Defer sort seed checks until adjust-order startup
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.trellis/spec/guides/cross-layer-thinking-guide.md
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.trellis/spec/guides/cross-layer-thinking-guide.md
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# Cross-Layer Thinking Guide
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> **Purpose**: Think through data flow across layers before implementing.
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---
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## The Problem
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**Most bugs happen at layer boundaries**, not within layers.
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Common cross-layer bugs:
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- API returns format A, frontend expects format B
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- Database stores X, service transforms to Y, but loses data
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- Multiple layers implement the same logic differently
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---
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## Before Implementing Cross-Layer Features
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### Step 1: Map the Data Flow
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Draw out how data moves:
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```
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Source → Transform → Store → Retrieve → Transform → Display
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```
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For each arrow, ask:
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- What format is the data in?
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- What could go wrong?
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- Who is responsible for validation?
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### Step 2: Identify Boundaries
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| Boundary | Common Issues |
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|----------|---------------|
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| API ↔ Service | Type mismatches, missing fields |
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| Service ↔ Database | Format conversions, null handling |
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| Backend ↔ Frontend | Serialization, date formats |
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| Component ↔ Component | Props shape changes |
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### Step 3: Define Contracts
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For each boundary:
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- What is the exact input format?
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- What is the exact output format?
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- What errors can occur?
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---
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## Common Cross-Layer Mistakes
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### Mistake 1: Implicit Format Assumptions
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**Bad**: Assuming date format without checking
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**Good**: Explicit format conversion at boundaries
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### Mistake 2: Scattered Validation
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**Bad**: Validating the same thing in multiple layers
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**Good**: Validate once at the entry point
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### Mistake 3: Leaky Abstractions
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**Bad**: Component knows about database schema
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**Good**: Each layer only knows its neighbors
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---
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## Checklist for Cross-Layer Features
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Before implementation:
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- [ ] Mapped the complete data flow
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- [ ] Identified all layer boundaries
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- [ ] Defined format at each boundary
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- [ ] Decided where validation happens
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After implementation:
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- [ ] Tested with edge cases (null, empty, invalid)
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- [ ] Verified error handling at each boundary
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- [ ] Checked data survives round-trip
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---
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## Cross-Platform Template Consistency
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In Trellis, command templates (e.g., `record-session.md`) exist in **multiple platforms** with identical or near-identical content. This is a cross-layer boundary.
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### Checklist: After Modifying Any Command Template
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- [ ] Find all platforms with the same command: `find src/templates/*/commands/trellis/ -name "<command>.*"`
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- [ ] Update all platform copies (Markdown `.md` and TOML `.toml`)
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- [ ] For Gemini TOML: adapt line continuations (`\\` vs `\`) and triple-quoted strings
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- [ ] Run `/trellis:check-cross-layer` to verify nothing was missed
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**Real-world example**: Updated `record-session.md` in Claude to use `--mode record`, but forgot iFlow, Kilo, OpenCode, and Gemini — caught by cross-layer check.
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---
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## Generated Runtime Template Upgrade Consistency
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Some generated files are both documentation and runtime input. In Trellis,
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`.trellis/workflow.md` is parsed by `get_context.py`, `workflow_phase.py`,
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SessionStart filters, and per-turn hooks. Template changes must be validated
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against both fresh init and upgrade paths.
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### Checklist: After Modifying A Runtime-Parsed Template
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- [ ] Identify every runtime parser that reads the template, not just the file
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writer that installs it
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- [ ] Check whether relevant syntax lives outside obvious managed regions
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such as tag blocks
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- [ ] Verify fresh `init` output and a versioned `update` scenario that writes
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the older `.trellis/.version`
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- [ ] Add an upgrade regression using an older pristine template fixture, then
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assert the installed file reaches the current packaged shape
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- [ ] Update the backend spec that owns the runtime contract
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**Real-world example**: Codex inline mode changed workflow platform markers from
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`[Codex]` / `[Kilo, Antigravity, Windsurf]` to `[codex-sub-agent]` /
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`[codex-inline, Kilo, Antigravity, Windsurf]`. Fresh init was correct, but
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`trellis update` only merged `[workflow-state:*]` blocks and preserved stale
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markers outside those blocks. Result: upgraded projects got new hook scripts
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but old workflow routing, so `get_context.py --mode phase --platform codex`
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could return empty Phase 2.1 detail.
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---
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## Mode-Detection Probe Checklist
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When a CLI auto-detects a mode by probing a remote resource (e.g., checking if `index.json` exists to decide marketplace vs direct download):
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### Before implementing:
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- [ ] Probe runs in **ALL** code paths that use the result (interactive, `-y`, `--flag` combos)
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- [ ] 404 vs transient error are distinguished — don't treat both as "not found"
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- [ ] Transient errors **abort or retry**, never silently switch modes
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- [ ] Shared state (caches, prefetched data) is **reset** when context changes (e.g., user switches source)
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- [ ] **Shortcut paths** (e.g., `--template` skipping picker) must have the same error-handling quality as the probed path — check that downstream functions don't call catch-all wrappers
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### After implementing:
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- [ ] Trace every path from probe result to the mode-decision branch — no fallthrough
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- [ ] External format contracts (giget URI, raw URLs) are tested or at least documented as comments
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- [ ] Metadata reads consume a complete response or use a streaming parser — never parse a fixed-size prefix as full JSON
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- [ ] When reconstructing a composite identifier from parsed parts, verify **all** fields are included and in the **correct position** (e.g., `provider:repo/path#ref` not `provider:repo#ref/path`)
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- [ ] Verify that **action functions** called after a shortcut don't internally use the old catch-all fetch — they must use the probe-quality variant when error distinction matters
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**Real-world example**: Custom registry flow had 8 bugs across 3 review rounds: (1) probe only ran in interactive mode, (2) transient errors fell through to wrong mode, (3) giget URI had `#ref` in wrong position, (4) prefetched templates leaked across source switches, (5) `--template` shortcut bypassed probe but `downloadTemplateById` internally used catch-all `fetchTemplateIndex`, turning timeouts into "Template not found".
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**Real-world example**: Agent-session update hints fetched npm `latest` metadata with `response.read(4096)` and then parsed it as complete JSON. The `@mindfoldhq/trellis` package metadata exceeded 4 KB, so the JSON was truncated, parse failed silently, and the first session injection showed no update hint. Fix: read the complete response before parsing, and add a regression where `version` is followed by an 8 KB metadata tail.
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---
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## When to Create Flow Documentation
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Create detailed flow docs when:
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- Feature spans 3+ layers
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- Multiple teams are involved
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- Data format is complex
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- Feature has caused bugs before
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