diff --git a/backend/src/__tests__/compose-network-inspector.test.ts b/backend/src/__tests__/compose-network-inspector.test.ts
index aa5a37b6..5cf33345 100644
--- a/backend/src/__tests__/compose-network-inspector.test.ts
+++ b/backend/src/__tests__/compose-network-inspector.test.ts
@@ -41,6 +41,9 @@ describe('normalized-model adapters', () => {
         backend: { name: 'myapp_backend', external: false, internal: false },
         shared: { name: 'shared_net', external: true, internal: false },
         custom: { name: 'custom_name', external: false, internal: false },
+        // External with no name override: the rendered model resolves the runtime
+        // name to the key verbatim, so the raw adapter must agree (not myapp_extnet).
+        extnet: { name: 'extnet', external: true, internal: false },
       },
       volumes: {},
     };
@@ -50,6 +53,7 @@ describe('normalized-model adapters', () => {
         backend: { external: false },
         shared: { external: true, name: 'shared_net' },
         custom: { external: false, name: 'custom_name' },
+        extnet: { external: true },
       },
       volumes: {},
     };
@@ -126,6 +130,16 @@ describe('runtimeResourceName', () => {
     expect(runtimeResourceName('myapp', 'backend', 'backend')).toBe('myapp_backend'); // name == key is not an override
     expect(runtimeResourceName('myapp', 'shared', 'shared_net')).toBe('shared_net');
   });
+
+  it('never project-prefixes an external resource (runtime name is the key, or a name override)', () => {
+    // Compose references an external network/volume by its real name and never
+    // prefixes the project, so an external resource with no name override keeps
+    // its key verbatim. Prefixing it invents a phantom `<project>_<key>` that no
+    // runtime resource matches, which then reads as foreign-network drift.
+    expect(runtimeResourceName('myapp', 'arr-net', undefined, true)).toBe('arr-net');
+    expect(runtimeResourceName('myapp', 'arr-net', 'arr-net', true)).toBe('arr-net');
+    expect(runtimeResourceName('myapp', 'shared', 'shared-prod', true)).toBe('shared-prod');
+  });
 });
 
 describe('parseAccessUrlPorts', () => {
@@ -207,6 +221,24 @@ describe('compareStackNetworks', () => {
     expect(compareStackNetworks(declaredFromCompose, snap, 'myapp').missingFromRuntime).toEqual(['edge_override']);
   });
 
+  it('does not flag an external network declared without a name override through the raw adapter', () => {
+    // The plex/arr-net case: an external network declared `external: true` with no
+    // name override. Its real runtime name is the key (arr-net), unprefixed, so an
+    // attachment to it reads as in-sync, not as a foreign network owned elsewhere.
+    const declaredFromCompose = fromDeclaredCompose({
+      services: [{ name: 'plex', dependsOn: [], networks: ['arr-net'], volumes: [], ports: [] }],
+      networks: { 'arr-net': { external: true } },
+      volumes: {},
+    }, 'plex');
+    const snap = snapshot(
+      [container({ name: 'plex', service: 'plex', composeProject: 'plex', stack: 'plex', networks: [{ name: 'arr-net', id: 'a', ip: '' }] })],
+      [depNet({ name: 'arr-net', composeProject: null, stack: null })],
+    );
+    const drift = compareStackNetworks(declaredFromCompose, snap, 'plex');
+    expect(drift.foreignNetworkAttachments).toEqual([]);
+    expect(drift.runtimeOnlyAttachments).toEqual([]);
+  });
+
   it('ignores system networks, the default network, and external networks', () => {
     const snap = snapshot(
       [container({ networks: [
diff --git a/backend/src/__tests__/drift-detection.test.ts b/backend/src/__tests__/drift-detection.test.ts
index e669ff85..69f42ad2 100644
--- a/backend/src/__tests__/drift-detection.test.ts
+++ b/backend/src/__tests__/drift-detection.test.ts
@@ -349,6 +349,21 @@ describe('assembleStackDrift - network drift', () => {
     });
     expect(report.findings.filter(f => f.kind.startsWith('network-'))).toEqual([]);
   });
+
+  it('does not flag an attachment to a declared external network (regression: shared arr-net)', () => {
+    // arr-net is declared `external: true` with no name override, so Docker attaches
+    // the container to the pre-existing network named "arr-net" (no project prefix).
+    // The runtime matches the compose, so this must read as in-sync rather than a
+    // foreign/undeclared-network finding.
+    const report = assembleStackDrift({
+      stack: 'plex',
+      declared: { services: [service({ name: 'plex', networks: ['arr-net'] })], networks: { 'arr-net': { external: true } }, volumes: {} },
+      containers: [container({ id: 'plex', service: 'plex', stack: 'plex', networks: [{ name: 'arr-net', id: 'a', ip: '' }] })],
+      networks: [depNet('arr-net', { composeProject: null, stack: null })],
+    });
+    expect(report.findings.filter(f => f.kind.startsWith('network-'))).toEqual([]);
+    expect(report.status).toBe('in-sync');
+  });
 });
 
 // ── normalizeImageRef ─────────────────────────────────────────────────────
diff --git a/backend/src/services/network/normalize.ts b/backend/src/services/network/normalize.ts
index ba4666d1..6e2d74c9 100644
--- a/backend/src/services/network/normalize.ts
+++ b/backend/src/services/network/normalize.ts
@@ -24,9 +24,13 @@ export function isLoopback(ip: string): boolean {
 }
 
 /** Resolved runtime name of a top-level network/volume: a `name:` override wins,
- *  otherwise compose prefixes the project (`<project>_<key>`). */
-export function runtimeResourceName(projectName: string, key: string, declaredName: string | undefined): string {
-  return declaredName && declaredName !== key ? declaredName : `${projectName}_${key}`;
+ *  otherwise compose prefixes the project (`<project>_<key>`). An external
+ *  resource is never project-prefixed: it references a pre-existing network/volume
+ *  by its real name (the key, or a `name:` override), so prefixing it would invent
+ *  a `<project>_<key>` that no runtime resource matches and read as foreign drift. */
+export function runtimeResourceName(projectName: string, key: string, declaredName: string | undefined, external = false): string {
+  if (declaredName && declaredName !== key) return declaredName;
+  return external ? key : `${projectName}_${key}`;
 }
 
 /** Extract host port numbers referenced by free-text access URLs, for the
@@ -67,7 +71,7 @@ export function fromEffectiveModel(m: EffectiveModel): NormalizedNetworkModel {
 export function fromDeclaredCompose(m: DeclaredCompose, projectName: string): NormalizedNetworkModel {
   const networks: NormalizedNetworkModel['networks'] = {};
   for (const [key, res] of Object.entries(m.networks)) {
-    networks[key] = { runtimeName: runtimeResourceName(projectName, key, res.name), external: res.external };
+    networks[key] = { runtimeName: runtimeResourceName(projectName, key, res.name, res.external), external: res.external };
   }
   return {
     projectName,