diff --git a/cmd/grounds/commands/cluster/cluster.go b/cmd/grounds/commands/cluster/cluster.go
index 36df37c..34cd4bf 100644
--- a/cmd/grounds/commands/cluster/cluster.go
+++ b/cmd/grounds/commands/cluster/cluster.go
@@ -3,6 +3,7 @@ package cluster
 import (
 	"context"
 	"net/http"
+	"os"
 	"time"
 
 	"github.com/spf13/cobra"
@@ -19,8 +20,9 @@ func NewClusterCommand() *cobra.Command {
 }
 
 // buildClient is the shared helper every cluster subcommand uses to
-// resolve config + auth + API client.
-func buildClient(ctx context.Context) (*api.Client, *config.Config, error) {
+// resolve config + auth + API client. The cobra command is passed in
+// so we can read the global --project flag.
+func buildClient(_ context.Context, cmd *cobra.Command) (*api.Client, *config.Config, error) {
 	cfg, err := config.Load("")
 	if err != nil {
 		return nil, nil, err
@@ -32,7 +34,28 @@ func buildClient(ctx context.Context) (*api.Client, *config.Config, error) {
 			Device: defaultDeviceClient(),
 		}
 	}
-	return api.New(cfg.APIURL, ts), cfg, nil
+	c := api.New(cfg.APIURL, ts)
+	c.ProjectID = resolveProjectID(cmd)
+	return c, cfg, nil
+}
+
+// resolveProjectID picks --project, falling back to the GROUNDS_PROJECT
+// env var. Empty string when neither is set, in which case forge falls
+// back to the caller's default project.
+func resolveProjectID(cmd *cobra.Command) string {
+	if cmd != nil {
+		if p, _ := cmd.Flags().GetString("project"); p != "" {
+			return p
+		}
+	}
+	return envOr("GROUNDS_PROJECT", "")
+}
+
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
 }
 
 // defaultDeviceClient mirrors login.go (same issuer, same client ID).
diff --git a/cmd/grounds/commands/cluster/delete.go b/cmd/grounds/commands/cluster/delete.go
index 4858afc..9bbb932 100644
--- a/cmd/grounds/commands/cluster/delete.go
+++ b/cmd/grounds/commands/cluster/delete.go
@@ -19,7 +19,7 @@ func newDelete() *cobra.Command {
 		Short: "Permanently delete the workspace and all its data",
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			ctx := context.Background()
-			c, _, err := buildClient(ctx)
+			c, _, err := buildClient(ctx, cmd)
 			if err != nil {
 				return err
 			}
diff --git a/cmd/grounds/commands/cluster/down.go b/cmd/grounds/commands/cluster/down.go
index f20663d..316fa41 100644
--- a/cmd/grounds/commands/cluster/down.go
+++ b/cmd/grounds/commands/cluster/down.go
@@ -15,7 +15,7 @@ func newDown() *cobra.Command {
 		Short: "Pause the workspace immediately",
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			ctx := context.Background()
-			c, _, err := buildClient(ctx)
+			c, _, err := buildClient(ctx, cmd)
 			if err != nil {
 				return err
 			}
diff --git a/cmd/grounds/commands/cluster/status.go b/cmd/grounds/commands/cluster/status.go
index 6b25ebf..946b804 100644
--- a/cmd/grounds/commands/cluster/status.go
+++ b/cmd/grounds/commands/cluster/status.go
@@ -16,7 +16,7 @@ func newStatus() *cobra.Command {
 		Short: "Show workspace state, deployments, quota",
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			ctx := context.Background()
-			c, _, err := buildClient(ctx)
+			c, _, err := buildClient(ctx, cmd)
 			if err != nil {
 				return err
 			}
diff --git a/cmd/grounds/commands/cluster/up.go b/cmd/grounds/commands/cluster/up.go
index 4b25156..2ae255d 100644
--- a/cmd/grounds/commands/cluster/up.go
+++ b/cmd/grounds/commands/cluster/up.go
@@ -15,7 +15,7 @@ func newUp() *cobra.Command {
 		Short: "Resume the paused workspace",
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			ctx := context.Background()
-			c, _, err := buildClient(ctx)
+			c, _, err := buildClient(ctx, cmd)
 			if err != nil {
 				return err
 			}
diff --git a/cmd/grounds/commands/push/list.go b/cmd/grounds/commands/push/list.go
index d3e20ca..0b797a3 100644
--- a/cmd/grounds/commands/push/list.go
+++ b/cmd/grounds/commands/push/list.go
@@ -29,6 +29,7 @@ func newList() *cobra.Command {
 				ts = &auth.FileTokenSource{Store: auth.NewStore(cfg.Dir), Device: defaultDevice()}
 			}
 			c := api.New(cfg.APIURL, ts)
+			c.ProjectID = projectIDFrom(cmd)
 			list, err := c.ListPushes(ctx, mine, limit)
 			if err != nil {
 				return err
diff --git a/cmd/grounds/commands/push/push.go b/cmd/grounds/commands/push/push.go
index 6e435ba..eccd65b 100644
--- a/cmd/grounds/commands/push/push.go
+++ b/cmd/grounds/commands/push/push.go
@@ -42,6 +42,18 @@ func newPush() *cobra.Command {
 	return cmd
 }
 
+// projectIDFrom resolves the global --project flag, falling back to
+// the GROUNDS_PROJECT env var. Empty string when neither is set —
+// forge then uses the caller's default project.
+func projectIDFrom(cmd *cobra.Command) string {
+	if cmd != nil {
+		if p, _ := cmd.Flags().GetString("project"); p != "" {
+			return p
+		}
+	}
+	return os.Getenv("GROUNDS_PROJECT")
+}
+
 // defaultDevice mirrors login.go (same issuer, same client ID).
 // Lifted here so subcommands don't depend on the commands package.
 func defaultDevice() *auth.DeviceClient {
diff --git a/cmd/grounds/commands/push/retry.go b/cmd/grounds/commands/push/retry.go
index 45410a3..3910229 100644
--- a/cmd/grounds/commands/push/retry.go
+++ b/cmd/grounds/commands/push/retry.go
@@ -31,6 +31,7 @@ func newRetry() *cobra.Command {
 				ts = &auth.FileTokenSource{Store: auth.NewStore(cfg.Dir), Device: defaultDevice()}
 			}
 			c := api.New(cfg.APIURL, ts)
+			c.ProjectID = projectIDFrom(cmd)
 			p, err := c.RetryPush(ctx, args[0])
 			if err != nil {
 				return err
diff --git a/cmd/grounds/commands/root.go b/cmd/grounds/commands/root.go
index 2471fec..cc0e7fb 100644
--- a/cmd/grounds/commands/root.go
+++ b/cmd/grounds/commands/root.go
@@ -14,5 +14,6 @@ func NewRootCommand() *cobra.Command {
 	cmd.PersistentFlags().BoolP("verbose", "v", false, "debug logging to stderr")
 	cmd.PersistentFlags().Bool("no-color", false, "disable colour output")
 	cmd.PersistentFlags().String("config", "", "alternative config directory (also GROUNDS_CONFIG_DIR)")
+	cmd.PersistentFlags().String("project", "", "project id to scope this call (also GROUNDS_PROJECT)")
 	return cmd
 }
diff --git a/internal/api/client.go b/internal/api/client.go
index 3b34378..e88e09f 100644
--- a/internal/api/client.go
+++ b/internal/api/client.go
@@ -7,6 +7,8 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
+	"strings"
 
 	"github.com/groundsgg/grounds-cli/internal/auth"
 )
@@ -15,6 +17,20 @@ type Client struct {
 	BaseURL string
 	HTTP    *http.Client
 	Tokens  TokenSource
+	// ProjectID, when set, is appended as `?projectId=...` to every
+	// project-scoped request. Driven by the global `--project` flag /
+	// GROUNDS_PROJECT env var. Empty string → forge falls back to the
+	// caller's default project (their auto-created Personal one).
+	ProjectID string
+}
+
+// WithProject returns a copy of the Client scoped to the given project id.
+// Used by command handlers that resolve --project before each call so the
+// underlying base client can be shared across goroutines.
+func (c *Client) WithProject(id string) *Client {
+	clone := *c
+	clone.ProjectID = id
+	return &clone
 }
 
 // TokenSource produces a fresh bearer token, refreshing on demand. The
@@ -36,7 +52,7 @@ func (c *Client) doRequest(ctx context.Context, method, path string, body any, o
 		}
 		rdr = bytes.NewReader(blob)
 	}
-	req, err := http.NewRequestWithContext(ctx, method, c.BaseURL+path, rdr)
+	req, err := http.NewRequestWithContext(ctx, method, c.BaseURL+c.scopedPath(path), rdr)
 	if err != nil {
 		return err
 	}
@@ -67,6 +83,25 @@ func (c *Client) doRequest(ctx context.Context, method, path string, body any, o
 	return nil
 }
 
+// scopedPath appends `?projectId=...` to the path when the client carries
+// a project id. Idempotent — does nothing when ProjectID is empty (forge
+// falls back to the caller's default project) or when the path already
+// contains a `projectId=` query.
+func (c *Client) scopedPath(path string) string {
+	if c.ProjectID == "" {
+		return path
+	}
+	// already scoped
+	if strings.Contains(path, "projectId=") {
+		return path
+	}
+	sep := "?"
+	if strings.Contains(path, "?") {
+		sep = "&"
+	}
+	return path + sep + "projectId=" + url.QueryEscape(c.ProjectID)
+}
+
 // envTokenSource uses GROUNDS_TOKEN verbatim, no refresh.
 type envTokenSource struct{ token string }
 
diff --git a/internal/api/client_test.go b/internal/api/client_test.go
index 8dfd17a..eaa4f72 100644
--- a/internal/api/client_test.go
+++ b/internal/api/client_test.go
@@ -33,6 +33,58 @@ func TestDoRequest_AuthHeader(t *testing.T) {
 	}
 }
 
+func TestScopedPath(t *testing.T) {
+	cases := []struct {
+		name      string
+		projectID string
+		in        string
+		want      string
+	}{
+		{name: "empty project leaves path untouched", projectID: "", in: "/v1/cluster", want: "/v1/cluster"},
+		{name: "appends as first query param", projectID: "p1", in: "/v1/cluster", want: "/v1/cluster?projectId=p1"},
+		{name: "appends with & when path already has a query", projectID: "p1", in: "/v1/pushes?cursor=x", want: "/v1/pushes?cursor=x&projectId=p1"},
+		{name: "url-escapes the project id", projectID: "with spaces", in: "/v1/cluster", want: "/v1/cluster?projectId=with+spaces"},
+		{name: "no double-append when projectId already present", projectID: "p1", in: "/v1/cluster?projectId=p2", want: "/v1/cluster?projectId=p2"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			c := &Client{ProjectID: tc.projectID}
+			got := c.scopedPath(tc.in)
+			if got != tc.want {
+				t.Errorf("got %q want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestProjectIDPropagatedThroughDoRequest(t *testing.T) {
+	var seenURL string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		seenURL = r.URL.String()
+		w.Write([]byte(`{}`))
+	}))
+	defer srv.Close()
+	c := New(srv.URL, nil)
+	c.ProjectID = "p-test"
+	if _, err := c.GetCluster(context.Background()); err != nil {
+		t.Fatalf("GetCluster: %v", err)
+	}
+	if seenURL != "/v1/cluster?projectId=p-test" {
+		t.Errorf("server saw %q", seenURL)
+	}
+}
+
+func TestWithProjectClonesIndependently(t *testing.T) {
+	base := &Client{BaseURL: "x", ProjectID: ""}
+	scoped := base.WithProject("p1")
+	if base.ProjectID != "" {
+		t.Errorf("WithProject mutated base.ProjectID = %q", base.ProjectID)
+	}
+	if scoped.ProjectID != "p1" {
+		t.Errorf("scoped.ProjectID = %q", scoped.ProjectID)
+	}
+}
+
 func TestDoRequest_ErrorMapping(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusConflict)
diff --git a/internal/api/cluster.go b/internal/api/cluster.go
index a0b790b..d0e989b 100644
--- a/internal/api/cluster.go
+++ b/internal/api/cluster.go
@@ -57,7 +57,7 @@ type ClusterDeleteResult struct {
 
 func (c *Client) ClusterDelete(ctx context.Context, namespace string) (*ClusterDeleteResult, error) {
 	// We can't reuse doRequest because we need a custom header. Inline.
-	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, c.BaseURL+"/v1/cluster", nil)
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, c.BaseURL+c.scopedPath("/v1/cluster"), nil)
 	if err != nil {
 		return nil, err
 	}