fix(upgrade): don't force-reinstall already installed versions

e2e/cli/test_upgrade

@@ -70,6 +70,44 @@ assert "ls $MISE_DATA_DIR/installs/dummy/1.0.0"
 mise up
 assert_contains "mise up --dry-run-code 2>&1" "All tools are up to date"
 
+# Test: upgrade should not re-download a version that is already installed
+# This verifies the fix for https://github.com/jdx/mise/discussions/8260
+cat <<EOF >mise.toml
+[tools]
+dummy = "1"
+EOF
+mise uninstall dummy --all
+# Install 1.0.0 first (old version), then also install 1.1.0 (the latest 1.x)
+mise install dummy@1.0.0
+mise install dummy@1.1.0
+assert_contains "mise ls --installed dummy" "1.0.0"
+assert_contains "mise ls --installed dummy" "1.1.0"
+# upgrade should report "All tools are up to date" since 1.1.0 is already installed
+assert_contains "mise up 2>&1" "All tools are up to date"
+
+# Test: upgrade with lockfile should update the lockfile to the new version
+cat <<EOF >mise.toml
+[tools]
+dummy = "1"
+EOF
+mise uninstall dummy --all
+mise install dummy@1.0.0
+# Write a lockfile pinning to 1.0.0
+cat <<EOF >mise.lock
+[[tools.dummy]]
+version = "1.0.0"
+backend = "asdf:dummy"
+EOF
+# Verify the lockfile pins to 1.0.0
+assert "mise ls dummy" "dummy  1.0.0  ~/workdir/mise.toml  1"
+# Upgrade should install 1.1.0 and update the lockfile
+mise up
+assert "mise ls dummy" "dummy  1.1.0  ~/workdir/mise.toml  1"
+# Verify the lockfile was updated to 1.1.0
+assert_contains "cat mise.lock" "1.1.0"
+assert_not_contains "cat mise.lock" "1.0.0"
+rm -f mise.lock
+
 # Test that upgrading a specific tool doesn't check other tools
 # This verifies the fix for https://github.com/jdx/mise/discussions/7328
 cat <<EOF >mise.toml

src/backend/mod.rs

@@ -563,6 +563,17 @@ pub trait Backend: Debug + Send + Sync {
                 if let Some(install_path) = tv.request.install_path(config)
                     && check_path(&install_path, true)
                 {
+                    // For Prefix requests, install_path finds any installed dir
+                    // matching the prefix (e.g., "1.0.0" for prefix "1"), but if
+                    // the ToolVersion resolved to a different version (e.g., "1.1.0"),
+                    // we must not treat it as installed.
+                    if let ToolRequest::Prefix { .. } = &tv.request
+                        && install_path
+                            .file_name()
+                            .is_some_and(|f| f.to_string_lossy() != tv.version)
+                    {
+                        return check_path(&tv.install_path(), check_symlink);
+                    }
                     return true;
                 }
                 check_path(&tv.install_path(), check_symlink)

src/cli/upgrade.rs

@@ -210,8 +210,7 @@ impl Upgrade {
 
         let opts = InstallOptions {
             reason: "upgrade".to_string(),
-            // TODO: can we remove this without breaking e2e/cli/test_upgrade? it may be causing tools to re-install
-            force: true,
+            force: false,
             jobs: self.jobs,
             raw: self.raw,
             resolve_options: ResolveOptions {