diff --git a/DESCRIPTION b/DESCRIPTION
index 34dc7d4e..4464e5c5 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: cmdstanr
 Title: R Interface to 'CmdStan'
-Version: 0.8.1
+Version: 0.8.1.9000
 Date: 2024-06-06
 Authors@R:
     c(person(given = "Jonah", family = "Gabry", role = "aut",
diff --git a/R/data.R b/R/data.R
index a1e4ab7c..ce5b9cf6 100644
--- a/R/data.R
+++ b/R/data.R
@@ -186,7 +186,18 @@ process_data <- function(data, model_variables = NULL) {
         # generating a decimal point in write_stan_json
         if (data_variables[[var_name]]$type == "int"
             && !is.integer(data[[var_name]])) {
-          mode(data[[var_name]]) <- "integer"
+          if (!isTRUE(all(is_wholenumber(data[[var_name]])))) {
+            # Don't warn for NULL/NA, as different warnings are used for those
+            if (!isTRUE(any(is.na(data[[var_name]])))) {
+              warning("A non-integer value was supplied for '", var_name, "'!",
+                      " It will be truncated to an integer.", call. = FALSE)
+            }
+            mode(data[[var_name]]) <- "integer"
+          } else {
+            # Round before setting mode to integer to avoid floating point errors
+            data[[var_name]] <- round(data[[var_name]])
+            mode(data[[var_name]]) <- "integer"
+          }
         }
       }
     }
diff --git a/R/utils.R b/R/utils.R
index ef36d0c7..ecac323c 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -36,6 +36,10 @@ matching_variables <- function(variable_filters, variables) {
   )
 }
 
+is_wholenumber <- function(x, tol = sqrt(.Machine$double.eps)) {
+  abs(x - round(x)) < tol
+}
+
 # checks for OS and hardware ----------------------------------------------
 
 os_is_windows <- function() {
diff --git a/tests/testthat/test-data.R b/tests/testthat/test-data.R
index 01fa7291..61ca79e3 100644
--- a/tests/testthat/test-data.R
+++ b/tests/testthat/test-data.R
@@ -355,3 +355,50 @@ test_that("process_data() corrrectly casts integers and floating point numbers",
     fixed = TRUE
   )
 })
+
+test_that("process_data warns on int coercion", {
+  stan_file <- write_stan_file("
+  data {
+    int a;
+    real b;
+  }
+  ")
+  mod <- cmdstan_model(stan_file, compile = FALSE)
+  expect_warning(
+    process_data(list(a = 1.1, b = 2.1), model_variables = mod$variables()),
+    "A non-integer value was supplied for 'a'! It will be truncated to an integer."
+  )
+
+  stan_file <- write_stan_file("
+  data {
+    array[3] int a;
+  }
+  ")
+  mod <- cmdstan_model(stan_file, compile = FALSE)
+  expect_warning(
+    process_data(list(a = c(1, 2.1, 3)), model_variables = mod$variables()),
+    "A non-integer value was supplied for 'a'! It will be truncated to an integer."
+  )
+
+  expect_no_warning(
+    process_data(list(a = c(1, 2, 3)), model_variables = mod$variables())
+  )
+})
+
+test_that("Floating-point differences do not cause truncation towards 0", {
+  stan_file <- write_stan_file("
+  data {
+    int a;
+    real b;
+  }
+  ")
+  mod <- cmdstan_model(stan_file, compile = FALSE)
+  a <- 10*(3-2.7)
+  expect_false(is.integer(a))
+  test_file <- process_data(list(a = a, b = 2.0), model_variables = mod$variables())
+  expect_match(
+    "  \"a\": 3,",
+    readLines(test_file)[2],
+    fixed = TRUE
+  )
+})