persisted error message.
2. Signal choice (medium): move all duration logging to signposts so Instruments and MetricKit can aggregate them.

End of example.

References

• references/fundamentals.md - the observability model: three pillars on Apple platforms, unified logging architecture (binary store, memory vs disk), levels and persistence, who can see what in dev vs production, the signal map.
• references/logger-api.md - Logger and legacy os_log: setup patterns, levels, privacy annotations and masking, formatting/alignment, lazy interpolation, wrapper and multi-destination facade design.
• references/log-cli-and-console.md - Console.app workflow, log stream/show/collect/config, predicate syntax, logging configuration profiles, sysdiagnose, scripted capture workflows for agent-assisted debugging (simulator stream-to-file, devicectl console launch with an env-gated print mirror, timestamped sudo log collect device archives).
• references/oslogstore.md - reading your own logs at runtime, scope and persistence limits, building a support-log export.
• references/signposts.md - OSSignposter intervals/events/animation intervals, signpost IDs, legacy ossignpost, Instruments ossignpost and Points of Interest, xctrace, XCTest metrics, DTrace probes for un-instrumentable processes.
• references/activity-tracing.md - os_activity in Swift, scopes, labelUserAction, correlation IDs, activities vs signposts.
• references/metrickit.md - MXMetricManager/MXMetricPayload/MXDiagnosticPayload (iOS 13-25), the iOS 27 MetricManager async API, payload cadence, histograms, custom mxSignpost metrics, extended launch tracking, testing.
• references/crash-reporting.md - what leaves a report and what does not: crash pipeline, exception types, watchdog codes, exit reasons, OOM, the hang tooling matrix (Thread Performance Checker, on-device detection, Organizer hang reports), Xcode Organizer panes and regressions.
• references/symbolication.md - dSYMs, UUID matching, atos, debug-info tiers and stripped-symbol behavior, Swift demangling, the dyld shared cache, symbolicating .ips files and MetricKit call stack trees, SDK symbol upload.
• references/production-monitoring.md - shipping telemetry: the App Store Connect Power and Performance API (metrics, smart insights, diagnostic signatures/logs), OpenTelemetry on iOS (opentelemetry-swift, OTLP exporters, gauge/counter/histogram semantics, TaskLocal span propagation, withSpan facade design, verbosity gating), export pipeline design, units and buckets, lab vs field, launch-metric skew (prewarming), score indexes, percentiles vs means, alerting.
• references/network-telemetry.md - request-level network observability: why lab conditions undersell the field, the report/count/drop error taxonomy, non-fatal report design (readable custom errors, userInfo context, embedded breadcrumbs), URLSessionTaskMetrics phases and diagnostics (keep-alive, server processing time, latency physics), aggregate monitoring (Firebase Performance / OTel), targeted full-dump collection.
• references/third-party-sdks.md - Sentry/Crashlytics/Datadog/Firebase vs the native stack, crash-handler exclusivity, bridging unified logging, dSYM upload automation, privacy manifests and cost.
• references/anti-patterns.md - common mistakes LLMs make when generating logging and telemetry code.