diff --git a/docs/coverage/aws/sfn.md b/docs/coverage/aws/sfn.md
index fdca184dc..2cd249989 100644
--- a/docs/coverage/aws/sfn.md
+++ b/docs/coverage/aws/sfn.md
@@ -47,4 +47,4 @@ AWS's `sfn` service · portable interface `driver.SFN` · [AWS index](./README.m
 
 ## Not in scope
 
-_Not documented yet. See the [emulator boundary](../../../README.md) for cloudemu-wide non-goals._
+- `StopExecution` on an execution that has already closed returns 200 with its original `stopDate` and changes nothing. The API reference documents no error for this case, and the behavior is not verified against live AWS.
diff --git a/docs/coverage/nongoals/sfn.md b/docs/coverage/nongoals/sfn.md
new file mode 100644
index 000000000..339ac04fb
--- /dev/null
+++ b/docs/coverage/nongoals/sfn.md
@@ -0,0 +1 @@
+- `StopExecution` on an execution that has already closed returns 200 with its original `stopDate` and changes nothing. The API reference documents no error for this case, and the behavior is not verified against live AWS.
diff --git a/providers/aws/sfn/errors.go b/providers/aws/sfn/errors.go
index 45eb1c1d9..b2246ff77 100644
--- a/providers/aws/sfn/errors.go
+++ b/providers/aws/sfn/errors.go
@@ -99,6 +99,12 @@ func invalidExecutionInput(msg string) error {
 	return &driver.APIError{Exception: driver.ExInvalidExecutionInput, Err: errors.New(errors.InvalidArgument, msg)}
 }
 
+// smTypeNotSupported builds the StateMachineTypeNotSupported error an
+// operation returns for a state machine type it does not support.
+func smTypeNotSupported(msg string) error {
+	return &driver.APIError{Exception: driver.ExStateMachineTypeNotSupported, Err: errors.New(errors.FailedPrecondition, msg)}
+}
+
 // invalidToken builds an InvalidToken-tagged error.
 func invalidToken(format string, args ...any) error {
 	return &driver.APIError{Exception: driver.ExInvalidToken, Err: errors.Newf(errors.InvalidArgument, format, args...)}
diff --git a/providers/aws/sfn/executions.go b/providers/aws/sfn/executions.go
index 1023e2dc6..6ed5f10f8 100644
--- a/providers/aws/sfn/executions.go
+++ b/providers/aws/sfn/executions.go
@@ -218,7 +218,23 @@ func (m *Mock) StartExternal(ctx context.Context, stateMachineARN, input string)
 	return err
 }
 
+// StartSyncExecution runs an EXPRESS workflow synchronously. Real Step
+// Functions does not offer it for STANDARD workflows and rejects them with
+// StateMachineTypeNotSupported.
 func (m *Mock) StartSyncExecution(ctx context.Context, in driver.StartExecutionInput) (*driver.Execution, error) {
+	sd, err := m.getSM(in.StateMachineArn)
+	if err != nil {
+		return nil, err
+	}
+
+	sd.mu.RLock()
+	smType := sd.sm.Type
+	sd.mu.RUnlock()
+
+	if smType != driver.TypeExpress {
+		return nil, smTypeNotSupported("This operation is not supported by this type of state machine")
+	}
+
 	return m.runExecution(ctx, in, false)
 }
 
@@ -310,6 +326,10 @@ func (m *Mock) abortExecution(ed *execData, errCode, cause string) (time.Time, *
 
 	now := m.now()
 
+	// Pinned: StopExecution on an already-closed execution returns 200 with the
+	// stored stop date and changes nothing. The API reference documents no
+	// error for this case and the behavior is not verified against live AWS.
+	//
 	// While an execution is still settling (observably RUNNING under AsyncSettle),
 	// Stop aborts it: ABORTED with a stop date, no output, the caller's error/cause
 	// persisted, and the history trimmed to the events already visible plus a
diff --git a/providers/aws/sfn/interpreter_test.go b/providers/aws/sfn/interpreter_test.go
index f20bf0a6d..769c8d367 100644
--- a/providers/aws/sfn/interpreter_test.go
+++ b/providers/aws/sfn/interpreter_test.go
@@ -11,12 +11,27 @@ import (
 	"github.com/stackshy/cloudemu/v2/services/sfn/driver"
 )
 
-// createDef registers a state machine with the given verbatim ASL definition.
+// createDef registers a STANDARD state machine with the given verbatim ASL
+// definition.
 func createDef(t *testing.T, m *sfn.Mock, name, def string) string {
 	t.Helper()
 
+	return createTypedDef(t, m, name, def, driver.TypeStandard)
+}
+
+// createExpressDef registers an EXPRESS state machine, the only type
+// StartSyncExecution accepts.
+func createExpressDef(t *testing.T, m *sfn.Mock, name, def string) string {
+	t.Helper()
+
+	return createTypedDef(t, m, name, def, driver.TypeExpress)
+}
+
+func createTypedDef(t *testing.T, m *sfn.Mock, name, def, smType string) string {
+	t.Helper()
+
 	arn, _, _, err := m.CreateStateMachine(context.Background(), driver.CreateStateMachineInput{
-		Name: name, Definition: def, RoleArn: "arn:aws:iam::000000000000:role/r",
+		Name: name, Definition: def, RoleArn: "arn:aws:iam::000000000000:role/r", Type: smType,
 	})
 	if err != nil {
 		t.Fatalf("CreateStateMachine(%s): %v", name, err)
@@ -25,11 +40,11 @@ func createDef(t *testing.T, m *sfn.Mock, name, def string) string {
 	return arn
 }
 
-// runSync creates a machine and runs one synchronous (instant) execution.
+// runSync creates an EXPRESS machine and runs one synchronous execution.
 func runSync(t *testing.T, m *sfn.Mock, name, def, input string) *driver.Execution {
 	t.Helper()
 
-	arn := createDef(t, m, name, def)
+	arn := createExpressDef(t, m, name, def)
 
 	exec, err := m.StartSyncExecution(context.Background(), driver.StartExecutionInput{
 		StateMachineArn: arn, Name: "run", Input: input,
diff --git a/providers/aws/sfn/parallel_map_test.go b/providers/aws/sfn/parallel_map_test.go
index d42147f57..0a0537821 100644
--- a/providers/aws/sfn/parallel_map_test.go
+++ b/providers/aws/sfn/parallel_map_test.go
@@ -171,7 +171,7 @@ func TestParallelTaskLambdaRecursionBounded(t *testing.T) {
 
 	def := `{"StartAt":"P","States":{"P":{"Type":"Parallel","End":true,
 		"Branches":[{"StartAt":"T","States":{"T":{"Type":"Task","Resource":"` + lambdaFuncARN + `","End":true}}}]}}}`
-	arn := createDef(t, m, "par-recurse", def)
+	arn := createExpressDef(t, m, "par-recurse", def)
 
 	maxDepth := 0
 	m.SetLambdaSyncInvoker(fakeLambda{fn: func(ctx context.Context, _ string, payload []byte) ([]byte, string, error) {
diff --git a/providers/aws/sfn/sfn_test.go b/providers/aws/sfn/sfn_test.go
index 058a386c2..a4bb031b9 100644
--- a/providers/aws/sfn/sfn_test.go
+++ b/providers/aws/sfn/sfn_test.go
@@ -641,9 +641,10 @@ func TestAsyncSettleSyncExecutionAndHistory(t *testing.T) {
 		config.WithAccountID("000000000000"), config.WithAsyncSettle()))
 	ctx := context.Background()
 	arn := createSM(t, m, "sm")
+	expressArn := createExpressDef(t, m, "sm-express", definition)
 
 	// Synchronous execution returns terminal SUCCEEDED with output, not RUNNING.
-	sync, err := m.StartSyncExecution(ctx, driver.StartExecutionInput{StateMachineArn: arn, Name: "sync1", Input: `{"k":1}`})
+	sync, err := m.StartSyncExecution(ctx, driver.StartExecutionInput{StateMachineArn: expressArn, Name: "sync1", Input: `{"k":1}`})
 	if err != nil {
 		t.Fatalf("StartSyncExecution: %v", err)
 	}
diff --git a/providers/aws/sfn/start_sync_execution_test.go b/providers/aws/sfn/start_sync_execution_test.go
new file mode 100644
index 000000000..b70c3bd15
--- /dev/null
+++ b/providers/aws/sfn/start_sync_execution_test.go
@@ -0,0 +1,93 @@
+package sfn_test
+
+import (
+	"context"
+	stderrors "errors"
+	"testing"
+
+	"github.com/stackshy/cloudemu/v2/services/sfn/driver"
+)
+
+// TestStartSyncExecutionRejectsStandard pins that StartSyncExecution is only
+// offered for EXPRESS workflows: a STANDARD state machine is rejected with
+// StateMachineTypeNotSupported and no execution is recorded.
+func TestStartSyncExecutionRejectsStandard(t *testing.T) {
+	m := newMock(t)
+	ctx := context.Background()
+	arn := createSM(t, m, "std")
+
+	_, err := m.StartSyncExecution(ctx, driver.StartExecutionInput{StateMachineArn: arn, Name: "s1"})
+
+	var apiErr *driver.APIError
+	if !stderrors.As(err, &apiErr) || apiErr.Exception != driver.ExStateMachineTypeNotSupported {
+		t.Fatalf("StartSyncExecution(STANDARD) = %v, want StateMachineTypeNotSupported", err)
+	}
+
+	execs, err := m.ListExecutions(ctx, arn, "")
+	if err != nil || len(execs) != 0 {
+		t.Fatalf("ListExecutions = %d, %v; want no execution recorded", len(execs), err)
+	}
+}
+
+// TestStartSyncExecutionExpress runs an EXPRESS workflow to completion.
+func TestStartSyncExecutionExpress(t *testing.T) {
+	m := newMock(t)
+	arn := createExpressDef(t, m, "exp", definition)
+
+	exec, err := m.StartSyncExecution(context.Background(),
+		driver.StartExecutionInput{StateMachineArn: arn, Name: "s1", Input: `{"a":1}`})
+	if err != nil {
+		t.Fatalf("StartSyncExecution(EXPRESS): %v", err)
+	}
+
+	if exec.Status != driver.ExecStatusSucceeded || exec.Output != `{"a":1}` {
+		t.Fatalf("execution = %s %q, want SUCCEEDED {\"a\":1}", exec.Status, exec.Output)
+	}
+}
+
+// TestStartSyncExecutionMissingStateMachine checks existence before type.
+func TestStartSyncExecutionMissingStateMachine(t *testing.T) {
+	m := newMock(t)
+
+	_, err := m.StartSyncExecution(context.Background(), driver.StartExecutionInput{
+		StateMachineArn: "arn:aws:states:us-east-1:000000000000:stateMachine:missing",
+	})
+
+	var apiErr *driver.APIError
+	if !stderrors.As(err, &apiErr) || apiErr.Exception != driver.ExStateMachineDoesNotExist {
+		t.Fatalf("StartSyncExecution(missing) = %v, want StateMachineDoesNotExist", err)
+	}
+}
+
+// TestStopExecutionTerminalKeepsStopDate pins StopExecution on a closed
+// execution: 200 with the original stop date and no state change. The API
+// reference documents no error for this case, and the behavior is unverified
+// against live AWS.
+func TestStopExecutionTerminalKeepsStopDate(t *testing.T) {
+	m := newMock(t)
+	ctx := context.Background()
+	arn := createSM(t, m, "stop-closed")
+
+	exec, err := m.StartExecution(ctx, driver.StartExecutionInput{StateMachineArn: arn, Name: "done"})
+	if err != nil || exec.Status != driver.ExecStatusSucceeded {
+		t.Fatalf("StartExecution = %v, %v; want SUCCEEDED", exec, err)
+	}
+
+	stopDate, err := m.StopExecution(ctx, exec.ARN, "Halt", "late")
+	if err != nil {
+		t.Fatalf("StopExecution(closed): %v", err)
+	}
+
+	if !stopDate.Equal(exec.StopDate) {
+		t.Fatalf("stopDate = %v, want the original %v", stopDate, exec.StopDate)
+	}
+
+	got, err := m.DescribeExecution(ctx, exec.ARN)
+	if err != nil {
+		t.Fatalf("DescribeExecution: %v", err)
+	}
+
+	if got.Status != driver.ExecStatusSucceeded || got.Error != "" || got.Cause != "" {
+		t.Fatalf("after stop = %s %q/%q, want unchanged SUCCEEDED", got.Status, got.Error, got.Cause)
+	}
+}
diff --git a/providers/aws/sfn/task_test.go b/providers/aws/sfn/task_test.go
index 6e23596d1..e0eb8bede 100644
--- a/providers/aws/sfn/task_test.go
+++ b/providers/aws/sfn/task_test.go
@@ -281,7 +281,7 @@ func TestTaskRecursionGuardTerminates(t *testing.T) {
 	m := newMock(t)
 
 	def := `{"StartAt":"T","States":{"T":{"Type":"Task","Resource":"` + lambdaFuncARN + `","End":true}}}`
-	arn := createDef(t, m, "recurse", def)
+	arn := createExpressDef(t, m, "recurse", def)
 
 	maxDepth := 0
 	m.SetLambdaSyncInvoker(fakeLambda{fn: func(ctx context.Context, _ string, payload []byte) ([]byte, string, error) {
diff --git a/server/aws/sfn/start_sync_execution_test.go b/server/aws/sfn/start_sync_execution_test.go
new file mode 100644
index 000000000..c7bb3f832
--- /dev/null
+++ b/server/aws/sfn/start_sync_execution_test.go
@@ -0,0 +1,63 @@
+package sfn_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	awssfn "github.com/aws/aws-sdk-go-v2/service/sfn"
+	sfntypes "github.com/aws/aws-sdk-go-v2/service/sfn/types"
+	"github.com/aws/smithy-go"
+)
+
+// syncOpts lets StartSyncExecution reach the local server (it uses the
+// "sync-" endpoint host prefix).
+func syncOpts(o *awssfn.Options) { o.APIOptions = append(o.APIOptions, disableHostPrefix) }
+
+// TestSDKStartSyncExecutionStandardTypeNotSupported pins the wire error a real
+// SDK sees for StartSyncExecution on a STANDARD state machine.
+func TestSDKStartSyncExecutionStandardTypeNotSupported(t *testing.T) {
+	ctx := context.Background()
+	c := newSFNClient(t)
+	arn := createSM(t, c, "std-sync")
+
+	_, err := c.StartSyncExecution(ctx, &awssfn.StartSyncExecutionInput{StateMachineArn: aws.String(arn)}, syncOpts)
+
+	var notSupported *sfntypes.StateMachineTypeNotSupported
+	if !errors.As(err, &notSupported) {
+		var apiErr smithy.APIError
+		if errors.As(err, &apiErr) {
+			t.Fatalf("want StateMachineTypeNotSupported, got %q", apiErr.ErrorCode())
+		}
+
+		t.Fatalf("want StateMachineTypeNotSupported, got %v", err)
+	}
+}
+
+// TestSDKStartSyncExecutionExpress runs an EXPRESS workflow over the wire.
+func TestSDKStartSyncExecutionExpress(t *testing.T) {
+	ctx := context.Background()
+	c := newSFNClient(t)
+
+	created, err := c.CreateStateMachine(ctx, &awssfn.CreateStateMachineInput{
+		Name:       aws.String("exp-sync"),
+		Definition: aws.String(definition),
+		RoleArn:    aws.String("arn:aws:iam::123456789012:role/svc"),
+		Type:       sfntypes.StateMachineTypeExpress,
+	})
+	if err != nil {
+		t.Fatalf("CreateStateMachine: %v", err)
+	}
+
+	out, err := c.StartSyncExecution(ctx, &awssfn.StartSyncExecutionInput{
+		StateMachineArn: created.StateMachineArn, Input: aws.String(`{"k":1}`),
+	}, syncOpts)
+	if err != nil {
+		t.Fatalf("StartSyncExecution(EXPRESS): %v", err)
+	}
+
+	if out.Status != sfntypes.SyncExecutionStatusSucceeded || aws.ToString(out.Output) != `{"k":1}` {
+		t.Fatalf("got %s %q, want SUCCEEDED {\"k\":1}", out.Status, aws.ToString(out.Output))
+	}
+}
diff --git a/services/sfn/driver/errors.go b/services/sfn/driver/errors.go
index 99f309e8b..1d93169bb 100644
--- a/services/sfn/driver/errors.go
+++ b/services/sfn/driver/errors.go
@@ -22,6 +22,9 @@ const (
 	ExConflict                  = "ConflictException"
 	ExValidation                = "ValidationException"
 	ExMissingRequiredParameter  = "MissingRequiredParameter"
+	// ExStateMachineTypeNotSupported is returned when an operation does not
+	// support the state machine's type, e.g. StartSyncExecution on STANDARD.
+	ExStateMachineTypeNotSupported = "StateMachineTypeNotSupported"
 )
 
 // APIError tags a canonical cloudemu error with the SFN exception name it