Add three new files to pkg/routing/:
features.go — ExtractFeatures(msg, history) → Features
Computes five structural dimensions with zero keyword matching:
- TokenEstimate: rune_count/3 (CJK-safe token proxy)
- CodeBlockCount: ``` pairs in the message
- RecentToolCalls: tool call count in the last 6 history entries
- ConversationDepth: total messages in session
- HasAttachments: data URIs or media file extensions
classifier.go — Classifier interface + RuleClassifier
RuleClassifier uses a weighted sum that is capped at 1.0:
code block → +0.40 (triggers heavy model alone at 0.35 threshold)
token > 200 → +0.35 (triggers heavy model alone)
tool calls > 3 → +0.25
token 50-200 → +0.15
conversation depth > 10 → +0.10
attachment → 1.00 (hard gate, always heavy)
router.go — Router wraps config + Classifier
Router.SelectModel(msg, history, primaryModel) returns either the
configured light_model or the primary model depending on whether
the complexity score clears the threshold. Threshold defaults to
0.35 when zero/negative to prevent misconfiguration.
router_test.go — 34 tests covering all branches and edge cases
1943c3e660