Use spread semantics for extendsMergeMode override

Override merge now fully replaces each set top-level property from the
child config instead of shallow-merging object maps and hostRequirements.

Author: SamuelFrost

CHANGELOG.md

@@ -4,7 +4,7 @@ Notable changes.
 
 ## Unreleased
 
-- Add support for the `extends` keyword so one `devcontainer.json` can inherit another, with optional `extendsMergeMode` (`combine` uses image metadata merge logic; `override` replaces arrays/scalars from the child). (https://github.com/devcontainers/spec/issues/22, https://github.com/devcontainers/cli/pull/311)
+- Add support for the `extends` keyword so one `devcontainer.json` can inherit another, with optional `extendsMergeMode` (`combine` uses image metadata merge logic; `override` uses `{ ...base, ...overlay }` so each set top-level property from the child fully replaces the inherited value). (https://github.com/devcontainers/spec/issues/22, https://github.com/devcontainers/cli/pull/311)
 
 ## August 2026
 

src/spec-node/imageMetadata.ts

@@ -272,8 +272,8 @@ export function mergeDevContainerConfigs(base: DevContainerConfig, overlay: DevC
 }
 
 /**
- * Overlay-style merge: scalars and arrays from the overlay replace the base when set;
- * object maps and `hostRequirements` are shallow-merged with overlay keys winning.
+ * Overlay-style merge: each top-level property from the overlay replaces the base value when set;
+ * omitted overlay keys keep the inherited base value ({ ...base, ...overlay }).
  */
 function mergeDevContainerConfigsOverride(base: DevContainerConfig, overlay: DevContainerConfig): DevContainerConfig {
 	const merged = {
@@ -282,20 +282,6 @@ function mergeDevContainerConfigsOverride(base: DevContainerConfig, overlay: Dev
 	} as DevContainerConfig;
 	delete merged.extends;
 	delete merged.extendsMergeMode;
-
-	const remoteEnv = Object.assign({}, base.remoteEnv, overlay.remoteEnv);
-	assignOrDelete(merged, 'remoteEnv', Object.keys(remoteEnv).length ? remoteEnv : undefined);
-	const containerEnv = Object.assign({}, base.containerEnv, overlay.containerEnv);
-	assignOrDelete(merged, 'containerEnv', Object.keys(containerEnv).length ? containerEnv : undefined);
-	const portsAttributes = Object.assign({}, base.portsAttributes, overlay.portsAttributes);
-	assignOrDelete(merged, 'portsAttributes', Object.keys(portsAttributes).length ? portsAttributes : undefined);
-	const features = Object.assign({}, base.features, overlay.features);
-	assignOrDelete(merged, 'features', Object.keys(features).length ? features : undefined);
-	const customizations = Object.assign({}, base.customizations, overlay.customizations);
-	assignOrDelete(merged, 'customizations', Object.keys(customizations).length ? customizations : undefined);
-	const hostRequirements = Object.assign({}, base.hostRequirements, overlay.hostRequirements);
-	assignOrDelete(merged, 'hostRequirements', Object.keys(hostRequirements).length ? hostRequirements : undefined);
-
 	return merged;
 }
 

src/test/cli.extends.up.test.ts

@@ -65,7 +65,10 @@ describe('Dev Containers CLI extends', function () {
 				containerId = response.containerId;
 				assert.ok(containerId, 'Container id not found.');
 				assert.equal(response.configuration?.name, 'extends-up-override');
-				assert.equal(response.configuration?.remoteEnv?.EXTENDS_UP, 'override');
+				assert.deepEqual(response.configuration?.remoteEnv, {
+					OVERRIDE_ME: 'child',
+					EXTENDS_UP: 'override',
+				});
 				assert.deepEqual(response.configuration?.forwardPorts, [9999]);
 			});
 

src/test/configContainer.test.ts

@@ -94,8 +94,14 @@ describe('readDevContainerConfigFile', function () {
 		assert.strictEqual(raw.name, 'Override merge');
 		assert.deepEqual(raw.forwardPorts, [443]);
 		assert.strictEqual(raw.init, false);
-		assert.strictEqual(raw.hostRequirements?.cpus, 2);
-		assert.strictEqual(raw.hostRequirements?.memory, '4gb');
+		assert.deepEqual(raw.remoteEnv, { OVERRIDE_ME: 'child' });
+		assert.deepEqual(raw.features, {
+			'ghcr.io/devcontainers/features/docker-in-docker:1': {
+				version: 'latest',
+				moby: true,
+			},
+		});
+		assert.deepEqual(raw.hostRequirements, { memory: '4gb' });
 		assert.notProperty(raw as any, 'extends');
 		assert.notProperty(raw as any, 'extendsMergeMode');
 	});
@@ -182,8 +188,6 @@ describe('mergeDevContainerConfigs', function () {
 		assert.strictEqual(merged.init, false);
 		assert.strictEqual(merged.privileged, true);
 		assert.deepEqual(merged.forwardPorts, [443]);
-		assert.strictEqual(merged.hostRequirements?.cpus, 4);
-		assert.strictEqual(merged.hostRequirements?.memory, '4gb');
-		assert.strictEqual(merged.hostRequirements?.storage, '32gb');
+		assert.deepEqual(merged.hostRequirements, { memory: '4gb' });
 	});
 });