feat(model): llm rate limiting (#2198)

* feat(model): rate limiting

* fix(agent): preserve per-model identity in rate limiting and fallback

* fix test

pkg/agent/loop.go

@@ -119,9 +119,18 @@ func NewAgentLoop(
 ) *AgentLoop {
 	registry := NewAgentRegistry(cfg, provider)
 
-	// Set up shared fallback chain
+	// Set up shared fallback chain with rate limiting.
 	cooldown := providers.NewCooldownTracker()
-	fallbackChain := providers.NewFallbackChain(cooldown)
+	rl := providers.NewRateLimiterRegistry()
+	// Register rate limiters for all agents' candidates so that RPM limits
+	// configured in ModelConfig are enforced before each LLM call.
+	for _, agentID := range registry.ListAgentIDs() {
+		if agent, ok := registry.GetAgent(agentID); ok {
+			rl.RegisterCandidates(agent.Candidates)
+			rl.RegisterCandidates(agent.LightCandidates)
+		}
+	}
+	fallbackChain := providers.NewFallbackChain(cooldown, rl)
 
 	// Create state manager using default agent's workspace for channel recording
 	defaultAgent := registry.GetDefaultAgent()
@@ -1032,8 +1041,15 @@ func (al *AgentLoop) ReloadProviderAndConfig(
 	al.cfg = cfg
 	al.registry = registry
 
-	// Also update fallback chain with new config
-	al.fallback = providers.NewFallbackChain(providers.NewCooldownTracker())
+	// Also update fallback chain with new config; rebuild rate limiter registry.
+	newRL := providers.NewRateLimiterRegistry()
+	for _, agentID := range registry.ListAgentIDs() {
+		if agent, ok := registry.GetAgent(agentID); ok {
+			newRL.RegisterCandidates(agent.Candidates)
+			newRL.RegisterCandidates(agent.LightCandidates)
+		}
+	}
+	al.fallback = providers.NewFallbackChain(providers.NewCooldownTracker(), newRL)
 
 	al.mu.Unlock()
 
@@ -3229,7 +3245,7 @@ func (al *AgentLoop) buildCommandsRuntime(agent *AgentInstance, opts *processOpt
 				return "", fmt.Errorf("failed to initialize model %q: %w", value, err)
 			}
 
-			nextCandidates := resolveModelCandidates(cfg, cfg.Agents.Defaults.Provider, modelCfg.Model, agent.Fallbacks)
+			nextCandidates := resolveModelCandidates(cfg, cfg.Agents.Defaults.Provider, value, agent.Fallbacks)
 			if len(nextCandidates) == 0 {
 				return "", fmt.Errorf("model %q did not resolve to any provider candidates", value)
 			}