diff --git a/migrations/20260406_fix_legacy_version_semver.sql b/migrations/20260406_fix_legacy_version_semver.sql
new file mode 100644
index 0000000..277e42d
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+++ b/migrations/20260406_fix_legacy_version_semver.sql
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+-- Fix for api#40: Assign correct sequential semver to legacy versions
+-- 
+-- The previous migration (20260406_backfill_legacy_versions.sql) had a bug where it used
+-- a correlated subquery that referenced the same table being updated with the same
+-- WHERE condition (bv2.semver IS NULL). During execution, this caused all rows to
+-- incorrectly get assigned '1.0.0-beta.1' because the subquery couldn't properly
+-- evaluate the relative ordering during the UPDATE.
+--
+-- This migration fixes the issue by using a safer approach with ROW_NUMBER() window function
+-- in a subquery that doesn't reference the table being modified.
+
+-- Reset any rows that have the incorrect '1.0.0-beta.1' value
+-- (or any semver starting with '1.0.0-beta.' where we need to reassign)
+UPDATE benchmark_versions
+SET semver = NULL, label = NULL
+WHERE semver LIKE '1.0.0-beta.%';
+
+-- Now assign correct sequential semver values
+-- First version gets beta.1, second gets beta.2, etc.
+UPDATE benchmark_versions
+SET 
+  semver = (
+    SELECT new_semver FROM (
+      SELECT 
+        id,
+        '1.0.0-beta.' || CAST(ROW_NUMBER() OVER (ORDER BY created_at ASC) AS TEXT) as new_semver
+      FROM benchmark_versions
+      WHERE semver IS NULL
+    ) numbered
+    WHERE numbered.id = benchmark_versions.id
+  ),
+  label = (
+    SELECT new_label FROM (
+      SELECT 
+        id,
+        '1.0.0-beta.' || CAST(ROW_NUMBER() OVER (ORDER BY created_at ASC) AS TEXT) as new_label
+      FROM benchmark_versions
+      WHERE semver IS NULL
+    ) numbered
+    WHERE numbered.id = benchmark_versions.id
+  )
+WHERE semver IS NULL;
+
+-- Verification query:
+-- SELECT id, semver, label, created_at FROM benchmark_versions ORDER BY created_at ASC;