diff --git a/CHANGELOG.md b/CHANGELOG.md
index d08f8fc..eeed56f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Fixed
+- `src/diff.ts` — Upgrade detection now works for versioned purls. Component identity was keyed on the full purl (which embeds the version, e.g. `pkg:npm/lodash@4.17.21`), so a version bump was reported as a removal + addition and `upgraded` stayed empty for essentially every real-world SBOM. Identity is now derived from the version-independent purl coordinates.
+
 ### Added
 - `src/cli.ts` — `--fail-on none|any|low|medium|high|critical` flag: turns the diff into a CI/CD gate that exits with code `3` when new CVEs meet the chosen severity policy (default `none` preserves prior always-exit-`0` behaviour)
 - Real devDependencies: `typescript`, `vitest`, `@vitest/coverage-v8`, `typescript-eslint`, `@types/node`
diff --git a/src/__tests__/diff.test.ts b/src/__tests__/diff.test.ts
index 0f8587c..ad781d4 100644
--- a/src/__tests__/diff.test.ts
+++ b/src/__tests__/diff.test.ts
@@ -42,14 +42,34 @@ describe('diff', () => {
     expect(report.removed[0].name).toBe('moment');
   });
 
-  it('detects version upgrades', () => {
+  it('detects version upgrades when matched by versioned purl', () => {
+    // Real SBOM generators embed the version in the purl, so identity must be
+    // derived from the version-independent purl coordinates — otherwise a bump
+    // looks like a remove + add rather than an upgrade.
     const a = makesbom([{ name: 'lodash', version: '4.17.20', purl: 'pkg:npm/lodash@4.17.20' }]);
     const b = makesbom([{ name: 'lodash', version: '4.17.21', purl: 'pkg:npm/lodash@4.17.21' }]);
     const report = diff(a, b);
-    // Different purl = treated as add/remove (purl includes version)
-    // With our current purl-based key: 4.17.20 -> removed, 4.17.21 -> added
-    // This is correct behavior — different purls are different packages
-    expect(report.added.length + report.removed.length + report.upgraded.length).toBeGreaterThan(0);
+    expect(report.added).toHaveLength(0);
+    expect(report.removed).toHaveLength(0);
+    expect(report.upgraded).toHaveLength(1);
+    expect(report.upgraded[0].from).toBe('4.17.20');
+    expect(report.upgraded[0].to).toBe('4.17.21');
+  });
+
+  it('matches versioned purls across purl qualifiers and subpaths', () => {
+    const a = makesbom([{ name: 'lodash', version: '4.17.20', purl: 'pkg:npm/lodash@4.17.20?arch=x64' }]);
+    const b = makesbom([{ name: 'lodash', version: '4.17.21', purl: 'pkg:npm/lodash@4.17.21?arch=x64' }]);
+    const report = diff(a, b);
+    expect(report.upgraded).toHaveLength(1);
+    expect(report.upgraded[0].isMajorBump).toBe(false);
+  });
+
+  it('matches scoped npm purls (percent-encoded @ in the scope)', () => {
+    const a = makesbom([{ name: '@angular/core', version: '15.0.0', purl: 'pkg:npm/%40angular/core@15.0.0' }]);
+    const b = makesbom([{ name: '@angular/core', version: '16.0.0', purl: 'pkg:npm/%40angular/core@16.0.0' }]);
+    const report = diff(a, b);
+    expect(report.upgraded).toHaveLength(1);
+    expect(report.upgraded[0].isMajorBump).toBe(true);
   });
 
   it('detects version upgrades when matched by name (no purl)', () => {
diff --git a/src/diff.ts b/src/diff.ts
index 6540b50..3aa8b76 100644
--- a/src/diff.ts
+++ b/src/diff.ts
@@ -63,13 +63,37 @@ export function diff(a: SBOM, b: SBOM): ChangeReport {
 function buildComponentMap(components: Component[]): Map<string, Component> {
   const map = new Map<string, Component>();
   for (const comp of components) {
-    // Prefer purl as key, fall back to name
-    const key = comp.purl ?? comp.name;
-    map.set(key, comp);
+    map.set(componentKey(comp), comp);
   }
   return map;
 }
 
+/**
+ * Derive a version-independent identity key for a component.
+ *
+ * A purl normally embeds the component's version (e.g. "pkg:npm/lodash@4.17.21"),
+ * so keying on the raw purl makes an upgraded dependency look like a removal plus
+ * an addition instead of an upgrade — defeating upgrade detection for essentially
+ * every real-world SBOM, since generators (syft, cdxgen, Trivy, …) emit versioned
+ * purls. Strip the version so the same package across two SBOMs shares a key and
+ * version changes surface as upgrades. Falls back to the name when no purl exists.
+ */
+function componentKey(comp: Component): string {
+  return comp.purl ? purlWithoutVersion(comp.purl) : comp.name;
+}
+
+/**
+ * Strip the `@version` suffix (plus any `?qualifiers` / `#subpath`) from a purl,
+ * leaving the version-independent coordinates. Per the purl spec any literal `@`
+ * within the name must be percent-encoded, so the last `@` unambiguously marks
+ * the version separator.
+ */
+function purlWithoutVersion(purl: string): string {
+  const base = purl.split('#')[0].split('?')[0];
+  const at = base.lastIndexOf('@');
+  return at > 0 ? base.slice(0, at) : base;
+}
+
 /**
  * Returns true if the major version changed (semver-style).
  * Handles versions like "1.2.3", "2.0.0-beta", etc.