# Implicit File Heuristics Turn Small Theme Changes Into Mode Switches ## Claim When WordPress infers a theme's operating mode from one file's presence, adding or inheriting that file can trigger system-wide behavior without an explicit declaration. <!-- atomicity: 1 idea ✓ --> ## Scope & conditions - Applies to `wp_is_block_theme()` and behavior gated by its result. - Parent-theme inheritance means the decisive file may sit outside the child-theme author's direct changes. - The risk grows when new editor behavior reuses the same classification heuristic. ## Evidence [[3. Resources/Readwise/Articles/How WordPress decides a theme is a block theme - Gutenberg Times]] traces block-theme classification to the existence of `templates/index.html` or its legacy path, including the parent-theme chain. ## Links - [[Type Compatibility Can Hide Runtime Removal]] — parallels: a compatible-looking surface can conceal a runtime mode change. - [[Zet · Technical constraints shape feature implementation paths]] — supports: a narrow implementation check can govern much broader product behavior. - [[Block Theme Development Has Outpaced Block Theme Adoption]] — applies-in: technical classification and actual ecosystem adoption measure different things. - [[3. Resources/MOCs/Open Source Governance MOC]] — applies-in: the MOC lists this note under Synthesis additions (2026-08-11 — implicit platform classification).