diff --git a/authbridge/authlib/plugins/litellm_budgettrack/plugin.go b/authbridge/authlib/plugins/litellm_budgettrack/plugin.go
index cfc883f0d..8a020c99f 100644
--- a/authbridge/authlib/plugins/litellm_budgettrack/plugin.go
+++ b/authbridge/authlib/plugins/litellm_budgettrack/plugin.go
@@ -1,4 +1,4 @@
-// Package litellm_budgettrack provides an inbound pipeline plugin that tracks
+// Package litellm_budgettrack provides a pipeline plugin that tracks
 // per-request cost via the x-litellm-response-cost response header and
 // enforces a daily spending budget, rejecting requests with HTTP 429
 // when the budget is exceeded.
@@ -17,6 +17,19 @@ import (
 	"github.com/rossoctl/cortex/authbridge/authlib/plugins"
 )
 
+// Response cost headers emitted by LiteLLM.
+//
+// responseCostHeader is the effective (post-discount) cost and is present on
+// OpenAI-style /v1/chat/completions responses. Newer LiteLLM releases — and the
+// Anthropic /v1/messages endpoint that Claude Code uses — do not emit it, only
+// the pre-discount "-original" variant, so we fall back to that when the bare
+// header is absent. Without the fallback, budget tracking silently records $0
+// for Anthropic-format traffic.
+const (
+	responseCostHeader         = "X-Litellm-Response-Cost"
+	responseCostOriginalHeader = "X-Litellm-Response-Cost-Original"
+)
+
 type budgetTrackConfig struct {
 	SpendFile string  `json:"spend_file" required:"true" description:"Path to the JSON spend ledger file."`
 	MaxBudget float64 `json:"max_budget" required:"true" description:"Daily budget in USD."`
@@ -80,7 +93,11 @@ func (p *BudgetTrack) OnRequest(_ context.Context, pctx *pipeline.Context) pipel
 
 // OnResponse reads x-litellm-response-cost and accumulates the spend.
 func (p *BudgetTrack) OnResponse(_ context.Context, pctx *pipeline.Context) pipeline.Action {
-	costStr := pctx.Headers.Get("X-Litellm-Response-Cost")
+	costStr := pctx.ResponseHeaders.Get(responseCostHeader)
+	if costStr == "" {
+		// Anthropic /v1/messages (and newer LiteLLM) omit the bare header.
+		costStr = pctx.ResponseHeaders.Get(responseCostOriginalHeader)
+	}
 	if costStr == "" {
 		return pipeline.Action{Type: pipeline.Continue}
 	}
diff --git a/authbridge/authlib/plugins/litellm_budgettrack/plugin_test.go b/authbridge/authlib/plugins/litellm_budgettrack/plugin_test.go
new file mode 100644
index 000000000..ee1b1d22c
--- /dev/null
+++ b/authbridge/authlib/plugins/litellm_budgettrack/plugin_test.go
@@ -0,0 +1,255 @@
+package litellm_budgettrack
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/rossoctl/cortex/authbridge/authlib/pipeline"
+)
+
+// configure builds a BudgetTrack with a temp-dir spend file and the given budget.
+func configure(t *testing.T, maxBudget float64) *BudgetTrack {
+	t.Helper()
+	p := New()
+	cfg := budgetTrackConfig{
+		SpendFile: filepath.Join(t.TempDir(), "spend.json"),
+		MaxBudget: maxBudget,
+	}
+	raw, _ := json.Marshal(cfg)
+	if err := p.Configure(raw); err != nil {
+		t.Fatalf("Configure() error = %v", err)
+	}
+	return p
+}
+
+// TestOnResponseReadsResponseHeader is the regression guard for the core fix:
+// the cost must be read from ResponseHeaders, not the request Headers.
+func TestOnResponseReadsResponseHeader(t *testing.T) {
+	p := configure(t, 5.00)
+	pctx := &pipeline.Context{
+		ResponseHeaders: http.Header{responseCostHeader: {"0.0025"}},
+	}
+
+	if action := p.OnResponse(context.Background(), pctx); action.Type != pipeline.Continue {
+		t.Fatalf("OnResponse() = %v, want Continue", action.Type)
+	}
+	if p.ledger.TotalSpend != 0.0025 {
+		t.Errorf("TotalSpend = %v, want 0.0025", p.ledger.TotalSpend)
+	}
+	if p.ledger.TotalCalls != 1 {
+		t.Errorf("TotalCalls = %d, want 1", p.ledger.TotalCalls)
+	}
+}
+
+// TestOnResponseIgnoresRequestHeader guards against the original bug: the cost
+// header on the request side (pctx.Headers) must NOT be accumulated.
+func TestOnResponseIgnoresRequestHeader(t *testing.T) {
+	p := configure(t, 5.00)
+	pctx := &pipeline.Context{
+		Headers:         http.Header{responseCostHeader: {"0.0025"}}, // wrong place; must be ignored
+		ResponseHeaders: http.Header{},
+	}
+
+	p.OnResponse(context.Background(), pctx)
+	if p.ledger.TotalSpend != 0 {
+		t.Errorf("TotalSpend = %v, want 0 (request-header cost must be ignored)", p.ledger.TotalSpend)
+	}
+}
+
+// TestOnResponseFallsBackToOriginal covers the Anthropic /v1/messages case where
+// only the pre-discount "-original" header is present.
+func TestOnResponseFallsBackToOriginal(t *testing.T) {
+	p := configure(t, 5.00)
+	pctx := &pipeline.Context{
+		ResponseHeaders: http.Header{responseCostOriginalHeader: {"2.204e-05"}},
+	}
+
+	p.OnResponse(context.Background(), pctx)
+	if p.ledger.TotalSpend != 2.204e-05 {
+		t.Errorf("TotalSpend = %v, want 2.204e-05 (fallback header)", p.ledger.TotalSpend)
+	}
+}
+
+// TestOnResponseBareHeaderWins verifies the effective (post-discount) header
+// takes precedence over "-original" when both are present.
+func TestOnResponseBareHeaderWins(t *testing.T) {
+	p := configure(t, 5.00)
+	pctx := &pipeline.Context{
+		ResponseHeaders: http.Header{
+			responseCostHeader:         {"0.001"},
+			responseCostOriginalHeader: {"0.002"},
+		},
+	}
+
+	p.OnResponse(context.Background(), pctx)
+	if p.ledger.TotalSpend != 0.001 {
+		t.Errorf("TotalSpend = %v, want 0.001 (bare header must win)", p.ledger.TotalSpend)
+	}
+}
+
+// TestOnResponseIgnoresMissingOrInvalid verifies absent / non-positive / unparseable
+// costs are skipped rather than corrupting the ledger.
+func TestOnResponseIgnoresMissingOrInvalid(t *testing.T) {
+	for _, tc := range []struct {
+		name    string
+		headers http.Header
+	}{
+		{"missing", http.Header{}},
+		{"zero", http.Header{responseCostHeader: {"0"}}},
+		{"negative", http.Header{responseCostHeader: {"-1"}}},
+		{"unparseable", http.Header{responseCostHeader: {"abc"}}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			p := configure(t, 5.00)
+			pctx := &pipeline.Context{ResponseHeaders: tc.headers}
+			if action := p.OnResponse(context.Background(), pctx); action.Type != pipeline.Continue {
+				t.Fatalf("OnResponse() = %v, want Continue", action.Type)
+			}
+			if p.ledger.TotalSpend != 0 || p.ledger.TotalCalls != 0 {
+				t.Errorf("ledger mutated: spend=%v calls=%d, want 0/0", p.ledger.TotalSpend, p.ledger.TotalCalls)
+			}
+		})
+	}
+}
+
+// TestOnRequestEnforcesBudget verifies OnRequest denies with 429 once the
+// accumulated spend reaches the daily budget, and allows before that.
+func TestOnRequestEnforcesBudget(t *testing.T) {
+	p := configure(t, 0.001)
+
+	// Under budget: allowed.
+	if action := p.OnRequest(context.Background(), &pipeline.Context{}); action.Type != pipeline.Continue {
+		t.Fatalf("OnRequest() under budget = %v, want Continue", action.Type)
+	}
+
+	// Accumulate past the budget via a response.
+	p.OnResponse(context.Background(), &pipeline.Context{
+		ResponseHeaders: http.Header{responseCostHeader: {"0.002"}},
+	})
+
+	// Over budget: rejected with 429 / budget.exceeded.
+	action := p.OnRequest(context.Background(), &pipeline.Context{})
+	if action.Type != pipeline.Reject {
+		t.Fatalf("OnRequest() over budget = %v, want Reject", action.Type)
+	}
+	if action.Violation == nil || action.Violation.Status != http.StatusTooManyRequests {
+		t.Errorf("Violation = %+v, want Status 429", action.Violation)
+	}
+	if action.Violation.Code != "budget.exceeded" {
+		t.Errorf("Violation.Code = %q, want budget.exceeded", action.Violation.Code)
+	}
+}
+
+// TestLedgerPersistsAcrossInstances verifies the spend file is reloaded, so a
+// restart on the same day resumes the accumulated total.
+func TestLedgerPersistsAcrossInstances(t *testing.T) {
+	spendFile := filepath.Join(t.TempDir(), "spend.json")
+	raw, _ := json.Marshal(budgetTrackConfig{SpendFile: spendFile, MaxBudget: 5.00})
+
+	p1 := New()
+	if err := p1.Configure(raw); err != nil {
+		t.Fatalf("Configure() error = %v", err)
+	}
+	p1.OnResponse(context.Background(), &pipeline.Context{
+		ResponseHeaders: http.Header{responseCostHeader: {"0.01"}},
+	})
+
+	p2 := New()
+	if err := p2.Configure(raw); err != nil {
+		t.Fatalf("Configure() error = %v", err)
+	}
+	if p2.ledger.TotalSpend != 0.01 {
+		t.Errorf("reloaded TotalSpend = %v, want 0.01", p2.ledger.TotalSpend)
+	}
+}
+
+// TestConfigureRejectsBadConfig verifies required-field and JSON validation.
+func TestConfigureRejectsBadConfig(t *testing.T) {
+	spend := filepath.Join(t.TempDir(), "spend.json")
+	for _, tc := range []struct {
+		name string
+		raw  string
+	}{
+		{"empty spend_file", `{"max_budget": 5.0}`},
+		{"zero max_budget", fmt.Sprintf(`{"spend_file": %q, "max_budget": 0}`, spend)},
+		{"negative max_budget", fmt.Sprintf(`{"spend_file": %q, "max_budget": -1}`, spend)},
+		{"invalid json", `{`},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if err := New().Configure(json.RawMessage(tc.raw)); err == nil {
+				t.Errorf("Configure(%s) = nil, want error", tc.raw)
+			}
+		})
+	}
+}
+
+// TestLoadLedgerResetsStaleDay verifies a spend file left over from a previous
+// day is discarded on Configure rather than counted against today's budget.
+func TestLoadLedgerResetsStaleDay(t *testing.T) {
+	spend := filepath.Join(t.TempDir(), "spend.json")
+	stale := `{"date":"2000-01-01","total_spend":9.99,"total_calls":42}`
+	if err := os.WriteFile(spend, []byte(stale), 0o644); err != nil {
+		t.Fatalf("seed spend file: %v", err)
+	}
+
+	p := New()
+	raw, _ := json.Marshal(budgetTrackConfig{SpendFile: spend, MaxBudget: 5.00})
+	if err := p.Configure(raw); err != nil {
+		t.Fatalf("Configure() error = %v", err)
+	}
+
+	today := time.Now().UTC().Format("2006-01-02")
+	if p.ledger.Date != today {
+		t.Errorf("ledger.Date = %q, want %q", p.ledger.Date, today)
+	}
+	if p.ledger.TotalSpend != 0 || p.ledger.TotalCalls != 0 {
+		t.Errorf("stale ledger not reset: spend=%v calls=%d", p.ledger.TotalSpend, p.ledger.TotalCalls)
+	}
+
+	// A same-day ledger, by contrast, is preserved.
+	sameDay := fmt.Sprintf(`{"date":%q,"total_spend":1.25,"total_calls":3}`, today)
+	if err := os.WriteFile(spend, []byte(sameDay), 0o644); err != nil {
+		t.Fatalf("seed same-day file: %v", err)
+	}
+	p2 := New()
+	if err := p2.Configure(raw); err != nil {
+		t.Fatalf("Configure() error = %v", err)
+	}
+	if p2.ledger.TotalSpend != 1.25 || p2.ledger.TotalCalls != 3 {
+		t.Errorf("same-day ledger not preserved: spend=%v calls=%d", p2.ledger.TotalSpend, p2.ledger.TotalCalls)
+	}
+}
+
+// TestConcurrentOnResponse exercises the mutex under concurrent responses.
+// Run with -race to catch data races on the ledger.
+func TestConcurrentOnResponse(t *testing.T) {
+	p := configure(t, 1000.0) // high budget so nothing is rejected
+	const goroutines = 50
+
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			p.OnResponse(context.Background(), &pipeline.Context{
+				ResponseHeaders: http.Header{responseCostHeader: {"0.01"}},
+			})
+		}()
+	}
+	wg.Wait()
+
+	if p.ledger.TotalCalls != goroutines {
+		t.Errorf("TotalCalls = %d, want %d", p.ledger.TotalCalls, goroutines)
+	}
+	// 50 × 0.01 = 0.50, within float tolerance.
+	if got := p.ledger.TotalSpend; got < 0.4999 || got > 0.5001 {
+		t.Errorf("TotalSpend = %v, want ~0.50", got)
+	}
+}
diff --git a/authbridge/docs/litellm-budgettrack-plugin.md b/authbridge/docs/litellm-budgettrack-plugin.md
index 6e1e21ff9..da47bf177 100644
--- a/authbridge/docs/litellm-budgettrack-plugin.md
+++ b/authbridge/docs/litellm-budgettrack-plugin.md
@@ -90,7 +90,12 @@ The spend file (`spend-authbridge.json`) is a simple JSON object:
 
 ### OnResponse (cost accumulation)
 
-1. Read `X-Litellm-Response-Cost` header from upstream response
+1. Read the cost from the **response** headers (`pctx.ResponseHeaders`):
+   `X-Litellm-Response-Cost`, falling back to `X-Litellm-Response-Cost-Original`
+   when the bare header is absent. The bare (effective, post-discount) header is
+   present on OpenAI `/v1/chat/completions` responses; the Anthropic `/v1/messages`
+   endpoint used by Claude Code — and newer LiteLLM releases — emit only the
+   pre-discount `-original` variant.
 2. If missing or non-positive → continue (no cost to track)
 3. Lock mutex, reset if new day
 4. Add cost to `total_spend`, increment `total_calls`