diff --git a/cmd/audit_stop_2103_test.go b/cmd/audit_stop_2103_test.go
index bab5c0ca0..7d66f60e9 100644
--- a/cmd/audit_stop_2103_test.go
+++ b/cmd/audit_stop_2103_test.go
@@ -43,7 +43,7 @@ func failAuditWrites(t *testing.T) {
 func auditCfg(t *testing.T) Config {
 	t.Helper()
 	dir := t.TempDir()
-	return Config{AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
+	return Config{AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv"), CallerAccountID: defaultTestAccount}
 }
 
 func TestExecuteAndReport_AuditWriteFailureStopsFurtherPurchases(t *testing.T) {
diff --git a/cmd/cancellation_test.go b/cmd/cancellation_test.go
index edb67e668..043ccd6b0 100644
--- a/cmd/cancellation_test.go
+++ b/cmd/cancellation_test.go
@@ -112,7 +112,7 @@ func TestExecuteAndReport_CancelDuringPurchaseFinishesInFlightAndReportsWhatRan(
 	slowDelay(t)
 	out := captureAppLog(t)
 	dir := t.TempDir()
-	cfg := Config{AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -153,7 +153,7 @@ func TestExecuteAndReport_ErroredPurchaseOnInterruptSaysVerifyInAWS(t *testing.T
 	slowDelay(t)
 	out := captureAppLog(t)
 	dir := t.TempDir()
-	cfg := Config{AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	h := &hookTransport{}
@@ -176,7 +176,7 @@ func TestExecuteAndReport_ErroredPurchaseOnInterruptSaysVerifyInAWS(t *testing.T
 func TestProcessPurchaseLoop_AuditRecordWrittenBeforeDelay(t *testing.T) {
 	slowDelay(t)
 	captureAppLog(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	recs := []common.Recommendation{validEC2Rec(), validEC2Rec()}
@@ -207,7 +207,7 @@ func TestProcessPurchaseLoop_AuditRecordWrittenBeforeDelay(t *testing.T) {
 func TestExecutePurchasePipeline_AuditRecordWrittenBeforeDelay(t *testing.T) {
 	slowDelay(t)
 	captureAppLog(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	h := &hookTransport{handle: func(r *http.Request) (int, string) {
@@ -283,7 +283,7 @@ func TestInvocationContext_FurtherSigintsAreAbsorbed(t *testing.T) {
 func TestProcessPurchaseLoop_CancelDuringPurchaseDoesNotCancelTheInFlightCall(t *testing.T) {
 	slowDelay(t)
 	captureAppLog(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	recs := []common.Recommendation{validEC2Rec(), validEC2Rec()}
diff --git a/cmd/ec2_zonal_az_test.go b/cmd/ec2_zonal_az_test.go
index 7d6a46df5..abb8d4207 100644
--- a/cmd/ec2_zonal_az_test.go
+++ b/cmd/ec2_zonal_az_test.go
@@ -62,7 +62,7 @@ func purchaseEC2WithRecorder(t *testing.T, rec common.Recommendation) (common.Pu
 		Credentials: aws.AnonymousCredentials{},
 		Retryer:     func() aws.Retryer { return aws.NopRetryer{} },
 	}
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	result, _ := purchaseSingleRec(context.Background(), awsCfg, rec, 1, false, cfg)
 	return result, recorder
 }
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..edc7d6a0e 100644
--- a/cmd/main_test.go
+++ b/cmd/main_test.go
@@ -14,6 +14,29 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+// defaultTestAccount is the account the stubbed credentials belong to.
+const defaultTestAccount = "111111111111"
+
+// wantPurchaseOutcome is the Success and Error report columns for a dry-run
+// row owned by account: the stubbed credentials belong to defaultTestAccount,
+// and rows of any other account are refused.
+func wantPurchaseOutcome(account string) (success, errMsg string) {
+	if account == defaultTestAccount {
+		return "true", ""
+	}
+	return "false", fmt.Sprintf("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", account, defaultTestAccount, account)
+}
+
+// useCallerAccount gives toolCfg the stubbed credentials' account for one test,
+// for tests that call the purchase loops directly instead of through an entry
+// point (which resolves the account itself).
+func useCallerAccount(t *testing.T) {
+	t.Helper()
+	prev := toolCfg.CallerAccountID
+	toolCfg.CallerAccountID = defaultTestAccount
+	t.Cleanup(func() { toolCfg.CallerAccountID = prev })
+}
+
 // TestMain overrides toolCfg.AuditLog's cobra-registered default
 // ("./cudly-audit.jsonl", relative to the process working directory) before
 // any test in this package runs. Since #1609, processPurchaseLoop (shared by
@@ -31,6 +54,9 @@ func TestMain(m *testing.M) {
 	engineVersionFetcher = func(context.Context, Config, bool) (engineVersionData, error) {
 		return engineVersionData{}, nil
 	}
+	callerAccountFetcher = func(context.Context, aws.Config) (string, error) {
+		return defaultTestAccount, nil
+	}
 	code := m.Run()
 	_ = os.Remove(toolCfg.AuditLog)
 	os.Exit(code)
diff --git a/cmd/multi_service.go b/cmd/multi_service.go
index 1636cf2aa..780370edd 100644
--- a/cmd/multi_service.go
+++ b/cmd/multi_service.go
@@ -16,6 +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"
 )
 
@@ -239,6 +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")
@@ -303,6 +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
@@ -579,7 +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"
 	}
@@ -722,6 +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)
 }
 
@@ -991,7 +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/multi_service_test.go b/cmd/multi_service_test.go
index ed5cef8aa..fadc4c585 100644
--- a/cmd/multi_service_test.go
+++ b/cmd/multi_service_test.go
@@ -207,6 +207,7 @@ func TestProcessService_EdgeCases(t *testing.T) {
 
 // TestProcessServiceWithMocks tests the processService function using mocks.
 func TestProcessServiceWithMocks(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	awsCfg := aws.Config{Region: "us-east-1"}
 
@@ -1258,6 +1259,7 @@ func TestWriteMultiServiceCSVReport_MultipleResults(t *testing.T) {
 // ==================== Additional ProcessPurchaseLoop Tests ====================
 
 func TestProcessPurchaseLoopPurchaseFailure(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	origCfg := toolCfg
 	defer func() { toolCfg = origCfg }()
@@ -1308,6 +1310,7 @@ func TestProcessPurchaseLoopEmptyRecommendations(t *testing.T) {
 }
 
 func TestProcessServicePurchasesUserCancellation(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	origCfg := toolCfg
 	defer func() { toolCfg = origCfg }()
@@ -1340,6 +1343,7 @@ func TestProcessServicePurchasesUserCancellation(t *testing.T) {
 }
 
 func TestProcessServicePurchasesDryRunMultiple(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	origCfg := toolCfg
 	defer func() { toolCfg = origCfg }()
@@ -1412,6 +1416,7 @@ func TestExecutePurchaseWithEmptyPurchaseID(t *testing.T) {
 }
 
 func TestProcessPurchaseLoopDryRun(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	// Save original values
 	origCfg := toolCfg
@@ -1447,6 +1452,7 @@ func TestProcessPurchaseLoopDryRun(t *testing.T) {
 }
 
 func TestProcessPurchaseLoopActualPurchase(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	// Save original values
 	origCfg := toolCfg
@@ -1492,6 +1498,7 @@ func TestProcessPurchaseLoopActualPurchase(t *testing.T) {
 }
 
 func TestProcessPurchaseLoopWithConfirmation(t *testing.T) {
+	useCallerAccount(t)
 	ctx := context.Background()
 	// Save original values
 	origCfg := toolCfg
@@ -1693,8 +1700,8 @@ func TestRunToolFromCSV(t *testing.T) {
 	// silently ignored, so every row loaded with Count=0 and the whole run
 	// processed nothing while the NotPanics assertion stayed green.
 	csvPath := writeTestRecommendationsCSV(t, `Service,Region,ResourceType,Engine,Count,Term,PaymentOption,Account
-rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
-elasticache,us-west-2,cache.t3.micro,redis,1,1yr,All Upfront,123456789012
+rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,111111111111
+elasticache,us-west-2,cache.t3.micro,redis,1,1yr,All Upfront,111111111111
 `)
 
 	tests := []struct {
@@ -1761,7 +1768,7 @@ func TestRunToolFromCSV_DuplicateCheckFailureRefusesToPurchase(t *testing.T) {
 	isolateAWSEnv(t)
 
 	csvPath := writeTestRecommendationsCSV(t, `Service,Region,ResourceType,Engine,Count,Term,PaymentOption,Account
-rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
+rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,111111111111
 `)
 	reportPath := filepath.Join(t.TempDir(), "report.csv")
 	auditPath := filepath.Join(t.TempDir(), "audit.jsonl")
@@ -1910,7 +1917,7 @@ func TestRunToolFromCSV_WritesAuditRecordsOnDryRun(t *testing.T) {
 	isolateAWSEnv(t)
 
 	csvPath := writeTestRecommendationsCSV(t, `Service,Region,ResourceType,Engine,Count,Term,PaymentOption,Account
-rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
+rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,111111111111
 `)
 	auditPath := filepath.Join(t.TempDir(), "audit.jsonl")
 
@@ -1952,6 +1959,7 @@ rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
 // TestProcessPurchaseLoopActualPurchase already uses to test this loop's
 // purchase behavior without live AWS credentials.
 func TestProcessPurchaseLoop_WritesAuditRecordForRealPurchase(t *testing.T) {
+	useCallerAccount(t)
 	tests := []struct {
 		result     common.PurchaseResult
 		name       string
@@ -2017,7 +2025,7 @@ func TestRunToolFromCSV_ChecksAuditLogWritability(t *testing.T) {
 	defer func() { toolCfg = origCfg }()
 
 	csvPath := writeTestRecommendationsCSV(t, `Service,Region,ResourceType,Engine,Count,Term,PaymentOption,Account
-rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
+rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,111111111111
 `)
 
 	toolCfg.CSVInput = csvPath
@@ -2052,7 +2060,7 @@ func TestRunToolFromCSV_ConfirmsOnceBeforeAnyRegion(t *testing.T) {
 	t.Cleanup(func() { log.SetOutput(oldWriter) })
 
 	csvPath := writeTestRecommendationsCSV(t, `Service,Region,ResourceType,Engine,Count,Term,PaymentOption,Account
-rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,123456789012
+rds,us-east-1,db.t3.small,postgres,2,1yr,All Upfront,111111111111
 rds,us-west-2,db.t3.medium,postgres,3,1yr,All Upfront,123456789012
 `)
 	reportPath := filepath.Join(t.TempDir(), "report.csv")
diff --git a/cmd/purchase_account_guard_test.go b/cmd/purchase_account_guard_test.go
new file mode 100644
index 000000000..b9dbb51b7
--- /dev/null
+++ b/cmd/purchase_account_guard_test.go
@@ -0,0 +1,92 @@
+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 = "444444444444"
+	otherAccount      = "555555555555"
+)
+
+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)
+
+	rec.Account = testCallerAccount
+	require.Error(t, checkPurchaseAccount(rec, ""), "an unresolved caller account must not disable the check")
+}
+
+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 "444444444444", nil }
+	cfg, err := withCallerAccount(context.Background(), aws.Config{}, Config{})
+	require.NoError(t, err)
+	assert.Equal(t, "444444444444", 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/purchase_confirmation_test.go b/cmd/purchase_confirmation_test.go
index d0c5062a6..079adfed1 100644
--- a/cmd/purchase_confirmation_test.go
+++ b/cmd/purchase_confirmation_test.go
@@ -72,7 +72,7 @@ func TestRunPurchaseAndReportRequiresTerminal(t *testing.T) {
 		for _, dryRun := range []bool{false, true} {
 			t.Run(fmt.Sprintf("%s/dryRun=%t", service, dryRun), func(t *testing.T) {
 				dir := t.TempDir()
-				cfg := Config{AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
+				cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(dir, "audit.jsonl"), CSVOutput: filepath.Join(dir, "report.csv")}
 				recs := []common.Recommendation{
 					{Service: service, Region: "us-east-1", ResourceType: "test-instance", Count: 2, EstimatedSavings: 50},
 					{Service: service, Region: "us-west-2", ResourceType: "test-instance", Count: 3, EstimatedSavings: 75},
diff --git a/cmd/purchase_safeguards_2121_test.go b/cmd/purchase_safeguards_2121_test.go
index efe474628..3489b2cec 100644
--- a/cmd/purchase_safeguards_2121_test.go
+++ b/cmd/purchase_safeguards_2121_test.go
@@ -244,14 +244,14 @@ func TestExecutePurchasePipeline_InterruptsFailClosed_2121(t *testing.T) {
 		auditPath := filepath.Join(t.TempDir(), "audit.jsonl")
 		shutdownRequested.Store(true)
 		defer shutdownRequested.Store(false)
-		results, _ := executePurchasePipeline(ctx, aws.Config{}, recs, true, "run-2121-int", Config{AuditLog: auditPath})
+		results, _ := executePurchasePipeline(ctx, aws.Config{}, recs, true, "run-2121-int", Config{CallerAccountID: defaultTestAccount, AuditLog: auditPath})
 		assert.Empty(t, results, "a requested shutdown must stop the pipeline before the first purchase")
 		_, err := os.Stat(auditPath)
 		assert.True(t, os.IsNotExist(err), "no audit records without purchase attempts")
 	})
 	t.Run("uninterrupted dry run audits every rec as skipped", func(t *testing.T) {
 		auditPath := filepath.Join(t.TempDir(), "audit.jsonl")
-		results, _ := executePurchasePipeline(ctx, aws.Config{}, recs, true, "run-2121-dry", Config{AuditLog: auditPath})
+		results, _ := executePurchasePipeline(ctx, aws.Config{}, recs, true, "run-2121-dry", Config{CallerAccountID: defaultTestAccount, AuditLog: auditPath})
 		require.Len(t, results, len(recs))
 		for _, result := range results {
 			assert.True(t, result.Success)
diff --git a/cmd/rds_family_expiry_command_test.go b/cmd/rds_family_expiry_command_test.go
index 77ef1d4cc..a82cecddc 100644
--- a/cmd/rds_family_expiry_command_test.go
+++ b/cmd/rds_family_expiry_command_test.go
@@ -256,11 +256,12 @@ func assertRDSExpiryCSV(t *testing.T, output, name, logs string) {
 			continue
 		}
 		require.True(t, present, "scenario=%s rec %d missing; rows=%v\nlogs=%s", name, i, rows, logs)
+		wantSuccess, wantErr := wantPurchaseOutcome(columns["Account"])
 		want := map[string]string{
 			"Service": "rds", "Region": "us-east-1", "ResourceType": rec.class, "Term": "1yr", "PaymentOption": "partial-upfront",
 			"Count": strconv.Itoa(count), "ExistingCoverage": fmt.Sprintf("%.1f", s.existing),
 			"UpfrontPayment": fmt.Sprintf("%.2f", float64(count)*100), "EstimatedSavings": fmt.Sprintf("%.2f", float64(count)*100),
-			"RecurringMonthlyCost": fmt.Sprintf("%.2f", float64(count)*10), "Success": "true", "Error": "",
+			"RecurringMonthlyCost": fmt.Sprintf("%.2f", float64(count)*10), "Success": wantSuccess, "Error": wantErr,
 		}
 		for field, value := range want {
 			require.Equal(t, value, columns[field], "scenario=%s rec=%d field=%s (parent count %d)", name, i, field, s.parent[i])
diff --git a/cmd/reservation_expiry_command_test.go b/cmd/reservation_expiry_command_test.go
index 837475ecc..7ddd009cc 100644
--- a/cmd/reservation_expiry_command_test.go
+++ b/cmd/reservation_expiry_command_test.go
@@ -97,13 +97,14 @@ func assertReservationExpiryCSV(t *testing.T, output, name, logs string) {
 		seen[account] = true
 		index := slices.Index([]string{"111111111111", "222222222222", "333333333333"}[:count], account)
 		require.NotEqual(t, -1, index, "unexpected account %s", account)
+		wantSuccess, wantErr := wantPurchaseOutcome(account)
 		want := map[string]string{
 			"Service": "ec2", "Region": "us-east-1", "ResourceType": "m5.large", "Term": "1yr", "PaymentOption": "partial-upfront",
 			"Count": strconv.Itoa(s.count), "RecommendedCount": "30", "AccountName": fmt.Sprintf("expiry-account-%d", index+1),
 			"Instances": fmt.Sprintf("%.1f", avg), "CoveredInstances": fmt.Sprintf("%.1f", avg*s.existing/100),
 			"ExistingCoverage": fmt.Sprintf("%.1f", s.existing), "ProjectedCoverage": fmt.Sprintf("%.1f", s.existing+float64(s.count)*100/avg),
 			"UpfrontPayment": fmt.Sprintf("%.2f", float64(s.count)*100), "EstimatedSavings": fmt.Sprintf("%.2f", float64(s.count)*100),
-			"RecurringMonthlyCost": fmt.Sprintf("%.2f", float64(s.count)*10), "Success": "true", "Error": "",
+			"RecurringMonthlyCost": fmt.Sprintf("%.2f", float64(s.count)*10), "Success": wantSuccess, "Error": wantErr,
 		}
 		for field, value := range want {
 			require.Equal(t, value, columns[field], "scenario=%s account=%s field=%s", name, account, field)
diff --git a/cmd/savings_plan_purchase.go b/cmd/savings_plan_purchase.go
index 80eab2452..1f795f7d8 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 +114,23 @@ 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. An unresolved caller account refuses too, so the check
+// cannot be skipped. A recommendation with no account (CSV rows may omit it)
+// carries nothing to compare.
+func checkPurchaseAccount(rec common.Recommendation, callerAccount string) error {
+	if callerAccount == "" {
+		return fmt.Errorf("cannot purchase %s %s: the AWS account of the credentials is unknown", rec.Service, rec.ResourceType)
+	}
+	if 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 fcb28beea..3ee9ae850 100644
--- a/cmd/savings_plan_purchase_test.go
+++ b/cmd/savings_plan_purchase_test.go
@@ -169,7 +169,7 @@ func parserShapedSP() common.Recommendation {
 
 func TestPurchaseSingleRec_SavingsPlanFromParserReachesAPIInSPRegion(t *testing.T) {
 	awsCfg, stub := stubAWSConfig(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 
 	result, status := purchaseSingleRec(context.Background(), awsCfg, parserShapedSP(), 1, false, cfg)
 
@@ -183,7 +183,7 @@ func TestPurchaseSingleRec_SavingsPlanFromParserReachesAPIInSPRegion(t *testing.
 
 func TestProcessCSVRegionPurchases_SavingsPlanRowWithEmptyRegionAndProvider(t *testing.T) {
 	awsCfg, stub := stubAWSConfig(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	// The real loader shape: no Provider, no Region (group key "").
 	rec := common.Recommendation{
 		Service: common.ServiceSavingsPlansCompute, Count: 1, Account: "111111111111", Term: "1yr", PaymentOption: "no-upfront",
@@ -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, defaultTestAccount), name)
 	}
 	bad := map[string]common.Recommendation{
 		"sp nil details":   mut(parserShapedSP(), func(r *common.Recommendation) { r.Details = nil }),
@@ -282,12 +282,12 @@ 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, defaultTestAccount), name)
 	}
 }
 
 func TestPurchaseSingleRec_DryRunReportsPreconditionFailure(t *testing.T) {
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	broken := parserShapedSP()
 	broken.Details = nil
 
@@ -303,7 +303,7 @@ func TestPurchaseSingleRec_DryRunReportsPreconditionFailure(t *testing.T) {
 }
 
 func TestProcessPurchaseLoop_DryRunReportsPreconditionFailureOnCSVShapedRows(t *testing.T) {
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	noDetailsSP := common.Recommendation{Service: common.ServiceSavingsPlansCompute, Count: 1}
 	csvEC2 := validEC2Rec()
 	csvEC2.Details = &common.ComputeDetails{InstanceType: "m5.large", Platform: "Linux/UNIX"} // loader leaves Tenancy/Scope blank
@@ -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, defaultTestAccount), "%s", r.Service)
 	}
 }
 
@@ -384,7 +384,7 @@ func TestCSVReader_MalformedHourlyCommitmentRejected(t *testing.T) {
 
 func TestProcessCSVRegionPurchases_SavingsPlanRowFromWrittenCSV(t *testing.T) {
 	awsCfg, stub := stubAWSConfig(t)
-	cfg := Config{AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
+	cfg := Config{CallerAccountID: defaultTestAccount, AuditLog: filepath.Join(t.TempDir(), "audit.jsonl")}
 	path := filepath.Join(t.TempDir(), "report.csv")
 	sp := parserShapedSP()
 	require.NoError(t, writeMultiServiceCSVReport([]common.PurchaseResult{{Recommendation: sp, Timestamp: time.Now()}}, path))
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