diff --git a/NEWS.md b/NEWS.md
index 2f942ccc..1bae0243 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -6,6 +6,9 @@ keywords (@VisruthSK, #1154)
 set to `TRUE` for an entire R session via new global options. (#1159)
 * `cmdstan_model()` no longer fails when `MAKEFLAGS` enables directory-printing 
 output while reading `STANCFLAGS` from `make`. (#1163)
+* `laplace()` no longer overwrites the internally generated optimizer CSV when
+`mode = NULL` and `output_basename` is supplied. The internally generated
+optimizer CSV now uses the filename `<output_basename>-mode-1.csv`.
 
 * CmdStanModel objects created using `compile_model_methods = TRUE` that are
 then saved and reloaded no longer error in model fitting methods. Model methods
diff --git a/R/model.R b/R/model.R
index 0359678a..f432ccdb 100644
--- a/R/model.R
+++ b/R/model.R
@@ -1622,6 +1622,10 @@ laplace <- function(data = NULL,
     }
   } else { # mode = NULL, run optimize()
     checkmate::assert_list(opt_args, any.missing = FALSE, names = "unique", null.ok = TRUE)
+    mode_output_basename <- output_basename
+    if (!is.null(mode_output_basename)) {
+      mode_output_basename <- paste0(mode_output_basename, "-mode")
+    }
     args <- list(
       data = data,
       seed = seed,
@@ -1629,7 +1633,7 @@ laplace <- function(data = NULL,
       init = init,
       save_latent_dynamics = FALSE,
       output_dir = output_dir,
-      output_basename = output_basename,
+      output_basename = mode_output_basename,
       sig_figs = sig_figs,
       threads = threads,
       opencl_ids = opencl_ids,
diff --git a/tests/testthat/test-model-compile-user_header.R b/tests/testthat/test-model-compile-user_header.R
index 6799c0f9..7c9d6edb 100644
--- a/tests/testthat/test-model-compile-user_header.R
+++ b/tests/testthat/test-model-compile-user_header.R
@@ -1,9 +1,5 @@
 skip_if(os_is_macos())
 
-file_that_exists <- withr::local_tempfile(pattern = "placeholder_exists")
-file.create(file_that_exists)
-file_that_doesnt_exist <- withr::local_tempfile(pattern = "placeholder_doesnt_exist")
-
 w_path <- function(f) {
   x <- sapply(f, function(fi) wsl_safe_path(absolute_path(fi)))
   names(x) <- NULL
@@ -36,7 +32,16 @@ namespace bernoulli_external_model_namespace
 }"
 
 test_that("cmdstan_model works with user_header with mock", {
+  file_that_exists <- withr::local_tempfile(pattern = "placeholder_exists")
+  file_that_doesnt_exist <- withr::local_tempfile(pattern = "placeholder_doesnt_exist")
   tmpfile <- withr::local_tempfile(lines = hpp, fileext = ".hpp")
+  file.create(file_that_exists)
+  header_mtime <- Sys.time()
+  # On GHA Windows/R 4.1 files created close together sometimes compared equal
+  # and skipped the mocked recompile, so set the header mtime to be in the past
+  # and ensure the exe mtime is newer
+  Sys.setFileTime(file_that_exists, header_mtime - 10)
+  Sys.setFileTime(tmpfile, header_mtime)
 
   with_mocked_cli(
     compile_ret = list(status = 0),
@@ -64,7 +69,10 @@ test_that("cmdstan_model works with user_header with mock", {
   )
 
   # Check recompilation upon changing header
+  exe_mtime <- header_mtime + 10
+  # Mocked compile does not create the executable that real compilation writes.
   file.create(file_that_exists)
+  Sys.setFileTime(file_that_exists, exe_mtime)
   with_mocked_cli(
     compile_ret = list(status = 0),
     info_ret = list(),
@@ -73,7 +81,8 @@ test_that("cmdstan_model works with user_header with mock", {
     })
   )
 
-  Sys.setFileTime(tmpfile, Sys.time() + 1) # touch file to trigger recompile
+  header_mtime <- exe_mtime + 10
+  Sys.setFileTime(tmpfile, header_mtime) # touch file to trigger recompile
   with_mocked_cli(
     compile_ret = list(status = 0),
     info_ret = list(),
@@ -82,8 +91,9 @@ test_that("cmdstan_model works with user_header with mock", {
     })
   )
 
-  # mock does not automatically update file mtime
-  Sys.setFileTime(mod$exe_file(), Sys.time() + 1) # touch file to trigger recompile
+  # Mocked compile does not create the executable that real compilation writes.
+  file.create(mod$exe_file())
+  Sys.setFileTime(mod$exe_file(), header_mtime + 10) # make exe newer than header
 
   # Alternative spec of user header
   with_mocked_cli(
diff --git a/tests/testthat/test-model-laplace.R b/tests/testthat/test-model-laplace.R
index 961883c8..3d4def90 100644
--- a/tests/testthat/test-model-laplace.R
+++ b/tests/testthat/test-model-laplace.R
@@ -62,6 +62,28 @@ test_that("laplace() runs when all arguments specified validly", {
   expect_s3_class(fit2, "CmdStanLaplace")
 })
 
+test_that("laplace() avoids output_basename conflict with internal optimize()", {
+  output_dir <- withr::local_tempdir("laplace-output-basename")
+
+  utils::capture.output(
+    fit <- mod$laplace(
+      data = data_list,
+      seed = 123,
+      refresh = 0,
+      output_dir = output_dir,
+      output_basename = "custom-laplace",
+      draws = 10
+    )
+  )
+
+  expect_equal(basename(fit$output_files()), "custom-laplace-1.csv")
+  expect_equal(basename(fit$mode()$output_files()), "custom-laplace-mode-1.csv")
+  expect_setequal(
+    list.files(output_dir, pattern = "\\.csv$"),
+    c("custom-laplace-1.csv", "custom-laplace-mode-1.csv")
+  )
+})
+
 test_that("laplace() all valid 'mode' inputs give same results", {
   utils::capture.output({
     mode <- mod$optimize(data = data_list, jacobian = TRUE, seed = 100, refresh = 0)