# Cross-Layer Thinking Guide

Use for changes crossing data, authority or state boundaries. Trace the affected
path from input through transformation/storage to display or export. Identify
consumers and the format, ownership, errors and invariants at each boundary.
Do not require a diagram or new document when a short task note suffices.

## Validation And Round Trips

Client validation helps the user; the server still enforces authentication,
authorization and untrusted input. Mutable business facts may need rechecking
under the transaction/lock that protects the write. Reuse pure validation rules
where semantics agree, but never remove a trust-boundary check merely because
another layer validated earlier.

Check null/omitted/empty distinctions, dates/time zones, precision, serialization
and error handling on affected consumers. Verify meaningful round trips and
negative cases; test depth follows the changed contract, not a fixed layer count.
The owning domain spec defines exact business behavior.

## Generated Runtime Inputs

For generated files that are also parsed at runtime, identify readers as well
as writers. Workflow tag/platform markers outside a managed block may affect
phase routing even when the block itself updates correctly.

In this installed project, use the pinned target CLI, preserve overlays and
exercise the actual context/hook parsers; see
[upgrade compatibility](../hyb-oa/project-governance.md#scenario-trellis-project-upgrade-compatibility).
Only an authorized upstream-template task should modify other platform copies
or test upstream init/update generation with older pristine fixtures. Do not
assume this repository contains `src/templates` or enable other platforms.

## Probe-Dependent Modes

When a mode depends on an external probe, check normal, non-interactive and
shortcut paths. Distinguish missing resources from transient failures; fail
clearly or retry within the authorized budget, never silently switch modes
after a timeout or transport error. Preserve that error
quality in downstream helpers and reset cached state when the source changes.

Parse complete metadata or use a real streaming parser; a fixed-length prefix
is not a complete JSON document. For reconstructed identifiers, preserve all
fields and their ordering (such as path before fragment). Exercise a large
metadata response and failure/shortcut cases only when those mechanisms are
part of the change. Do not contact real external systems merely to read this guide.