diff --git a/DESCRIPTION b/DESCRIPTION
index 2b14576..d5a6797 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -20,6 +20,7 @@ Imports:
     terra,
     tibble
 Suggests: 
-    testthat (>= 3.0.0)
+    testthat (>= 3.0.0),
+    withr
 Config/testthat/edition: 3
 LazyData: true
diff --git a/R/check_type_and_length.R b/R/check_type_and_length.R
new file mode 100644
index 0000000..2bec7df
--- /dev/null
+++ b/R/check_type_and_length.R
@@ -0,0 +1,51 @@
+check_type_and_length <- function(...,
+                                  call = rlang::caller_env(),
+                                  env = rlang::caller_env()) {
+  dots <- list(...)
+  if (length(dots) == 0) {
+    return(invisible(TRUE))
+  }
+
+  if (is.null(names(dots)) || any(names(dots) == "")) {
+    rlang::abort("All arguments to `check_type_and_length()` must be named.")
+  }
+
+  problem_args <- character()
+  for (dot in names(dots)) {
+    arg <- get(dot, envir = env)
+    if (is.null(arg)) next
+
+    arg_class <- class(arg)
+    dot_class <- class(dots[[dot]])
+    if (arg_class != dot_class) {
+      if ("integer" %in% arg_class && "numeric" %in% dot_class) {
+        next # Purposefully doing nothing -- rely on implicit conversion
+      } else if ("integer" %in% dot_class && rlang::is_integerish(arg)) {
+        next # Purposefully doing nothing -- rely on implicit conversion
+      } else {
+        problem_args <- c(
+          problem_args,
+          glue::glue("{dot} should be a {class(dots[[dot]])}, but is a {class(arg)}.")
+        )
+      }
+    }
+    if (length(dots[[dot]]) && length(arg) != length(dots[[dot]])) {
+      problem_args <- c(
+        problem_args,
+        glue::glue("{dot} should be of length {length(dots[[dot]])}, but is length {length(arg)}.")
+      )
+    }
+  }
+
+  if (length(problem_args)) {
+    names(problem_args) <- rep("*", length(problem_args))
+    rlang::abort(
+      c(
+        "Some input arguments weren't the right class or length:",
+        problem_args
+      ),
+      call = call
+    )
+  }
+  return(invisible(TRUE))
+}
diff --git a/R/get_stac_data.R b/R/get_stac_data.R
index e4d8dcc..b8da1ca 100644
--- a/R/get_stac_data.R
+++ b/R/get_stac_data.R
@@ -1,11 +1,15 @@
-#' Retrieve images (or composite images) from STAC endpoints
+#' Retrieve and composite images from STAC endpoints
 #'
-#' This function retrieves composites of satellite images from STAC endpoints.
+#' These function retrieves composites of satellite images from STAC endpoints.
+#' `get_stac_data()` is a generic function which should be able to download and
+#' composite images from a variety of data sources, while the other helper
+#' functions have useful defaults for downloading common data sets from standard
+#' STAC sources.
 #'
 #' @section Usage Tips:
 #' It's often useful to buffer your `aoi` object slightly, on the order of 1-2
 #' cell widths, in order to ensure that data is downloaded for your entire AOI
-#' even after accounting for any reprojection necessary to compare your AOI to
+#' even after accounting for any reprojection needed to compare your AOI to
 #' the data on the STAC server.
 #'
 #' Setting the following GDAL configuration variables via `Sys.setenv()` or in
@@ -75,17 +79,11 @@
 #' @param mask_function A function that takes a raster and returns a boolean
 #' raster, where `TRUE` pixels will be preserved and `FALSE` or `NA` pixels will
 #' be masked out. See [sentinel2_mask_function()].
-#' @param output_filename The filename to write the output raster to. If
-#' `composite_function` is `NULL`, something will be appended to the end of the
-#' filename (before the extension) to create a unique file for each downloaded
-#' image. Currently this means sequential numbers are added to each file, but
-#' this may change in the future -- do not depend on the resulting filenames
-#' staying the same across versions!
-#' @param composite_function The (quoted) name of a function from
-#' `terra` (for instance, [terra::median]) used to combine downloaded images
-#' into a single composite (i.e., to aggregate pixel values from multiple images
-#' into a single value). Set to `NULL` to not composite images, but instead
-#' save each image as a separate file.
+#' @param output_filename The filename to write the output raster to.
+#' @param composite_function Character of length 1: The (quoted) name of a
+#' function from `terra` (for instance, [terra::median]) used to combine
+#' downloaded images into a single composite (i.e., to aggregate pixel values
+#' from multiple images into a single value).
 #' @inheritParams rstac::stac_search
 #' @param gdalwarp_options Options passed to `gdalwarp` through the `options`
 #' argument of [sf::gdal_utils()]. The same set of options are used for all
@@ -139,6 +137,29 @@ get_stac_data <- function(aoi,
                             "-co", "PREDICTOR=2",
                             "-co", "NUM_THREADS=ALL_CPUS"
                           )) {
+
+  if (!(inherits(aoi, "sf") || inherits(aoi, "sfc"))) {
+    rlang::abort(
+      "`aoi` must be an sf or sfc object.",
+      class = "rsi_aoi_not_sf"
+    )
+  }
+
+  check_type_and_length(
+    start_date = character(1),
+    end_date = character(1),
+    pixel_x_size = numeric(1),
+    pixel_y_size = numeric(1),
+    stac_source = character(1),
+    collection = character(1),
+    rescale_bands = logical(1),
+    mask_band = character(1),
+    output_filename = character(1),
+    composite_function = character(1),
+    limit = numeric(1),
+    gdalwarp_options = character()
+  )
+
   gdalwarp_options <- process_gdalwarp_options(
     gdalwarp_options = gdalwarp_options,
     aoi = aoi,
@@ -248,54 +269,52 @@ get_stac_data <- function(aoi,
     )
   }
 
-  if (!is.null(composite_function)) {
-    download_dir <- file.path(tempdir(), "composite_dir")
-    if (!dir.exists(download_dir)) dir.create(download_dir)
-
-    composited_bands <- vapply(
-      names(downloaded_bands),
-      function(band_name) {
-        out_file <- file.path(download_dir, paste0(toupper(band_name), ".tif"))
-
-        do.call(
-          getFromNamespace(composite_function, "terra"),
-          list(
-            x = terra::rast(downloaded_bands[[band_name]]),
-            na.rm = TRUE,
-            filename = out_file,
-            overwrite = TRUE
-          )
+  download_dir <- file.path(tempdir(), "composite_dir")
+  if (!dir.exists(download_dir)) dir.create(download_dir)
+
+  composited_bands <- vapply(
+    names(downloaded_bands),
+    function(band_name) {
+      out_file <- file.path(download_dir, paste0(toupper(band_name), ".tif"))
+
+      do.call(
+        getFromNamespace(composite_function, "terra"),
+        list(
+          x = terra::rast(downloaded_bands[[band_name]]),
+          na.rm = TRUE,
+          filename = out_file,
+          overwrite = TRUE
         )
+      )
 
-        if (rescale_bands) {
-          out_file <- rescale_band(downloaded_bands, band_name, out_file)
-        }
+      if (rescale_bands) {
+        out_file <- rescale_band(downloaded_bands, band_name, out_file)
+      }
 
-        out_file
-      },
-      character(1)
-    )
-    on.exit(file.remove(composited_bands), add = TRUE)
-
-    out_vrt <- tempfile(fileext = ".vrt")
-    invisible(
-      stack_rasters(
-        composited_bands,
-        out_vrt,
-        band_names = remap_band_names(names(items_urls), asset_names)
-      )
-    )
-    on.exit(file.remove(out_vrt), add = TRUE)
+      out_file
+    },
+    character(1)
+  )
+  on.exit(file.remove(composited_bands), add = TRUE)
 
-    sf::gdal_utils(
-      "warp",
+  out_vrt <- tempfile(fileext = ".vrt")
+  invisible(
+    stack_rasters(
+      composited_bands,
       out_vrt,
-      output_filename,
-      options = gdalwarp_options
+      band_names = remap_band_names(names(items_urls), asset_names)
     )
+  )
+  on.exit(file.remove(out_vrt), add = TRUE)
 
-    output_filename
-  }
+  sf::gdal_utils(
+    "warp",
+    out_vrt,
+    output_filename,
+    options = gdalwarp_options
+  )
+
+  output_filename
 }
 
 #' @rdname get_stac_data
diff --git a/R/spectral_indices.R b/R/spectral_indices.R
index f8bb026..054206f 100644
--- a/R/spectral_indices.R
+++ b/R/spectral_indices.R
@@ -50,7 +50,7 @@ spectral_indices <- function(..., url = spectral_indices_url(), update_cache = N
 
   if (update_cache) {
     tryCatch(
-      download_indices(url),
+      update_cached_indices(url),
       error = function(e) {
         rlang::warn(c(
           "Failed to update the cache of indices.",
@@ -99,7 +99,7 @@ download_indices <- function(url = spectral_indices_url()) {
 
 update_cached_indices <- function(url = spectral_indices_url()) {
   if (!dir.exists(tools::R_user_dir("rsi"))) {
-    dir.create(tools::R_user_dir("rsi"))
+    dir.create(tools::R_user_dir("rsi"), recursive = TRUE)
   }
   indices_path <- file.path(
     tools::R_user_dir("rsi"),
diff --git a/R/stack_rasters.R b/R/stack_rasters.R
index 7a3137f..e9f174c 100644
--- a/R/stack_rasters.R
+++ b/R/stack_rasters.R
@@ -102,31 +102,31 @@ stack_rasters <- function(rasters,
 
   ref_crs <- terra::crs(ref_rast)
 
-  if (missing(band_names) || is.function(band_names)) {
-    var_names <- unlist(
-      lapply(
-        rasters,
-        function(r) {
-          r <- terra::rast(r)
-          # this is the only place we instantiate these rasters, so may as well
-          # check CRS alignment while we're here...
-          if (terra::crs(r) != ref_crs) {
-            rlang::abort(c(
-              "Rasters do not all share the reference raster's CRS.",
-              i = "Reproject rasters to all share the same CRS."
-            ),
-            class = "rsi_multiple_crs"
-            )
-          }
-          names(r)
+  var_names <- unlist(
+    lapply(
+      rasters,
+      function(r) {
+        r <- terra::rast(r)
+        # this is the only place we instantiate these rasters, so may as well
+        # check CRS alignment while we're here...
+        if (terra::crs(r) != ref_crs) {
+          rlang::abort(c(
+            "Rasters do not all share the reference raster's CRS.",
+            i = "Reproject rasters to all share the same CRS."
+          ),
+          class = "rsi_multiple_crs",
+          call = rlang::caller_env()
+          )
         }
-      )
+        names(r)
+      }
     )
-  }
+  )
+
   if (!missing(band_names) && is.function(band_names)) {
     var_names <- band_names(var_names)
   }
-  if (is.character(band_names)) {
+  if (!missing(band_names) && is.character(band_names)) {
     var_names <- band_names
   }
 
diff --git a/data-raw/band_mappings.R b/data-raw/band_mappings.R
index ff0d8d8..83bd520 100644
--- a/data-raw/band_mappings.R
+++ b/data-raw/band_mappings.R
@@ -1,3 +1,4 @@
+# devtools::install()
 library(rsi)
 sentinel2_band_mapping <- list(
   aws_v0 = structure(
diff --git a/man/get_stac_data.Rd b/man/get_stac_data.Rd
index 1e49451..2d796e3 100644
--- a/man/get_stac_data.Rd
+++ b/man/get_stac_data.Rd
@@ -6,7 +6,7 @@
 \alias{get_sentinel2_imagery}
 \alias{get_landsat_imagery}
 \alias{get_dem}
-\title{Retrieve images (or composite images) from STAC endpoints}
+\title{Retrieve and composite images from STAC endpoints}
 \usage{
 get_stac_data(
   aoi,
@@ -169,18 +169,12 @@ STAC source to use to mask the data. Set to \code{NULL} to not mask. See the
 raster, where \code{TRUE} pixels will be preserved and \code{FALSE} or \code{NA} pixels will
 be masked out. See \code{\link[=sentinel2_mask_function]{sentinel2_mask_function()}}.}
 
-\item{output_filename}{The filename to write the output raster to. If
-\code{composite_function} is \code{NULL}, something will be appended to the end of the
-filename (before the extension) to create a unique file for each downloaded
-image. Currently this means sequential numbers are added to each file, but
-this may change in the future -- do not depend on the resulting filenames
-staying the same across versions!}
+\item{output_filename}{The filename to write the output raster to.}
 
 \item{composite_function}{The (quoted) name of a function from
 \code{terra} (for instance, \link[terra:summarize-generics]{terra::median}) used to combine downloaded images
 into a single composite (i.e., to aggregate pixel values from multiple images
-into a single value). Set to \code{NULL} to not composite images, but instead
-save each image as a separate file.}
+into a single value).}
 
 \item{limit}{an \code{integer} defining the maximum number of results
 to return. If not informed, it defaults to the service implementation.}
@@ -195,13 +189,17 @@ varying data types.}
 \code{output_filename}, unchanged.
 }
 \description{
-This function retrieves composites of satellite images from STAC endpoints.
+These function retrieves composites of satellite images from STAC endpoints.
+\code{get_stac_data()} is a generic function which should be able to download and
+composite images from a variety of data sources, while the other helper
+functions have useful defaults for downloading common data sets from standard
+STAC sources.
 }
 \section{Usage Tips}{
 
 It's often useful to buffer your \code{aoi} object slightly, on the order of 1-2
 cell widths, in order to ensure that data is downloaded for your entire AOI
-even after accounting for any reprojection necessary to compare your AOI to
+even after accounting for any reprojection needed to compare your AOI to
 the data on the STAC server.
 
 Setting the following GDAL configuration variables via \code{Sys.setenv()} or in
diff --git a/tests/testthat/test-calculate_indices.R b/tests/testthat/test-calculate_indices.R
index 18e3d3e..3029192 100644
--- a/tests/testthat/test-calculate_indices.R
+++ b/tests/testthat/test-calculate_indices.R
@@ -17,3 +17,25 @@ test_that("Index calculation is stable", {
     terra::values(terra::rast(system.file("rasters/dpdd.tif", package = "rsi")))
   )
 })
+
+test_that("Index calculations fail when missing a column", {
+  expect_error(
+    calculate_indices(
+      system.file("rasters/example_sentinel1.tif", package = "rsi"),
+      filter_platforms(platforms = "Sentinel-1 (Dual Polarisation VV-VH)")["formula"],
+      index_out
+    ),
+    class = "rsi_missing_column"
+  )
+})
+
+test_that("Index calculations fail when missing bands", {
+  expect_error(
+    calculate_indices(
+      system.file("rasters/example_sentinel1.tif", package = "rsi"),
+      filter_platforms(platforms = "Landsat-OLI"),
+      index_out
+    ),
+    class = "rsi_missing_indices"
+  )
+})
diff --git a/tests/testthat/test-get_stac_data.R b/tests/testthat/test-get_stac_data.R
new file mode 100644
index 0000000..e136e12
--- /dev/null
+++ b/tests/testthat/test-get_stac_data.R
@@ -0,0 +1,97 @@
+test_that("get_landsat_imagery() is stable", {
+  skip_on_cran()
+  aoi <- sf::st_point(c(-74.912131, 44.080410)) |>
+    sf::st_sfc() |>
+    sf::st_set_crs(4326) |>
+    sf::st_transform(3857) |>
+    sf::st_buffer(1000)
+
+  expect_no_error(
+    out <- get_landsat_imagery(
+      aoi,
+      "2022-06-01",
+      "2022-06-30",
+      output_filename = tempfile(fileext = ".tif")
+    )
+  )
+  expect_no_error(terra::rast(out))
+  expect_true(
+    all(
+      names(terra::rast(out)) %in%
+        as.vector(landsat_band_mapping$planetary_computer_v1)
+    )
+  )
+})
+
+test_that("get_sentinel1_imagery() is stable", {
+  skip_on_cran()
+  aoi <- sf::st_point(c(-74.912131, 44.080410)) |>
+    sf::st_sfc() |>
+    sf::st_set_crs(4326) |>
+    sf::st_transform(3857) |>
+    sf::st_buffer(1000)
+
+  expect_no_error(
+    out <- get_sentinel1_imagery(
+      aoi,
+      "2022-06-01",
+      "2022-06-30",
+      output_filename = tempfile(fileext = ".tif")
+    )
+  )
+  expect_no_error(terra::rast(out))
+  expect_true(
+    all(
+      names(terra::rast(out)) %in%
+        as.vector(sentinel1_band_mapping$planetary_computer_v1)
+    )
+  )
+})
+
+test_that("get_sentinel2_imagery() is stable", {
+  skip_on_cran()
+  aoi <- sf::st_point(c(-74.912131, 44.080410)) |>
+    sf::st_sfc() |>
+    sf::st_set_crs(4326) |>
+    sf::st_transform(3857) |>
+    sf::st_buffer(1000)
+
+  expect_no_error(
+    out <- get_sentinel2_imagery(
+      aoi,
+      "2022-06-01",
+      "2022-06-30",
+      output_filename = tempfile(fileext = ".tif")
+    )
+  )
+  expect_no_error(terra::rast(out))
+  expect_true(
+    all(
+      names(terra::rast(out)) %in%
+        as.vector(sentinel2_band_mapping$planetary_computer_v1)
+    )
+  )
+})
+
+test_that("get_dem() is stable", {
+  skip_on_cran()
+  aoi <- sf::st_point(c(-74.912131, 44.080410)) |>
+    sf::st_sfc() |>
+    sf::st_set_crs(4326) |>
+    sf::st_transform(3857) |>
+    sf::st_buffer(1000)
+
+  expect_no_error(
+    out <- get_dem(
+      aoi,
+      output_filename = tempfile(fileext = ".tif")
+    )
+  )
+  expect_no_error(terra::rast(out))
+  expect_true(
+    all(
+      names(terra::rast(out)) %in%
+        as.vector(dem_band_mapping$planetary_computer_v1)
+    )
+  )
+})
diff --git a/tests/testthat/test-spectral_indices.R b/tests/testthat/test-spectral_indices.R
index 7545717..fd4ae27 100644
--- a/tests/testthat/test-spectral_indices.R
+++ b/tests/testthat/test-spectral_indices.R
@@ -18,6 +18,27 @@ test_that("spectral_indices() works", {
   # we should be able to update our cache:
   skip_on_cran()
   expect_no_warning(spectral_indices())
+  expect_no_warning(spectral_indices(update_cache = TRUE))
 })
 
+test_that("spectral_indices_url() respects options", {
+  skip_if_not_installed("withr")
+  expect_identical(
+    withr::with_options(
+      list("rsi_url" = "example"),
+      spectral_indices_url()
+    ),
+    "example"
+  )
+})
 
+test_that("spectral_indices_url() respects environment variables", {
+  skip_if_not_installed("withr")
+  expect_identical(
+    withr::with_envvar(
+      list("rsi_url" = "example"),
+      spectral_indices_url()
+    ),
+    "example"
+  )
+})
diff --git a/tests/testthat/test-stack_rasters.R b/tests/testthat/test-stack_rasters.R
index d3f66d0..25836ea 100644
--- a/tests/testthat/test-stack_rasters.R
+++ b/tests/testthat/test-stack_rasters.R
@@ -12,3 +12,78 @@ test_that("stack_rasters works", {
       terra::values()
   )
 })
+
+test_that("stack_rasters fails when reference_raster isn't in the vector", {
+  expect_error(
+    stack_rasters(
+      list(
+        system.file("rasters/example_sentinel1.tif", package = "rsi")
+      ),
+      tempfile(fileext = ".vrt"),
+      reference_raster = 2
+    ),
+    class = "rsi_not_in_vec"
+  )
+
+  expect_error(
+    stack_rasters(
+      list(
+        system.file("rasters/example_sentinel1.tif", package = "rsi")
+      ),
+      tempfile(fileext = ".vrt"),
+      reference_raster = "some_raster"
+    ),
+    class = "rsi_not_in_vec"
+  )
+})
+
+test_that("stack_rasters fails when extent isn't four numbers", {
+  expect_error(
+    stack_rasters(
+      list(
+        system.file("rasters/example_sentinel1.tif", package = "rsi")
+      ),
+      tempfile(fileext = ".vrt"),
+      extent = 20
+    ),
+    class = "rsi_bad_extent"
+  )
+})
+
+test_that("stack_rasters fails when resolution isn't 1-2 numbers", {
+  expect_error(
+    stack_rasters(
+      list(
+        system.file("rasters/example_sentinel1.tif", package = "rsi")
+      ),
+      tempfile(fileext = ".vrt"),
+      resolution = c(20, 30, 40)
+    ),
+    class = "rsi_bad_resolution"
+  )
+})
+
+test_that("stack_rasters fails when rasters don't share a CRS", {
+  s1 <- tempfile(fileext = ".tif")
+  terra::writeRaster(
+    terra::project(
+      terra::rast(
+        system.file("rasters/example_sentinel1.tif", package = "rsi")
+      ),
+      "EPSG:4326"
+    ),
+    s1
+  )
+
+  expect_error(
+    stack_rasters(
+      list(
+        system.file("rasters/example_sentinel1.tif", package = "rsi"),
+        s1
+      ),
+      tempfile(fileext = ".vrt")
+    ),
+    class = "rsi_multiple_crs"
+  )
+})
+