diff --git a/NEWS.md b/NEWS.md
index bf6aaef8..a7537c16 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -10,6 +10,8 @@
 * Fixed test in `test-ppc-distributions.R` that incorrectly used `ppc_dens()` instead of `ppd_dens()` when testing PPD functions
 * New functions `mcmc_dots` and `mcmc_dots_by_chain` for dot plots of MCMC draws by @behramulukir (#402)
 * Default to `quantiles=100` for all dot plots by @behramulukir (#402)
+* Make diagnostic color scale helpers handle `"neff"` and `"neff_ratio"` explicitly, avoiding reliance on partial matching.
+* Replace `stopifnot()` checks with `abort()` and descriptive input-validation messages in helper and diagnostics internals.
 * Use `"neff_ratio"` consistently in diagnostic color scale helpers to avoid relying on partial matching of `"neff"`.
 
 # bayesplot 1.15.0
diff --git a/R/helpers-mcmc.R b/R/helpers-mcmc.R
index 41e2c4ee..c133d35c 100644
--- a/R/helpers-mcmc.R
+++ b/R/helpers-mcmc.R
@@ -24,7 +24,9 @@ prepare_mcmc_array <- function(x,
     x <- as.array(x)
   }
 
-  stopifnot(is.matrix(x) || is.array(x))
+  if (!(is.matrix(x) || is.array(x))) {
+    abort("'x' must be a matrix or array.")
+  }
   if (is.array(x) && !(length(dim(x)) %in% c(2,3))) {
     abort("Arrays should have 2 or 3 dimensions. See help('MCMC-overview').")
   }
@@ -80,9 +82,11 @@ select_parameters <-
            patterns = character(),
            complete_pars = character()) {
 
-    stopifnot(is.character(explicit),
-              is.character(patterns),
-              is.character(complete_pars))
+    if (!is.character(explicit) ||
+        !is.character(patterns) ||
+        !is.character(complete_pars)) {
+      abort("'explicit', 'patterns', and 'complete_pars' must be character vectors.")
+    }
 
     if (!length(explicit) && !length(patterns)) {
       return(complete_pars)
@@ -132,7 +136,9 @@ melt_mcmc.mcmc_array <- function(x,
                                  value.name = "Value",
                                  as.is = TRUE,
                                  ...) {
-  stopifnot(is_mcmc_array(x))
+  if (!is_mcmc_array(x)) {
+    abort("'x' must be an mcmc_array.")
+  }
 
   long <- reshape2::melt(
     data = x,
@@ -168,7 +174,9 @@ melt_mcmc.matrix <- function(x,
 #' @param parnames Character vector of parameter names
 #' @return x with a modified dimnames.
 set_mcmc_dimnames <- function(x, parnames) {
-  stopifnot(is_3d_array(x))
+  if (!is_3d_array(x)) {
+    abort("'x' must be a 3-D array.")
+  }
   dimnames(x) <- list(
     Iteration = seq_len(nrow(x)),
     Chain = seq_len(ncol(x)),
@@ -201,7 +209,9 @@ is_df_with_chain <- function(x) {
 }
 
 validate_df_with_chain <- function(x) {
-  stopifnot(is_df_with_chain(x))
+  if (!is_df_with_chain(x)) {
+    abort("'x' must be a data frame with a chain column.")
+  }
   x <- as.data.frame(x)
   if (!is.null(x$chain)) {
     if (is.null(x$Chain)) {
@@ -311,7 +321,9 @@ parameter_names <- function(x) UseMethod("parameter_names")
 
 #' @export
 parameter_names.array <- function(x) {
-  stopifnot(is_3d_array(x))
+  if (!is_3d_array(x)) {
+    abort("'x' must be a 3-D array.")
+  }
   dimnames(x)[[3]] %||% abort("No parameter names found.")
 }
 #' @export
@@ -350,12 +362,16 @@ is_mcmc_array <- function(x) {
 
 # Check if 3-D array has multiple chains
 has_multiple_chains <- function(x) {
-  stopifnot(is_3d_array(x))
+  if (!is_3d_array(x)) {
+    abort("'x' must be a 3-D array.")
+  }
   isTRUE(dim(x)[2] > 1)
 }
 # Check if 3-D array has multiple parameters
 has_multiple_params <- function(x) {
-  stopifnot(is_3d_array(x))
+  if (!is_3d_array(x)) {
+    abort("'x' must be a 3-D array.")
+  }
   isTRUE(dim(x)[3] > 1)
 }
 
@@ -412,7 +428,9 @@ apply_transformations.matrix <- function(x, ..., transformations = list()) {
 
 #' @export
 apply_transformations.array <- function(x, ..., transformations = list()) {
-  stopifnot(length(dim(x)) == 3)
+  if (length(dim(x)) != 3) {
+    abort("'x' must be a 3-D array.")
+  }
   pars <- dimnames(x)[[3]]
   x_transforms <- validate_transformations(transformations, pars)
   for (p in names(x_transforms)) {
@@ -423,7 +441,9 @@ apply_transformations.array <- function(x, ..., transformations = list()) {
 }
 
 rename_transformed_pars <- function(pars, transformations) {
-  stopifnot(is.character(pars), is.list(transformations))
+  if (!is.character(pars) || !is.list(transformations)) {
+    abort("'pars' must be a character vector and 'transformations' must be a list.")
+  }
   has_names <- sapply(transformations, is.character)
   if (any(has_names)) {
     nms <- names(which(has_names))
@@ -456,18 +476,24 @@ num_chains.mcmc_array <- function(x, ...) dim(x)[2]
 num_iters.mcmc_array <- function(x, ...) dim(x)[1]
 #' @export
 num_params.data.frame <- function(x, ...) {
-  stopifnot("Parameter" %in% colnames(x))
+  if (!("Parameter" %in% colnames(x))) {
+    abort("'x' must contain a 'Parameter' column.")
+  }
   length(unique(x$Parameter))
 }
 #' @export
 num_chains.data.frame <- function(x, ...) {
-  stopifnot("Chain" %in% colnames(x))
+  if (!("Chain" %in% colnames(x))) {
+    abort("'x' must contain a 'Chain' column.")
+  }
   length(unique(x$Chain))
 }
 #' @export
 num_iters.data.frame <- function(x, ...) {
   cols <- colnames(x)
-  stopifnot("Iteration" %in% cols || "Draws" %in% cols)
+  if (!("Iteration" %in% cols || "Draws" %in% cols)) {
+    abort("'x' must contain an 'Iteration' or 'Draws' column.")
+  }
 
   if ("Iteration" %in% cols) {
     n <- length(unique(x$Iteration))
diff --git a/R/helpers-ppc.R b/R/helpers-ppc.R
index 5206b9af..c52764db 100644
--- a/R/helpers-ppc.R
+++ b/R/helpers-ppc.R
@@ -38,7 +38,9 @@ all_counts <- function(x, ...) {
 #' @return Either throws an error or returns a numeric vector.
 #' @noRd
 validate_y <- function(y) {
-  stopifnot(is.numeric(y))
+  if (!is.numeric(y)) {
+    abort("'y' must be numeric.")
+  }
 
   if (!(inherits(y, "ts") && is.null(dim(y)))) {
     if (!is_vector_or_1Darray(y)) {
@@ -67,9 +69,13 @@ validate_y <- function(y) {
 #' @noRd
 validate_predictions <- function(predictions, n_obs = NULL) {
   # sanity checks
-  stopifnot(is.matrix(predictions), is.numeric(predictions))
+  if (!is.matrix(predictions) || !is.numeric(predictions)) {
+    abort("'predictions' must be a numeric matrix.")
+  }
   if (!is.null(n_obs)) {
-    stopifnot(length(n_obs) == 1, n_obs == as.integer(n_obs))
+    if (length(n_obs) != 1 || !identical(n_obs, as.integer(n_obs))) {
+      abort("'n_obs' must be a single integer.")
+    }
   }
 
   if (is.integer(predictions)) {
@@ -111,7 +117,9 @@ validate_pit <- function(pit) {
     abort("NAs not allowed in 'pit'.")
   }
 
-  stopifnot(is.numeric(pit))
+  if (!is.numeric(pit)) {
+    abort("'pit' must be numeric.")
+  }
 
   if (!is_vector_or_1Darray(pit)) {
     abort("'pit' must be a vector or 1D array.")
@@ -137,8 +145,12 @@ validate_pit <- function(pit) {
 #' @noRd
 validate_group <- function(group, n_obs) {
   # sanity checks
-  stopifnot(is.vector(group) || is.factor(group),
-            length(n_obs) == 1, n_obs == as.integer(n_obs))
+  if (!(is.vector(group) || is.factor(group))) {
+    abort("'group' must be a vector or factor.")
+  }
+  if (length(n_obs) != 1 || !identical(n_obs, as.integer(n_obs))) {
+    abort("'n_obs' must be a single integer.")
+  }
 
   if (!is.factor(group)) {
     group <- as.factor(group)
@@ -175,7 +187,9 @@ validate_x <- function(x = NULL, y, unique_x = FALSE) {
     }
   }
 
-  stopifnot(is.numeric(x))
+  if (!is.numeric(x)) {
+    abort("'x' must be numeric.")
+  }
 
   if (!is_vector_or_1Darray(x)) {
     abort("'x' must be a vector or 1D array.")
@@ -191,7 +205,9 @@ validate_x <- function(x = NULL, y, unique_x = FALSE) {
   }
 
   if (unique_x) {
-    stopifnot(identical(length(x), length(unique(x))))
+    if (!identical(length(x), length(unique(x)))) {
+      abort("'x' must contain only unique values.")
+    }
   }
 
   unname(x)
diff --git a/R/mcmc-diagnostics.R b/R/mcmc-diagnostics.R
index 2a71b8e0..4692d3e8 100644
--- a/R/mcmc-diagnostics.R
+++ b/R/mcmc-diagnostics.R
@@ -389,7 +389,9 @@ diagnostic_factor.neff_ratio <- function(x, ..., breaks = c(0.1, 0.5)) {
 
 diagnostic_data_frame <- function(x) {
   x <- auto_name(sort(x))
-  stopifnot(!anyDuplicated(names(x)))
+  if (anyDuplicated(names(x))) {
+    abort("Diagnostic values must have unique parameter names.")
+  }
   diagnostic <- class(x)[1]
 
   d <- tibble::tibble(
@@ -584,7 +586,9 @@ drop_NAs_and_warn <- function(x) {
 # @param x object returned by prepare_mcmc_array
 # @param lags user's 'lags' argument
 acf_data <- function(x, lags) {
-  stopifnot(is_mcmc_array(x))
+  if (!is_mcmc_array(x)) {
+    abort("'x' must be an mcmc_array.")
+  }
   n_iter <- num_iters(x)
   n_chain <- num_chains(x)
   n_param <- num_params(x)
@@ -628,7 +632,9 @@ new_rhat <- function(x) {
 }
 
 validate_rhat <- function(x) {
-  stopifnot(is.numeric(x), !is.list(x), !is.array(x))
+  if (!is.numeric(x) || is.list(x) || is.array(x)) {
+    abort("'rhat' must be a numeric vector.")
+  }
   if (any(x < 0, na.rm = TRUE)) {
     abort("All 'rhat' values must be positive.")
   }
@@ -656,7 +662,9 @@ new_neff_ratio <- function(x) {
 }
 
 validate_neff_ratio <- function(x) {
-  stopifnot(is.numeric(x), !is.list(x), !is.array(x))
+  if (!is.numeric(x) || is.list(x) || is.array(x)) {
+    abort("'neff_ratio' must be a numeric vector.")
+  }
   if (any(x < 0, na.rm = TRUE)) {
     abort("All neff ratios must be positive.")
   }