diff --git a/DESCRIPTION b/DESCRIPTION
index 4785036..77e93fd 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -22,6 +22,7 @@ BugReports: https://github.com/rstudio/reactlog/issues
 Depends:
     R (>= 3.0.2)
 Imports:
+    cli,
     jsonlite (>= 0.9.16),
     rlang,
     rmarkdown
@@ -36,6 +37,7 @@ Suggests:
     withr
 VignetteBuilder: 
     knitr
+Config/testthat/edition: 3
 Config/roxygen2/version: 8.1.1
 Encoding: UTF-8
 Language: en-US
diff --git a/NEWS.md b/NEWS.md
index 05f4605..b170546 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # reactlog (development)
 
+- Error and warning messages now use cli and name the function you called. Missing or too-old shiny now gives a clear message from `rlang::check_installed()`, including when `reactlog_show()` or `reactlog_widget()` is called with no `log`. cli is now an imported package (#164)
+
 - Added `reactlog_widget()`, which shows the reactlog as an htmlwidget so it can be embedded in R Markdown or Quarto documents. Like `reactlog_show()`, `log` defaults to `shiny::reactlog()`. To show it in a Shiny app, use `reactlog_widget_module_ui()` and `reactlog_widget_module_server()`, or `reactlog_widget_output()` and `render_reactlog_widget()`. Requires the htmlwidgets package (#160)
 
 - `reactlog_show()` now uses `shiny::reactlog()` as its default `log`, so `reactlog_show()` works with no arguments after running an app (#91)
diff --git a/R/renderReactLog.R b/R/renderReactLog.R
index f19c043..53c2076 100644
--- a/R/renderReactLog.R
+++ b/R/renderReactLog.R
@@ -11,10 +11,20 @@ inst_reactlog_assets <- function() {
 #' @param session_token token to be used to subset which session is displayed.
 #'   Defaults to all sessions.
 reactlog_render <- function(log, session_token = NULL, time = TRUE) {
-  html <- reactlog_page_html(log, session_token = session_token, time = time)
+  reactlog_render_file(log, session_token = session_token, time = time)
+}
+
+# `call` names the user-facing function in error messages
+reactlog_render_file <- function(
+  log,
+  session_token = NULL,
+  time = TRUE,
+  call = rlang::caller_env()
+) {
+  html <- reactlog_page_html(log, session_token = session_token, time = time, call = call)
 
   # check if shiny is even available
-  if (requireNamespace("shiny", quietly = TRUE)) {
+  if (rlang::is_installed("shiny")) {
     # add resource path to pkg files for shiny
     # (avoids warning of tmp folder moving around)
     shiny::addResourcePath(
@@ -42,8 +52,13 @@ fixed_sub <- function(...) {
 }
 
 # The reactlog page as one string. Assets are still linked from `reactlogAsset/`.
-reactlog_page_html <- function(log, session_token = NULL, time = TRUE) {
-  log <- reactlog_upgrade(log)
+reactlog_page_html <- function(
+  log,
+  session_token = NULL,
+  time = TRUE,
+  call = rlang::caller_env()
+) {
+  log <- reactlog_upgrade(log, call = call)
 
   template_file <- inst_reactlog_file("reactlog.html")
   # does not need utf8 support
diff --git a/R/setReactLog.R b/R/setReactLog.R
index de6c27c..37ca2b9 100644
--- a/R/setReactLog.R
+++ b/R/setReactLog.R
@@ -15,12 +15,14 @@ reactlog_enable <- function() {
 #' @export
 #' @rdname setReactLog
 reactlog_disable <- function(warn = TRUE) {
-  if (!isTRUE(warn) && !identical(warn, FALSE)) {
-    stop("'warn' must be either 'TRUE' or 'FALSE'")
+  if (!rlang::is_bool(warn)) {
+    cli::cli_abort(
+      "{.arg warn} must be {.code TRUE} or {.code FALSE}, not {.obj_type_friendly {warn}}."
+    )
   }
 
   options(shiny.reactlog = FALSE)
   if (isTRUE(warn)) {
-    warning("Please restart R to free up 'reactlog' memory", call. = FALSE)
+    cli::cli_warn("Please restart R to free up {.pkg reactlog} memory.")
   }
 }
diff --git a/R/shinyModule.R b/R/shinyModule.R
index 440eac7..78c89fc 100644
--- a/R/shinyModule.R
+++ b/R/shinyModule.R
@@ -144,10 +144,14 @@ test_shiny_version <- function() {
     FALSE
   })
 }
-assert_shiny_version <- function() {
-  if (!test_shiny_version()) {
-    stop("`shiny` v", shiny_version_required(), " or greater must be installed")
-  }
+assert_shiny_version <- function(call = rlang::caller_env()) {
+  # Fail instead of asking to install; this runs at the start of a live session
+  rlang::local_options(rlib_restart_package_not_found = FALSE)
+  rlang::check_installed(
+    "shiny",
+    version = as.character(shiny_version_required()),
+    call = call
+  )
 }
 
 
diff --git a/R/showReactLog.R b/R/showReactLog.R
index 80650cd..9c68c2d 100644
--- a/R/showReactLog.R
+++ b/R/showReactLog.R
@@ -61,7 +61,8 @@
 #' }
 
 reactlog_show <- function(log = shiny::reactlog(), time = TRUE, ...) {
-  file <- reactlog_render(log, time = as.logical(time), ...)
+  if (missing(log)) rlang::check_installed("shiny")
+  file <- reactlog_render_file(log, time = as.logical(time), ...)
   utils::browseURL(file)
 }
 
diff --git a/R/version.R b/R/version.R
index c04eca1..49df0a5 100644
--- a/R/version.R
+++ b/R/version.R
@@ -1,17 +1,19 @@
 
-reactlog_upgrade <- function(log) {
+reactlog_upgrade <- function(log, call = rlang::caller_env()) {
   version <- attr(log, "version")
   if (is.null(version)) {
-    stop("'log' is missing a 'version' attribute.  This is required.")
+    cli::cli_abort(c(
+      "{.arg log} is missing a {.field version} attribute.",
+      "i" = "Use {.fn shiny::reactlog} to get a log."
+    ), call = call)
   }
   version <- as.character(version)
   if (identical(version, "1")) {
     return(log)
   }
 
-  stop(
-    "Versions available: 1\n",
-    "Latest reactlog version: 1\n",
-    "Version provided: ", version
-  )
+  cli::cli_abort(c(
+    "x" = "{.arg log} has version {.val {version}}.",
+    "i" = "{.pkg reactlog} supports version {.val 1}."
+  ), call = call)
 }
diff --git a/R/widget.R b/R/widget.R
index d722cde..54a6848 100644
--- a/R/widget.R
+++ b/R/widget.R
@@ -33,6 +33,7 @@ reactlog_widget <- function(
   elementId = NULL # nolint: object_name_linter.
 ) {
   rlang::check_installed("htmlwidgets")
+  if (missing(log)) rlang::check_installed("shiny")
 
   html <- reactlog_page_html(log, time = as.logical(time), ...)
 
diff --git a/tests/testthat/_snaps/errors.md b/tests/testthat/_snaps/errors.md
new file mode 100644
index 0000000..c4c7cb4
--- /dev/null
+++ b/tests/testthat/_snaps/errors.md
@@ -0,0 +1,43 @@
+# reactlog_disable() checks `warn` and warns about memory
+
+    Code
+      reactlog_disable(warn = "yes")
+    Condition
+      Error in `reactlog_disable()`:
+      ! `warn` must be `TRUE` or `FALSE`, not a string.
+
+---
+
+    Code
+      reactlog_disable()
+    Condition
+      Warning:
+      Please restart R to free up reactlog memory.
+
+# bad log versions are explained by the function the user called
+
+    Code
+      reactlog_render(list())
+    Condition
+      Error in `reactlog_render()`:
+      ! `log` is missing a version attribute.
+      i Use `shiny::reactlog()` to get a log.
+
+---
+
+    Code
+      reactlog_render(structure(list(), version = "2"))
+    Condition
+      Error in `reactlog_render()`:
+      x `log` has version "2".
+      i reactlog supports version "1".
+
+---
+
+    Code
+      reactlog_show(list())
+    Condition
+      Error in `reactlog_show()`:
+      ! `log` is missing a version attribute.
+      i Use `shiny::reactlog()` to get a log.
+
diff --git a/tests/testthat/test-errors.R b/tests/testthat/test-errors.R
new file mode 100644
index 0000000..3ca93b9
--- /dev/null
+++ b/tests/testthat/test-errors.R
@@ -0,0 +1,34 @@
+test_that("reactlog_disable() checks `warn` and warns about memory", {
+  withr::local_options(shiny.reactlog = TRUE)
+  expect_snapshot(error = TRUE, reactlog_disable(warn = "yes"))
+  expect_snapshot(reactlog_disable())
+  expect_silent(reactlog_disable(warn = FALSE))
+})
+
+test_that("bad log versions are explained by the function the user called", {
+  expect_snapshot(error = TRUE, reactlog_render(list()))
+  expect_snapshot(error = TRUE, reactlog_render(structure(list(), version = "2")))
+  expect_snapshot(error = TRUE, reactlog_show(list()))
+})
+
+test_that("assert_shiny_version() uses rlang::check_installed() without asking to install", {
+  local_mocked_bindings(shiny_version_required = function(...) package_version("999.0.0"))
+  withr::local_options(rlang_interactive = TRUE)
+  expect_error(assert_shiny_version(), class = "rlib_error_package_not_found")
+})
+
+test_that("reactlog_show() and reactlog_widget() check for shiny when `log` is missing", {
+  local_mocked_bindings(
+    check_installed = function(pkg, ...) stop("checked ", pkg),
+    .package = "rlang"
+  )
+  # if the check is skipped, don't open a browser
+  withr::local_options(browser = function(...) NULL)
+  expect_error(reactlog_show(), "checked shiny")
+  skip_if_not_installed("htmlwidgets")
+  local_mocked_bindings(
+    check_installed = function(pkg, ...) if (pkg == "shiny") stop("checked ", pkg),
+    .package = "rlang"
+  )
+  expect_error(reactlog_widget(), "checked shiny")
+})