Session log: Claude Code model selection Q&A + complexity-based sub-agent routing system

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/COMPLEXITY_ROUTING.md

@@ -0,0 +1,74 @@
+# Complexity-Based Model Routing
+
+When spawning an agent, pick a tier based on the request signals below, then pass `model` accordingly.
+
+---
+
+## Tier 1 — Haiku (fast/cheap)
+
+**Signals:** single lookup, no code changes, classification, formatting, summarization, status check, documentation
+
+**Examples:**
+- "What's the status of X?"
+- Summarize or format a session log
+- Search/grep for a value
+- Convert or extract data
+- Write/update a markdown doc
+
+**Agents that default here:** documentation-squire, explore (quick searches), photo
+
+**Agent call:** `model: "haiku"`
+
+---
+
+## Tier 2 — Sonnet (default, inherit)
+
+**Signals:** standard code generation, routine DB queries, test execution, API work, multi-file reads, git operations
+
+**Examples:**
+- Add or modify an endpoint
+- Run tests and report results
+- Write a DB migration
+- Fetch credentials, configure a service
+- Commit and push changes
+
+**Agents that default here:** coding, database, testing, gitea, general-purpose, deep-explore (standard search)
+
+**Agent call:** omit `model` (inherits session model)
+
+---
+
+## Tier 3 — Opus (high-stakes reasoning)
+
+**Signals:** architectural decision, security/auth, 3+ interacting systems, ambiguous root cause, production data risk, anything that fails badly if wrong
+
+**Examples:**
+- Redesign an auth or data flow
+- Security or code review of a critical PR
+- Debug a multi-service race condition
+- Schema migration on production data
+- Evaluate competing architectural approaches
+
+**Agents that default here:** code-review (when Sequential Thinking triggers), deep-explore (architecture questions)
+
+**Agent call:** `model: "opus"`
+
+---
+
+## Bump Rule
+
+If the request contains ANY of these keywords, bump one tier up regardless of other signals:
+
+`security`, `auth`, `token`, `credential`, `migration`, `production`, `race condition`, `data loss`, `breach`, `encrypt`
+
+---
+
+## Quick Reference
+
+| Tier | Model | Typical cost | Use when |
+|------|-------|-------------|----------|
+| 1 | `haiku` | ~10x cheaper | Lookup, format, summarize, doc |
+| 2 | (inherit) | baseline | Standard code, DB, tests |
+| 3 | `opus` | ~5x more expensive | Architecture, security, ambiguous failures |
+
+Err toward Tier 2 when uncertain. Only use Opus when the reasoning stakes justify the cost.