diff --git a/iter_test.go b/iter_test.go
index aaa59be..f353e16 100644
--- a/iter_test.go
+++ b/iter_test.go
@@ -31,13 +31,19 @@ func TestIterators(t *testing.T) {
 		assert.Contains(t, models, "model-a")
 		assert.Contains(t, models, "model-b")
 		assert.Contains(t, models, "model-c")
-		
+
 		// Check sorting of our specific models
 		foundA, foundB, foundC := -1, -1, -1
 		for i, m := range models {
-			if m == "model-a" { foundA = i }
-			if m == "model-b" { foundB = i }
-			if m == "model-c" { foundC = i }
+			if m == "model-a" {
+				foundA = i
+			}
+			if m == "model-b" {
+				foundB = i
+			}
+			if m == "model-c" {
+				foundC = i
+			}
 		}
 		assert.True(t, foundA < foundB && foundB < foundC, "models should be sorted: a < b < c")
 	})
@@ -57,9 +63,15 @@ func TestIterators(t *testing.T) {
 		// Check sorting
 		foundA, foundB, foundC := -1, -1, -1
 		for i, m := range models {
-			if m == "model-a" { foundA = i }
-			if m == "model-b" { foundB = i }
-			if m == "model-c" { foundC = i }
+			if m == "model-a" {
+				foundA = i
+			}
+			if m == "model-b" {
+				foundB = i
+			}
+			if m == "model-c" {
+				foundC = i
+			}
 		}
 		assert.True(t, foundA < foundB && foundB < foundC, "iter should be sorted: a < b < c")
 	})
@@ -99,9 +111,8 @@ func TestIterEarlyBreak(t *testing.T) {
 }
 
 func TestCountTokensFull(t *testing.T) {
-	t.Run("invalid URL/network error", func(t *testing.T) {
-		// Using an invalid character in model name to trigger URL error or similar
-		_, err := CountTokens(context.Background(), "key", "invalid model", "text")
+	t.Run("empty model is rejected", func(t *testing.T) {
+		_, err := CountTokens(context.Background(), "key", "", "text")
 		assert.Error(t, err)
 	})
 
@@ -112,15 +123,15 @@ func TestCountTokensFull(t *testing.T) {
 		}))
 		defer server.Close()
 
-		// We can't easily override the URL in CountTokens without changing the code,
-		// but we can test the logic if we make it slightly more testable.
-		// For now, I've already updated ratelimit_test.go with some of this.
+		_, err := countTokensWithClient(context.Background(), server.Client(), server.URL, "key", "model", "text")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "status 400")
 	})
 
 	t.Run("context cancelled", func(t *testing.T) {
 		ctx, cancel := context.WithCancel(context.Background())
 		cancel()
-		_, err := CountTokens(ctx, "key", "model", "text")
+		_, err := countTokensWithClient(ctx, http.DefaultClient, "https://generativelanguage.googleapis.com", "key", "model", "text")
 		assert.Error(t, err)
 		assert.Contains(t, err.Error(), "do request")
 	})
diff --git a/ratelimit.go b/ratelimit.go
index d2d0007..0e34709 100644
--- a/ratelimit.go
+++ b/ratelimit.go
@@ -9,9 +9,11 @@ import (
 	"iter"
 	"maps"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"slices"
+	"strings"
 	"sync"
 	"time"
 
@@ -34,6 +36,16 @@ const (
 	ProviderLocal Provider = "local"
 )
 
+const (
+	defaultStateDirName         = ".core"
+	defaultYAMLStateFile        = "ratelimits.yaml"
+	defaultSQLiteStateFile      = "ratelimits.db"
+	backendYAML                 = "yaml"
+	backendSQLite               = "sqlite"
+	countTokensErrorBodyLimit   = 8 * 1024
+	countTokensSuccessBodyLimit = 1 * 1024 * 1024
+)
+
 // ModelQuota defines the rate limits for a specific model.
 type ModelQuota struct {
 	MaxRPM int `yaml:"max_rpm"` // Requests per minute (0 = unlimited)
@@ -143,39 +155,30 @@ func New() (*RateLimiter, error) {
 // NewWithConfig creates a RateLimiter from explicit configuration.
 // If no providers or quotas are specified, Gemini defaults are used.
 func NewWithConfig(cfg Config) (*RateLimiter, error) {
+	backend, err := normaliseBackend(cfg.Backend)
+	if err != nil {
+		return nil, err
+	}
+
 	filePath := cfg.FilePath
 	if filePath == "" {
-		home, err := os.UserHomeDir()
+		filePath, err = defaultStatePath(backend)
 		if err != nil {
 			return nil, err
 		}
-		filePath = filepath.Join(home, ".core", "ratelimits.yaml")
 	}
 
-	rl := &RateLimiter{
-		Quotas:   make(map[string]ModelQuota),
-		State:    make(map[string]*UsageStats),
-		filePath: filePath,
-	}
-
-	// Load provider profiles
-	profiles := DefaultProfiles()
-	providers := cfg.Providers
-
-	// If nothing specified at all, default to Gemini
-	if len(providers) == 0 && len(cfg.Quotas) == 0 {
-		providers = []Provider{ProviderGemini}
-	}
-
-	for _, p := range providers {
-		if profile, ok := profiles[p]; ok {
-			maps.Copy(rl.Quotas, profile.Models)
+	if backend == backendSQLite {
+		if cfg.FilePath == "" {
+			if err := os.MkdirAll(filepath.Dir(filePath), 0o755); err != nil {
+				return nil, coreerr.E("ratelimit.NewWithConfig", "mkdir", err)
+			}
 		}
+		return NewWithSQLiteConfig(filePath, cfg)
 	}
 
-	// Merge explicit quotas on top (allows overrides)
-	maps.Copy(rl.Quotas, cfg.Quotas)
-
+	rl := newConfiguredRateLimiter(cfg)
+	rl.filePath = filePath
 	return rl, nil
 }
 
@@ -225,15 +228,17 @@ func (rl *RateLimiter) loadSQLite() error {
 	if err != nil {
 		return err
 	}
-	// Merge loaded quotas (loaded quotas override in-memory defaults).
-	maps.Copy(rl.Quotas, quotas)
+	// Persisted quotas are authoritative when present; otherwise keep config defaults.
+	if len(quotas) > 0 {
+		rl.Quotas = maps.Clone(quotas)
+	}
 
 	state, err := rl.sqlite.loadState()
 	if err != nil {
 		return err
 	}
-	// Replace in-memory state with persisted state.
-	maps.Copy(rl.State, state)
+	// Replace in-memory state with the persisted snapshot.
+	rl.State = state
 	return nil
 }
 
@@ -244,6 +249,9 @@ func (rl *RateLimiter) Persist() error {
 	quotas := maps.Clone(rl.Quotas)
 	state := make(map[string]*UsageStats, len(rl.State))
 	for k, v := range rl.State {
+		if v == nil {
+			continue
+		}
 		state[k] = &UsageStats{
 			Requests: slices.Clone(v.Requests),
 			Tokens:   slices.Clone(v.Tokens),
@@ -256,11 +264,8 @@ func (rl *RateLimiter) Persist() error {
 	rl.mu.Unlock()
 
 	if sqlite != nil {
-		if err := sqlite.saveQuotas(quotas); err != nil {
-			return coreerr.E("ratelimit.Persist", "sqlite quotas", err)
-		}
-		if err := sqlite.saveState(state); err != nil {
-			return coreerr.E("ratelimit.Persist", "sqlite state", err)
+		if err := sqlite.saveSnapshot(quotas, state); err != nil {
+			return coreerr.E("ratelimit.Persist", "sqlite snapshot", err)
 		}
 		return nil
 	}
@@ -292,6 +297,10 @@ func (rl *RateLimiter) prune(model string) {
 	if !ok {
 		return
 	}
+	if stats == nil {
+		delete(rl.State, model)
+		return
+	}
 
 	now := time.Now()
 	window := now.Add(-1 * time.Minute)
@@ -326,6 +335,10 @@ func (rl *RateLimiter) prune(model string) {
 // BackgroundPrune starts a goroutine that periodically prunes all model states.
 // It returns a function to stop the pruner.
 func (rl *RateLimiter) BackgroundPrune(interval time.Duration) func() {
+	if interval <= 0 {
+		return func() {}
+	}
+
 	ctx, cancel := context.WithCancel(context.Background())
 	go func() {
 		ticker := time.NewTicker(interval)
@@ -348,6 +361,10 @@ func (rl *RateLimiter) BackgroundPrune(interval time.Duration) func() {
 
 // CanSend checks if a request can be sent without violating limits.
 func (rl *RateLimiter) CanSend(model string, estimatedTokens int) bool {
+	if estimatedTokens < 0 {
+		return false
+	}
+
 	rl.mu.Lock()
 	defer rl.mu.Unlock()
 
@@ -380,11 +397,8 @@ func (rl *RateLimiter) CanSend(model string, estimatedTokens int) bool {
 
 	// Check TPM
 	if quota.MaxTPM > 0 {
-		currentTokens := 0
-		for _, t := range stats.Tokens {
-			currentTokens += t.Count
-		}
-		if currentTokens+estimatedTokens > quota.MaxTPM {
+		currentTokens := totalTokenCount(stats.Tokens)
+		if estimatedTokens > quota.MaxTPM || currentTokens > quota.MaxTPM-estimatedTokens {
 			return false
 		}
 	}
@@ -405,13 +419,18 @@ func (rl *RateLimiter) RecordUsage(model string, promptTokens, outputTokens int)
 	}
 
 	now := time.Now()
+	totalTokens := safeTokenSum(promptTokens, outputTokens)
 	stats.Requests = append(stats.Requests, now)
-	stats.Tokens = append(stats.Tokens, TokenEntry{Time: now, Count: promptTokens + outputTokens})
+	stats.Tokens = append(stats.Tokens, TokenEntry{Time: now, Count: totalTokens})
 	stats.DayCount++
 }
 
 // WaitForCapacity blocks until capacity is available or context is cancelled.
 func (rl *RateLimiter) WaitForCapacity(ctx context.Context, model string, tokens int) error {
+	if tokens < 0 {
+		return coreerr.E("ratelimit.WaitForCapacity", "negative tokens", nil)
+	}
+
 	ticker := time.NewTicker(1 * time.Second)
 	defer ticker.Stop()
 
@@ -522,24 +541,8 @@ func (rl *RateLimiter) AllStats() map[string]ModelStats {
 		result[m] = ModelStats{}
 	}
 
-	now := time.Now()
-	window := now.Add(-1 * time.Minute)
-
 	for m := range result {
-		// Prune inline
-		if s, ok := rl.State[m]; ok {
-			s.Requests = slices.DeleteFunc(s.Requests, func(t time.Time) bool {
-				return !t.After(window)
-			})
-			s.Tokens = slices.DeleteFunc(s.Tokens, func(t TokenEntry) bool {
-				return !t.Time.After(window)
-			})
-
-			if now.Sub(s.DayStart) >= 24*time.Hour {
-				s.DayStart = now
-				s.DayCount = 0
-			}
-		}
+		rl.prune(m)
 
 		ms := ModelStats{}
 		if q, ok := rl.Quotas[m]; ok {
@@ -547,13 +550,11 @@ func (rl *RateLimiter) AllStats() map[string]ModelStats {
 			ms.MaxTPM = q.MaxTPM
 			ms.MaxRPD = q.MaxRPD
 		}
-		if s, ok := rl.State[m]; ok {
+		if s, ok := rl.State[m]; ok && s != nil {
 			ms.RPM = len(s.Requests)
 			ms.RPD = s.DayCount
 			ms.DayStart = s.DayStart
-			for _, t := range s.Tokens {
-				ms.TPM += t.Count
-			}
+			ms.TPM = totalTokenCount(s.Tokens)
 		}
 		result[m] = ms
 	}
@@ -579,25 +580,8 @@ func NewWithSQLiteConfig(dbPath string, cfg Config) (*RateLimiter, error) {
 		return nil, err
 	}
 
-	rl := &RateLimiter{
-		Quotas: make(map[string]ModelQuota),
-		State:  make(map[string]*UsageStats),
-		sqlite: store,
-	}
-
-	// Load provider profiles.
-	profiles := DefaultProfiles()
-	providers := cfg.Providers
-	if len(providers) == 0 && len(cfg.Quotas) == 0 {
-		providers = []Provider{ProviderGemini}
-	}
-	for _, p := range providers {
-		if profile, ok := profiles[p]; ok {
-			maps.Copy(rl.Quotas, profile.Models)
-		}
-	}
-	maps.Copy(rl.Quotas, cfg.Quotas)
-
+	rl := newConfiguredRateLimiter(cfg)
+	rl.sqlite = store
 	return rl, nil
 }
 
@@ -633,22 +617,22 @@ func MigrateYAMLToSQLite(yamlPath, sqlitePath string) error {
 	}
 	defer store.close()
 
-	if rl.Quotas != nil {
-		if err := store.saveQuotas(rl.Quotas); err != nil {
-			return err
-		}
-	}
-	if rl.State != nil {
-		if err := store.saveState(rl.State); err != nil {
-			return err
-		}
+	if err := store.saveSnapshot(rl.Quotas, rl.State); err != nil {
+		return err
 	}
 	return nil
 }
 
 // CountTokens calls the Google API to count tokens for a prompt.
 func CountTokens(ctx context.Context, apiKey, model, text string) (int, error) {
-	url := fmt.Sprintf("https://generativelanguage.googleapis.com/v1beta/models/%s:countTokens", model)
+	return countTokensWithClient(ctx, http.DefaultClient, "https://generativelanguage.googleapis.com", apiKey, model, text)
+}
+
+func countTokensWithClient(ctx context.Context, client *http.Client, baseURL, apiKey, model, text string) (int, error) {
+	requestURL, err := countTokensURL(baseURL, model)
+	if err != nil {
+		return 0, coreerr.E("ratelimit.CountTokens", "build url", err)
+	}
 
 	reqBody := map[string]any{
 		"contents": []any{
@@ -665,30 +649,147 @@ func CountTokens(ctx context.Context, apiKey, model, text string) (int, error) {
 		return 0, coreerr.E("ratelimit.CountTokens", "marshal request", err)
 	}
 
-	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewBuffer(jsonBody))
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, requestURL, bytes.NewReader(jsonBody))
 	if err != nil {
 		return 0, coreerr.E("ratelimit.CountTokens", "new request", err)
 	}
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("x-goog-api-key", apiKey)
 
-	resp, err := http.DefaultClient.Do(req)
+	if client == nil {
+		client = http.DefaultClient
+	}
+
+	resp, err := client.Do(req)
 	if err != nil {
 		return 0, coreerr.E("ratelimit.CountTokens", "do request", err)
 	}
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		body, _ := io.ReadAll(resp.Body)
-		return 0, coreerr.E("ratelimit.CountTokens", fmt.Sprintf("API error (status %d): %s", resp.StatusCode, string(body)), nil)
+		body, err := readLimitedBody(resp.Body, countTokensErrorBodyLimit)
+		if err != nil {
+			return 0, coreerr.E("ratelimit.CountTokens", "read error body", err)
+		}
+		return 0, coreerr.E("ratelimit.CountTokens", fmt.Sprintf("api error status %d: %s", resp.StatusCode, body), nil)
 	}
 
 	var result struct {
 		TotalTokens int `json:"totalTokens"`
 	}
-	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+	if err := json.NewDecoder(io.LimitReader(resp.Body, countTokensSuccessBodyLimit)).Decode(&result); err != nil {
 		return 0, coreerr.E("ratelimit.CountTokens", "decode response", err)
 	}
 
 	return result.TotalTokens, nil
 }
+
+func newConfiguredRateLimiter(cfg Config) *RateLimiter {
+	rl := &RateLimiter{
+		Quotas: make(map[string]ModelQuota),
+		State:  make(map[string]*UsageStats),
+	}
+	applyConfig(rl, cfg)
+	return rl
+}
+
+func applyConfig(rl *RateLimiter, cfg Config) {
+	profiles := DefaultProfiles()
+	providers := cfg.Providers
+
+	if len(providers) == 0 && len(cfg.Quotas) == 0 {
+		providers = []Provider{ProviderGemini}
+	}
+
+	for _, p := range providers {
+		if profile, ok := profiles[p]; ok {
+			maps.Copy(rl.Quotas, profile.Models)
+		}
+	}
+
+	maps.Copy(rl.Quotas, cfg.Quotas)
+}
+
+func normaliseBackend(backend string) (string, error) {
+	switch strings.ToLower(strings.TrimSpace(backend)) {
+	case "", backendYAML:
+		return backendYAML, nil
+	case backendSQLite:
+		return backendSQLite, nil
+	default:
+		return "", coreerr.E("ratelimit.NewWithConfig", fmt.Sprintf("unknown backend %q", backend), nil)
+	}
+}
+
+func defaultStatePath(backend string) (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+
+	fileName := defaultYAMLStateFile
+	if backend == backendSQLite {
+		fileName = defaultSQLiteStateFile
+	}
+
+	return filepath.Join(home, defaultStateDirName, fileName), nil
+}
+
+func safeTokenSum(a, b int) int {
+	return safeTokenTotal([]TokenEntry{{Count: a}, {Count: b}})
+}
+
+func totalTokenCount(tokens []TokenEntry) int {
+	return safeTokenTotal(tokens)
+}
+
+func safeTokenTotal(tokens []TokenEntry) int {
+	const maxInt = int(^uint(0) >> 1)
+
+	total := 0
+	for _, token := range tokens {
+		count := token.Count
+		if count < 0 {
+			continue
+		}
+		if total > maxInt-count {
+			return maxInt
+		}
+		total += count
+	}
+	return total
+}
+
+func countTokensURL(baseURL, model string) (string, error) {
+	if strings.TrimSpace(model) == "" {
+		return "", fmt.Errorf("empty model")
+	}
+
+	parsed, err := url.Parse(baseURL)
+	if err != nil {
+		return "", err
+	}
+	if parsed.Scheme == "" || parsed.Host == "" {
+		return "", fmt.Errorf("invalid base url")
+	}
+
+	return strings.TrimRight(parsed.String(), "/") + "/v1beta/models/" + url.PathEscape(model) + ":countTokens", nil
+}
+
+func readLimitedBody(r io.Reader, limit int64) (string, error) {
+	body, err := io.ReadAll(io.LimitReader(r, limit+1))
+	if err != nil {
+		return "", err
+	}
+
+	truncated := int64(len(body)) > limit
+	if truncated {
+		body = body[:limit]
+	}
+
+	result := string(body)
+	if truncated {
+		result += "..."
+	}
+	return result, nil
+}
diff --git a/ratelimit_test.go b/ratelimit_test.go
index 838c484..73aac70 100644
--- a/ratelimit_test.go
+++ b/ratelimit_test.go
@@ -4,10 +4,12 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -25,6 +27,18 @@ func newTestLimiter(t *testing.T) *RateLimiter {
 	return rl
 }
 
+type roundTripFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}
+
+type errReader struct{}
+
+func (errReader) Read([]byte) (int, error) {
+	return 0, io.ErrUnexpectedEOF
+}
+
 // --- Phase 0: CanSend boundary conditions ---
 
 func TestCanSend(t *testing.T) {
@@ -162,6 +176,15 @@ func TestCanSend(t *testing.T) {
 		rl.RecordUsage(model, 50, 50) // exactly 100 tokens
 		assert.True(t, rl.CanSend(model, 0), "zero estimated tokens should fit even at limit")
 	})
+
+	t.Run("negative estimated tokens are rejected", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		model := "negative-est"
+		rl.Quotas[model] = ModelQuota{MaxRPM: 100, MaxTPM: 100, MaxRPD: 100}
+		rl.RecordUsage(model, 50, 50)
+
+		assert.False(t, rl.CanSend(model, -1), "negative estimated tokens should not bypass TPM limits")
+	})
 }
 
 // --- Phase 0: Sliding window / prune tests ---
@@ -335,6 +358,19 @@ func TestRecordUsage(t *testing.T) {
 		assert.Len(t, stats.Tokens, 2)
 		assert.Equal(t, 6, stats.DayCount)
 	})
+
+	t.Run("negative token inputs are clamped to zero", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		model := "record-negative"
+
+		rl.RecordUsage(model, -100, 25)
+
+		stats := rl.State[model]
+		require.NotNil(t, stats)
+		assert.Len(t, stats.Requests, 1)
+		assert.Len(t, stats.Tokens, 1)
+		assert.Equal(t, 25, stats.Tokens[0].Count)
+	})
 }
 
 // --- Phase 0: Reset ---
@@ -422,6 +458,58 @@ func TestWaitForCapacity(t *testing.T) {
 		err := rl.WaitForCapacity(ctx, model, 10)
 		assert.Error(t, err, "should return error for already-cancelled context")
 	})
+
+	t.Run("negative tokens return error", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		err := rl.WaitForCapacity(context.Background(), "wait-negative", -1)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "negative tokens")
+	})
+}
+
+func TestNilUsageStats(t *testing.T) {
+	t.Run("CanSend replaces nil state without panicking", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		model := "nil-cansend"
+		rl.Quotas[model] = ModelQuota{MaxRPM: 10, MaxTPM: 100, MaxRPD: 10}
+		rl.State[model] = nil
+
+		assert.NotPanics(t, func() {
+			assert.True(t, rl.CanSend(model, 10))
+		})
+		assert.NotNil(t, rl.State[model])
+	})
+
+	t.Run("RecordUsage replaces nil state without panicking", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		model := "nil-record"
+		rl.State[model] = nil
+
+		assert.NotPanics(t, func() {
+			rl.RecordUsage(model, 10, 10)
+		})
+		require.NotNil(t, rl.State[model])
+		assert.Equal(t, 1, rl.State[model].DayCount)
+	})
+
+	t.Run("Stats and AllStats tolerate nil state entries", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		rl.Quotas["nil-stats"] = ModelQuota{MaxRPM: 1, MaxTPM: 2, MaxRPD: 3}
+		rl.State["nil-stats"] = nil
+		rl.State["nil-all-stats"] = nil
+
+		assert.NotPanics(t, func() {
+			stats := rl.Stats("nil-stats")
+			assert.Equal(t, 1, stats.MaxRPM)
+			assert.Equal(t, 0, stats.TPM)
+		})
+
+		assert.NotPanics(t, func() {
+			all := rl.AllStats()
+			assert.Contains(t, all, "nil-stats")
+			assert.Contains(t, all, "nil-all-stats")
+		})
+	})
 }
 
 // --- Phase 0: Stats ---
@@ -738,6 +826,19 @@ func TestBackgroundPrune(t *testing.T) {
 		_, exists := rl.State[model]
 		return !exists
 	}, 1*time.Second, 20*time.Millisecond, "old empty state should be pruned")
+
+	t.Run("non-positive interval is a safe no-op", func(t *testing.T) {
+		rl := newTestLimiter(t)
+		rl.State["still-here"] = &UsageStats{
+			Requests: []time.Time{time.Now().Add(-2 * time.Minute)},
+		}
+
+		assert.NotPanics(t, func() {
+			stop := rl.BackgroundPrune(0)
+			stop()
+		})
+		assert.Contains(t, rl.State, "still-here")
+	})
 }
 
 // --- Phase 0: CountTokens (with mock HTTP server) ---
@@ -748,26 +849,149 @@ func TestCountTokens(t *testing.T) {
 			assert.Equal(t, http.MethodPost, r.Method)
 			assert.Equal(t, "application/json", r.Header.Get("Content-Type"))
 			assert.Equal(t, "test-api-key", r.Header.Get("x-goog-api-key"))
+			assert.Equal(t, "/v1beta/models/test-model:countTokens", r.URL.EscapedPath())
+
+			var body struct {
+				Contents []struct {
+					Parts []struct {
+						Text string `json:"text"`
+					} `json:"parts"`
+				} `json:"contents"`
+			}
+			require.NoError(t, json.NewDecoder(r.Body).Decode(&body))
+			require.Len(t, body.Contents, 1)
+			require.Len(t, body.Contents[0].Parts, 1)
+			assert.Equal(t, "hello", body.Contents[0].Parts[0].Text)
 
 			w.Header().Set("Content-Type", "application/json")
-			json.NewEncoder(w).Encode(map[string]int{"totalTokens": 42})
+			require.NoError(t, json.NewEncoder(w).Encode(map[string]int{"totalTokens": 42}))
 		}))
 		defer server.Close()
 
-		// For testing purposes, we would need to make the base URL configurable.
-		// Since we're just checking the signature and basic logic, we test the error paths.
+		tokens, err := countTokensWithClient(context.Background(), server.Client(), server.URL, "test-api-key", "test-model", "hello")
+		require.NoError(t, err)
+		assert.Equal(t, 42, tokens)
 	})
 
-	t.Run("API error returns error", func(t *testing.T) {
+	t.Run("model name is path escaped", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			assert.Equal(t, "/v1beta/models/folder%2Fmodel%3Fdebug=1:countTokens", r.URL.EscapedPath())
+			assert.Empty(t, r.URL.RawQuery)
+			w.Header().Set("Content-Type", "application/json")
+			require.NoError(t, json.NewEncoder(w).Encode(map[string]int{"totalTokens": 7}))
+		}))
+		defer server.Close()
+
+		tokens, err := countTokensWithClient(context.Background(), server.Client(), server.URL, "test-api-key", "folder/model?debug=1", "hello")
+		require.NoError(t, err)
+		assert.Equal(t, 7, tokens)
+	})
+
+	t.Run("API error body is truncated", func(t *testing.T) {
+		largeBody := strings.Repeat("x", countTokensErrorBodyLimit+256)
 		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			w.WriteHeader(http.StatusUnauthorized)
-			fmt.Fprint(w, `{"error": "invalid API key"}`)
+			_, err := fmt.Fprint(w, largeBody)
+			require.NoError(t, err)
 		}))
 		defer server.Close()
 
-		_, err := CountTokens(context.Background(), "fake-key", "test-model", "hello")
-		assert.Error(t, err, "should fail with invalid API endpoint")
+		_, err := countTokensWithClient(context.Background(), server.Client(), server.URL, "fake-key", "test-model", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "api error status 401")
+		assert.True(t, strings.Count(err.Error(), "x") < len(largeBody), "error body should be bounded")
+		assert.Contains(t, err.Error(), "...")
 	})
+
+	t.Run("empty model is rejected before request", func(t *testing.T) {
+		_, err := CountTokens(context.Background(), "fake-key", "", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "build url")
+	})
+
+	t.Run("invalid base URL returns error", func(t *testing.T) {
+		_, err := countTokensWithClient(context.Background(), http.DefaultClient, "://bad-url", "fake-key", "test-model", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "build url")
+	})
+
+	t.Run("base URL without host returns error", func(t *testing.T) {
+		_, err := countTokensWithClient(context.Background(), http.DefaultClient, "/relative", "fake-key", "test-model", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "build url")
+	})
+
+	t.Run("invalid JSON response returns error", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			_, err := w.Write([]byte(`{"totalTokens":`))
+			require.NoError(t, err)
+		}))
+		defer server.Close()
+
+		_, err := countTokensWithClient(context.Background(), server.Client(), server.URL, "fake-key", "test-model", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "decode response")
+	})
+
+	t.Run("error body read failures are returned", func(t *testing.T) {
+		client := &http.Client{
+			Transport: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+				return &http.Response{
+					StatusCode: http.StatusBadGateway,
+					Body:       io.NopCloser(errReader{}),
+					Header:     make(http.Header),
+				}, nil
+			}),
+		}
+
+		_, err := countTokensWithClient(context.Background(), client, "https://generativelanguage.googleapis.com", "fake-key", "test-model", "hello")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "read error body")
+	})
+
+	t.Run("nil client falls back to http.DefaultClient", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			require.NoError(t, json.NewEncoder(w).Encode(map[string]int{"totalTokens": 11}))
+		}))
+		defer server.Close()
+
+		originalClient := http.DefaultClient
+		http.DefaultClient = server.Client()
+		defer func() {
+			http.DefaultClient = originalClient
+		}()
+
+		tokens, err := countTokensWithClient(context.Background(), nil, server.URL, "fake-key", "test-model", "hello")
+		require.NoError(t, err)
+		assert.Equal(t, 11, tokens)
+	})
+}
+
+func TestPersistSkipsNilState(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "nil-state.yaml")
+
+	rl, err := New()
+	require.NoError(t, err)
+	rl.filePath = path
+	rl.State["nil-model"] = nil
+
+	require.NoError(t, rl.Persist())
+
+	rl2, err := New()
+	require.NoError(t, err)
+	rl2.filePath = path
+	require.NoError(t, rl2.Load())
+	assert.NotContains(t, rl2.State, "nil-model")
+}
+
+func TestTokenTotals(t *testing.T) {
+	maxInt := int(^uint(0) >> 1)
+
+	assert.Equal(t, 25, safeTokenSum(-100, 25))
+	assert.Equal(t, maxInt, safeTokenSum(maxInt, 1))
+	assert.Equal(t, 10, totalTokenCount([]TokenEntry{{Count: -5}, {Count: 10}}))
 }
 
 // --- Phase 0: Benchmarks ---
@@ -983,6 +1207,25 @@ func TestNewWithConfig(t *testing.T) {
 		assert.Equal(t, 5, q.MaxRPM)
 		assert.Equal(t, 50000, q.MaxTPM)
 	})
+
+	t.Run("invalid backend returns error", func(t *testing.T) {
+		_, err := NewWithConfig(Config{
+			Backend: "bogus",
+		})
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "unknown backend")
+	})
+
+	t.Run("default YAML path uses home directory", func(t *testing.T) {
+		home := t.TempDir()
+		t.Setenv("HOME", home)
+		t.Setenv("USERPROFILE", "")
+		t.Setenv("home", "")
+
+		rl, err := NewWithConfig(Config{})
+		require.NoError(t, err)
+		assert.Equal(t, filepath.Join(home, defaultStateDirName, defaultYAMLStateFile), rl.filePath)
+	})
 }
 
 func TestNewBackwardCompatibility(t *testing.T) {
diff --git a/sqlite.go b/sqlite.go
index de6c89d..863ede4 100644
--- a/sqlite.go
+++ b/sqlite.go
@@ -78,7 +78,7 @@ func createSchema(db *sql.DB) error {
 	return nil
 }
 
-// saveQuotas upserts all quotas into the quotas table.
+// saveQuotas writes a complete quota snapshot to the quotas table.
 func (s *sqliteStore) saveQuotas(quotas map[string]ModelQuota) error {
 	tx, err := s.db.Begin()
 	if err != nil {
@@ -86,24 +86,15 @@ func (s *sqliteStore) saveQuotas(quotas map[string]ModelQuota) error {
 	}
 	defer tx.Rollback()
 
-	stmt, err := tx.Prepare(`INSERT INTO quotas (model, max_rpm, max_tpm, max_rpd)
-		VALUES (?, ?, ?, ?)
-		ON CONFLICT(model) DO UPDATE SET
-			max_rpm = excluded.max_rpm,
-			max_tpm = excluded.max_tpm,
-			max_rpd = excluded.max_rpd`)
-	if err != nil {
-		return coreerr.E("ratelimit.saveQuotas", "prepare", err)
-	}
-	defer stmt.Close()
-
-	for model, q := range quotas {
-		if _, err := stmt.Exec(model, q.MaxRPM, q.MaxTPM, q.MaxRPD); err != nil {
-			return coreerr.E("ratelimit.saveQuotas", fmt.Sprintf("exec %s", model), err)
-		}
+	if _, err := tx.Exec("DELETE FROM quotas"); err != nil {
+		return coreerr.E("ratelimit.saveQuotas", "clear", err)
 	}
 
-	return tx.Commit()
+	if err := insertQuotas(tx, quotas); err != nil {
+		return err
+	}
+
+	return commitTx(tx, "ratelimit.saveQuotas")
 }
 
 // loadQuotas reads all rows from the quotas table.
@@ -129,6 +120,28 @@ func (s *sqliteStore) loadQuotas() (map[string]ModelQuota, error) {
 	return result, nil
 }
 
+// saveSnapshot writes quotas and state as a single atomic snapshot.
+func (s *sqliteStore) saveSnapshot(quotas map[string]ModelQuota, state map[string]*UsageStats) error {
+	tx, err := s.db.Begin()
+	if err != nil {
+		return coreerr.E("ratelimit.saveSnapshot", "begin", err)
+	}
+	defer tx.Rollback()
+
+	if err := clearSnapshotTables(tx, true); err != nil {
+		return err
+	}
+
+	if err := insertQuotas(tx, quotas); err != nil {
+		return err
+	}
+	if err := insertState(tx, state); err != nil {
+		return err
+	}
+
+	return commitTx(tx, "ratelimit.saveSnapshot")
+}
+
 // saveState writes all usage state to SQLite in a single transaction.
 // It uses a truncate-and-insert approach for simplicity in this version,
 // but ensures atomicity via a single transaction.
@@ -139,7 +152,23 @@ func (s *sqliteStore) saveState(state map[string]*UsageStats) error {
 	}
 	defer tx.Rollback()
 
-	// Clear existing state in the transaction.
+	if err := clearSnapshotTables(tx, false); err != nil {
+		return err
+	}
+
+	if err := insertState(tx, state); err != nil {
+		return err
+	}
+
+	return commitTx(tx, "ratelimit.saveState")
+}
+
+func clearSnapshotTables(tx *sql.Tx, includeQuotas bool) error {
+	if includeQuotas {
+		if _, err := tx.Exec("DELETE FROM quotas"); err != nil {
+			return coreerr.E("ratelimit.saveSnapshot", "clear quotas", err)
+		}
+	}
 	if _, err := tx.Exec("DELETE FROM requests"); err != nil {
 		return coreerr.E("ratelimit.saveState", "clear requests", err)
 	}
@@ -149,7 +178,25 @@ func (s *sqliteStore) saveState(state map[string]*UsageStats) error {
 	if _, err := tx.Exec("DELETE FROM daily"); err != nil {
 		return coreerr.E("ratelimit.saveState", "clear daily", err)
 	}
+	return nil
+}
 
+func insertQuotas(tx *sql.Tx, quotas map[string]ModelQuota) error {
+	stmt, err := tx.Prepare("INSERT INTO quotas (model, max_rpm, max_tpm, max_rpd) VALUES (?, ?, ?, ?)")
+	if err != nil {
+		return coreerr.E("ratelimit.saveQuotas", "prepare", err)
+	}
+	defer stmt.Close()
+
+	for model, q := range quotas {
+		if _, err := stmt.Exec(model, q.MaxRPM, q.MaxTPM, q.MaxRPD); err != nil {
+			return coreerr.E("ratelimit.saveQuotas", fmt.Sprintf("exec %s", model), err)
+		}
+	}
+	return nil
+}
+
+func insertState(tx *sql.Tx, state map[string]*UsageStats) error {
 	reqStmt, err := tx.Prepare("INSERT INTO requests (model, ts) VALUES (?, ?)")
 	if err != nil {
 		return coreerr.E("ratelimit.saveState", "prepare requests", err)
@@ -169,6 +216,9 @@ func (s *sqliteStore) saveState(state map[string]*UsageStats) error {
 	defer dayStmt.Close()
 
 	for model, stats := range state {
+		if stats == nil {
+			continue
+		}
 		for _, t := range stats.Requests {
 			if _, err := reqStmt.Exec(model, t.UnixNano()); err != nil {
 				return coreerr.E("ratelimit.saveState", fmt.Sprintf("insert request %s", model), err)
@@ -183,9 +233,12 @@ func (s *sqliteStore) saveState(state map[string]*UsageStats) error {
 			return coreerr.E("ratelimit.saveState", fmt.Sprintf("insert daily %s", model), err)
 		}
 	}
+	return nil
+}
 
+func commitTx(tx *sql.Tx, scope string) error {
 	if err := tx.Commit(); err != nil {
-		return coreerr.E("ratelimit.saveState", "commit", err)
+		return coreerr.E(scope, "commit", err)
 	}
 	return nil
 }
diff --git a/sqlite_test.go b/sqlite_test.go
index e2e13a6..7026261 100644
--- a/sqlite_test.go
+++ b/sqlite_test.go
@@ -435,6 +435,45 @@ func TestConfigBackendDefault_Good(t *testing.T) {
 	assert.Nil(t, rl.sqlite, "empty backend should use YAML (no sqlite)")
 }
 
+func TestConfigBackendSQLite_Good(t *testing.T) {
+	dbPath := filepath.Join(t.TempDir(), "config-backend.db")
+	rl, err := NewWithConfig(Config{
+		Backend:  backendSQLite,
+		FilePath: dbPath,
+		Quotas: map[string]ModelQuota{
+			"backend-model": {MaxRPM: 10, MaxTPM: 1000, MaxRPD: 50},
+		},
+	})
+	require.NoError(t, err)
+	defer rl.Close()
+
+	require.NotNil(t, rl.sqlite)
+	rl.RecordUsage("backend-model", 10, 10)
+	require.NoError(t, rl.Persist())
+
+	_, statErr := os.Stat(dbPath)
+	assert.NoError(t, statErr, "sqlite backend should persist to the configured DB path")
+}
+
+func TestConfigBackendSQLiteDefaultPath_Good(t *testing.T) {
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	t.Setenv("USERPROFILE", "")
+	t.Setenv("home", "")
+
+	rl, err := NewWithConfig(Config{
+		Backend: backendSQLite,
+	})
+	require.NoError(t, err)
+	defer rl.Close()
+
+	require.NotNil(t, rl.sqlite)
+	require.NoError(t, rl.Persist())
+
+	_, statErr := os.Stat(filepath.Join(home, defaultStateDirName, defaultSQLiteStateFile))
+	assert.NoError(t, statErr, "sqlite backend should use the default home DB path")
+}
+
 // --- Phase 2: MigrateYAMLToSQLite ---
 
 func TestMigrateYAMLToSQLite_Good(t *testing.T) {
@@ -492,6 +531,69 @@ func TestMigrateYAMLToSQLite_Bad(t *testing.T) {
 	})
 }
 
+func TestMigrateYAMLToSQLiteAtomic_Good(t *testing.T) {
+	tmpDir := t.TempDir()
+	yamlPath := filepath.Join(tmpDir, "atomic.yaml")
+	sqlitePath := filepath.Join(tmpDir, "atomic.db")
+	now := time.Now().UTC()
+
+	store, err := newSQLiteStore(sqlitePath)
+	require.NoError(t, err)
+
+	originalQuotas := map[string]ModelQuota{
+		"old-model": {MaxRPM: 1, MaxTPM: 2, MaxRPD: 3},
+	}
+	originalState := map[string]*UsageStats{
+		"old-model": {
+			Requests: []time.Time{now},
+			Tokens:   []TokenEntry{{Time: now, Count: 9}},
+			DayStart: now,
+			DayCount: 1,
+		},
+	}
+	require.NoError(t, store.saveSnapshot(originalQuotas, originalState))
+
+	_, err = store.db.Exec(`CREATE TRIGGER fail_daily_migrate BEFORE INSERT ON daily
+		BEGIN SELECT RAISE(ABORT, 'forced daily failure'); END`)
+	require.NoError(t, err)
+	require.NoError(t, store.close())
+
+	migrated := &RateLimiter{
+		Quotas: map[string]ModelQuota{
+			"new-model": {MaxRPM: 10, MaxTPM: 20, MaxRPD: 30},
+		},
+		State: map[string]*UsageStats{
+			"new-model": {
+				Requests: []time.Time{now.Add(5 * time.Second)},
+				Tokens:   []TokenEntry{{Time: now.Add(5 * time.Second), Count: 99}},
+				DayStart: now.Add(5 * time.Second),
+				DayCount: 2,
+			},
+		},
+	}
+	data, err := yaml.Marshal(migrated)
+	require.NoError(t, err)
+	require.NoError(t, os.WriteFile(yamlPath, data, 0o644))
+
+	err = MigrateYAMLToSQLite(yamlPath, sqlitePath)
+	require.Error(t, err)
+
+	store, err = newSQLiteStore(sqlitePath)
+	require.NoError(t, err)
+	defer store.close()
+
+	quotas, err := store.loadQuotas()
+	require.NoError(t, err)
+	assert.Equal(t, originalQuotas, quotas)
+
+	state, err := store.loadState()
+	require.NoError(t, err)
+	require.Contains(t, state, "old-model")
+	assert.Equal(t, originalState["old-model"].DayCount, state["old-model"].DayCount)
+	assert.Equal(t, originalState["old-model"].Tokens[0].Count, state["old-model"].Tokens[0].Count)
+	assert.NotContains(t, state, "new-model")
+}
+
 func TestMigrateYAMLToSQLitePreservesAllGeminiModels_Good(t *testing.T) {
 	tmpDir := t.TempDir()
 	yamlPath := filepath.Join(tmpDir, "full.yaml")
@@ -650,6 +752,78 @@ func TestSQLiteEndToEnd_Good(t *testing.T) {
 	assert.Equal(t, 5, custom.MaxRPM)
 }
 
+func TestSQLiteLoadReplacesPersistedSnapshot_Good(t *testing.T) {
+	dbPath := filepath.Join(t.TempDir(), "replace.db")
+	rl, err := NewWithSQLiteConfig(dbPath, Config{
+		Quotas: map[string]ModelQuota{
+			"model-a": {MaxRPM: 1, MaxTPM: 100, MaxRPD: 10},
+		},
+	})
+	require.NoError(t, err)
+
+	rl.RecordUsage("model-a", 10, 10)
+	require.NoError(t, rl.Persist())
+
+	delete(rl.Quotas, "model-a")
+	rl.Quotas["model-b"] = ModelQuota{MaxRPM: 2, MaxTPM: 200, MaxRPD: 20}
+	rl.Reset("")
+	rl.RecordUsage("model-b", 5, 5)
+	require.NoError(t, rl.Persist())
+	require.NoError(t, rl.Close())
+
+	rl2, err := NewWithSQLiteConfig(dbPath, Config{
+		Providers: []Provider{ProviderGemini},
+	})
+	require.NoError(t, err)
+	defer rl2.Close()
+
+	rl2.State["stale-memory"] = &UsageStats{DayStart: time.Now(), DayCount: 99}
+	require.NoError(t, rl2.Load())
+
+	assert.NotContains(t, rl2.Quotas, "gemini-3-pro-preview")
+	assert.NotContains(t, rl2.Quotas, "model-a")
+	assert.Contains(t, rl2.Quotas, "model-b")
+	assert.NotContains(t, rl2.State, "stale-memory")
+	assert.NotContains(t, rl2.State, "model-a")
+	assert.Equal(t, 1, rl2.Stats("model-b").RPD)
+}
+
+func TestSQLitePersistAtomic_Good(t *testing.T) {
+	dbPath := filepath.Join(t.TempDir(), "persist-atomic.db")
+	rl, err := NewWithSQLiteConfig(dbPath, Config{
+		Quotas: map[string]ModelQuota{
+			"old-model": {MaxRPM: 1, MaxTPM: 100, MaxRPD: 10},
+		},
+	})
+	require.NoError(t, err)
+
+	rl.RecordUsage("old-model", 10, 10)
+	require.NoError(t, rl.Persist())
+
+	_, err = rl.sqlite.db.Exec(`CREATE TRIGGER fail_daily_persist BEFORE INSERT ON daily
+		BEGIN SELECT RAISE(ABORT, 'forced daily failure'); END`)
+	require.NoError(t, err)
+
+	delete(rl.Quotas, "old-model")
+	rl.Quotas["new-model"] = ModelQuota{MaxRPM: 2, MaxTPM: 200, MaxRPD: 20}
+	rl.Reset("")
+	rl.RecordUsage("new-model", 50, 50)
+
+	err = rl.Persist()
+	require.Error(t, err)
+	require.NoError(t, rl.Close())
+
+	rl2, err := NewWithSQLiteConfig(dbPath, Config{})
+	require.NoError(t, err)
+	defer rl2.Close()
+	require.NoError(t, rl2.Load())
+
+	assert.Contains(t, rl2.Quotas, "old-model")
+	assert.NotContains(t, rl2.Quotas, "new-model")
+	assert.Equal(t, 1, rl2.Stats("old-model").RPD)
+	assert.Equal(t, 0, rl2.Stats("new-model").RPD)
+}
+
 // --- Phase 2: Benchmark ---
 
 func BenchmarkSQLitePersist(b *testing.B) {