diff --git a/internal/proxy/handler.go b/internal/proxy/handler.go
index caaff03..146d92e 100644
--- a/internal/proxy/handler.go
+++ b/internal/proxy/handler.go
@@ -14,10 +14,11 @@ import (
 
 	"github.com/fujin/anthropic-proxy/internal/auth"
 	"github.com/fujin/anthropic-proxy/internal/logging"
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 	"github.com/fujin/anthropic-proxy/internal/telemetry"
 )
 
-func HandleMessages(pool *auth.Pool, profile *SniffedProfile, getSanitizer func() *Sanitizer) gin.HandlerFunc {
+func HandleMessages(pool *auth.Pool, profile *SniffedProfile, getSanitizer func() *Sanitizer, tracker *ratelimit.Tracker) gin.HandlerFunc {
 	upstream := NewUpstreamClient(profile)
 
 	return func(c *gin.Context) {
@@ -49,14 +50,14 @@ func HandleMessages(pool *auth.Pool, profile *SniffedProfile, getSanitizer func(
 		isStream := gjson.GetBytes(body, "stream").Bool()
 
 		if isStream {
-			handleStream(c, upstream, san, pool, cred, body, originalBody)
+			handleStream(c, upstream, san, pool, cred, body, originalBody, tracker)
 		} else {
-			handleNonStream(c, upstream, san, pool, cred, body, originalBody)
+			handleNonStream(c, upstream, san, pool, cred, body, originalBody, tracker)
 		}
 	}
 }
 
-func handleNonStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool *auth.Pool, cred *auth.Credential, body []byte, originalBody []byte) {
+func handleNonStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool *auth.Pool, cred *auth.Credential, body []byte, originalBody []byte, tracker *ratelimit.Tracker) {
 	startTime := time.Now()
 	model := gjson.GetBytes(body, "model").String()
 	ctx := c.Request.Context()
@@ -147,6 +148,10 @@ func handleNonStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, p
 		)
 		telemetry.TokensInput.Add(ctx, inputTokens, tokenAttrs)
 		telemetry.TokensOutput.Add(ctx, outputTokens, tokenAttrs)
+		if tracker != nil {
+			tracker.RecordTokens(inputTokens, outputTokens)
+			tracker.UpdateFromHeaders(headers)
+		}
 
 		log.Info().
 			Int("status", statusCode).
@@ -166,7 +171,7 @@ func handleNonStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, p
 	c.Data(statusCode, headers.Get("Content-Type"), respBody)
 }
 
-func handleStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool *auth.Pool, cred *auth.Credential, body []byte, originalBody []byte) {
+func handleStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool *auth.Pool, cred *auth.Credential, body []byte, originalBody []byte, tracker *ratelimit.Tracker) {
 	startTime := time.Now()
 	model := gjson.GetBytes(body, "model").String()
 	ctx := c.Request.Context()
@@ -299,6 +304,10 @@ func handleStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool
 		)
 		telemetry.TokensInput.Add(ctx, inputTokens, tokenAttrs)
 		telemetry.TokensOutput.Add(ctx, outputTokens, tokenAttrs)
+		if tracker != nil {
+			tracker.RecordTokens(inputTokens, outputTokens)
+			tracker.UpdateFromHeaders(resp.Header)
+		}
 	}
 
 	log.Info().
diff --git a/internal/ratelimit/tracker.go b/internal/ratelimit/tracker.go
new file mode 100644
index 0000000..5b25634
--- /dev/null
+++ b/internal/ratelimit/tracker.go
@@ -0,0 +1,213 @@
+package ratelimit
+
+import (
+	"context"
+	"net/http"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/rs/zerolog/log"
+)
+
+// Window holds per-window usage state.
+type Window struct {
+	Utilization float64   // 0-100 from API
+	ResetsAt    time.Time // when window resets
+	TokensIn    atomic.Int64
+	TokensOut   atomic.Int64
+}
+
+// Snapshot is a read-only copy of a Window's state.
+type Snapshot struct {
+	Utilization float64
+	ResetsAt    time.Time
+	TokensIn    int64
+	TokensOut   int64
+}
+
+// Tracker polls /api/oauth/usage and tracks per-window token usage.
+type Tracker struct {
+	tokenFn  func() string // returns current OAuth access token
+	mu       sync.RWMutex
+	fiveHour Window
+	sevenDay Window
+	sonnet   Window
+	extra    ExtraUsage
+}
+
+// NewTracker creates a tracker. tokenFn should return the current access token.
+func NewTracker(tokenFn func() string) *Tracker {
+	return &Tracker{tokenFn: tokenFn}
+}
+
+// Start begins the background poll loop.
+func (t *Tracker) Start(ctx context.Context) {
+	go func() {
+		// Poll immediately on start
+		t.poll(ctx)
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-time.After(5 * time.Minute):
+				t.poll(ctx)
+			}
+		}
+	}()
+}
+
+// UpdateFromHeaders extracts rate limit data from /v1/messages response headers.
+// This provides real-time utilization updates without polling the usage API.
+func (t *Tracker) UpdateFromHeaders(h http.Header) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	if v := h.Get("Anthropic-Ratelimit-Unified-5h-Utilization"); v != "" {
+		if f, err := strconv.ParseFloat(v, 64); err == nil {
+			t.fiveHour.Utilization = f * 100 // header is 0-1, we store 0-100
+		}
+	}
+	if v := h.Get("Anthropic-Ratelimit-Unified-5h-Reset"); v != "" {
+		if ts, err := strconv.ParseInt(v, 10, 64); err == nil {
+			t.fiveHour.ResetsAt = time.Unix(ts, 0).UTC().Truncate(time.Minute)
+		}
+	}
+	if v := h.Get("Anthropic-Ratelimit-Unified-7d-Utilization"); v != "" {
+		if f, err := strconv.ParseFloat(v, 64); err == nil {
+			t.sevenDay.Utilization = f * 100
+		}
+	}
+	if v := h.Get("Anthropic-Ratelimit-Unified-7d-Reset"); v != "" {
+		if ts, err := strconv.ParseInt(v, 10, 64); err == nil {
+			t.sevenDay.ResetsAt = time.Unix(ts, 0).UTC().Truncate(time.Minute)
+		}
+	}
+}
+
+// RecordTokens adds tokens to all active windows.
+func (t *Tracker) RecordTokens(inputTokens, outputTokens int64) {
+	t.fiveHour.TokensIn.Add(inputTokens)
+	t.fiveHour.TokensOut.Add(outputTokens)
+	t.sevenDay.TokensIn.Add(inputTokens)
+	t.sevenDay.TokensOut.Add(outputTokens)
+}
+
+// FiveHour returns a snapshot of the 5-hour window.
+func (t *Tracker) FiveHour() Snapshot {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return Snapshot{
+		Utilization: t.fiveHour.Utilization,
+		ResetsAt:    t.fiveHour.ResetsAt,
+		TokensIn:    t.fiveHour.TokensIn.Load(),
+		TokensOut:   t.fiveHour.TokensOut.Load(),
+	}
+}
+
+// SevenDay returns a snapshot of the 7-day window.
+func (t *Tracker) SevenDay() Snapshot {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return Snapshot{
+		Utilization: t.sevenDay.Utilization,
+		ResetsAt:    t.sevenDay.ResetsAt,
+		TokensIn:    t.sevenDay.TokensIn.Load(),
+		TokensOut:   t.sevenDay.TokensOut.Load(),
+	}
+}
+
+// Sonnet returns a snapshot of the 7-day sonnet window.
+func (t *Tracker) Sonnet() Snapshot {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return Snapshot{
+		Utilization: t.sonnet.Utilization,
+		ResetsAt:    t.sonnet.ResetsAt,
+	}
+}
+
+// Extra returns the current extra usage state.
+func (t *Tracker) Extra() ExtraUsage {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return t.extra
+}
+
+func (t *Tracker) poll(ctx context.Context) {
+	token := t.tokenFn()
+	if token == "" {
+		return
+	}
+
+	usage, err := fetchUsage(ctx, token)
+	if err != nil {
+		log.Warn().Err(err).Msg("usage poll failed")
+		return
+	}
+
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	if usage.FiveHour != nil {
+		t.updateWindow(&t.fiveHour, usage.FiveHour, "5h")
+	}
+	if usage.SevenDay != nil {
+		t.updateWindow(&t.sevenDay, usage.SevenDay, "7d")
+	}
+	if usage.SevenDaySonnet != nil {
+		t.updateWindow(&t.sonnet, usage.SevenDaySonnet, "7d_sonnet")
+	}
+	if usage.ExtraUsage != nil {
+		t.extra = *usage.ExtraUsage
+	}
+
+	log.Debug().
+		Float64("5h_util", t.fiveHour.Utilization).
+		Time("5h_resets", t.fiveHour.ResetsAt).
+		Int64("5h_tokens_in", t.fiveHour.TokensIn.Load()).
+		Int64("5h_tokens_out", t.fiveHour.TokensOut.Load()).
+		Float64("7d_util", t.sevenDay.Utilization).
+		Time("7d_resets", t.sevenDay.ResetsAt).
+		Int64("7d_tokens_in", t.sevenDay.TokensIn.Load()).
+		Int64("7d_tokens_out", t.sevenDay.TokensOut.Load()).
+		Msg("usage poll")
+
+	// Warn on high utilization
+	if t.fiveHour.Utilization > 80 {
+		log.Warn().Float64("utilization", t.fiveHour.Utilization).Time("resets_at", t.fiveHour.ResetsAt).Msg("5h window utilization high")
+	}
+	if t.sevenDay.Utilization > 80 {
+		log.Warn().Float64("utilization", t.sevenDay.Utilization).Time("resets_at", t.sevenDay.ResetsAt).Msg("7d window utilization high")
+	}
+}
+
+func (t *Tracker) updateWindow(w *Window, rl *RateLimit, name string) {
+	if rl.Utilization != nil {
+		w.Utilization = *rl.Utilization
+	}
+	if rl.ResetsAt != nil {
+		parsed, err := time.Parse(time.RFC3339Nano, *rl.ResetsAt)
+		if err != nil {
+			// Fallback to RFC3339 without fractional seconds
+			parsed, err = time.Parse(time.RFC3339, *rl.ResetsAt)
+		}
+		parsed = parsed.UTC().Truncate(time.Minute)
+		if err == nil && parsed != w.ResetsAt && !w.ResetsAt.IsZero() {
+			// Window reset detected — zero token counters
+			log.Info().
+				Str("window", name).
+				Int64("prev_tokens_in", w.TokensIn.Load()).
+				Int64("prev_tokens_out", w.TokensOut.Load()).
+				Time("old_reset", w.ResetsAt).
+				Time("new_reset", parsed).
+				Msg("window reset detected")
+			w.TokensIn.Store(0)
+			w.TokensOut.Store(0)
+		}
+		if err == nil {
+			w.ResetsAt = parsed
+		}
+	}
+}
diff --git a/internal/ratelimit/usage.go b/internal/ratelimit/usage.go
new file mode 100644
index 0000000..5aa8586
--- /dev/null
+++ b/internal/ratelimit/usage.go
@@ -0,0 +1,62 @@
+package ratelimit
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+const usageURL = "https://api.anthropic.com/api/oauth/usage"
+
+type RateLimit struct {
+	Utilization *float64 `json:"utilization"` // 0-100
+	ResetsAt    *string  `json:"resets_at"`   // ISO 8601
+}
+
+type ExtraUsage struct {
+	IsEnabled   bool     `json:"is_enabled"`
+	MonthlyLimit *float64 `json:"monthly_limit"`
+	UsedCredits  *float64 `json:"used_credits"`
+	Utilization  *float64 `json:"utilization"`
+}
+
+type UsageResponse struct {
+	FiveHour     *RateLimit  `json:"five_hour"`
+	SevenDay     *RateLimit  `json:"seven_day"`
+	SevenDaySonnet *RateLimit `json:"seven_day_sonnet"`
+	ExtraUsage   *ExtraUsage `json:"extra_usage"`
+}
+
+func fetchUsage(ctx context.Context, token string) (*UsageResponse, error) {
+	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, usageURL, nil)
+	if err != nil {
+		return nil, fmt.Errorf("build request: %w", err)
+	}
+	req.Header.Set("Authorization", "Bearer "+token)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("anthropic-beta", "oauth-2025-04-20")
+	req.Header.Set("User-Agent", "claude-cli/2.1.92")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("usage returned %d: %s", resp.StatusCode, string(body))
+	}
+
+	var usage UsageResponse
+	if err := json.Unmarshal(body, &usage); err != nil {
+		return nil, fmt.Errorf("decode: %w", err)
+	}
+	return &usage, nil
+}
diff --git a/internal/server/server.go b/internal/server/server.go
index 64c097c..28d7492 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -14,6 +14,7 @@ import (
 	"github.com/fujin/anthropic-proxy/internal/config"
 	"github.com/fujin/anthropic-proxy/internal/logging"
 	"github.com/fujin/anthropic-proxy/internal/proxy"
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 	"go.opentelemetry.io/contrib/instrumentation/github.com/gin-gonic/gin/otelgin"
 )
 
@@ -25,7 +26,7 @@ type Server struct {
 	apiKeys    atomic.Pointer[map[string]struct{}]
 }
 
-func New(cfg *config.Config, pool *auth.Pool, profile *proxy.SniffedProfile) *Server {
+func New(cfg *config.Config, pool *auth.Pool, profile *proxy.SniffedProfile, tracker *ratelimit.Tracker) *Server {
 	s := &Server{configPath: "config.yaml"}
 
 	san := proxy.NewSanitizer(cfg.Sanitize)
@@ -46,7 +47,7 @@ func New(cfg *config.Config, pool *auth.Pool, profile *proxy.SniffedProfile) *Se
 
 	handler := proxy.HandleMessages(pool, profile, func() *proxy.Sanitizer {
 		return s.sanitizer.Load()
-	})
+	}, tracker)
 	engine.POST("/v1/messages", handler)
 	engine.POST("/messages", handler)
 
diff --git a/internal/telemetry/metrics.go b/internal/telemetry/metrics.go
index 1f7abd6..a5f0662 100644
--- a/internal/telemetry/metrics.go
+++ b/internal/telemetry/metrics.go
@@ -1,7 +1,12 @@
 package telemetry
 
 import (
+	"context"
+
+	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/metric"
+
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 )
 
 var (
@@ -17,7 +22,8 @@ var (
 )
 
 // InitMetrics creates all metric instruments from the given meter.
-func InitMetrics(meter metric.Meter) {
+// If tracker is non-nil, registers observable gauges for per-window usage.
+func InitMetrics(meter metric.Meter, tracker *ratelimit.Tracker) {
 	RequestCounter, _ = meter.Int64Counter("proxy.request.count",
 		metric.WithDescription("Total proxy requests"),
 	)
@@ -47,4 +53,50 @@ func InitMetrics(meter metric.Meter) {
 	StreamRequests, _ = meter.Int64Counter("proxy.stream.requests",
 		metric.WithDescription("Streaming request count"),
 	)
+
+	if tracker == nil {
+		return
+	}
+
+	attr5h := attribute.String("window", "5h")
+	attr7d := attribute.String("window", "7d")
+	attrSonnet := attribute.String("window", "7d_sonnet")
+
+	meter.Float64ObservableGauge("proxy.usage.utilization",
+		metric.WithDescription("Current utilization % from API"),
+		metric.WithFloat64Callback(func(_ context.Context, o metric.Float64Observer) error {
+			o.Observe(tracker.FiveHour().Utilization, metric.WithAttributes(attr5h))
+			o.Observe(tracker.SevenDay().Utilization, metric.WithAttributes(attr7d))
+			o.Observe(tracker.Sonnet().Utilization, metric.WithAttributes(attrSonnet))
+			return nil
+		}),
+	)
+
+	meter.Int64ObservableGauge("proxy.usage.resets_at",
+		metric.WithDescription("Unix seconds when window resets"),
+		metric.WithInt64Callback(func(_ context.Context, o metric.Int64Observer) error {
+			o.Observe(tracker.FiveHour().ResetsAt.Unix(), metric.WithAttributes(attr5h))
+			o.Observe(tracker.SevenDay().ResetsAt.Unix(), metric.WithAttributes(attr7d))
+			o.Observe(tracker.Sonnet().ResetsAt.Unix(), metric.WithAttributes(attrSonnet))
+			return nil
+		}),
+	)
+
+	meter.Int64ObservableGauge("proxy.usage.window_tokens.input",
+		metric.WithDescription("Proxy input tokens in current window"),
+		metric.WithInt64Callback(func(_ context.Context, o metric.Int64Observer) error {
+			o.Observe(tracker.FiveHour().TokensIn, metric.WithAttributes(attr5h))
+			o.Observe(tracker.SevenDay().TokensIn, metric.WithAttributes(attr7d))
+			return nil
+		}),
+	)
+
+	meter.Int64ObservableGauge("proxy.usage.window_tokens.output",
+		metric.WithDescription("Proxy output tokens in current window"),
+		metric.WithInt64Callback(func(_ context.Context, o metric.Int64Observer) error {
+			o.Observe(tracker.FiveHour().TokensOut, metric.WithAttributes(attr5h))
+			o.Observe(tracker.SevenDay().TokensOut, metric.WithAttributes(attr7d))
+			return nil
+		}),
+	)
 }
diff --git a/internal/telemetry/telemetry.go b/internal/telemetry/telemetry.go
index 0e5f7c9..d836e93 100644
--- a/internal/telemetry/telemetry.go
+++ b/internal/telemetry/telemetry.go
@@ -5,6 +5,7 @@ import (
 	"io"
 
 	"github.com/fujin/anthropic-proxy/internal/config"
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc"
 	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc"
@@ -21,7 +22,7 @@ import (
 // so metrics can be recorded in-process. When cfg.ExportEnabled(), OTLP gRPC
 // exporters are additionally configured to push to the LGTM stack.
 // Returns a shutdown function and an optional io.Writer for the log bridge.
-func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(context.Context) error, logWriter io.Writer, err error) {
+func Setup(ctx context.Context, cfg config.TelemetryConfig, tracker *ratelimit.Tracker) (shutdown func(context.Context) error, logWriter io.Writer, err error) {
 	res, err := resource.New(ctx,
 		resource.WithAttributes(
 			semconv.ServiceName(cfg.ServiceName),
@@ -36,7 +37,7 @@ func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(conte
 		// instruments are valid (they just don't export anywhere).
 		mp := sdkmetric.NewMeterProvider(sdkmetric.WithResource(res))
 		otel.SetMeterProvider(mp)
-		InitMetrics(mp.Meter(cfg.ServiceName))
+		InitMetrics(mp.Meter(cfg.ServiceName), tracker)
 		return func(ctx context.Context) error { return mp.Shutdown(ctx) }, nil, nil
 	}
 
@@ -74,7 +75,7 @@ func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(conte
 		sdkmetric.WithResource(res),
 	)
 	otel.SetMeterProvider(mp)
-	InitMetrics(mp.Meter(cfg.ServiceName))
+	InitMetrics(mp.Meter(cfg.ServiceName), tracker)
 
 	// Log exporter
 	logExp, err := otlploggrpc.New(ctx, logOpts...)
diff --git a/main.go b/main.go
index eaffa31..4974ce9 100644
--- a/main.go
+++ b/main.go
@@ -14,6 +14,7 @@ import (
 	"github.com/fujin/anthropic-proxy/internal/config"
 	"github.com/fujin/anthropic-proxy/internal/logging"
 	"github.com/fujin/anthropic-proxy/internal/proxy"
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 	"github.com/fujin/anthropic-proxy/internal/server"
 	"github.com/fujin/anthropic-proxy/internal/telemetry"
 	"github.com/rs/zerolog/log"
@@ -25,8 +26,17 @@ func run() error {
 		return fmt.Errorf("load config: %w", err)
 	}
 
+	// Create usage tracker (started later once credential is loaded)
+	var credForTracker *auth.Credential
+	tracker := ratelimit.NewTracker(func() string {
+		if credForTracker == nil {
+			return ""
+		}
+		return credForTracker.Token()
+	})
+
 	// Initialize telemetry (metrics always active; OTLP export when endpoint set)
-	telemetryShutdown, logBridge, err := telemetry.Setup(context.Background(), cfg.Telemetry)
+	telemetryShutdown, logBridge, err := telemetry.Setup(context.Background(), cfg.Telemetry, tracker)
 	if err != nil {
 		return fmt.Errorf("telemetry setup: %w", err)
 	}
@@ -85,6 +95,8 @@ func run() error {
 
 	log.Info().Str("credential", cred.Email).Msg("credential loaded")
 
+	credForTracker = cred
+
 	pool := auth.NewPool([]*auth.Credential{cred})
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -92,6 +104,7 @@ func run() error {
 
 	pool.RefreshExpiring(context.Background())
 	auth.StartBackgroundRefresh(ctx, pool)
+	tracker.Start(ctx)
 
 	var profile *proxy.SniffedProfile
 	if cfg.ClaudeBinary != "" {
@@ -103,7 +116,7 @@ func run() error {
 	}
 
 	log.Info().Int("port", cfg.Port).Msg("starting server")
-	srv := server.New(cfg, pool, profile)
+	srv := server.New(cfg, pool, profile, tracker)
 
 	quit := make(chan os.Signal, 1)
 	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)