diff --git a/cli/lib/cli/engine.ex b/cli/lib/cli/engine.ex
index d63783f..a459f90 100644
--- a/cli/lib/cli/engine.ex
+++ b/cli/lib/cli/engine.ex
@@ -100,7 +100,8 @@ defmodule Cli.Engine do
     |> Map.keys()
     |> Enum.filter(fn name ->
       name == "CALLER_PWD" or String.ends_with?(name, "_CALLER_PWD") or
-        String.starts_with?(name, "MISE_") or String.starts_with?(name, "usage_")
+        (String.starts_with?(name, "MISE_") and name != "MISE_DATA_DIR") or
+        String.starts_with?(name, "usage_")
     end)
     |> Enum.map(&{String.to_charlist(&1), false})
   end
diff --git a/cli/test/engine_test.exs b/cli/test/engine_test.exs
index 1b8aec3..89213ca 100644
--- a/cli/test/engine_test.exs
+++ b/cli/test/engine_test.exs
@@ -136,6 +136,7 @@ defmodule Cli.EngineTest do
         printf 'EXECUTABLE=%s\\n' "$0"
         printf 'CWD=%s\\n' "$(pwd -P)"
         printf 'PATH=%s\\n' "$PATH"
+        printf 'MISE_DATA_DIR=%s\\n' "${MISE_DATA_DIR-}"
         printf 'MISE_CONFIG_ROOT=%s\\n' "${MISE_CONFIG_ROOT-}"
         printf 'MISE_TASK_NAME=%s\\n' "${MISE_TASK_NAME-}"
         printf 'usage_message=%s\\n' "${usage_message-}"
@@ -184,6 +185,7 @@ defmodule Cli.EngineTest do
       assert child =~ "EXECUTABLE=#{executable}"
       assert child =~ "CWD=#{String.trim(physical_cwd)}"
       assert child =~ "PATH=#{shim_bin}:#{ordinary_bin}"
+      assert child =~ "MISE_DATA_DIR=#{mise_data}"
       assert child =~ "MISE_CONFIG_ROOT=\n"
       assert child =~ "MISE_TASK_NAME=\n"
       assert child =~ "usage_message=\n"
diff --git a/lib/harness-env.sh b/lib/harness-env.sh
index 85320e9..852e567 100644
--- a/lib/harness-env.sh
+++ b/lib/harness-env.sh
@@ -15,6 +15,9 @@ sessions_scrub_task_env() {
   local name
   while IFS= read -r name; do
     case "$name" in
+      # MISE_DATA_DIR is user-owned tool storage. Retained shims need it to
+      # resolve target-project tools from a non-default mise installation.
+      MISE_DATA_DIR) ;;
       MISE_*|usage_*) unset "$name" ;;
     esac
   done < <(compgen -e)
diff --git a/test/helpers.bash b/test/helpers.bash
index 1f08b7e..98b4943 100644
--- a/test/helpers.bash
+++ b/test/helpers.bash
@@ -72,6 +72,7 @@ stub_pi_capture_env() {
 printf 'CALLER_PWD=%s\n' "\${CALLER_PWD-}" > "$env_capture"
 printf 'SESSIONS_CALLER_PWD=%s\n' "\${SESSIONS_CALLER_PWD-}" >> "$env_capture"
 printf 'OTHER_CALLER_PWD=%s\n' "\${OTHER_CALLER_PWD-}" >> "$env_capture"
+printf 'MISE_DATA_DIR=%s\n' "\${MISE_DATA_DIR-}" >> "$env_capture"
 printf 'MISE_CONFIG_ROOT=%s\n' "\${MISE_CONFIG_ROOT-}" >> "$env_capture" # codebase:ignore - fixture proves inherited MCR is scrubbed
 printf 'MISE_TASK_NAME=%s\n' "\${MISE_TASK_NAME-}" >> "$env_capture"
 printf 'usage_message=%s\n' "\${usage_message-}" >> "$env_capture"
diff --git a/test/run.bats b/test/run.bats
index 8e152bc..647dfca 100644
--- a/test/run.bats
+++ b/test/run.bats
@@ -676,6 +676,7 @@ STUB
   export CALLER_PWD="/stale/caller"
   export SESSIONS_CALLER_PWD="/stale/sessions"
   export OTHER_CALLER_PWD="/stale/other"
+  export MISE_DATA_DIR="$mise_data"
   export usage_stale_probe="stale task value"
 
   PATH="$stale_bin:$pi_bin:$shim_bin:$fresh_bin:$PATH" run sessions run \
@@ -688,6 +689,7 @@ STUB
   grep -q '^CALLER_PWD=$' "$env_capture"
   grep -q '^SESSIONS_CALLER_PWD=$' "$env_capture"
   grep -q '^OTHER_CALLER_PWD=$' "$env_capture"
+  grep -q "^MISE_DATA_DIR=$mise_data$" "$env_capture"
   grep -q '^MISE_CONFIG_ROOT=$' "$env_capture"
   grep -q '^MISE_TASK_NAME=$' "$env_capture"
   grep -q '^usage_message=$' "$env_capture"