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+# TaxonRouter Architecture
+
+## Overview
+
+TaxonRouter is a dual-binary Go project that classifies GitHub PRs and applies labels automatically.
+
+```
+┌─────────────────────────────────────────────────────────────────────┐
+│                         TaxonRouter                                  │
+├─────────────────────────────┬───────────────────────────────────────┤
+│   cmd/taxonrouter-mcp      │    cmd/taxonrouter-auto-tagger         │
+│   (MCP server)              │    (Webhook daemon)                    │
+│   Port 3000                 │    Port 3001                           │
+├─────────────────────────────┼───────────────────────────────────────┤
+│   internal/mcp/             │    internal/webhook/                  │
+│   - tools_read.go           │    - webhook.go                        │
+│   - tools_write.go          │    internal/classifier/                │
+│   internal/llm/             │    - classifier.go                     │
+│   - hybrid.go               │    - rules.go                          │
+│   - providers.go            │    internal/apply/                     │
+│   - prompt.go               │    - pipeline.go                        │
+├─────────────────────────────┴───────────────────────────────────────┤
+│                          pkg/                                        │
+│   domain/types.go  — shared pure types (no I/O)                      │
+│   github/client.go  — GitHub API client                              │
+│   rules/classifier.go — rule-based classification                     │
+│   rules/config.go — configuration types                              │
+└─────────────────────────────────────────────────────────────────────┘
+```
+
+## Binary: MCP Server (`cmd/taxonrouter-mcp`)
+
+An MCP server that LLMs can call to:
+- Classify a PR description and diff
+- Get suggested labels
+- Get project field values
+
+### Tool: `classify_pr`
+
+**Input**: `ClassifyInput { description, diff, context? }`
+
+**Output**: `Classification { labels[], project_fields{}, confidence, reasons[], manual_review }`
+
+### Tool: `get_rules`
+
+Returns the current rule set used for classification.
+
+## Binary: Auto-Tagger (`cmd/taxonrouter-auto-tagger`)
+
+A webhook daemon that listens for GitHub webhook events (`pull_request`, `pull_request_target`) and applies labels automatically.
+
+### Pipeline
+
+```
+GitHub webhook
+    │
+    ▼
+webhook.Validate()        ← verify signature
+    │
+    ▼
+classifier.Classify()      ← rule-based + LLM hybrid
+    │
+    ▼
+pipeline.ApplyLabels()     ← batch label + project field application
+    │
+    ▼
+GitHub API
+```
+
+## Classification Strategy
+
+### Rule-Based (Primary)
+
+Rules in `pkg/rules/classifier.go` match file patterns, keywords, and code structures:
+
+```go
+type Rule struct {
+    Name        string
+    Labels      []string
+    FilePatterns []string  // glob patterns
+    Keywords    []string   // in description or diff
+    Confidence  float64
+}
+```
+
+### LLM-Assisted (Secondary)
+
+When rules are uncertain (`confidence < 0.8`), the hybrid classifier calls an LLM:
+- `internal/llm/hybrid.go` — orchestrates rule-first, LLM-second
+- `internal/llm/prompt.go` — prompt templates
+- `internal/llm/providers.go` — OpenAI / Anthropic / Azure OpenAI / Local
+
+### Project Fields
+
+GitHub Projects V2 support single-select fields. The classifier can set:
+- `Area`: which subsystem the PR affects
+- `Priority`: low / medium / high / critical
+- `Effort`: estimated review time
+
+## Key Types
+
+See `pkg/domain/types.go` for the canonical type definitions:
+
+- `Classification` — output of classification
+- `PR` — minimum viable input
+- `Rule` — rule definition
+- `ClassifierConfig` — configuration
+
+## Configuration
+
+Environment variables (see `.env.example`):
+- `GITHUB_TOKEN` — API token
+- `LLM_PROVIDER` — which LLM to use
+- `WEBHOOK_SECRET` — webhook signature verification