feat: Add support for `workspace list -json`

tfexec/internal/testutil/tfcache.go

diff --git a/tfexec/internal/testutil/tfcache.go b/tfexec/internal/testutil/tfcache.go
index 60dacd6..ec5ac21 100644
--- a/tfexec/internal/testutil/tfcache.go
+++ b/tfexec/internal/testutil/tfcache.go
@@ -38,8 +38,10 @@
 	Latest_v1_11       = "1.11.4"
 	Latest_v1_12       = "1.12.2"
 	Latest_Alpha_v1_14 = "1.14.0-alpha20250903"
+	Latest_v1_15       = "1.15.1"
+	Latest_v1_16       = "1.16.0"
 
-	Latest_v1 = "1.14.0"
+	Latest_v1 = "1.16.1"
 )
 
 const appendUserAgent = "tfexec-testutil"

tfexec/version.go

diff --git a/tfexec/version.go b/tfexec/version.go
index aa0926a..751c9b2 100644
--- a/tfexec/version.go
+++ b/tfexec/version.go
@@ -39,6 +39,7 @@
 	tf1_10_0 = version.Must(version.NewVersion("1.10.0"))
 	tf1_13_0 = version.Must(version.NewVersion("1.13.0"))
 	tf1_14_0 = version.Must(version.NewVersion("1.14.0"))
+	tf1_16_0 = version.Must(version.NewVersion("1.16.0"))
 )
 
 // Version returns structured output from the terraform version command including both the Terraform CLI version

tfexec/workspace_list.go

diff --git a/tfexec/workspace_list.go b/tfexec/workspace_list.go
index 6162fd8..a12751c 100644
--- a/tfexec/workspace_list.go
+++ b/tfexec/workspace_list.go
@@ -5,8 +5,12 @@
 
 import (
 	"context"
+	"fmt"
+	"io"
 	"os/exec"
 	"strings"
+
+	tfjson "github.com/hashicorp/terraform-json"
 )
 
 type workspaceListConfig struct {
@@ -79,6 +83,55 @@
 	return workspaces, current
 }
 
+// WorkspaceListJSON represents the terraform workspace list subcommand with the `-json` flag.
+// Using the `-json` flag will result in
+// [machine-readable](https://developer.hashicorp.com/terraform/internals/machine-readable-ui)
+// JSON being written to the supplied `io.Writer`. WorkspaceListJSON is likely to be
+// removed in a future major version in favour of WorkspaceList returning JSON by default.
+func (tf *Terraform) WorkspaceListJSON(ctx context.Context, w io.Writer, opts ...WorkspaceListOption) (*tfjson.WorkspaceListOutput, error) {
+	err := tf.compatible(ctx, tf1_16_0, nil)
+	if err != nil {
+		return nil, fmt.Errorf("terraform workspace list -json was added in 1.16.0: %w", err)
+	}
+
+	tf.SetStdout(w)
+
+	cmd, err := tf.workspaceListJSONCmd(ctx, opts...)
+	if err != nil {
+		return nil, err
+	}
+
+	// Here we need the JSON representation of the workspace list JSON output.
+	var output tfjson.WorkspaceListOutput
+
+	err = tf.runTerraformCmdJSON(ctx, cmd, &output)
+	if err != nil {
+		return nil, err
+	}
+
+	return &output, nil
+}
+
+func (tf *Terraform) workspaceListJSONCmd(ctx context.Context, opts ...WorkspaceListOption) (*exec.Cmd, error) {
+	c := defaultWorkspaceListOptions
+
+	for _, o := range opts {
+		o.configureWorkspaceList(&c)
+	}
+
+	mergeEnv := map[string]string{}
+	if c.reattachInfo != nil {
+		reattachStr, err := c.reattachInfo.marshalString()
+		if err != nil {
+			return nil, err
+		}
+		mergeEnv[reattachEnvVar] = reattachStr
+	}
+
+	args := []string{"workspace", "list", "-json"}
+	return tf.buildWorkspaceListCmd(ctx, c, args)
+}
+
 func (tf *Terraform) buildWorkspaceListCmd(ctx context.Context, c workspaceListConfig, args []string) (*exec.Cmd, error) {
 	mergeEnv := map[string]string{}
 	if c.reattachInfo != nil {

tfexec/workspace_list_test.go

diff --git a/tfexec/workspace_list_test.go b/tfexec/workspace_list_test.go
index 917cea1..9862614 100644
--- a/tfexec/workspace_list_test.go
+++ b/tfexec/workspace_list_test.go
@@ -59,6 +59,53 @@
 	})
 }
 
+func TestWorkspaceListCmdJSON(t *testing.T) {
+	tf, err := NewTerraform(t.TempDir(), tfVersion(t, testutil.Latest_v1))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// empty env, to avoid environ mismatch in testing
+	tf.SetEnv(map[string]string{})
+
+	t.Run("defaults", func(t *testing.T) {
+		workspaceListCmd, err := tf.workspaceListJSONCmd(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		assertCmd(t, []string{
+			"workspace", "list",
+			"-json",
+		}, nil, workspaceListCmd)
+	})
+
+	t.Run("reattach config", func(t *testing.T) {
+		workspaceListCmd, err := tf.workspaceListJSONCmd(context.Background(), Reattach(map[string]ReattachConfig{
+			"registry.terraform.io/hashicorp/examplecloud": {
+				Protocol:        "grpc",
+				ProtocolVersion: 6,
+				Pid:             1234,
+				Test:            true,
+				Addr: ReattachConfigAddr{
+					Network: "unix",
+					String:  "/fake_folder/T/plugin123",
+				},
+			},
+		}))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		assertCmd(t, []string{
+			"workspace", "list",
+			"-json",
+		}, map[string]string{
+			"TF_REATTACH_PROVIDERS": `{"registry.terraform.io/hashicorp/examplecloud":{"Protocol":"grpc","ProtocolVersion":6,"Pid":1234,"Test":true,"Addr":{"Network":"unix","String":"/fake_folder/T/plugin123"}}}`,
+		}, workspaceListCmd)
+	})
+}
+
 func TestParseWorkspaceListHumanOutput(t *testing.T) {
 	for i, c := range []struct {
 		expected        []string