diff --git a/packages/bundlecheck/README.md b/packages/bundlecheck/README.md
index 987ba2eb..b57fa8ff 100644
--- a/packages/bundlecheck/README.md
+++ b/packages/bundlecheck/README.md
@@ -11,7 +11,7 @@ A CLI tool to check the bundle size of npm packages, similar to [bundlephobia.co
 - Interactive version selection with `--versions` flag
 - Bundle size trend analysis with `--trend` flag (bar graph across versions)
 - Support for checking specific exports (tree-shaking)
-- Automatic externalization of React and React-DOM
+- Smart externalization of React and React-DOM (only when declared as dependencies)
 - Raw and gzip sizes with configurable compression level
 - **Platform support**: target `browser` (default) or `node` with smart auto-detection
 - Custom npm registry support (for private registries)
@@ -122,7 +122,7 @@ bundlecheck @versini/ui-panel@1.0.0
 # Check specific exports from @mantine/core
 bundlecheck @mantine/core "ScrollArea,Button"
 
-# Check react itself (without marking it as external)
+# Check react with all dependencies bundled (no externals)
 bundlecheck react -n
 
 # Add vue and svelte as additional externals
@@ -181,7 +181,7 @@ console.log(stats);
 //   rawSizeFormatted: "229.07 kB",
 //   gzipSizeFormatted: "44.61 kB",
 //   exports: [],
-//   externals: ["react", "react-dom"],
+//   externals: ["react", "react-dom"],  // Only present because @mantine/core has these as peerDependencies
 //   dependencies: ["@floating-ui/react", ...],
 //   platform: "browser",
 //   gzipLevel: 5,
@@ -241,7 +241,7 @@ Get bundle size statistics for a single package.
 | `package`    | `string`                      | (required) | Package name with optional version (e.g., `lodash@4.17.0`) |
 | `exports`    | `string[]`                    | `undefined`| Specific exports to measure (tree-shaking)               |
 | `external`   | `string[]`                    | `undefined`| Additional packages to mark as external                  |
-| `noExternal` | `boolean`                     | `false`    | Bundle everything including default externals            |
+| `noExternal` | `boolean`                     | `false`    | Bundle everything (no externals, even react/react-dom)   |
 | `gzipLevel`  | `number`                      | `5`        | Gzip compression level (1-9)                             |
 | `registry`   | `string`                      | `undefined`| Custom npm registry URL                                  |
 | `platform`   | `"browser" \| "node" \| "auto"` | `"auto"`   | Target platform                                          |
@@ -386,9 +386,11 @@ bundlecheck fastify -p server  # "server" is an alias for "node"
 
 ## Default Externals
 
-By default, `react` and `react-dom` are marked as external (not included in the bundle size) since most React-based packages expect these as peer dependencies. This matches how these packages would typically be used in a real application.
+When a package declares `react` or `react-dom` in its `dependencies` or `peerDependencies`, they are automatically marked as external (not included in the bundle size). This matches how these packages would typically be used in a real application where React is provided by the host application.
 
-To include React/React-DOM in the bundle size calculation, use the `--no-external` flag.
+For packages that don't depend on React, these are not automatically externalized.
+
+To include all dependencies (including React when present) in the bundle size calculation, use the `--no-external` flag.
 
 ## Custom Registry
 
diff --git a/packages/bundlecheck/src/__tests__/bundler.test.ts b/packages/bundlecheck/src/__tests__/bundler.test.ts
index f1b5de78..b15dc29d 100644
--- a/packages/bundlecheck/src/__tests__/bundler.test.ts
+++ b/packages/bundlecheck/src/__tests__/bundler.test.ts
@@ -199,8 +199,9 @@ describe("checkBundleSize", () => {
 		});
 		expect(result.rawSize).toBeGreaterThan(0);
 		expect(result.gzipSize).toBeGreaterThan(0);
-		expect(result.externals).toContain("react");
-		expect(result.externals).toContain("react-dom");
+		// react and react-dom are NOT external since the package doesn't have them as dependencies
+		expect(result.externals).not.toContain("react");
+		expect(result.externals).not.toContain("react-dom");
 	});
 
 	it("should respect noExternal option", async () => {
@@ -218,27 +219,61 @@ describe("checkBundleSize", () => {
 			additionalExternals: ["lodash", "dayjs"],
 		});
 
-		expect(result.externals).toContain("react");
-		expect(result.externals).toContain("react-dom");
+		// react and react-dom are NOT external since the package doesn't have them as dependencies
+		expect(result.externals).not.toContain("react");
+		expect(result.externals).not.toContain("react-dom");
+		// Additional externals are added
 		expect(result.externals).toContain("lodash");
 		expect(result.externals).toContain("dayjs");
 	});
 
-	it("should exclude react from externals when checking react itself", async () => {
+	it("should not mark react as external when checking react itself", async () => {
 		const result = await checkBundleSize({
 			packageName: "react",
 		});
 
+		// When checking react, it should never be external (even if it were somehow in deps)
 		expect(result.externals).not.toContain("react");
+	});
+
+	it("should include react/react-dom as external when package has them as peer dependencies", async () => {
+		vi.mocked(fs.readFileSync).mockReturnValue(
+			JSON.stringify({
+				name: "test-package",
+				version: "1.0.0",
+				main: "./dist/index.js",
+				dependencies: {},
+				peerDependencies: { react: "^18.0.0", "react-dom": "^18.0.0" },
+			}),
+		);
+
+		const result = await checkBundleSize({
+			packageName: "test-package",
+		});
+
+		// react and react-dom ARE external since they're in peerDependencies
+		expect(result.externals).toContain("react");
 		expect(result.externals).toContain("react-dom");
 	});
 
-	it("should exclude react-dom from externals when checking react-dom itself", async () => {
+	it("should include react as external when package has it as prod dependency", async () => {
+		vi.mocked(fs.readFileSync).mockReturnValue(
+			JSON.stringify({
+				name: "test-package",
+				version: "1.0.0",
+				main: "./dist/index.js",
+				dependencies: { react: "^18.0.0" },
+				peerDependencies: {},
+			}),
+		);
+
 		const result = await checkBundleSize({
-			packageName: "react-dom",
+			packageName: "test-package",
 		});
 
+		// react IS external since it's in dependencies
 		expect(result.externals).toContain("react");
+		// react-dom is NOT external since it's not in dependencies
 		expect(result.externals).not.toContain("react-dom");
 	});
 
@@ -275,6 +310,8 @@ describe("checkBundleSize", () => {
 		});
 
 		expect(result.dependencies).toContain("react");
+		// react should be external since it's in peerDependencies
+		expect(result.externals).toContain("react");
 	});
 
 	it("should respect custom gzip level", async () => {
diff --git a/packages/bundlecheck/src/bundler.ts b/packages/bundlecheck/src/bundler.ts
index 2ca2e2b6..79489a99 100644
--- a/packages/bundlecheck/src/bundler.ts
+++ b/packages/bundlecheck/src/bundler.ts
@@ -273,25 +273,40 @@ function generateEntryContent(options: EntryContentOptions): string {
 }
 
 /**
- * Get externals list based on options.
+ * Get externals list based on options and package dependencies. react and
+ * react-dom are only marked as external if they are declared in the package's
+ * dependencies or peerDependencies.
  */
 export function getExternals(
 	packageName: string,
 	externals?: string[],
 	noExternal?: boolean,
+	packageDependencies?: string[],
 ): string[] {
 	if (noExternal) {
 		return [];
 	}
 
-	// Start with default externals (react, react-dom).
-	let result = [...DEFAULT_EXTERNALS];
+	/**
+	 * Start with empty result - we'll only add react/react-dom if they're in
+	 * package deps.
+	 */
+	let result: string[] = [];
 
-	// If checking react or react-dom themselves, don't mark them as external.
-	if (packageName === "react") {
-		result = result.filter((e) => e !== "react");
-	} else if (packageName === "react-dom") {
-		result = result.filter((e) => e !== "react-dom");
+	/**
+	 * Only include react/react-dom if they're in the package's dependencies or
+	 * peerDependencies.
+	 */
+	if (packageDependencies && packageDependencies.length > 0) {
+		for (const dep of DEFAULT_EXTERNALS) {
+			// Don't mark as external if we're checking the package itself.
+			if (dep === packageName) {
+				continue;
+			}
+			if (packageDependencies.includes(dep)) {
+				result.push(dep);
+			}
+		}
 	}
 
 	// Add any additional externals.
@@ -626,11 +641,15 @@ export async function checkBundleSize(
 		const entryFile = path.join(tmpDir, "entry.js");
 		fs.writeFileSync(entryFile, entryContent);
 
-		// Get externals.
+		/**
+		 * Get externals based on package dependencies. Only include react/react-dom
+		 * if they're in the package's dependencies or peerDependencies.
+		 */
 		const externals = getExternals(
 			packageName,
 			additionalExternals,
 			noExternal,
+			allDependencies,
 		);
 
 		// Bundle with esbuild.
diff --git a/packages/bundlecheck/src/index.ts b/packages/bundlecheck/src/index.ts
index e8a92e25..8dd8b10a 100644
--- a/packages/bundlecheck/src/index.ts
+++ b/packages/bundlecheck/src/index.ts
@@ -9,7 +9,6 @@ import {
 	type BundleResult,
 	checkBundleSize,
 	formatBytes,
-	getExternals,
 	parsePackageSpecifier,
 } from "./bundler.js";
 import {
@@ -45,7 +44,7 @@ export type GetBundleStatsOptions = {
 	 */
 	external?: string[];
 	/**
-	 * Bundle everything including default externals (react, react-dom).
+	 * Bundle everything including dependencies that would normally be external.
 	 */
 	noExternal?: boolean;
 	/**
@@ -142,7 +141,7 @@ export type GetBundleTrendOptions = {
 	 */
 	external?: string[];
 	/**
-	 * Bundle everything including default externals (react, react-dom).
+	 * Bundle everything including dependencies that would normally be external.
 	 */
 	noExternal?: boolean;
 	/**
@@ -329,17 +328,19 @@ export async function getBundleStats(
 		resolvedVersion = tags.latest || requestedVersion;
 	}
 
-	// Compute externals for cache key.
-	const externals = getExternals(baseName, additionalExternals, noExternal);
-
-	// Build cache key.
+	/**
+	 * Build cache key.
+	 * NOTE: We use additionalExternals here, not computed externals, because the default externals (react, react-dom) are determined at bundle time based on the package's dependencies.
+	 * The cache key captures the user's intent (additionalExternals + noExternal
+	 * flag), and the actual externals are stored in the cached result.
+	 */
 	const cacheKey = normalizeCacheKey({
 		packageName: baseName,
 		version: resolvedVersion,
 		exports: exportsList,
 		platform,
 		gzipLevel,
-		externals,
+		externals: additionalExternals || [],
 		noExternal: noExternal ?? false,
 	});
 
diff --git a/packages/bundlecheck/src/parse.ts b/packages/bundlecheck/src/parse.ts
index 9b180bfd..01ee2eca 100644
--- a/packages/bundlecheck/src/parse.ts
+++ b/packages/bundlecheck/src/parse.ts
@@ -1,10 +1,6 @@
 /* istanbul ignore file */
 import { parser } from "@node-cli/parser";
-import {
-	DEFAULT_EXTERNALS,
-	defaultFlags,
-	isValidPlatform,
-} from "./defaults.js";
+import { defaultFlags, isValidPlatform } from "./defaults.js";
 
 export type Flags = {
 	boring?: boolean;
@@ -101,7 +97,8 @@ export const config: Configuration = parser({
 		external: {
 			shortFlag: "e",
 			default: defaultFlags.external,
-			description: `Comma-separated additional externals (default externals: ${DEFAULT_EXTERNALS.join(", ")})`,
+			description:
+				"Comma-separated additional externals (react, react-dom auto-external when in package deps)",
 			type: "string",
 		},
 		noExternal: {