diff --git a/CHANGELOG.md b/CHANGELOG.md
index a45f3248..388fe91a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - `--jobs auto` / `-j auto` caps parallel concurrency at the CPU core count (portable across Linux/macOS/BSD); the default stays unlimited (#766)
 
 ### Changed
+- Multi-file runs are no longer quadratic in file count: each file's test functions are unset once the file has been processed, so test subshells stop forking an ever-growing shell. bashunit's own 63-file unit suite: ~64s -> ~22s sequential, ~26s -> ~7s parallel (#829)
 - Major performance work with no behaviour change: assertions, per-test execution, per-file discovery, cold start and parallel result publishing are now (near) fork-free, snapshots and `--tag` scans are cached, and quadratic failure rendering is single-pass. Benchmarks on bash 3.2: 100x10 `assert_equals` ~1.50s -> ~0.76s, 500 snapshot assertions ~7.5s -> ~3.0s, 100 tagged tests ~2.92s -> ~0.68s, and bashunit's own acceptance suite ~61s -> ~17s (#761-#764, #772-#775, #798, #801-#807, #809, #810, #813, #817)
 - Per-test timing now defaults to `auto` (`BASHUNIT_SHOW_EXECUTION_TIME=true|false|auto`): shown only when the clock is fork-free, avoiding `perl` forks on bash 3.2; `--profile`/`--verbose`/reports still measure (see `adrs/adr-008-auto-skip-per-test-timing.md`) (#765)
 - `assert_equals`/`assert_same` failures with multiline values now render a git word-diff below the header (requires git, opt out with `BASHUNIT_NO_DIFF=true`, respects `--no-color`); machine reports keep the raw values (#777)
diff --git a/src/runner.sh b/src/runner.sh
index 1a2d914d..bb1b08da 100755
--- a/src/runner.sh
+++ b/src/runner.sh
@@ -430,6 +430,9 @@ function bashunit::runner::load_test_files() {
     filtered_functions=$(bashunit::helper::get_functions_to_run "test" "$filter" "$_BASHUNIT_CACHED_ALL_FUNCTIONS")
     local functions_for_script
     functions_for_script=$(bashunit::runner::functions_for_script "$test_file" "$filtered_functions")
+    # Full pre-tag/rerun list: these are unset once the file has been
+    # processed, whatever subset actually runs (#829).
+    local _script_fns_to_clean="$functions_for_script"
     # Apply tag filtering to the early check as well
     if [ -n "$tag_filter" ] || [ -n "$exclude_tag_filter" ]; then
       bashunit::helper::build_tags_map "$test_file"
@@ -448,6 +451,7 @@ function bashunit::runner::load_test_files() {
       functions_for_script=$(bashunit::rerun::filter_functions "$test_file" "$functions_for_script")
     fi
     if [ -z "$functions_for_script" ]; then
+      bashunit::runner::clean_script_test_functions "$_script_fns_to_clean"
       bashunit::runner::clean_set_up_and_tear_down_after_script
       bashunit::runner::restore_workdir
       continue
@@ -492,6 +496,7 @@ function bashunit::runner::load_test_files() {
         "$exclude_tag_filter" "$_cached_fns"
     fi
     bashunit::runner::run_tear_down_after_script "$test_file"
+    bashunit::runner::clean_script_test_functions "$_script_fns_to_clean"
     bashunit::runner::clean_set_up_and_tear_down_after_script
     if ! bashunit::parallel::is_enabled; then
       bashunit::cleanup_script_temp_files
@@ -2066,6 +2071,24 @@ function bashunit::runner::run_tear_down_after_script() {
   return $status
 }
 
+##
+# Unset a file's test functions once the file has been processed.
+#
+# Test files are sourced into the main shell, and their functions used to stay
+# defined for the whole run: every test's $() subshell then forked an
+# ever-growing shell, making multi-file runs quadratic in file count (#829).
+# In parallel mode the file's workers have already forked (with their own copy
+# of the functions) by the time this runs, so unsetting here is race-free.
+# Arguments: $1 - whitespace-separated test function names
+##
+function bashunit::runner::clean_script_test_functions() {
+  local IFS=$' \t\n'
+  local fn
+  for fn in $1; do
+    unset -f "$fn" 2>/dev/null || true
+  done
+}
+
 function bashunit::runner::clean_set_up_and_tear_down_after_script() {
   bashunit::internal_log "clean_set_up_and_tear_down_after_script"
   bashunit::helper::unset_if_exists 'set_up'
diff --git a/tests/acceptance/bashunit_fn_cleanup_test.sh b/tests/acceptance/bashunit_fn_cleanup_test.sh
new file mode 100644
index 00000000..0fa3ce8d
--- /dev/null
+++ b/tests/acceptance/bashunit_fn_cleanup_test.sh
@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Regression test for https://github.com/TypedDevs/bashunit/issues/829
+# Test functions must be unset once their file has been processed: they stay
+# defined in the main shell otherwise, and every test's $() subshell forks an
+# ever-fatter process, making multi-file runs quadratic in file count.
+
+function set_up_before_script() {
+  TEST_ENV_FILE="tests/acceptance/fixtures/.env.default"
+}
+
+function test_test_functions_are_unset_after_their_file_ran() {
+  local first_file=./tests/acceptance/fixtures/test_fn_cleanup_first.sh
+  local second_file=./tests/acceptance/fixtures/test_fn_cleanup_second.sh
+
+  local actual
+  actual="$(./bashunit --no-parallel --env "$TEST_ENV_FILE" "$first_file" "$second_file" | strip_ansi)"
+
+  assert_contains "2 passed, 2 total" "$actual"
+}
+
+function test_test_functions_are_unset_after_their_file_ran_in_parallel() {
+  local first_file=./tests/acceptance/fixtures/test_fn_cleanup_first.sh
+  local second_file=./tests/acceptance/fixtures/test_fn_cleanup_second.sh
+
+  local actual
+  actual="$(./bashunit --parallel --env "$TEST_ENV_FILE" "$first_file" "$second_file" | strip_ansi)"
+
+  assert_contains "2 passed, 2 total" "$actual"
+}
diff --git a/tests/acceptance/fixtures/test_fn_cleanup_first.sh b/tests/acceptance/fixtures/test_fn_cleanup_first.sh
new file mode 100644
index 00000000..36d477eb
--- /dev/null
+++ b/tests/acceptance/fixtures/test_fn_cleanup_first.sh
@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Regression fixture for https://github.com/TypedDevs/bashunit/issues/829
+# First file: defines a marker test function that must not leak into the
+# main shell once this file has been processed.
+
+function test_fn_cleanup_marker_from_first_file() {
+  assert_equals "first" "first"
+}
diff --git a/tests/acceptance/fixtures/test_fn_cleanup_second.sh b/tests/acceptance/fixtures/test_fn_cleanup_second.sh
new file mode 100644
index 00000000..53707af3
--- /dev/null
+++ b/tests/acceptance/fixtures/test_fn_cleanup_second.sh
@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Regression fixture for https://github.com/TypedDevs/bashunit/issues/829
+# Second file: the first file's test functions must already be unset, so the
+# main shell does not grow (and slow down every fork) as files accumulate.
+
+function test_previous_file_test_functions_are_unset() {
+  local defined="no"
+  if declare -F test_fn_cleanup_marker_from_first_file >/dev/null 2>&1; then
+    defined="yes"
+  fi
+  assert_equals "no" "$defined"
+}