feat(cli): enrich user-agent header for client requests (#21483)

Adds the following information to CLI User-Agent headers to aid
deployment administrators in troubleshooting where requests are coming
from.

Before: `Go-http-client/1.1`
After: `coder-cli/v2.34.5 (linux/amd64; coder whoami)`

🤖 These changes were generated by Claude Sonnet 4.5 but reviewed and
edited manually by me.

cli/root.go

@@ -684,6 +684,7 @@ func (r *RootCmd) HeaderTransport(ctx context.Context, serverURL *url.URL) (*cod
 func (r *RootCmd) createHTTPClient(ctx context.Context, serverURL *url.URL, inv *serpent.Invocation) (*http.Client, error) {
 	transport := http.DefaultTransport
 	transport = wrapTransportWithTelemetryHeader(transport, inv)
+	transport = wrapTransportWithUserAgentHeader(transport, inv)
 	if !r.noVersionCheck {
 		transport = wrapTransportWithVersionMismatchCheck(transport, inv, buildinfo.Version(), func(ctx context.Context) (codersdk.BuildInfoResponse, error) {
 			// Create a new client without any wrapped transport
@@ -1497,6 +1498,22 @@ func wrapTransportWithTelemetryHeader(transport http.RoundTripper, inv *serpent.
 	})
 }
 
+// wrapTransportWithUserAgentHeader sets a User-Agent header for all CLI requests
+// that includes the CLI version, os/arch, and the specific command being run.
+func wrapTransportWithUserAgentHeader(transport http.RoundTripper, inv *serpent.Invocation) http.RoundTripper {
+	var (
+		userAgent string
+		once      sync.Once
+	)
+	return roundTripper(func(req *http.Request) (*http.Response, error) {
+		once.Do(func() {
+			userAgent = fmt.Sprintf("coder-cli/%s (%s/%s; %s)", buildinfo.Version(), runtime.GOOS, runtime.GOARCH, inv.Command.FullName())
+		})
+		req.Header.Set("User-Agent", userAgent)
+		return transport.RoundTrip(req)
+	})
+}
+
 type roundTripper func(req *http.Request) (*http.Response, error)
 
 func (r roundTripper) RoundTrip(req *http.Request) (*http.Response, error) {

cli/root_test.go

@@ -380,3 +380,59 @@ func agentClientCommand(clientRef **agentsdk.Client) *serpent.Command {
 	agentAuth.AttachOptions(cmd, false)
 	return cmd
 }
+
+func TestWrapTransportWithUserAgentHeader(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name                    string
+		cmdArgs                 []string
+		cmdEnv                  map[string]string
+		expectedUserAgentHeader string
+	}{
+		{
+			name:                    "top-level command",
+			cmdArgs:                 []string{"login"},
+			expectedUserAgentHeader: fmt.Sprintf("coder-cli/%s (%s/%s; coder login)", buildinfo.Version(), runtime.GOOS, runtime.GOARCH),
+		},
+		{
+			name:                    "nested commands",
+			cmdArgs:                 []string{"templates", "list"},
+			expectedUserAgentHeader: fmt.Sprintf("coder-cli/%s (%s/%s; coder templates list)", buildinfo.Version(), runtime.GOOS, runtime.GOARCH),
+		},
+		{
+			name:                    "does not include positional args, flags, or env",
+			cmdArgs:                 []string{"templates", "push", "my-template", "-d", "/path/to/template", "--yes", "--var", "myvar=myvalue"},
+			cmdEnv:                  map[string]string{"SECRET_KEY": "secret_value"},
+			expectedUserAgentHeader: fmt.Sprintf("coder-cli/%s (%s/%s; coder templates push)", buildinfo.Version(), runtime.GOOS, runtime.GOARCH),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ch := make(chan string, 1)
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				select {
+				case ch <- r.Header.Get("User-Agent"):
+				default: // already sent
+				}
+			}))
+			t.Cleanup(srv.Close)
+
+			args := append([]string{}, tc.cmdArgs...)
+			inv, _ := clitest.New(t, args...)
+			inv.Environ.Set("CODER_URL", srv.URL)
+			for k, v := range tc.cmdEnv {
+				inv.Environ.Set(k, v)
+			}
+
+			ctx := testutil.Context(t, testutil.WaitShort)
+			_ = inv.WithContext(ctx).Run() // Ignore error as we only care about headers.
+
+			actual := testutil.RequireReceive(ctx, t, ch)
+			require.Equal(t, tc.expectedUserAgentHeader, actual, "User-Agent should match expected format exactly")
+		})
+	}
+}