diff --git a/cmd/main.go b/cmd/main.go
index 8a25aa9a8..eb77a29be 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -37,6 +37,10 @@ const (
 
 // Config holds all configuration for the RI helper tool.
 type Config struct {
+	// CallerAccountID is the account the credentials belong to (STS
+	// GetCallerIdentity), set at run start. Purchases are refused for
+	// recommendations owned by any other account.
+	CallerAccountID        string
 	AuditLog               string
 	CSVInput               string
 	IdempotencyWindow      string
diff --git a/cmd/main_test.go b/cmd/main_test.go
index 87dc3ada0..41e5036db 100644
--- a/cmd/main_test.go
+++ b/cmd/main_test.go
@@ -31,6 +31,12 @@ func TestMain(m *testing.M) {
 	engineVersionFetcher = func(context.Context, Config, bool) (engineVersionData, error) {
 		return engineVersionData{}, nil
 	}
+	// An empty account turns the cross-account purchase check off for tests
+	// that do not set CallerAccountID; the check is tested explicitly in
+	// purchase_account_guard_test.go.
+	callerAccountFetcher = func(context.Context, aws.Config) (string, error) {
+		return "", nil
+	}
 	code := m.Run()
 	_ = os.Remove(toolCfg.AuditLog)
 	os.Exit(code)
diff --git a/cmd/multi_service.go b/cmd/multi_service.go
index 2085869b6..f80cc05fd 100644
--- a/cmd/multi_service.go
+++ b/cmd/multi_service.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 	"sync/atomic"
@@ -15,6 +16,7 @@ import (
 	"github.com/LeanerCloud/cloud-commitments-go/providers/aws/recommendations"
 	"github.com/aws/aws-sdk-go-v2/aws"
 	awsconfig "github.com/aws/aws-sdk-go-v2/config"
+	"github.com/aws/aws-sdk-go-v2/service/sts"
 	"github.com/google/uuid"
 )
 
@@ -238,6 +240,10 @@ func exitIfInterrupted(ctx context.Context, phase string) {
 // runToolMultiService within the cyclomatic-complexity limit.
 func runPurchaseAndReport(ctx context.Context, awsCfg aws.Config, scoredResult scorer.ScoredResult, isDryRun bool, cfg Config, drops *common.DropSummary) error {
 	runID := uuid.New().String()
+	cfg, err := withCallerAccount(ctx, awsCfg, cfg)
+	if err != nil {
+		return err
+	}
 	if !confirmPurchaseRun(scoredResult.Passed, isDryRun) {
 		printDropSummary(drops)
 		AppLogger.Printf("\n❌ Purchase canceled.\n")
@@ -302,6 +308,32 @@ func printDropSummary(drops *common.DropSummary) {
 	}
 }
 
+// callerAccountFetcher is the seam tests replace so they never reach STS.
+var callerAccountFetcher = fetchCallerAccount
+
+func fetchCallerAccount(ctx context.Context, awsCfg aws.Config) (string, error) {
+	out, err := sts.NewFromConfig(awsCfg).GetCallerIdentity(ctx, &sts.GetCallerIdentityInput{})
+	if err != nil {
+		return "", err
+	}
+	if aws.ToString(out.Account) == "" {
+		return "", errors.New("GetCallerIdentity returned no account")
+	}
+	return aws.ToString(out.Account), nil
+}
+
+// withCallerAccount returns cfg with CallerAccountID resolved, so the purchase
+// preconditions can refuse recommendations that belong to another account. It
+// runs on dry runs too, so a dry run predicts what a real run will refuse.
+func withCallerAccount(ctx context.Context, awsCfg aws.Config, cfg Config) (Config, error) {
+	account, err := callerAccountFetcher(ctx, awsCfg)
+	if err != nil {
+		return cfg, fmt.Errorf("cannot determine the AWS account of the credentials (needed to check purchases): %w", err)
+	}
+	cfg.CallerAccountID = account
+	return cfg, nil
+}
+
 // loadAWSConfig builds an aws.Config from the tool config.
 func loadAWSConfig(ctx context.Context, cfg Config) (aws.Config, error) {
 	var opts []func(*awsconfig.LoadOptions) error
@@ -578,7 +610,7 @@ func purchaseAuditStatus(result common.PurchaseResult) string {
 func purchaseSingleRec(ctx context.Context, awsCfg aws.Config, rec common.Recommendation, index int, isDryRun bool, cfg Config) (purchaseResult common.PurchaseResult, auditStatus string) {
 	label := purchaseRegionLabel(rec, rec.Region)
 	AppLogger.Printf("  [%d] %s %s %s (count=%d)\n", index, rec.Service, label, rec.ResourceType, rec.Count)
-	if err := validatePurchasePreconditions(rec, rec.Region); err != nil {
+	if err := validatePurchasePreconditions(rec, rec.Region, cfg.CallerAccountID); err != nil {
 		AppLogger.Printf("    ❌ cannot purchase: %v\n", err)
 		return preconditionFailure(rec, err, isDryRun), "error"
 	}
@@ -721,6 +753,10 @@ func runToolFromCSV(ctx context.Context, cfg Config) error {
 		return nil
 	}
 
+	cfg, err = withCallerAccount(ctx, awsCfg, cfg)
+	if err != nil {
+		return err
+	}
 	return runCSVPurchases(ctx, awsCfg, recs, isDryRun, cfg, runID)
 }
 
@@ -990,7 +1026,7 @@ func processPurchaseLoop(ctx context.Context, recs []common.Recommendation, regi
 // runOnePurchase validates and then dry-runs or executes one CSV-path
 // recommendation. attempted reports whether a real purchase call was made.
 func runOnePurchase(ctx context.Context, rec common.Recommendation, label, region string, index int, isDryRun bool, serviceClient provider.ServiceClient, cfg Config) (result common.PurchaseResult, status string, attempted bool) {
-	if err := validatePurchasePreconditions(rec, region); err != nil {
+	if err := validatePurchasePreconditions(rec, region, cfg.CallerAccountID); err != nil {
 		return preconditionFailure(rec, err, isDryRun), "error", false
 	}
 	if isDryRun {
diff --git a/cmd/purchase_account_guard_test.go b/cmd/purchase_account_guard_test.go
new file mode 100644
index 000000000..b3c1e1073
--- /dev/null
+++ b/cmd/purchase_account_guard_test.go
@@ -0,0 +1,90 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"path/filepath"
+	"testing"
+
+	"github.com/LeanerCloud/cloud-commitments-go/pkg/common"
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	testCallerAccount = "640848491082"
+	otherAccount      = "726468838177"
+)
+
+func TestCheckPurchaseAccount(t *testing.T) {
+	rec := common.Recommendation{Service: common.ServiceRDS, Region: "us-east-1", Account: testCallerAccount}
+
+	require.NoError(t, checkPurchaseAccount(rec, testCallerAccount))
+
+	rec.Account = ""
+	require.NoError(t, checkPurchaseAccount(rec, testCallerAccount), "a row with no account has nothing to compare")
+
+	rec.Account = otherAccount
+	err := checkPurchaseAccount(rec, testCallerAccount)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), otherAccount)
+	assert.Contains(t, err.Error(), testCallerAccount)
+
+}
+
+func TestPurchaseSingleRec_RefusesOtherAccountsRecommendation(t *testing.T) {
+	captureAppLog(t)
+	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl"), CallerAccountID: testCallerAccount}
+	rec := validEC2Rec()
+	rec.Account = otherAccount
+
+	for _, dry := range []bool{true, false} {
+		// A real run must refuse before it builds a service client: aws.Config{}
+		// has no credentials, so reaching the purchase call would fail
+		// differently, not with the account message.
+		result, status := purchaseSingleRec(context.Background(), aws.Config{}, rec, 1, dry, cfg)
+		assert.False(t, result.Success, "dryRun=%v", dry)
+		require.Error(t, result.Error, "dryRun=%v", dry)
+		assert.Contains(t, result.Error.Error(), "cross-account purchases are not supported", "dryRun=%v", dry)
+		assert.Equal(t, "error", status)
+	}
+}
+
+func TestProcessPurchaseLoop_RefusesOtherAccountsRecommendationWithoutPurchasing(t *testing.T) {
+	captureAppLog(t)
+	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl"), CallerAccountID: testCallerAccount}
+	other := validEC2Rec()
+	other.Account = otherAccount
+	own := validEC2Rec()
+	own.Account = testCallerAccount
+	client := &MockServiceClient{}
+	client.On("PurchaseCommitment", mock.Anything, mock.MatchedBy(func(r common.Recommendation) bool { return r.Account == testCallerAccount }), mock.Anything).
+		Return(common.PurchaseResult{Success: true, CommitmentID: "ri-own"}, nil).Once()
+
+	results, err := processPurchaseLoop(context.Background(), []common.Recommendation{other, own}, "us-east-1", false, client, cfg, "run-1")
+
+	require.NoError(t, err)
+	require.Len(t, results, 2)
+	assert.False(t, results[0].Success)
+	require.Error(t, results[0].Error)
+	assert.Contains(t, results[0].Error.Error(), otherAccount)
+	assert.True(t, results[1].Success)
+	client.AssertNumberOfCalls(t, "PurchaseCommitment", 1)
+}
+
+func TestWithCallerAccount(t *testing.T) {
+	orig := callerAccountFetcher
+	t.Cleanup(func() { callerAccountFetcher = orig })
+
+	callerAccountFetcher = func(context.Context, aws.Config) (string, error) { return "640848491082", nil }
+	cfg, err := withCallerAccount(context.Background(), aws.Config{}, Config{})
+	require.NoError(t, err)
+	assert.Equal(t, "640848491082", cfg.CallerAccountID)
+
+	callerAccountFetcher = func(context.Context, aws.Config) (string, error) { return "", errors.New("expired token") }
+	_, err = withCallerAccount(context.Background(), aws.Config{}, Config{})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "expired token")
+}
diff --git a/cmd/savings_plan_purchase.go b/cmd/savings_plan_purchase.go
index 80eab2452..6fe98e168 100644
--- a/cmd/savings_plan_purchase.go
+++ b/cmd/savings_plan_purchase.go
@@ -92,7 +92,10 @@ func purchaseRegionLabel(rec common.Recommendation, groupRegion string) string {
 // what the library would reject for a missing or malformed field; it does not
 // call any API. region is the one the client will be built from: the
 // recommendation's own on the main path, the CSV group's on the CSV path.
-func validatePurchasePreconditions(rec common.Recommendation, region string) error {
+func validatePurchasePreconditions(rec common.Recommendation, region, callerAccount string) error {
+	if err := checkPurchaseAccount(rec, callerAccount); err != nil {
+		return err
+	}
 	if _, err := clientRegionFor(rec.Service, region); err != nil {
 		return err
 	}
@@ -111,6 +114,22 @@ func validatePurchasePreconditions(rec common.Recommendation, region string) err
 	return nil
 }
 
+// checkPurchaseAccount refuses a recommendation owned by an account other than
+// the one the credentials belong to. Cost Explorer returns linked-account
+// recommendations to a payer, but a purchase call buys in the caller's own
+// account, so buying one would commit the caller's account to another
+// account's usage. callerAccount is set by withCallerAccount, which both
+// purchase entry points run first and which fails the run when the account
+// cannot be resolved; empty here means a caller below those entry points (unit
+// tests) did not resolve it. A recommendation with no account (CSV rows may
+// omit it) carries nothing to compare.
+func checkPurchaseAccount(rec common.Recommendation, callerAccount string) error {
+	if callerAccount != "" && rec.Account != "" && rec.Account != callerAccount {
+		return fmt.Errorf("recommendation is for account %s but the credentials are for account %s; cross-account purchases are not supported, so run with credentials for account %s", rec.Account, callerAccount, rec.Account)
+	}
+	return nil
+}
+
 func validateSavingsPlanPreconditions(rec common.Recommendation) error {
 	d, ok := rec.Details.(*common.SavingsPlanDetails)
 	if !ok || d == nil {
diff --git a/cmd/savings_plan_purchase_test.go b/cmd/savings_plan_purchase_test.go
index d2b804c8e..5dd01b624 100644
--- a/cmd/savings_plan_purchase_test.go
+++ b/cmd/savings_plan_purchase_test.go
@@ -262,7 +262,7 @@ func TestValidatePurchasePreconditions(t *testing.T) {
 		"rds":             {Service: common.ServiceRDS, Region: "us-east-1"},
 	}
 	for name, rec := range ok {
-		assert.NoError(t, validatePurchasePreconditions(rec, rec.Region), name)
+		assert.NoError(t, validatePurchasePreconditions(rec, rec.Region, ""), name)
 	}
 	bad := map[string]common.Recommendation{
 		"sp nil details":   mut(parserShapedSP(), func(r *common.Recommendation) { r.Details = nil }),
@@ -282,7 +282,7 @@ func TestValidatePurchasePreconditions(t *testing.T) {
 		"rds no region": {Service: common.ServiceRDS},
 	}
 	for name, rec := range bad {
-		assert.Error(t, validatePurchasePreconditions(rec, rec.Region), name)
+		assert.Error(t, validatePurchasePreconditions(rec, rec.Region, ""), name)
 	}
 }
 
@@ -351,7 +351,7 @@ func TestCSVRoundTrip_SavingsPlanAndEC2DetailsSurviveWriteThenRead(t *testing.T)
 	assert.Equal(t, ec2.Details, byService[ec2.Service].Details)
 	assert.Equal(t, rds.Details, byService[rds.Service].Details)
 	for _, r := range out {
-		assert.NoError(t, validatePurchasePreconditions(r, r.Region), "%s", r.Service)
+		assert.NoError(t, validatePurchasePreconditions(r, r.Region, ""), "%s", r.Service)
 	}
 }
 
diff --git a/go.mod b/go.mod
index 9f915c246..41f8ee0c6 100644
--- a/go.mod
+++ b/go.mod
@@ -88,6 +88,7 @@ require (
 	github.com/LeanerCloud/cloud-commitments-go/providers/gcp v0.0.0-20261006205158-7ff8c1aee1bb
 	github.com/aws/aws-sdk-go-v2/service/organizations v1.45.3
 	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.40.3
+	github.com/aws/aws-sdk-go-v2/service/sts v1.33.17
 	github.com/google/uuid v1.6.0
 	github.com/microsoftgraph/msgraph-sdk-go v1.99.0
 	golang.org/x/term v0.46.0
@@ -107,7 +108,6 @@ require (
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.33.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric v0.54.0 // indirect
 	github.com/GoogleCloudPlatform/opentelemetry-operations-go/internal/resourcemapping v0.54.0 // indirect
-	github.com/aws/aws-sdk-go-v2/service/sts v1.33.17 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cncf/xds/go v0.0.0-20260202195803-dba9d589def2 // indirect
 	github.com/envoyproxy/go-control-plane/envoy v1.37.0 // indirect