diff --git a/maintainers/flake-module.nix b/maintainers/flake-module.nix
index 225b0b300..5bf7837f7 100644
--- a/maintainers/flake-module.nix
+++ b/maintainers/flake-module.nix
@@ -547,7 +547,6 @@
             ''^tests/functional/flakes/absolute-paths\.sh$''
             ''^tests/functional/flakes/check\.sh$''
             ''^tests/functional/flakes/config\.sh$''
-            ''^tests/functional/flakes/develop\.sh$''
             ''^tests/functional/flakes/flakes\.sh$''
             ''^tests/functional/flakes/follow-paths\.sh$''
             ''^tests/functional/flakes/prefetch\.sh$''
diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index 295d290c5..2d9731ca6 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -321,11 +321,7 @@ MixEnvironment::MixEnvironment() : ignoreEnvironment(false)
     addFlag({
         .longName = "set-env-var",
         .shortName = 's',
-        .description = "Add/override an environment variable *name* with *value*.\n\n"
-                       "> **Notes**\n"
-                       ">\n"
-                       "> Duplicate definitions will be overwritten, last one wins.\n\n"
-                       "> Cancles out with `--unset-env-var`.\n\n",
+        .description = "Sets an environment variable *name* with *value*.",
         .category = environmentVariablesCategory,
         .labels = {"name", "value"},
         .handler = {[&](std::string name, std::string value) {
diff --git a/tests/functional/flakes/develop.sh b/tests/functional/flakes/develop.sh
index ba41a5feb..2e75081d4 100755
--- a/tests/functional/flakes/develop.sh
+++ b/tests/functional/flakes/develop.sh
@@ -5,11 +5,11 @@ source ../common.sh
 TODO_NixOS
 
 clearStore
-rm -rf $TEST_HOME/.cache $TEST_HOME/.config $TEST_HOME/.local
+rm -rf "$TEST_HOME/.cache" "$TEST_HOME/.config" "$TEST_HOME/.local"
 
 # Create flake under test.
-cp ../shell-hello.nix "${config_nix}" $TEST_HOME/
-cat <<EOF >$TEST_HOME/flake.nix
+cp ../shell-hello.nix "$config_nix" "$TEST_HOME/"
+cat <<EOF >"$TEST_HOME/flake.nix"
 {
     inputs.nixpkgs.url = "$TEST_HOME/nixpkgs";
     outputs = {self, nixpkgs}: {
@@ -24,13 +24,13 @@ cat <<EOF >$TEST_HOME/flake.nix
 EOF
 
 # Create fake nixpkgs flake.
-mkdir -p $TEST_HOME/nixpkgs
-cp "${config_nix}" ../shell.nix $TEST_HOME/nixpkgs
+mkdir -p "$TEST_HOME/nixpkgs"
+cp "${config_nix}" ../shell.nix "$TEST_HOME/nixpkgs"
 
 # `config.nix` cannot be gotten via build dir / env var (runs afoul pure eval). Instead get from flake.
 removeBuildDirRef "$TEST_HOME/nixpkgs"/*.nix
 
-cat <<EOF >$TEST_HOME/nixpkgs/flake.nix
+cat <<EOF >"$TEST_HOME/nixpkgs/flake.nix"
 {
     outputs = {self}: {
       legacyPackages.$system.bashInteractive = (import ./shell.nix {}).bashInteractive;
@@ -38,7 +38,7 @@ cat <<EOF >$TEST_HOME/nixpkgs/flake.nix
 }
 EOF
 
-cd $TEST_HOME
+cd "$TEST_HOME"
 
 # Test whether `nix develop` passes through environment variables.
 [[ "$(
@@ -54,6 +54,79 @@ echo "\$ENVVAR"
 EOF
 )" ]]
 
+# Test wether `--keep-env-var` keeps the environment variable.
+(
+  expect='BAR'
+  got="$(FOO='BAR' nix develop --ignore-env --keep-env-var FOO --no-write-lock-file .#hello <<EOF
+echo "\$FOO"
+EOF
+)"
+  [[ "$got" == "$expect" ]]
+)
+
+# Test wether duplicate `--keep-env-var` keeps the environment variable.
+(
+  expect='BAR'
+  got="$(FOO='BAR' nix develop --ignore-env --keep-env-var FOO --keep-env-var FOO --no-write-lock-file .#hello <<EOF
+echo "\$FOO"
+EOF
+)"
+  [[ "$got" == "$expect" ]]
+)
+
+# Test wether `--set-env-var` sets the environment variable.
+(
+  expect='BAR'
+  got="$(nix develop --ignore-env --set-env-var FOO 'BAR' --no-write-lock-file .#hello <<EOF
+echo "\$FOO"
+EOF
+)"
+  [[ "$got" == "$expect" ]]
+)
+
+# Test that `--set-env-var` overwrites previously set variables.
+(
+  expect='BLA'
+  got="$(FOO='BAR' nix develop --set-env-var FOO 'BLA' --no-write-lock-file .#hello <<EOF
+echo "\$FOO"
+EOF
+)"
+  [[ "$got" == "$expect" ]]
+)
+
+# Test that multiple `--set-env-var` work.
+(
+  expect='BARFOO'
+  got="$(nix develop --set-env-var FOO 'BAR' --set-env-var BAR 'FOO' --no-write-lock-file .#hello <<EOF | tr -d '\n'
+echo "\$FOO"
+echo "\$BAR"
+EOF
+)"
+  [[ "$got" == "$expect" ]]
+)
+
+# Check that we throw an error when `--keep-env-var` is used without `--ignore-env`.
+expectStderr 1 nix develop --keep-env-var FOO .#hello |
+  grepQuiet "error: --keep-env-var does not make sense without --ignore-env"
+
+# Check that we throw an error when `--unset-env-var` is used with `--ignore-env`.
+expectStderr 1 nix develop --ignore-env --unset-env-var FOO .#hello |
+  grepQuiet "error: --unset-env-var does not make sense with --ignore-env"
+
+# Test wether multiple occurances of `--set-env-var` throws.
+expectStderr 1 nix develop --set-env-var FOO 'BAR' --set-env-var FOO 'BLA' --no-write-lock-file .#hello |
+  grepQuiet "error: Duplicate definition of environment variable 'FOO' with '--set-env-var' is ambiguous"
+
+# Test wether similar `--unset-env-var` and `--set-env-var` throws.
+expectStderr 1 nix develop --set-env-var FOO 'BAR' --unset-env-var FOO --no-write-lock-file .#hello |
+  grepQuiet "error: Cannot unset environment variable 'FOO' that is set with '--set-env-var'"
+
+expectStderr 1 nix develop --unset-env-var FOO --set-env-var FOO 'BAR' --no-write-lock-file .#hello |
+  grepQuiet "error: Cannot set environment variable 'FOO' that is unset with '--unset-env-var'"
+
+# Check that multiple `--ignore-env`'s are okay.
+expectStderr 0 nix develop --ignore-env --set-env-var FOO 'BAR' --ignore-env .#hello
+
 # Determine the bashInteractive executable.
 nix build --no-write-lock-file './nixpkgs#bashInteractive' --out-link ./bash-interactive
 BASH_INTERACTIVE_EXECUTABLE="$PWD/bash-interactive/bin/bash"