snapshot of https://github.com/grpc/grpc-go.git (master, 2026-09-15; blobs > 131072B omitted)

credentials/jwt/doc.go

@@ -0,0 +1,50 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package jwt implements JWT token file-based call credentials.
+//
+// This package provides support for A97 JWT Call Credentials, allowing gRPC
+// clients to authenticate using JWT tokens read from files. While originally
+// designed for xDS environments, these credentials are general-purpose.
+//
+// The credentials can be used directly in gRPC clients or configured via xDS.
+//
+// # Token Requirements
+//
+// JWT tokens must:
+//   - Be valid, well-formed JWT tokens with header, payload, and signature
+//   - Include an "exp" (expiration) claim
+//   - Be readable from the specified file path
+//
+// # Considerations
+//
+// - Tokens are cached until expiration to avoid excessive file I/O
+// - Transport security is required (RequireTransportSecurity returns true)
+// - Errors in reading tokens or parsing JWTs will result in RPC UNAVAILALBE or
+// UNAUTHENTICATED errors. The errors are cached and retried with exponential
+// backoff.
+//
+// This implementation is originally intended for use in service mesh
+// environments like Istio where JWT tokens are provisioned and rotated by the
+// infrastructure.
+//
+// # Experimental
+//
+// Notice: All APIs in this package are experimental and may be removed in a
+// later release.
+package jwt

credentials/jwt/file_reader.go

@@ -0,0 +1,118 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+var (
+	errTokenFileAccess = errors.New("token file access error")
+	errJWTValidation   = errors.New("invalid JWT")
+)
+
+// jwtClaims represents the JWT claims structure for extracting expiration time.
+type jwtClaims struct {
+	Exp int64 `json:"exp"`
+}
+
+// jwtFileReader handles reading and parsing JWT tokens from files.
+// It is safe to call methods on this type concurrently as no state is stored.
+type jwtFileReader struct {
+	tokenFilePath string
+}
+
+// readToken reads and parses a JWT token from the configured file.
+// Returns the token string, expiration time, and any error encountered.
+func (r *jwtFileReader) readToken() (string, time.Time, error) {
+	tokenBytes, err := os.ReadFile(r.tokenFilePath)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("%v: %w", err, errTokenFileAccess)
+	}
+
+	token := strings.TrimSpace(string(tokenBytes))
+	if token == "" {
+		return "", time.Time{}, fmt.Errorf("token file %q is empty: %w", r.tokenFilePath, errJWTValidation)
+	}
+
+	exp, err := r.extractExpiration(token)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("token file %q: %v: %w", r.tokenFilePath, err, errJWTValidation)
+	}
+
+	return token, exp, nil
+}
+
+const tokenDelim = "."
+
+// extractClaimsRaw returns the JWT's claims part as raw string. Even though the
+// header and signature are not used, it still expects that the input string to
+// be well-formed (ie comprised of exactly three parts, separated by a dot
+// character).
+func extractClaimsRaw(s string) (string, bool) {
+	_, s, ok := strings.Cut(s, tokenDelim)
+	if !ok { // no period found
+		return "", false
+	}
+	claims, s, ok := strings.Cut(s, tokenDelim)
+	if !ok { // only one period found
+		return "", false
+	}
+	_, _, ok = strings.Cut(s, tokenDelim)
+	if ok { // three periods found
+		return "", false
+	}
+	return claims, true
+}
+
+// extractExpiration parses the JWT token to extract the expiration time.
+func (r *jwtFileReader) extractExpiration(token string) (time.Time, error) {
+	claimsRaw, ok := extractClaimsRaw(token)
+	if !ok {
+		return time.Time{}, fmt.Errorf("expected 3 parts in token")
+	}
+	payloadBytes, err := base64.RawURLEncoding.DecodeString(claimsRaw)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("decode error: %v", err)
+	}
+
+	var claims jwtClaims
+	if err := json.Unmarshal(payloadBytes, &claims); err != nil {
+		return time.Time{}, fmt.Errorf("unmarshal error: %v", err)
+	}
+
+	if claims.Exp == 0 {
+		return time.Time{}, fmt.Errorf("no expiration claims")
+	}
+
+	expTime := time.Unix(claims.Exp, 0)
+
+	// Check if token is already expired.
+	if expTime.Before(time.Now()) {
+		return time.Time{}, fmt.Errorf("expired token")
+	}
+
+	return expTime, nil
+}

credentials/jwt/file_reader_test.go

@@ -0,0 +1,172 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+func (s) TestJWTFileReader_ReadToken_FileErrors(t *testing.T) {
+	tests := []struct {
+		name     string
+		create   bool
+		contents string
+		wantErr  error
+	}{
+		{
+			name:     "nonexistent_file",
+			create:   false,
+			contents: "",
+			wantErr:  errTokenFileAccess,
+		},
+		{
+			name:     "empty_file",
+			create:   true,
+			contents: "",
+			wantErr:  errJWTValidation,
+		},
+		{
+			name:     "file_with_whitespace_only",
+			create:   true,
+			contents: "   \n\t  ",
+			wantErr:  errJWTValidation,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var tokenFile string
+			if !tt.create {
+				tokenFile = "/does-not-exist"
+			} else {
+				tokenFile = writeTempFile(t, "token", tt.contents)
+			}
+
+			reader := jwtFileReader{tokenFilePath: tokenFile}
+			if _, _, err := reader.readToken(); err == nil {
+				t.Fatal("ReadToken() expected error, got nil")
+			} else if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ReadToken() error = %v, want error %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func (s) TestJWTFileReader_ReadToken_InvalidJWT(t *testing.T) {
+	now := time.Now().Truncate(time.Second)
+	tests := []struct {
+		name         string
+		tokenContent string
+		wantErr      error
+	}{
+		{
+			name:         "valid_token_without_expiration",
+			tokenContent: createTestJWT(t, time.Time{}),
+			wantErr:      errJWTValidation,
+		},
+		{
+			name:         "expired_token",
+			tokenContent: createTestJWT(t, now.Add(-time.Hour)),
+			wantErr:      errJWTValidation,
+		},
+		{
+			name:         "malformed_JWT_not_enough_parts",
+			tokenContent: "invalid.jwt",
+			wantErr:      errJWTValidation,
+		},
+		{
+			name:         "malformed_JWT_invalid_base64",
+			tokenContent: "header.invalid_base64!@#.signature",
+			wantErr:      errJWTValidation,
+		},
+		{
+			name:         "malformed_JWT_invalid_JSON",
+			tokenContent: createInvalidJWT(t),
+			wantErr:      errJWTValidation,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tokenFile := writeTempFile(t, "token", tt.tokenContent)
+
+			reader := jwtFileReader{tokenFilePath: tokenFile}
+			if _, _, err := reader.readToken(); err == nil {
+				t.Fatal("ReadToken() expected error, got nil")
+			} else if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ReadToken() error = %v, want error %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func (s) TestJWTFileReader_ReadToken_ValidToken(t *testing.T) {
+	now := time.Now().Truncate(time.Second)
+	tokenExp := now.Add(time.Hour)
+	token := createTestJWT(t, tokenExp)
+	tokenFile := writeTempFile(t, "token", token)
+
+	reader := jwtFileReader{tokenFilePath: tokenFile}
+	readToken, expiry, err := reader.readToken()
+	if err != nil {
+		t.Fatalf("ReadToken() unexpected error: %v", err)
+	}
+
+	if readToken != token {
+		t.Errorf("ReadToken() token = %q, want %q", readToken, token)
+	}
+
+	if !expiry.Equal(tokenExp) {
+		t.Errorf("ReadToken() expiry = %v, want %v", expiry, tokenExp)
+	}
+}
+
+// createInvalidJWT creates a JWT with invalid JSON in the payload.
+func createInvalidJWT(t *testing.T) string {
+	t.Helper()
+
+	header := map[string]any{
+		"typ": "JWT",
+		"alg": "HS256",
+	}
+
+	headerBytes, err := json.Marshal(header)
+	if err != nil {
+		t.Fatalf("Failed to marshal header: %v", err)
+	}
+
+	headerB64 := base64.URLEncoding.EncodeToString(headerBytes)
+	headerB64 = strings.TrimRight(headerB64, "=")
+
+	// Create invalid JSON payload
+	invalidJSON := "invalid json content"
+	payloadB64 := base64.URLEncoding.EncodeToString([]byte(invalidJSON))
+	payloadB64 = strings.TrimRight(payloadB64, "=")
+
+	signature := base64.URLEncoding.EncodeToString([]byte("fake_signature"))
+	signature = strings.TrimRight(signature, "=")
+
+	return fmt.Sprintf("%s.%s.%s", headerB64, payloadB64, signature)
+}

credentials/jwt/token_file_call_creds.go

@@ -0,0 +1,180 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/internal/backoff"
+	"google.golang.org/grpc/status"
+)
+
+const preemptiveRefreshThreshold = time.Minute
+
+// jwtTokenFileCallCreds provides JWT token-based PerRPCCredentials that reads
+// tokens from a file.
+// This implementation follows the A97 JWT Call Credentials specification.
+type jwtTokenFileCallCreds struct {
+	fileReader      *jwtFileReader
+	backoffStrategy backoff.Strategy
+
+	// cached data protected by mu
+	mu               sync.Mutex
+	cachedAuthHeader string    // "Bearer " + token
+	cachedExpiry     time.Time // Slightly less than actual expiration time
+	cachedError      error     // Error from last failed attempt
+	retryAttempt     int       // Current retry attempt number
+	nextRetryTime    time.Time // When next retry is allowed
+	pendingRefresh   bool      // Whether a refresh is currently in progress
+}
+
+// NewTokenFileCallCredentials creates PerRPCCredentials that reads JWT tokens
+// from the specified file path.
+func NewTokenFileCallCredentials(tokenFilePath string) (credentials.PerRPCCredentials, error) {
+	if tokenFilePath == "" {
+		return nil, fmt.Errorf("tokenFilePath cannot be empty")
+	}
+
+	creds := &jwtTokenFileCallCreds{
+		fileReader:      &jwtFileReader{tokenFilePath: tokenFilePath},
+		backoffStrategy: backoff.DefaultExponential,
+	}
+
+	return creds, nil
+}
+
+// GetRequestMetadata gets the current request metadata, refreshing tokens if
+// required. This implementation follows the PerRPCCredentials interface.  The
+// tokens will get automatically refreshed if they are about to expire or if
+// they haven't been loaded successfully yet.
+// If it's not possible to extract a token from the file, UNAVAILABLE is
+// returned.
+// If the token is extracted but invalid, then UNAUTHENTICATED is returned.
+// If errors are encoutered, a backoff is applied before retrying.
+func (c *jwtTokenFileCallCreds) GetRequestMetadata(ctx context.Context, _ ...string) (map[string]string, error) {
+	ri, _ := credentials.RequestInfoFromContext(ctx)
+	if err := credentials.CheckSecurityLevel(ri.AuthInfo, credentials.PrivacyAndIntegrity); err != nil {
+		return nil, fmt.Errorf("cannot send secure credentials on an insecure connection: %v", err)
+	}
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if c.isTokenValidLocked() {
+		needsPreemptiveRefresh := time.Until(c.cachedExpiry) < preemptiveRefreshThreshold
+		if needsPreemptiveRefresh && !c.pendingRefresh {
+			// Start refresh if not pending (handling the prior RPC may have
+			// just spawned a goroutine).
+			c.pendingRefresh = true
+			go c.refreshToken()
+		}
+		return map[string]string{
+			"authorization": c.cachedAuthHeader,
+		}, nil
+	}
+
+	// If in backoff state, just return the cached error.
+	if c.cachedError != nil && time.Now().Before(c.nextRetryTime) {
+		return nil, c.cachedError
+	}
+
+	// At this point, the token is either invalid or expired and we are no
+	// longer backing off from any encountered errors. So refresh it.
+	// NB: We are holding the lock while reading the token from file. This will
+	// cause other RPCs to block until the read completes (sucecssfully or not)
+	// and the cache is updated. Subsequent RPCs will end up using the cache.
+	// This is per A97.
+	token, expiry, err := c.fileReader.readToken()
+	c.updateCacheLocked(token, expiry, err)
+
+	if c.cachedError != nil {
+		return nil, c.cachedError
+	}
+	return map[string]string{
+		"authorization": c.cachedAuthHeader,
+	}, nil
+}
+
+// RequireTransportSecurity indicates whether the credentials requires
+// transport security.
+func (c *jwtTokenFileCallCreds) RequireTransportSecurity() bool {
+	return true
+}
+
+// isTokenValidLocked checks if the cached token is still valid.
+// Caller must hold c.mu lock.
+func (c *jwtTokenFileCallCreds) isTokenValidLocked() bool {
+	if c.cachedAuthHeader == "" {
+		return false
+	}
+	return c.cachedExpiry.After(time.Now())
+}
+
+// refreshToken reads the token from file and updates the cached data.
+func (c *jwtTokenFileCallCreds) refreshToken() {
+	// Deliberately not locking c.mu here. This way other RPCs can proceed
+	// while we read the token. This is per gRFC A97.
+	token, expiry, err := c.fileReader.readToken()
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.updateCacheLocked(token, expiry, err)
+	c.pendingRefresh = false
+}
+
+// updateCacheLocked updates the cached token, expiry, and error state.
+// If an error is provided, it determines whether to set it as an UNAVAILABLE
+// or UNAUTHENTICATED error based on the error type.
+// NOTE: This method (and its callers) do not queue up a token refresh/retry if
+// the expiration is soon / an error was encountered. Instead, this is done when
+// handling RPCs. This is as per gRFC A97, which states that it is
+// undesirable to retry loading the token if the channel is idle.
+// Caller must hold c.mu lock.
+func (c *jwtTokenFileCallCreds) updateCacheLocked(token string, expiry time.Time, err error) {
+	if err != nil {
+		// Convert to gRPC status codes
+		if errors.Is(err, errTokenFileAccess) {
+			c.cachedError = status.Error(codes.Unavailable, err.Error())
+		} else if errors.Is(err, errJWTValidation) {
+			c.cachedError = status.Error(codes.Unauthenticated, err.Error())
+		} else {
+			// Should not happen. Treat unknown errors as UNAUTHENTICATED.
+			c.cachedError = status.Error(codes.Unauthenticated, err.Error())
+		}
+		c.retryAttempt++
+		backoffDelay := c.backoffStrategy.Backoff(c.retryAttempt - 1)
+		c.nextRetryTime = time.Now().Add(backoffDelay)
+		return
+	}
+	// Success - clear any cached error and update token cache
+	c.cachedError = nil
+	c.retryAttempt = 0
+	c.nextRetryTime = time.Time{}
+
+	c.cachedAuthHeader = "Bearer " + token
+	// Per gRFC A97: consider token invalid if it expires within the next 30
+	// seconds to accommodate for clock skew and server processing time.
+	c.cachedExpiry = expiry.Add(-30 * time.Second)
+}

credentials/jwt/token_file_call_creds_ext_test.go

@@ -0,0 +1,265 @@
+/*
+ *
+ * Copyright 2026 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt_test
+
+import (
+	"context"
+	"crypto/tls"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/credentials/jwt"
+	"google.golang.org/grpc/internal/stubserver"
+	"google.golang.org/grpc/metadata"
+	"google.golang.org/grpc/testdata"
+
+	testgrpc "google.golang.org/grpc/interop/grpc_testing"
+	testpb "google.golang.org/grpc/interop/grpc_testing"
+)
+
+const defaultTestTimeout = 5 * time.Second
+
+const wantErr = "cannot send secure credentials on an insecure connection"
+
+// TestJWTCallCredentials_InsecureTransport_AsCallOption verifies that when JWT
+// call credentials are passed as a per-RPC call option over an insecure
+// transport, the RPC fails with a meaningful error.
+func TestJWTCallCredentials_InsecureTransport_AsCallOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	ss := &stubserver.StubServer{}
+	if err := ss.StartServer(grpc.Creds(insecure.NewCredentials())); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	cc, err := grpc.NewClient(ss.Address, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	_, err = client.EmptyCall(ctx, &testpb.Empty{}, grpc.PerRPCCredentials(jwtCreds))
+	if err == nil || !strings.Contains(err.Error(), wantErr) {
+		t.Fatalf("EmptyCall() error = %v; want error containing %q", err, wantErr)
+	}
+}
+
+// TestJWTCallCredentials_InsecureTransport_AsDialOption verifies that when JWT
+// call credentials are passed as a dial option over an insecure transport, the
+// client creation fails with a meaningful error.
+func TestJWTCallCredentials_InsecureTransport_AsDialOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	ss := &stubserver.StubServer{}
+	if err := ss.StartServer(grpc.Creds(insecure.NewCredentials())); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	_, err = grpc.NewClient(ss.Address, grpc.WithTransportCredentials(insecure.NewCredentials()), grpc.WithPerRPCCredentials(jwtCreds))
+	if err == nil || !strings.Contains(err.Error(), "the credentials require transport level security") {
+		t.Fatalf("grpc.NewClient() error = %v; want error containing %q", err, "the credentials require transport level security")
+	}
+}
+
+// TestJWTCallCredentials_SecureTransport_AsDialOption verifies that JWT call
+// credentials work correctly when passed as a dial option over a secure TLS
+// transport.
+func TestJWTCallCredentials_SecureTransport_AsDialOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	wantAuth := "Bearer " + token
+	ss := &stubserver.StubServer{
+		EmptyCallF: func(ctx context.Context, _ *testpb.Empty) (*testpb.Empty, error) {
+			md, ok := metadata.FromIncomingContext(ctx)
+			if !ok {
+				return nil, fmt.Errorf("no metadata received")
+			}
+			authHeaders := md.Get("authorization")
+			if len(authHeaders) != 1 || authHeaders[0] != wantAuth {
+				return nil, fmt.Errorf("authorization header mismatch: got %v, want %q", authHeaders, wantAuth)
+			}
+			return &testpb.Empty{}, nil
+		},
+	}
+
+	serverCert, err := tls.LoadX509KeyPair(testdata.Path("x509/server1_cert.pem"), testdata.Path("x509/server1_key.pem"))
+	if err != nil {
+		t.Fatalf("Failed to load server cert: %v", err)
+	}
+	if err := ss.StartServer(grpc.Creds(credentials.NewServerTLSFromCert(&serverCert))); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	clientCreds, err := credentials.NewClientTLSFromFile(testdata.Path("x509/server_ca_cert.pem"), "x.test.example.com")
+	if err != nil {
+		t.Fatalf("Failed to create client TLS credentials: %v", err)
+	}
+
+	cc, err := grpc.NewClient(ss.Address, grpc.WithTransportCredentials(clientCreds), grpc.WithPerRPCCredentials(jwtCreds))
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	if _, err := client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
+		t.Fatalf("EmptyCall() failed: %v", err)
+	}
+}
+
+// TestJWTCallCredentials_SecureTransport_AsCallOption verifies that JWT call
+// credentials work correctly when passed as a per-RPC call option over a secure
+// TLS transport.
+func TestJWTCallCredentials_SecureTransport_AsCallOption(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempTokenFile(t, token)
+
+	jwtCreds, err := jwt.NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials(%q) failed: %v", tokenFile, err)
+	}
+
+	wantAuth := "Bearer " + token
+	ss := &stubserver.StubServer{
+		EmptyCallF: func(ctx context.Context, _ *testpb.Empty) (*testpb.Empty, error) {
+			md, ok := metadata.FromIncomingContext(ctx)
+			if !ok {
+				return nil, fmt.Errorf("no metadata received")
+			}
+			authHeaders := md.Get("authorization")
+			if len(authHeaders) != 1 || authHeaders[0] != wantAuth {
+				return nil, fmt.Errorf("authorization header mismatch: got %v, want %q", authHeaders, wantAuth)
+			}
+			return &testpb.Empty{}, nil
+		},
+	}
+
+	serverCert, err := tls.LoadX509KeyPair(testdata.Path("x509/server1_cert.pem"), testdata.Path("x509/server1_key.pem"))
+	if err != nil {
+		t.Fatalf("Failed to load server cert: %v", err)
+	}
+	if err := ss.StartServer(grpc.Creds(credentials.NewServerTLSFromCert(&serverCert))); err != nil {
+		t.Fatalf("Failed to start server: %v", err)
+	}
+	defer ss.Stop()
+
+	clientCreds, err := credentials.NewClientTLSFromFile(testdata.Path("x509/server_ca_cert.pem"), "x.test.example.com")
+	if err != nil {
+		t.Fatalf("Failed to create client TLS credentials: %v", err)
+	}
+
+	cc, err := grpc.NewClient(ss.Address, grpc.WithTransportCredentials(clientCreds))
+	if err != nil {
+		t.Fatalf("grpc.NewClient(%q) failed: %v", ss.Address, err)
+	}
+	defer cc.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	client := testgrpc.NewTestServiceClient(cc)
+	if _, err := client.EmptyCall(ctx, &testpb.Empty{}, grpc.PerRPCCredentials(jwtCreds)); err != nil {
+		t.Fatalf("EmptyCall() failed: %v", err)
+	}
+}
+
+// createTestJWT creates a test JWT token with the specified expiration.
+func createTestJWT(t *testing.T, expiration time.Time) string {
+	t.Helper()
+
+	claims := map[string]any{}
+	if !expiration.IsZero() {
+		claims["exp"] = expiration.Unix()
+	}
+
+	header := map[string]any{
+		"typ": "JWT",
+		"alg": "HS256",
+	}
+	headerBytes, err := json.Marshal(header)
+	if err != nil {
+		t.Fatalf("Failed to marshal header: %v", err)
+	}
+
+	claimsBytes, err := json.Marshal(claims)
+	if err != nil {
+		t.Fatalf("Failed to marshal claims: %v", err)
+	}
+
+	headerB64 := base64.URLEncoding.EncodeToString(headerBytes)
+	claimsB64 := base64.URLEncoding.EncodeToString(claimsBytes)
+
+	// Remove padding for URL-safe base64.
+	headerB64 = strings.TrimRight(headerB64, "=")
+	claimsB64 = strings.TrimRight(claimsB64, "=")
+
+	// For testing, we use a fake signature.
+	signature := base64.URLEncoding.EncodeToString([]byte("fake_signature"))
+	signature = strings.TrimRight(signature, "=")
+
+	return fmt.Sprintf("%s.%s.%s", headerB64, claimsB64, signature)
+}
+
+// writeTempTokenFile writes the token to a temporary file and returns the path.
+func writeTempTokenFile(t *testing.T, token string) string {
+	t.Helper()
+	tempDir := t.TempDir()
+	filePath := filepath.Join(tempDir, "token")
+	if err := os.WriteFile(filePath, []byte(token), 0600); err != nil {
+		t.Fatalf("Failed to write temp token file: %v", err)
+	}
+	return filePath
+}

credentials/jwt/token_file_call_creds_test.go

@@ -0,0 +1,555 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package jwt
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/internal/grpctest"
+	"google.golang.org/grpc/status"
+)
+
+const defaultTestTimeout = 5 * time.Second
+
+type s struct {
+	grpctest.Tester
+}
+
+func Test(t *testing.T) {
+	grpctest.RunSubTests(t, s{})
+}
+
+func (s) TestNewTokenFileCallCredentialsValidFilepath(t *testing.T) {
+	creds, err := NewTokenFileCallCredentials("/path/to/token")
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() unexpected error: %v", err)
+	}
+	if creds == nil {
+		t.Fatal("NewTokenFileCallCredentials() returned nil credentials")
+	}
+}
+
+func (s) TestNewTokenFileCallCredentialsMissingFilepath(t *testing.T) {
+	if _, err := NewTokenFileCallCredentials(""); err == nil {
+		t.Fatalf("NewTokenFileCallCredentials() expected error, got nil")
+	}
+}
+
+func (s) TestTokenFileCallCreds_RequireTransportSecurity(t *testing.T) {
+	creds, err := NewTokenFileCallCredentials("/path/to/token")
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+	}
+
+	if !creds.RequireTransportSecurity() {
+		t.Error("RequireTransportSecurity() = false, want true")
+	}
+}
+
+func (s) TestTokenFileCallCreds_GetRequestMetadata(t *testing.T) {
+	now := time.Now().Truncate(time.Second)
+	tests := []struct {
+		name             string
+		invalidTokenPath bool
+		tokenContent     string
+		authInfo         credentials.AuthInfo
+		wantCode         codes.Code
+		wantMetadata     map[string]string
+	}{
+		{
+			name:         "valid_token_with_future_expiration",
+			tokenContent: createTestJWT(t, now.Add(time.Hour)),
+			authInfo:     &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+			wantCode:     codes.OK,
+			wantMetadata: map[string]string{"authorization": "Bearer " + createTestJWT(t, now.Add(time.Hour))},
+		},
+		{
+			name:         "insufficient_security_level",
+			tokenContent: createTestJWT(t, now.Add(time.Hour)),
+			authInfo:     &testAuthInfo{secLevel: credentials.NoSecurity},
+			wantCode:     codes.Unknown, // http2Client.getCallAuthData actually transforms such errors into into Unauthenticated
+		},
+		{
+			name:             "unreachable_token_file",
+			invalidTokenPath: true,
+			authInfo:         &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+			wantCode:         codes.Unavailable,
+		},
+		{
+			name:         "empty_file",
+			tokenContent: "",
+			authInfo:     &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+			wantCode:     codes.Unauthenticated,
+		},
+		{
+			name:         "malformed_JWT_token",
+			tokenContent: "bad contents",
+			authInfo:     &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+			wantCode:     codes.Unauthenticated,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var tokenFile string
+			if tt.invalidTokenPath {
+				tokenFile = "/does-not-exist"
+			} else {
+				tokenFile = writeTempFile(t, "token", tt.tokenContent)
+			}
+			creds, err := NewTokenFileCallCredentials(tokenFile)
+			if err != nil {
+				t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+			}
+
+			ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+			defer cancel()
+			ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+				AuthInfo: tt.authInfo,
+			})
+
+			metadata, err := creds.GetRequestMetadata(ctx)
+			if gotCode := status.Code(err); gotCode != tt.wantCode {
+				t.Fatalf("GetRequestMetadata() = %v, want %v", gotCode, tt.wantCode)
+			}
+
+			if diff := cmp.Diff(tt.wantMetadata, metadata); diff != "" {
+				t.Errorf("GetRequestMetadata() returned unexpected metadata (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func (s) TestTokenFileCallCreds_TokenCaching(t *testing.T) {
+	token := createTestJWT(t, time.Now().Add(time.Hour))
+	tokenFile := writeTempFile(t, "token", token)
+
+	creds, err := NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+		AuthInfo: &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+	})
+
+	// First call should read from file.
+	metadata1, err := creds.GetRequestMetadata(ctx)
+	if err != nil {
+		t.Fatalf("First GetRequestMetadata() failed: %v", err)
+	}
+	wantMetadata := map[string]string{"authorization": "Bearer " + token}
+	if diff := cmp.Diff(wantMetadata, metadata1); diff != "" {
+		t.Errorf("First GetRequestMetadata() returned unexpected metadata (-want +got):\n%s", diff)
+	}
+
+	// Update the file with a different token.
+	newToken := createTestJWT(t, time.Now().Add(2*time.Hour))
+	if err := os.WriteFile(tokenFile, []byte(newToken), 0600); err != nil {
+		t.Fatalf("Failed to update token file: %v", err)
+	}
+
+	// Second call should return cached token (not the updated one).
+	metadata2, err := creds.GetRequestMetadata(ctx)
+	if err != nil {
+		t.Fatalf("Second GetRequestMetadata() failed: %v", err)
+	}
+
+	if diff := cmp.Diff(metadata1, metadata2); diff != "" {
+		t.Errorf("Second GetRequestMetadata() returned unexpected metadata (-want +got):\n%s", diff)
+	}
+}
+
+// testAuthInfo implements credentials.AuthInfo for testing.
+type testAuthInfo struct {
+	secLevel credentials.SecurityLevel
+}
+
+func (t *testAuthInfo) AuthType() string {
+	return "test"
+}
+
+func (t *testAuthInfo) GetCommonAuthInfo() credentials.CommonAuthInfo {
+	return credentials.CommonAuthInfo{SecurityLevel: t.secLevel}
+}
+
+// Tests that cached token expiration is set to 30 seconds before actual token
+// expiration.
+// TODO: Refactor the test to avoid inspecting and mutating internal state.
+func (s) TestTokenFileCallCreds_CacheExpirationIsBeforeTokenExpiration(t *testing.T) {
+	// Create token that expires in 2 hours.
+	tokenExp := time.Now().Truncate(time.Second).Add(2 * time.Hour)
+	token := createTestJWT(t, tokenExp)
+	tokenFile := writeTempFile(t, "token", token)
+
+	creds, err := NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+		AuthInfo: &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+	})
+
+	// Get token to trigger caching.
+	if _, err = creds.GetRequestMetadata(ctx); err != nil {
+		t.Fatalf("GetRequestMetadata() failed: %v", err)
+	}
+
+	// Verify cached expiration is 30 seconds before actual token expiration.
+	impl := creds.(*jwtTokenFileCallCreds)
+	impl.mu.Lock()
+	cachedExp := impl.cachedExpiry
+	impl.mu.Unlock()
+
+	wantExp := tokenExp.Add(-30 * time.Second)
+	if !cachedExp.Equal(wantExp) {
+		t.Errorf("Cache expiration = %v, want %v", cachedExp, wantExp)
+	}
+}
+
+// Tests that pre-emptive refresh is triggered within 1 minute of expiration.
+// This is tested as follows:
+// * A token which expires "soon" is created.
+// * On the first call to GetRequestMetadata, the token will get loaded and returned.
+// * Another token is created and overwrites the file.
+// * On the second call we will still return the (valid) first token but also
+// detect that a refresh needs to happen and trigger it.
+// * On the third call we confirm the new token has been loaded and returned.
+func (s) TestTokenFileCallCreds_PreemptiveRefreshIsTriggered(t *testing.T) {
+	// Create token that expires in 80 seconds (=> cache expires in ~50s).
+	// This ensures pre-emptive refresh triggers since 50s < the 1 minute check.
+	tokenExp := time.Now().Add(80 * time.Second)
+	expiringToken := createTestJWT(t, tokenExp)
+	tokenFile := writeTempFile(t, "token", expiringToken)
+
+	creds, err := NewTokenFileCallCredentials(tokenFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+		AuthInfo: &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+	})
+
+	// First call should read from file synchronously.
+	metadata1, err := creds.GetRequestMetadata(ctx)
+	if err != nil {
+		t.Fatalf("GetRequestMetadata() failed: %v", err)
+	}
+	wantAuth1 := "Bearer " + expiringToken
+	gotAuth1 := metadata1["authorization"]
+	if gotAuth1 != wantAuth1 {
+		t.Fatalf("First call should return original token: got %q, want %q", gotAuth1, wantAuth1)
+	}
+
+	// Verify token was cached and confirm expectation that refresh should be
+	// triggered.
+	impl := creds.(*jwtTokenFileCallCreds)
+	impl.mu.Lock()
+	cacheExp := impl.cachedExpiry
+	tokenCached := impl.cachedAuthHeader != ""
+	shouldTriggerRefresh := time.Until(cacheExp) < preemptiveRefreshThreshold
+	impl.mu.Unlock()
+
+	if !tokenCached {
+		t.Fatal("Token should be cached after successful GetRequestMetadata")
+	}
+
+	if !shouldTriggerRefresh {
+		timeUntilExp := time.Until(cacheExp)
+		t.Fatalf("Cache expires in %v; test precondition requires that this triggers preemptive refresh", timeUntilExp)
+	}
+
+	// Create new token file with different expiration while refresh is
+	// happening.
+	newToken := createTestJWT(t, time.Now().Add(2*time.Hour))
+	if err := os.WriteFile(tokenFile, []byte(newToken), 0600); err != nil {
+		t.Fatalf("Failed to write updated token file: %v", err)
+	}
+
+	// Get token again - this call should trigger a refresh given that the first
+	// one was cached but expiring soon.
+	// However, the function should have returned right away with the current
+	// cached token because it is still valid and the preemptive refresh is
+	// meant to happen without blocking the RPC.
+	metadata2, err := creds.GetRequestMetadata(ctx)
+	if err != nil {
+		t.Fatalf("Second GetRequestMetadata() failed: %v", err)
+	}
+	wantAuth2 := wantAuth1
+	gotAuth2 := metadata2["authorization"]
+	if gotAuth2 != wantAuth2 {
+		t.Fatalf("Second call should return the original token: got %q, want %q", gotAuth2, wantAuth2)
+	}
+
+	// Now should get the new token which was refreshed in the background.
+	wantAuth3 := "Bearer " + newToken
+	ctx, cancel = context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+		AuthInfo: &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+	})
+	for ; ; <-time.After(time.Millisecond) {
+		if ctx.Err() != nil {
+			t.Fatal("Context deadline expired before pre-emptive refresh completed")
+		}
+		// If the newly returned metadata is different to the old one, verify
+		// that it matches the token from the updated file. If not, fail the
+		// test.
+		metadata3, err := creds.GetRequestMetadata(ctx)
+		if err != nil {
+			t.Fatalf("Second GetRequestMetadata() failed: %v", err)
+		}
+		// Pre-emptive refresh not completed yet, try again.
+		gotAuth3 := metadata3["authorization"]
+		if gotAuth3 == gotAuth2 {
+			continue
+		}
+		if gotAuth3 != wantAuth3 {
+			t.Fatalf("Third call should return the new token: got %q, want %q", gotAuth3, wantAuth3)
+		}
+		break
+	}
+}
+
+// Tests that backoff behavior handles file read errors correctly.
+// It has the following expectations:
+// First call to GetRequestMetadata() fails with UNAVAILABLE due to a
+// missing file.
+// Second call to GetRequestMetadata() fails with UNAVAILABLE due backoff.
+// Third call to GetRequestMetadata() fails with UNAVAILABLE due to retry.
+// Fourth call to GetRequestMetadata() fails with UNAVAILABLE due to backoff
+// even though file exists.
+// Fifth call to GetRequestMetadata() succeeds after reading the file and
+// backoff has expired.
+// TODO: Refactor the test to avoid inspecting and mutating internal state.
+func (s) TestTokenFileCallCreds_BackoffBehavior(t *testing.T) {
+	tempDir := t.TempDir()
+	nonExistentFile := filepath.Join(tempDir, "nonexistent")
+
+	creds, err := NewTokenFileCallCredentials(nonExistentFile)
+	if err != nil {
+		t.Fatalf("NewTokenFileCallCredentials() failed: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	ctx = credentials.NewContextWithRequestInfo(ctx, credentials.RequestInfo{
+		AuthInfo: &testAuthInfo{secLevel: credentials.PrivacyAndIntegrity},
+	})
+
+	// First call should fail with UNAVAILABLE.
+	beforeCallRetryTime := time.Now()
+	_, err = creds.GetRequestMetadata(ctx)
+	if err == nil {
+		t.Fatal("Expected error from nonexistent file")
+	}
+	if status.Code(err) != codes.Unavailable {
+		t.Fatalf("GetRequestMetadata() = %v, want UNAVAILABLE", status.Code(err))
+	}
+
+	// Verify error is cached internally.
+	impl := creds.(*jwtTokenFileCallCreds)
+	impl.mu.Lock()
+	retryAttempt := impl.retryAttempt
+	nextRetryTime := impl.nextRetryTime
+	impl.mu.Unlock()
+
+	if retryAttempt != 1 {
+		t.Errorf("Expected retry attempt to be 1, got %d", retryAttempt)
+	}
+	if !nextRetryTime.After(beforeCallRetryTime) {
+		t.Error("Next retry time should be set to a time after the first call")
+	}
+
+	// Second call should still return cached error and not retry.
+	// Set nextRetryTime far enough in the future to ensure that's the case.
+	impl.mu.Lock()
+	impl.nextRetryTime = time.Now().Add(1 * time.Minute)
+	wantNextRetryTime := impl.nextRetryTime
+	impl.mu.Unlock()
+	_, err = creds.GetRequestMetadata(ctx)
+	if err == nil {
+		t.Fatalf("creds.GetRequestMetadata() = %v, want non-nil", err)
+	}
+	if status.Code(err) != codes.Unavailable {
+		t.Fatalf("GetRequestMetadata() = %v, want cached UNAVAILABLE", status.Code(err))
+	}
+
+	impl.mu.Lock()
+	retryAttempt2 := impl.retryAttempt
+	nextRetryTime2 := impl.nextRetryTime
+	impl.mu.Unlock()
+	if !nextRetryTime2.Equal(wantNextRetryTime) {
+		t.Errorf("nextRetryTime should not change due to backoff. Got: %v, Want: %v", nextRetryTime2, wantNextRetryTime)
+	}
+	if retryAttempt2 != 1 {
+		t.Error("Retry attempt should not change due to backoff")
+	}
+
+	// Third call should retry but still fail with UNAVAILABLE.
+	// Set the backoff retry time in the past to allow next retry attempt.
+	impl.mu.Lock()
+	impl.nextRetryTime = time.Now().Add(-1 * time.Minute)
+	beforeCallRetryTime = impl.nextRetryTime
+	impl.mu.Unlock()
+	_, err = creds.GetRequestMetadata(ctx)
+	if err == nil {
+		t.Fatalf("creds.GetRequestMetadata() = %v, want non-nil", err)
+	}
+	if status.Code(err) != codes.Unavailable {
+		t.Fatalf("GetRequestMetadata() = %v, want cached UNAVAILABLE", status.Code(err))
+	}
+
+	impl.mu.Lock()
+	retryAttempt3 := impl.retryAttempt
+	nextRetryTime3 := impl.nextRetryTime
+	impl.mu.Unlock()
+
+	if !nextRetryTime3.After(beforeCallRetryTime) {
+		t.Error("nextRetryTime3 should have been updated after third call")
+	}
+	if retryAttempt3 != 2 {
+		t.Error("Expected retry attempt to increase after retry")
+	}
+
+	// Create valid token file.
+	validToken := createTestJWT(t, time.Now().Add(time.Hour))
+	if err := os.WriteFile(nonExistentFile, []byte(validToken), 0600); err != nil {
+		t.Fatalf("Failed to create valid token file: %v", err)
+	}
+
+	// Fourth call should still fail even though the file now exists due to backoff.
+	// Set nextRetryTime far enough in the future to ensure that's the case.
+	_, err = creds.GetRequestMetadata(ctx)
+	impl.mu.Lock()
+	impl.nextRetryTime = time.Now().Add(1 * time.Minute)
+	wantNextRetryTime = impl.nextRetryTime
+	impl.mu.Unlock()
+	if err == nil {
+		t.Fatalf("creds.GetRequestMetadata() = %v, want non-nil", err)
+	}
+	if status.Code(err) != codes.Unavailable {
+		t.Fatalf("GetRequestMetadata() = %v, want cached UNAVAILABLE", status.Code(err))
+	}
+
+	impl.mu.Lock()
+	retryAttempt4 := impl.retryAttempt
+	nextRetryTime4 := impl.nextRetryTime
+	impl.mu.Unlock()
+
+	if !nextRetryTime4.Equal(wantNextRetryTime) {
+		t.Errorf("nextRetryTime should not change due to backoff. Got: %v, Want: %v", nextRetryTime4, wantNextRetryTime)
+	}
+	if retryAttempt4 != retryAttempt3 {
+		t.Error("Retry attempt should not change due to backoff")
+	}
+
+	// Fifth call should succeed since the file now exists and the backoff has
+	// expired.
+	// Set the backoff retry time in the past to allow next retry attempt.
+	impl.mu.Lock()
+	impl.nextRetryTime = time.Now().Add(-1 * time.Minute)
+	impl.mu.Unlock()
+	_, err = creds.GetRequestMetadata(ctx)
+	if err != nil {
+		t.Fatalf("After creating valid token file, backoff should expire and trigger a token reload on the next RPC. GetRequestMetadata() should eventually succeed, but got: %v", err)
+	}
+	// If successful, verify error cache and backoff state were cleared.
+	impl.mu.Lock()
+	clearedErr := impl.cachedError
+	retryAttempt = impl.retryAttempt
+	nextRetryTime = impl.nextRetryTime
+	impl.mu.Unlock()
+
+	if clearedErr != nil {
+		t.Errorf("After successful retry, cached error should be cleared, got: %v", clearedErr)
+	}
+	if retryAttempt != 0 {
+		t.Errorf("After successful retry, retry attempt should be reset, got: %d", retryAttempt)
+	}
+	if !nextRetryTime.IsZero() {
+		t.Error("After successful retry, next retry time should be cleared")
+	}
+}
+
+// createTestJWT creates a test JWT token with the specified expiration.
+func createTestJWT(t *testing.T, expiration time.Time) string {
+	t.Helper()
+
+	claims := map[string]any{}
+	if !expiration.IsZero() {
+		claims["exp"] = expiration.Unix()
+	}
+
+	header := map[string]any{
+		"typ": "JWT",
+		"alg": "HS256",
+	}
+	headerBytes, err := json.Marshal(header)
+	if err != nil {
+		t.Fatalf("Failed to marshal header: %v", err)
+	}
+
+	claimsBytes, err := json.Marshal(claims)
+	if err != nil {
+		t.Fatalf("Failed to marshal claims: %v", err)
+	}
+
+	headerB64 := base64.URLEncoding.EncodeToString(headerBytes)
+	claimsB64 := base64.URLEncoding.EncodeToString(claimsBytes)
+
+	// Remove padding for URL-safe base64
+	headerB64 = strings.TrimRight(headerB64, "=")
+	claimsB64 = strings.TrimRight(claimsB64, "=")
+
+	// For testing, we'll use a fake signature
+	signature := base64.URLEncoding.EncodeToString([]byte("fake_signature"))
+	signature = strings.TrimRight(signature, "=")
+
+	return fmt.Sprintf("%s.%s.%s", headerB64, claimsB64, signature)
+}
+
+func writeTempFile(t *testing.T, name, content string) string {
+	t.Helper()
+	tempDir := t.TempDir()
+	filePath := filepath.Join(tempDir, name)
+	if err := os.WriteFile(filePath, []byte(content), 0600); err != nil {
+		t.Fatalf("Failed to write temp file: %v", err)
+	}
+	return filePath
+}