diff --git a/DESCRIPTION b/DESCRIPTION
index ce7ac01b..a0df548c 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: monty
 Title: Monte Carlo Models
-Version: 0.2.3
+Version: 0.2.4
 Authors@R: c(person("Rich", "FitzJohn", role = c("aut", "cre"),
                     email = "rich.fitzjohn@gmail.com"),
              person("Wes", "Hinsley", role = "aut"),
diff --git a/R/dsl-parse-error.R b/R/dsl-parse-error.R
index 44c196f9..b9019376 100644
--- a/R/dsl-parse-error.R
+++ b/R/dsl-parse-error.R
@@ -11,32 +11,15 @@
 ##' @param code The error code, as a string, in the form `Exxx` (a
 ##'   capital "E" followed by three numbers)
 ##'
+##' @param how How to explain the error. Options are `pretty` (render
+##'   pretty text in the console), `plain` (display plain text in the
+##'   console) and `link` (browse to the online help).
+##'
 ##' @return Nothing, this is called for its side effect only
 ##'
 ##' @export
-monty_dsl_error_explain <- function(code) {
-  ## See odin2 for the canonical implementation of this, we're just
-  ## shadowing it here really as our dsl is much simpler.  Note that
-  ## error codes from monty are three numbers long, whereas they are
-  ## four long in odin so that it's easy (for us) to tell who the
-  ## error belongs to.
-  assert_scalar_character(code)
-  if (!grepl("^E[0-9]{3}$", code)) {
-    cli::cli_abort("Invalid code '{code}', should match 'Exxx'",
-                   arg = "code")
-  }
-  txt <- dsl_errors[[code]]
-  if (is.null(txt)) {
-    cli::cli_abort(
-      c("Error '{code}' is undocumented",
-        i = paste("If you were directed here from an error message, please",
-                  "let us know (e.g., file an issue or send us a message)")),
-      arg = "code")
-  }
-  url <- sprintf(
-    "https://mrc-ide.github.io/monty/articles/dsl-errors.html#%s",
-    tolower(code))
-  utils::browseURL(url)
+monty_dsl_error_explain <- function(code, how = "pretty") {
+  error_explain(dsl_errors, code, how)
 }
 
 
@@ -70,7 +53,6 @@ cnd_footer.monty_parse_error <- function(cnd, ...) {
   ## to some number of spaces, probably.
   detail <- gsub(" ", "\u00a0", detail)
 
-
   code <- cnd$code
   ## See https://cli.r-lib.org/reference/links.html#click-to-run-code
   ## RStudio will only run code in namespaced form
diff --git a/R/import-standalone-error-render.R b/R/import-standalone-error-render.R
new file mode 100644
index 00000000..f2033077
--- /dev/null
+++ b/R/import-standalone-error-render.R
@@ -0,0 +1,75 @@
+# Standalone file: do not edit by hand
+# Source: https://github.com/reside-ic/reside.utils/blob/HEAD/R/standalone-error-render.R
+# Generated by: usethis::use_standalone("reside-ic/reside.utils", "error-render")
+# ----------------------------------------------------------------------
+#
+# ---
+# repo: reside/reside.utils
+# file: standalone-errors-render.R
+# dependencies: standalone-utils-assert.R
+# imports: cli
+# ---
+error_explain <- function(errors, code, how, call = parent.frame()) {
+  assert_scalar_character(code, name = "code", call = call)
+  how <- match_value(how, c("pretty", "plain", "link"),
+                     name = "how", call = call)
+
+  if (!grepl(errors$pattern$complete, code)) {
+    cli::cli_abort(
+      "Invalid code '{code}', should match '{errors$pattern$hint}'",
+      arg = "code", call = call)
+  }
+  err <- errors$errors[[code]]
+  if (is.null(err)) {
+    cli::cli_abort(
+      c("Error '{code}' is undocumented",
+        i = paste("If you were directed here from an error message, please",
+                  "let us know (e.g., file an issue or send us a message)")),
+      arg = "code", call = call)
+  }
+  if (how == "link") {
+    url <- sprintf("%s#%s",errors$url, tolower(code))
+    utils::browseURL(url)
+  } else {
+    error_render(err, how == "pretty")
+  }
+}
+
+
+error_render <- function(err, pretty) {
+  render_paragraph <- function(el) {
+    cli::cli_par()
+    cli::cli_text(el$text)
+  }
+
+  render_code_block <- function(el) {
+    cli::cli_par()
+    cli::cli_code(paste0("    ", el$text))
+  }
+
+  render_list <- function(el) {
+    cli::cli_par()
+    if (el$mode == "bullet") {
+      cli::cli_ul()
+    } else {
+      cli::cli_ol()
+    }
+    for (i in el$items) {
+      cli::cli_li(i$text)
+    }
+  }
+
+  if (pretty) {
+    cli::cli_h1(err$code)
+    for (el in err$parsed) {
+      switch(
+        el$type,
+        paragraph = render_paragraph(el),
+        code_block = render_code_block(el),
+        list = render_list(el),
+        cli::cli_abort("Unknown error block type '{el$type}' while rendering"))
+    }
+  } else {
+    writeLines(c(sprintf("# %s", err$code), "", err$plain))
+  }
+}
diff --git a/R/import-standalone-utils-assert.R b/R/import-standalone-utils-assert.R
index 67adf3bf..441503d8 100644
--- a/R/import-standalone-utils-assert.R
+++ b/R/import-standalone-utils-assert.R
@@ -1,6 +1,6 @@
 # Standalone file: do not edit by hand
-# Source: https://github.com/reside-ic/reside.utils/blob/prototype/R/standalone-utils-assert.R
-# Generated by: usethis::use_standalone("reside-ic/reside.utils", "utils-assert", ref = "prototype")
+# Source: https://github.com/reside-ic/reside.utils/blob/HEAD/R/standalone-utils-assert.R
+# Generated by: usethis::use_standalone("reside-ic/reside.utils", "utils-assert")
 # ----------------------------------------------------------------------
 #
 # ---
@@ -48,7 +48,7 @@ assert_integer <- function(x, name = deparse(substitute(x)),
     }
     if (!isTRUE(all.equal(x, rx, tolerance = tolerance))) {
       cli::cli_abort(
-        c("Exected '{name}' to be integer",
+        c("Expected '{name}' to be integer",
           i = paste("{cli::qty(length(x))}The provided",
                     "{?value was/values were} numeric, but not very close",
                     "to integer values")),
@@ -56,7 +56,7 @@ assert_integer <- function(x, name = deparse(substitute(x)),
     }
     x <- as.integer(rx)
   } else {
-    cli::cli_abort("Exected '{name}' to be integer", call = call, arg = arg)
+    cli::cli_abort("Expected '{name}' to be integer", call = call, arg = arg)
   }
   invisible(x)
 }
@@ -72,7 +72,7 @@ assert_logical <- function(x, name = deparse(substitute(x)),
 
 
 assert_nonmissing <- function(x, name = deparse(substitute(x)),
-                          arg = name, call = parent.frame()) {
+                              arg = name, call = parent.frame()) {
   if (anyNA(x)) {
     cli::cli_abort("Expected '{name}' to be non-NA", arg = arg, call = call)
   }
@@ -81,7 +81,11 @@ assert_nonmissing <- function(x, name = deparse(substitute(x)),
 
 
 assert_scalar_character <- function(x, name = deparse(substitute(x)),
-                                  arg = name, call = parent.frame()) {
+                                    allow_null = FALSE,
+                                    arg = name, call = parent.frame()) {
+  if (allow_null && is.null(x)) {
+    return(invisible(x))
+  }
   assert_scalar(x, name, arg = arg, call = call)
   assert_character(x, name, arg = arg, call = call)
   assert_nonmissing(x, name, arg = arg, call = call)
@@ -89,7 +93,11 @@ assert_scalar_character <- function(x, name = deparse(substitute(x)),
 
 
 assert_scalar_numeric <- function(x, name = deparse(substitute(x)),
+                                  allow_null = FALSE,
                                   arg = name, call = parent.frame()) {
+  if (allow_null && is.null(x)) {
+    return(invisible(x))
+  }
   assert_scalar(x, name, arg = arg, call = call)
   assert_numeric(x, name, arg = arg, call = call)
   assert_nonmissing(x, name, arg = arg, call = call)
@@ -97,7 +105,11 @@ assert_scalar_numeric <- function(x, name = deparse(substitute(x)),
 
 
 assert_scalar_integer <- function(x, name = deparse(substitute(x)),
-                                  tolerance = NULL, arg = name, call = parent.frame()) {
+                                  tolerance = NULL, allow_null = FALSE,
+                                  arg = name, call = parent.frame()) {
+  if (allow_null && is.null(x)) {
+    return(invisible(x))
+  }
   assert_scalar(x, name, arg = arg, call = call)
   assert_integer(x, name, tolerance = tolerance, arg = arg, call = call)
   assert_nonmissing(x, name, arg = arg, call = call)
@@ -105,16 +117,23 @@ assert_scalar_integer <- function(x, name = deparse(substitute(x)),
 
 
 assert_scalar_logical <- function(x, name = deparse(substitute(x)),
+                                  allow_null = FALSE,
                                   arg = name, call = parent.frame()) {
+  if (allow_null && is.null(x)) {
+    return(invisible(x))
+  }
   assert_scalar(x, name, arg = arg, call = call)
   assert_logical(x, name, arg = arg, call = call)
   assert_nonmissing(x, name, arg = arg, call = call)
 }
 
 
-assert_scalar_size <- function(x, allow_zero = TRUE,
+assert_scalar_size <- function(x, allow_zero = TRUE, allow_null = FALSE,
                                name = deparse(substitute(x)),
                                arg = name, call = parent.frame()) {
+  if (allow_null && is.null(x)) {
+    return(invisible(x))
+  }
   assert_scalar_integer(x, name = name, arg = arg, call = call)
   assert_nonmissing(x, name, arg = arg, call = call)
   min <- if (allow_zero) 0 else 1
@@ -156,6 +175,48 @@ assert_list <- function(x, name = deparse(substitute(x)), arg = name,
 }
 
 
+assert_scalar_positive_numeric <- function(x, allow_zero = TRUE,
+                                           name = deparse(substitute(x)),
+                                           arg = name, call = parent.frame()) {
+  assert_scalar_numeric(x, name = name, call = call)
+  if (allow_zero) {
+    if (x < 0) {
+      cli::cli_abort("'{name}' must be at least 0", arg = arg, call = call)
+    }
+  } else {
+    if (x <= 0) {
+      cli::cli_abort("'{name}' must be greater than 0", arg = arg, call = call)
+    }
+  }
+  invisible(x)
+}
+
+
+assert_scalar_positive_integer <- function(x, allow_zero = TRUE,
+                                           name = deparse(substitute(x)),
+                                           tolerance = NULL, arg = name,
+                                           call = parent.frame()) {
+  assert_scalar_integer(x, name, tolerance = tolerance, arg = arg, call = call)
+  min <- if (allow_zero) 0 else 1
+  if (x < min) {
+    cli::cli_abort("'{name}' must be at least {min}", arg = arg, call = call)
+  }
+  invisible(x)
+}
+
+
+assert_raw <- function(x, len = NULL, name = deparse(substitute(x)),
+                       arg = name, call = parent.frame()) {
+  if (!is.raw(x)) {
+    cli::cli_abort("'{name}' must be a raw vector", arg = arg, call = call)
+  }
+  if (!is.null(len)) {
+    assert_length(x, len, name = name, call = call)
+  }
+  invisible(x)
+}
+
+
 assert_named <- function(x, unique = FALSE, name = deparse(substitute(x)),
                          arg = name, call = parent.frame()) {
   nms <- names(x)
@@ -179,7 +240,7 @@ assert_named <- function(x, unique = FALSE, name = deparse(substitute(x)),
 
 match_value <- function(x, choices, name = deparse(substitute(x)), arg = name,
                         call = parent.frame()) {
-  assert_scalar_character(x, call = call, arg = arg)
+  assert_scalar_character(x, call = call, name = name, arg = arg)
   if (!(x %in% choices)) {
     choices_str <- paste(sprintf("'%s'", choices), collapse = ", ")
     cli::cli_abort(c("'{name}' must be one of {choices_str}",
diff --git a/R/sysdata.rda b/R/sysdata.rda
index 9c0dfbcf..4c9f4fd9 100644
Binary files a/R/sysdata.rda and b/R/sysdata.rda differ
diff --git a/man/monty_dsl_error_explain.Rd b/man/monty_dsl_error_explain.Rd
index 7e441c32..24727f2a 100644
--- a/man/monty_dsl_error_explain.Rd
+++ b/man/monty_dsl_error_explain.Rd
@@ -4,11 +4,15 @@
 \alias{monty_dsl_error_explain}
 \title{Explain monty error}
 \usage{
-monty_dsl_error_explain(code)
+monty_dsl_error_explain(code, how = "pretty")
 }
 \arguments{
 \item{code}{The error code, as a string, in the form \code{Exxx} (a
 capital "E" followed by three numbers)}
+
+\item{how}{How to explain the error. Options are \code{pretty} (render
+pretty text in the console), \code{plain} (display plain text in the
+console) and \code{link} (browse to the online help).}
 }
 \value{
 Nothing, this is called for its side effect only
diff --git a/scripts/update_parse_errors b/scripts/update_parse_errors
index 8af5d42c..436add51 100755
--- a/scripts/update_parse_errors
+++ b/scripts/update_parse_errors
@@ -1,34 +1,4 @@
 #!/usr/bin/env Rscript
-
-read_errors <- function() {
-  path_rmd <- "vignettes/dsl-errors.Rmd"
-  txt <- readLines(path_rmd)
-
-  re <- "^# `(E[0-9]{3})`$"
-  i <- grep(re, txt)
-  if (length(setdiff(grep("^# ", txt), i)) > 0) {
-    stop("Some headings don't match expected pattern")
-  }
-
-  f <- function(from, to) {
-    ret <- txt[from:to]
-    while (ret[[1]] == "") {
-      ret <- ret[-1]
-    }
-    while (ret[[length(ret)]] == "") {
-      ret <- ret[-length(ret)]
-    }
-    ret
-  }
-
-  ret <- Map(f, i + 1, c(i[-1] - 1, length(txt)))
-  names(ret) <- sub(re, "\\1", txt[i])
-  ret
-}
-
-
-## We might parse this further, e.g., with commonmark, so that we can
-## render this nicely to the console as cli would make this pretty
-## easy really.
-dsl_errors <- read_errors()
+dsl_errors <- reside.utils::errors_parse(
+  "vignettes/dsl-errors.Rmd", "E[0-9]{3}", "monty::monty_dsl_error_explain")
 save(list = "dsl_errors", file = file.path("R/sysdata.rda"), version = 2)
diff --git a/tests/testthat/test-dsl-parse.R b/tests/testthat/test-dsl-parse.R
index eb825d64..ef1af335 100644
--- a/tests/testthat/test-dsl-parse.R
+++ b/tests/testthat/test-dsl-parse.R
@@ -202,31 +202,11 @@ test_that("report back invalid distribution calls", {
 
 test_that("can explain an error", {
   skip_if_not_installed("mockery")
-  mock_browse <- mockery::mock()
-  mockery::stub(monty_dsl_error_explain, "utils::browseURL", mock_browse)
+  mock_explain <- mockery::mock()
+  mockery::stub(monty_dsl_error_explain, "error_explain", mock_explain)
   monty_dsl_error_explain("E101")
-  mockery::expect_called(mock_browse, 1)
+  mockery::expect_called(mock_explain, 1)
   expect_equal(
-    mockery::mock_args(mock_browse)[[1]],
-    list("https://mrc-ide.github.io/monty/articles/dsl-errors.html#e101"))
-})
-
-
-test_that("error if given invalid code", {
-  msg <- "Invalid code 'E01', should match 'Exxx'"
-  expect_error(monty_dsl_error_explain("E01"),
-               "Invalid code 'E01', should match 'Exxx'")
-  expect_error(monty_dsl_error_explain("e0001"),
-               "Invalid code 'e0001', should match 'Exxx'")
-  expect_error(monty_dsl_error_explain("E0001"),
-               "Invalid code 'E0001', should match 'Exxx'")
-  expect_error(monty_dsl_error_explain("anything"),
-               "Invalid code 'anything', should match 'Exxx'")
-})
-
-
-test_that("error if given unknown code", {
-  expect_error(
-    monty_dsl_error_explain("E999"),
-    "Error 'E999' is undocumented")
+    mockery::mock_args(mock_explain)[[1]],
+    list(dsl_errors, "E101", "pretty"))
 })