diff --git a/crates/socket-patch-cli/src/commands/vendor.rs b/crates/socket-patch-cli/src/commands/vendor.rs
index e7403890..f9d40028 100644
--- a/crates/socket-patch-cli/src/commands/vendor.rs
+++ b/crates/socket-patch-cli/src/commands/vendor.rs
@@ -40,7 +40,7 @@ use crate::commands::apply::{representative_file, result_to_event, variant_match
 use crate::commands::fetch_stage::{stage_vendor_sources_in_memory, MemStageOutcome};
 use crate::commands::lock_cli::acquire_or_emit;
 use crate::commands::vex::{generate_vex_from_manifest_path, VexEmbedArgs};
-use crate::ecosystem_dispatch::{find_packages_for_purls, partition_purls};
+use crate::ecosystem_dispatch::{find_packages_for_rollback, partition_purls};
 use crate::json_envelope::{
     Command, Envelope, EnvelopeError, PatchAction, PatchEvent, RunWarning, Status, VexSummary,
 };
@@ -681,7 +681,18 @@ pub(crate) async fn vendor_records(
         global: common.global,
         global_prefix: common.global_prefix.clone(),
     };
-    let mut all_packages = find_packages_for_purls(
+    // Resolve installed packages with the qualified-purl-aware resolver, NOT
+    // `find_packages_for_purls`: the manifest keys release-variant ecosystems
+    // (gem `?platform=`, pypi `?artifact_id=`, maven `?classifier=&ext=`) by
+    // *qualified* purls, but the crawler only knows the *base* purl.
+    // `find_packages_for_purls` keys the result map by the base purl, so the
+    // `missing`/`contains_key` check below would miss every installed
+    // qualified-purl package and falsely classify it "not installed" —
+    // triggering a spurious `vendor_fetched_missing`, a redundant per-run
+    // registry download, and (for gem) a HashMap-order platform coin-flip.
+    // The rollback variant fans each base path back out to every qualified
+    // manifest purl (same invariant as `find_manifest_package_paths`).
+    let mut all_packages = find_packages_for_rollback(
         &vendorable_partition,
         &crawler_options,
         common.silent || common.json,
diff --git a/crates/socket-patch-cli/src/ecosystem_dispatch.rs b/crates/socket-patch-cli/src/ecosystem_dispatch.rs
index d2b44ea4..e218227d 100644
--- a/crates/socket-patch-cli/src/ecosystem_dispatch.rs
+++ b/crates/socket-patch-cli/src/ecosystem_dispatch.rs
@@ -964,6 +964,53 @@ mod tests {
         assert_eq!(out.get(&qualified), Some(&pkg_dir));
     }
 
+    #[tokio::test]
+    async fn find_packages_for_rollback_resolves_installed_qualified_gem() {
+        // Regression for the vendor lookup path (vendor.rs): every real
+        // production gem/pypi patch PURL is QUALIFIED (`?platform=` /
+        // `?artifact_id=`), but the crawler only knows the BASE PURL.
+        // `vendor` must resolve installed packages via the qualified-aware
+        // rollback resolver so its `all_packages.contains_key(qualified)`
+        // check recognizes the installed gem. Using `find_packages_for_purls`
+        // (base-keyed) misses the qualified key, falsely classifying the
+        // installed gem "not installed" — the bug that produced spurious
+        // `vendor_fetched_missing` events and the gem platform coin-flip.
+        let tmp = tempfile::tempdir().unwrap();
+        // A platform gem installs into a `<name>-<version>` dir (with an
+        // optional `-<platform>` suffix); lay down the plain-platform case.
+        let gem_dir = tmp.path().join("activestorage-7.0.2.2");
+        std::fs::create_dir_all(gem_dir.join("lib")).unwrap();
+
+        // `global_prefix` makes the gem crawler treat `tmp` as the gems root
+        // directly (same shortcut the ruby crawler's own tests use).
+        let options = CrawlerOptions {
+            cwd: tmp.path().to_path_buf(),
+            global: false,
+            global_prefix: Some(tmp.path().to_path_buf()),
+        };
+
+        let qualified = "pkg:gem/activestorage@7.0.2.2?platform=ruby".to_string();
+        let partitioned = partition_purls(std::slice::from_ref(&qualified), None);
+
+        // The vendor lookup path: the qualified manifest PURL is resolved to
+        // the installed dir under its EXACT qualified key.
+        let rollback = find_packages_for_rollback(&partitioned, &options, true).await;
+        assert_eq!(
+            rollback.get(&qualified),
+            Some(&gem_dir),
+            "installed qualified gem must resolve under its qualified key"
+        );
+
+        // The old resolver keyed by the BASE PURL only, so a `contains_key`
+        // on the qualified PURL missed — the exact false "not installed".
+        let base_keyed = find_packages_for_purls(&partitioned, &options, true).await;
+        assert!(
+            !base_keyed.contains_key(&qualified),
+            "find_packages_for_purls must NOT be used by vendor: it keys by \
+             the base PURL, so the qualified lookup falsely misses"
+        );
+    }
+
     #[tokio::test]
     async fn dispatch_find_empty_partition_yields_empty_map() {
         let tmp = tempfile::tempdir().unwrap();