diff --git a/internal/router/heuristic.go b/internal/router/heuristic.go
new file mode 100644
index 00000000..f02cc75f
--- /dev/null
+++ b/internal/router/heuristic.go
@@ -0,0 +1,73 @@
+package router
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+
+	"gophergate/internal/db"
+)
+
+// HeuristicRule defines a pattern-based routing rule.
+type HeuristicRule struct {
+	Pattern       string `json:"pattern"`
+	TargetIdx     int    `json:"target"`
+	CaseSensitive bool   `json:"case_sensitive,omitempty"`
+}
+
+func routeHeuristic(group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	selected := targets[0]
+	reason := "default (first target)"
+
+	// If heuristic_rules is set, use them
+	if group.HeuristicRules != nil && *group.HeuristicRules != "" {
+		var rules []HeuristicRule
+		if err := json.Unmarshal([]byte(*group.HeuristicRules), &rules); err == nil {
+			searchMsg := userMessage
+			for _, rule := range rules {
+				pattern := rule.Pattern
+				msg := searchMsg
+				if !rule.CaseSensitive {
+					pattern = strings.ToLower(pattern)
+					msg = strings.ToLower(msg)
+				}
+				if strings.Contains(msg, pattern) {
+					if rule.TargetIdx >= 0 && rule.TargetIdx < len(targets) {
+						selected = targets[rule.TargetIdx]
+						reason = "matched heuristic rule: " + rule.Pattern
+						break
+					}
+				}
+			}
+		}
+	}
+
+	// Built-in fallback heuristics
+	if reason == "default (first target)" && len(targets) > 1 {
+		msgLower := strings.ToLower(userMessage)
+		complexIndicators := []string{
+			"step by step", "explain in detail", "reason through",
+			"think carefully", "analyze", "debug", "write code",
+			"implement", "refactor", "architecture",
+		}
+		for _, indicator := range complexIndicators {
+			if strings.Contains(msgLower, indicator) {
+				selected = targets[len(targets)-1]
+				reason = "complex task indicator: " + indicator
+				break
+			}
+		}
+	}
+
+	return &Decision{
+		SelectedModel: selected,
+		Strategy:      "heuristic",
+		Reason:        reason,
+	}, nil
+}
+
+// routeClassifier is a stub — real implementation in classifier.go (Task 3).
+// Falls back to heuristic routing for now.
+func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	return routeHeuristic(group, targets, userMessage)
+}
diff --git a/internal/router/router.go b/internal/router/router.go
new file mode 100644
index 00000000..2e512a83
--- /dev/null
+++ b/internal/router/router.go
@@ -0,0 +1,76 @@
+package router
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"gophergate/internal/db"
+)
+
+// Decision holds the result of a routing decision.
+type Decision struct {
+	SelectedModel string `json:"selected_model"`
+	Strategy      string `json:"strategy"` // "heuristic" or "classifier"
+	Reason        string `json:"reason"`
+}
+
+// ClassifierFunc is the callback for classifier-based routing.
+type ClassifierFunc func(ctx context.Context, selectorModel, systemPrompt, userMessage string) (string, error)
+
+// Router resolves model groups to concrete models.
+type Router struct {
+	groups   map[string]db.ModelGroup
+	classify ClassifierFunc
+}
+
+// New creates a Router. classify may be nil if no classifier groups exist.
+func New(groups []db.ModelGroup, classify ClassifierFunc) *Router {
+	r := &Router{
+		groups:   make(map[string]db.ModelGroup),
+		classify: classify,
+	}
+	for _, g := range groups {
+		r.groups[g.ID] = g
+	}
+	return r
+}
+
+// IsGroup returns true if the model name is a group ID.
+func (r *Router) IsGroup(modelID string) bool {
+	_, ok := r.groups[modelID]
+	return ok
+}
+
+// Route resolves a group to a concrete model.
+func (r *Router) Route(ctx context.Context, groupID string, userMessage string) (*Decision, error) {
+	group, ok := r.groups[groupID]
+	if !ok {
+		return nil, fmt.Errorf("unknown model group: %s", groupID)
+	}
+
+	var targets []string
+	if err := json.Unmarshal([]byte(group.Targets), &targets); err != nil || len(targets) == 0 {
+		return nil, fmt.Errorf("invalid or empty targets for group %s", groupID)
+	}
+
+	switch group.Strategy {
+	case "heuristic":
+		return routeHeuristic(group, targets, userMessage)
+	case "classifier":
+		if r.classify == nil {
+			return routeHeuristic(group, targets, userMessage)
+		}
+		return routeClassifier(ctx, r.classify, group, targets, userMessage)
+	default:
+		return nil, fmt.Errorf("unknown strategy: %s", group.Strategy)
+	}
+}
+
+// Reload replaces the group definitions without recreating the router.
+func (r *Router) Reload(groups []db.ModelGroup) {
+	r.groups = make(map[string]db.ModelGroup)
+	for _, g := range groups {
+		r.groups[g.ID] = g
+	}
+}