Hidden Assumptions in posthog
12 assumptions this code never checks · 3 critical · spanning Environment, Contract, Resource, Ordering, Shape, Temporal, Domain, Scale
Every codebase relies on things it never checks. Most of them are routine. CodeSea looked at posthog/posthog and picked out the few most likely to cause trouble. The full list is just below.
Most of what this code assumes is routine. These 3 are the ones most likely to cause trouble here. The rest are minor; they're under "Show everything".
If running in a browser environment or restricted Node.js context without stdin access, the plugin transpiler silently hangs waiting for input that never comes
If webpack.config.js changes its export structure or createEntry signature, Storybook build fails with property access errors on undefined objects
If events arrive out of order due to network delays or buffering, the replay timeline becomes scrambled, showing user actions in wrong sequence without any reordering logic
Show everything (9 more)
Assumes filesystem has sufficient space and permissions for webpack filesystem cache, and CI environment variable accurately indicates memory constraints
If this fails: In CI with limited disk space or wrong CI detection, either runs out of disk space with caching enabled or suffers slow builds with caching disabled unnecessarily
common/storybook/.storybook/main.ts:cache
Assumes event.type values 2,3,4,5 correspond exactly to specific transformer functions and that no new event types are introduced
If this fails: When new mobile event types are added or existing type numbers change, events silently pass through untransformed, causing replay corruption or missing functionality
common/replay-shared/src/mobile/index.ts:transformEventToWeb
Assumes 'pnpm' command is available in PATH and 'tsx' can execute TypeScript files, with shell access enabled
If this fails: In environments without pnpm (using npm/yarn) or restricted shell access, token rebuilds fail silently and CSS custom properties become stale until manual restart
packages/quill/apps/storybook/.storybook/main.ts:quillTokensWatcher
Assumes file change events arrive within reasonable intervals and 100ms debounce is sufficient to batch multiple file changes
If this fails: During rapid file changes or slow filesystem events, rebuild triggers repeatedly causing resource exhaustion, or changes get lost if debounce window is too short
packages/quill/apps/storybook/.storybook/main.ts:rebuildTimer
Assumes presets object has entries for 'site' and 'frontend' types with consistent structure containing wrapper and options properties
If this fails: If presets configuration is missing required types or has different property structure, transpilation fails with property access errors
common/plugin_transpiler/src/index.ts:presets
Assumes all valid replay events must have 'type' and 'timestamp' properties, and that these are sufficient to identify processable events
If this fails: Events with valid structure but missing these fields are discarded, causing gaps in replay data, while malformed events with only these fields may crash downstream processing
common/replay-shared/src/mobile/index.ts:couldBeEventWithTime
Assumes mobile replay data arrays fit in memory and that reduce operation can process all events synchronously without memory pressure
If this fails: For very long sessions with thousands of events, the transformation consumes excessive memory and may cause browser tab crashes or Node.js heap exhaustion
common/replay-shared/src/mobile/index.ts:transformToWeb
Assumes file watcher can monitor unlimited files in tokens directory and that filesystem events are reliable
If this fails: On systems with file watcher limits or unreliable filesystem events, token changes may not trigger rebuilds, leaving stale CSS in development
packages/quill/apps/storybook/.storybook/main.ts:server.watcher
Assumes CI environment is accurately detected by process.env.CI and that CI environments always have memory constraints requiring cache disabling
If this fails: In CI environments with ample memory but CI=true, builds run slower without filesystem cache, while memory-constrained non-CI environments may crash with caching enabled
common/storybook/.storybook/main.ts:process.env.CI
See the full structural analysis of posthog: the pipeline, data models, and system behavior that put these assumptions in context.
Full analysis of posthog/posthog →Frequently Asked Questions
What does posthog assume that could break in production?
The one most likely to cause trouble: Assumes stdin stream is available and readable in Node.js environment, with proper 'utf8' encoding support If this fails, If running in a browser environment or restricted Node.js context without stdin access, the plugin transpiler silently hangs waiting for input that never comes
How many hidden assumptions does posthog have?
CodeSea found 12 assumptions posthog relies on but never validates, 3 of them critical, spanning Environment, Contract, Resource, Ordering, Shape, Temporal, Domain, Scale. Most are routine — the analysis flags the two or three most likely to actually bite.
What is a hidden assumption?
Something the code depends on but never checks: a data shape, an ordering, an environment condition, a scale limit, or a contract with another service. It holds until the world it runs in changes, then fails silently.