diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index a0fbaff..2de2717 100755
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -356,6 +356,22 @@ $psa_settings = array(
 );
 ```
 
+### Schema versioning & automatic upgrades
+
+The plugin stores the schema version it last migrated to in a separate option, `psa_db_version`. This is distinct from `PSA_VERSION` (the running code's semver), and lets the plugin detect whether the database needs to catch up after an in-place file update.
+
+- **Option name:** `PSA_Upgrade::OPTION_NAME` (`psa_db_version`), string, semver.
+- **Default:** `PSA_Upgrade::DEFAULT_STORED_VERSION` (`'0.0.0'`) when the option has never been written — strictly less than any released version so fresh installs always trigger the upgrade path.
+- **Driver:** `PSA_Upgrade::maybe_run()` in `includes/class-psa-upgrade.php`, called on every `admin_init` from `psa_admin_init()`. The method:
+  1. Reads `psa_db_version`.
+  2. Calls the pure helper `PSA_Upgrade::is_needed( $stored, PSA_VERSION )`, a thin wrapper over `version_compare` (stored < current → upgrade).
+  3. If an upgrade is needed, re-runs `PSA_Cost_Tracker::create_table()` so `dbDelta` picks up any additive schema changes (new columns, new indexes).
+  4. Writes `update_option( 'psa_db_version', PSA_VERSION, false )` so the upgrade doesn't re-run on the next request.
+- **Why on `admin_init`:** dbDelta is safe-to-repeat but not free. Running it once when an admin visits after an update — rather than on every frontend page load — keeps the cost near zero without requiring users to deactivate/reactivate the plugin.
+- **Downgrade / rollback:** if the running code is older than `psa_db_version` (e.g., deploy rolled back), `is_needed()` returns false and the driver is a no-op. The newer schema is left intact; old code continues to work against it as long as changes are additive.
+- **Cleanup:** `uninstall.php` deletes `psa_db_version` alongside the other plugin options.
+- **Testability:** `PSA_Upgrade::is_needed()` is a pure static method (no WordPress API calls) and is covered by `tests/unit/UpgradeTest.php` across fresh-install, same-version, future-version, and older-version scenarios, plus constant-stability checks on `OPTION_NAME` and `DEFAULT_STORED_VERSION`.
+
 ## Frontend Architecture
 
 ### JavaScript (jQuery-based)
diff --git a/includes/class-psa-upgrade.php b/includes/class-psa-upgrade.php
new file mode 100644
index 0000000..643fa2b
--- /dev/null
+++ b/includes/class-psa-upgrade.php
@@ -0,0 +1,83 @@
+<?php
+/**
+ * Schema upgrade helpers for Peptide Search AI.
+ *
+ * What: Pure-function helpers that decide whether a database schema upgrade
+ * is required, plus the driver that runs the upgrade when the installed
+ * code version is newer than the stored psa_db_version option.
+ *
+ * Who triggers it: psa_admin_init() in peptide-search-ai.php calls
+ * PSA_Upgrade::maybe_run() on every WordPress admin_init hook.
+ *
+ * Dependencies: PSA_Cost_Tracker (for re-running dbDelta on the
+ * cost-tracker table), WordPress options API (get_option / update_option),
+ * and the PSA_VERSION constant defined in peptide-search-ai.php.
+ *
+ * @package PeptideSearchAI
+ */
+
+declare( strict_types=1 );
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * PSA_Upgrade — thin wrapper around version_compare + dbDelta.
+ *
+ * The class exists so the decision logic (is_needed) can be unit-tested as a
+ * pure function, while the impure driver (maybe_run) stays isolated.
+ */
+class PSA_Upgrade {
+
+	/**
+	 * Option name used to persist the schema version last migrated to.
+	 *
+	 * @var string
+	 */
+	const OPTION_NAME = 'psa_db_version';
+
+	/**
+	 * Default stored version for fresh installs. Strictly less than any
+	 * real released version so version_compare treats it as "older".
+	 *
+	 * @var string
+	 */
+	const DEFAULT_STORED_VERSION = '0.0.0';
+
+	/**
+	 * Decide whether a schema upgrade is needed given a stored version and
+	 * the currently-installed code version. Pure function — no side effects,
+	 * no WordPress API calls — so it can be exercised in isolation.
+	 *
+	 * @param string $stored_version  Version string recorded in psa_db_version.
+	 * @param string $current_version Version string of the running code.
+	 * @return bool True if an upgrade must run; false otherwise.
+	 */
+	public static function is_needed( string $stored_version, string $current_version ): bool {
+		return version_compare( $stored_version, $current_version, '<' );
+	}
+
+	/**
+	 * Run database upgrades when the installed code version is newer than
+	 * the stored psa_db_version option. Keeps users from needing to
+	 * deactivate/reactivate the plugin after a schema change.
+	 *
+	 * Side effects: reads psa_db_version option; on upgrade, calls
+	 * PSA_Cost_Tracker::create_table() and writes psa_db_version.
+	 *
+	 * @return void
+	 */
+	public static function maybe_run(): void {
+		$stored = get_option( self::OPTION_NAME, self::DEFAULT_STORED_VERSION );
+		if ( ! self::is_needed( $stored, PSA_VERSION ) ) {
+			return;
+		}
+
+		// Re-run CREATE TABLE via dbDelta — additive schema changes (new
+		// columns, new indexes) are applied automatically.
+		PSA_Cost_Tracker::create_table();
+
+		update_option( self::OPTION_NAME, PSA_VERSION, false );
+	}
+}
diff --git a/peptide-search-ai.php b/peptide-search-ai.php
index e6da0bb..e1661c3 100755
--- a/peptide-search-ai.php
+++ b/peptide-search-ai.php
@@ -33,6 +33,7 @@ require_once PSA_PLUGIN_DIR . 'includes/class-psa-admin.php';
 require_once PSA_PLUGIN_DIR . 'includes/class-psa-batch-enrichment.php';
 require_once PSA_PLUGIN_DIR . 'includes/class-psa-template.php';
 require_once PSA_PLUGIN_DIR . 'includes/class-psa-directory.php';
+require_once PSA_PLUGIN_DIR . 'includes/class-psa-upgrade.php';
 
 function psa_init() {
 	PSA_Post_Type::register_peptide_post_type();
@@ -48,33 +49,10 @@ add_action( 'init', 'psa_init' );
 
 function psa_admin_init() {
 	PSA_Post_Type::init_admin();
-	psa_maybe_run_upgrades();
+	PSA_Upgrade::maybe_run();
 }
 add_action( 'admin_init', 'psa_admin_init' );
 
-/**
- * Run database upgrades when the installed code version is newer than the
- * stored `psa_db_version` option. Keeps users from needing to
- * deactivate/reactivate the plugin after a schema change.
- *
- * Triggered on admin_init so dbDelta runs once per upgrade, not on every
- * frontend page load.
- *
- * @return void
- */
-function psa_maybe_run_upgrades() {
-	$stored = get_option( 'psa_db_version', '0.0.0' );
-	if ( version_compare( $stored, PSA_VERSION, '>=' ) ) {
-		return;
-	}
-
-	// Re-run CREATE TABLE via dbDelta — additive schema changes (new columns)
-	// will be applied automatically.
-	PSA_Cost_Tracker::create_table();
-
-	update_option( 'psa_db_version', PSA_VERSION, false );
-}
-
 /**
  * Check if current page is the Echo KB main page.
  * Uses is_page() which is reliable even in block templates.
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 34f8ad5..8dc8f02 100755
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -130,3 +130,4 @@ require_once ABSPATH . 'includes/class-psa-cost-tracker.php';
 require_once ABSPATH . 'includes/class-psa-openrouter.php';
 require_once ABSPATH . 'includes/class-psa-kb-builder.php';
 require_once ABSPATH . 'includes/class-psa-ai-generator.php';
+require_once ABSPATH . 'includes/class-psa-upgrade.php';
diff --git a/tests/unit/UpgradeTest.php b/tests/unit/UpgradeTest.php
new file mode 100644
index 0000000..999773a
--- /dev/null
+++ b/tests/unit/UpgradeTest.php
@@ -0,0 +1,87 @@
+<?php
+/**
+ * Tests for the schema-upgrade decision logic in PSA_Upgrade.
+ *
+ * Covers PSA_Upgrade::is_needed() across the four version-comparison
+ * scenarios: fresh install, already-upgraded, future-version (downgrade),
+ * and older stored version.
+ *
+ * @package PeptideSearchAI
+ */
+
+declare( strict_types=1 );
+
+use PHPUnit\Framework\TestCase;
+
+// PSA_Upgrade is already loaded by tests/bootstrap.php via class-psa-upgrade.php.
+// No plugin-file bootstrap needed — the decision helper is a pure function.
+
+/**
+ * Test case for PSA_Upgrade::is_needed().
+ *
+ * The helper is a pure static method (no WordPress API calls, no side
+ * effects), so it can be exercised directly without a live option store.
+ */
+class UpgradeTest extends TestCase {
+
+	/**
+	 * Fresh install: stored version defaults to '0.0.0', which is strictly
+	 * less than any real released version, so an upgrade must run.
+	 */
+	public function test_fresh_install_triggers_upgrade(): void {
+		$this->assertTrue(
+			PSA_Upgrade::is_needed( '0.0.0', '4.4.3' ),
+			'Fresh install (stored = 0.0.0) must trigger an upgrade against any released version.'
+		);
+	}
+
+	/**
+	 * Already-upgraded: stored version equals current. No upgrade needed.
+	 */
+	public function test_same_version_skips_upgrade(): void {
+		$this->assertFalse(
+			PSA_Upgrade::is_needed( '4.4.3', '4.4.3' ),
+			'Stored version equal to current version must not trigger an upgrade.'
+		);
+	}
+
+	/**
+	 * Future-version: stored version is ahead of the running code (e.g., a
+	 * downgrade, or a user running multiple sites out of sync). Do not run
+	 * upgrade — it could apply an older schema over a newer one.
+	 */
+	public function test_future_stored_version_skips_upgrade(): void {
+		$this->assertFalse(
+			PSA_Upgrade::is_needed( '5.0.0', '4.4.3' ),
+			'Stored version greater than current version must not trigger an upgrade.'
+		);
+	}
+
+	/**
+	 * Belt-and-braces: a strictly newer code version against an older
+	 * stored version must trigger an upgrade. Guards against an accidental
+	 * operator flip in version_compare() (e.g., '>=' vs '<').
+	 */
+	public function test_older_stored_version_triggers_upgrade(): void {
+		$this->assertTrue(
+			PSA_Upgrade::is_needed( '4.4.2', '4.4.3' ),
+			'Stored version older than current version must trigger an upgrade.'
+		);
+	}
+
+	/**
+	 * Constants used by the driver should be stable across releases so
+	 * existing installs keep pointing at the same option key.
+	 */
+	public function test_option_name_constant_is_stable(): void {
+		$this->assertSame( 'psa_db_version', PSA_Upgrade::OPTION_NAME );
+	}
+
+	/**
+	 * Default stored version must be strictly less than any future release
+	 * so fresh-install detection keeps working.
+	 */
+	public function test_default_stored_version_is_zero(): void {
+		$this->assertSame( '0.0.0', PSA_Upgrade::DEFAULT_STORED_VERSION );
+	}
+}
diff --git a/uninstall.php b/uninstall.php
index 6381564..8b81cd9 100755
--- a/uninstall.php
+++ b/uninstall.php
@@ -42,6 +42,7 @@ if ( ! is_wp_error( $terms ) && ! empty( $terms ) ) {
 // 3. Delete plugin options.
 delete_option( 'psa_settings' );
 delete_option( 'psa_search_cache_gen' );
+delete_option( 'psa_db_version' );
 
 // 4. Delete all plugin transients (validation cache, rate limits).
 $wpdb->query(