diff --git a/next-env.d.ts b/next-env.d.ts
deleted file mode 100644
index c4b7818f..00000000
--- a/next-env.d.ts
+++ /dev/null
@@ -1,6 +0,0 @@
-/// <reference types="next" />
-/// <reference types="next/image-types/global" />
-import "./.next/dev/types/routes.d.ts";
-
-// NOTE: This file should not be edited
-// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.
diff --git a/src/app/[...parts]/_page/DiffIntro/DiffIntro.tsx b/src/app/[...parts]/_page/DiffIntro/DiffIntro.tsx
index 27a1776b..d8aa27e5 100644
--- a/src/app/[...parts]/_page/DiffIntro/DiffIntro.tsx
+++ b/src/app/[...parts]/_page/DiffIntro/DiffIntro.tsx
@@ -35,6 +35,8 @@ const DiffIntro = forwardRef<ElementRef<typeof Stack>, DiffIntroProps>(
                         left={
                             <SpecBox
                                 pkg={aWithName}
+                                otherPkg={bWithName}
+                                side="a"
                                 pkgClassName="rounded-r-none"
                             />
                         }
@@ -46,6 +48,8 @@ const DiffIntro = forwardRef<ElementRef<typeof Stack>, DiffIntroProps>(
                         right={
                             <SpecBox
                                 pkg={bWithName}
+                                otherPkg={aWithName}
+                                side="b"
                                 pkgClassName="rounded-l-none"
                             />
                         }
diff --git a/src/app/[...parts]/_page/DiffIntro/SpecBox.tsx b/src/app/[...parts]/_page/DiffIntro/SpecBox.tsx
index 56aadf74..64db798c 100644
--- a/src/app/[...parts]/_page/DiffIntro/SpecBox.tsx
+++ b/src/app/[...parts]/_page/DiffIntro/SpecBox.tsx
@@ -1,5 +1,6 @@
 import { forwardRef, type HTMLAttributes } from "react";
 import Pkg from "^/components/ui/Pkg";
+import PkgWithVersionSelector from "^/components/ui/PkgWithVersionSelector";
 import { cx } from "^/lib/cva";
 import type SimplePackageSpec from "^/lib/SimplePackageSpec";
 import { simplePackageSpecToString } from "^/lib/SimplePackageSpec";
@@ -9,12 +10,25 @@ import ServiceLinks from "./ServiceLinks";
 interface SpecBoxProps extends HTMLAttributes<HTMLElement> {
     pkg: SimplePackageSpec;
     pkgClassName?: string;
+    /** The other package spec (for interactive version selection) */
+    otherPkg?: SimplePackageSpec;
+    /** Which side this is ('a' or 'b') - required for version selection */
+    side?: "a" | "b";
 }
 
 const SpecBox = forwardRef<HTMLElement, SpecBoxProps>(
-    ({ pkg, pkgClassName, ...props }, ref) => (
+    ({ pkg, pkgClassName, otherPkg, side, ...props }, ref) => (
         <section {...props} ref={ref}>
-            <Pkg pkg={pkg} className={cx("px-1", pkgClassName)} />
+            {otherPkg && side ? (
+                <PkgWithVersionSelector
+                    pkg={pkg}
+                    otherPkg={otherPkg}
+                    side={side}
+                    className={cx("px-1", pkgClassName)}
+                />
+            ) : (
+                <Pkg pkg={pkg} className={cx("px-1", pkgClassName)} />
+            )}
             <PublishDate
                 suspenseKey={"publishdate-" + simplePackageSpecToString(pkg)}
                 pkg={pkg}
diff --git a/src/app/[...parts]/_page/DiffIntro/VersionSelector.test.tsx b/src/app/[...parts]/_page/DiffIntro/VersionSelector.test.tsx
new file mode 100644
index 00000000..fd4e842a
--- /dev/null
+++ b/src/app/[...parts]/_page/DiffIntro/VersionSelector.test.tsx
@@ -0,0 +1,168 @@
+// @jest-environment jsdom
+
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { useRouter } from "next/navigation";
+import VersionSelector from "./VersionSelector";
+
+// Mock next/navigation
+jest.mock("next/navigation", () => ({
+    useRouter: jest.fn(),
+}));
+
+// Mock fetch
+global.fetch = jest.fn();
+
+describe("VersionSelector", () => {
+    const mockPush = jest.fn();
+    const mockFetch = global.fetch as jest.MockedFunction<typeof fetch>;
+
+    beforeEach(() => {
+        jest.clearAllMocks();
+        (useRouter as jest.Mock).mockReturnValue({
+            push: mockPush,
+        });
+    });
+
+    it("renders loading state initially", () => {
+        mockFetch.mockResolvedValueOnce({
+            ok: true,
+            json: async () => [],
+        } as Response);
+
+        render(
+            <VersionSelector
+                currentSpec={{ name: "react", version: "18.0.0" }}
+                otherSpec={{ name: "react", version: "17.0.0" }}
+                side="a"
+            />,
+        );
+
+        expect(screen.getByText("18.0.0")).toBeInTheDocument();
+    });
+
+    it("fetches and displays versions", async () => {
+        const mockVersions = [
+            { version: "18.2.0", time: "2023-01-01" },
+            { version: "18.1.0", time: "2022-12-01" },
+            { version: "18.0.0", time: "2022-11-01" },
+        ];
+
+        mockFetch.mockResolvedValueOnce({
+            ok: true,
+            json: async () => mockVersions,
+        } as Response);
+
+        render(
+            <VersionSelector
+                currentSpec={{ name: "react", version: "18.0.0" }}
+                otherSpec={{ name: "react", version: "17.0.0" }}
+                side="a"
+            />,
+        );
+
+        await waitFor(() => {
+            expect(screen.getByRole("combobox")).toBeInTheDocument();
+        });
+
+        const select = screen.getByRole("combobox") as HTMLSelectElement;
+        expect(select.value).toBe("18.0.0");
+    });
+
+    it("navigates to new diff when version is selected (side a)", async () => {
+        const mockVersions = [
+            { version: "18.2.0", time: "2023-01-01" },
+            { version: "18.0.0", time: "2022-11-01" },
+        ];
+
+        mockFetch.mockResolvedValueOnce({
+            ok: true,
+            json: async () => mockVersions,
+        } as Response);
+
+        render(
+            <VersionSelector
+                currentSpec={{ name: "react", version: "18.0.0" }}
+                otherSpec={{ name: "react", version: "17.0.0" }}
+                side="a"
+            />,
+        );
+
+        await waitFor(() => {
+            expect(screen.getByRole("combobox")).toBeInTheDocument();
+        });
+
+        const select = screen.getByRole("combobox") as HTMLSelectElement;
+        fireEvent.change(select, { target: { value: "18.2.0" } });
+
+        await waitFor(() => {
+            expect(mockPush).toHaveBeenCalledWith(
+                "/react@18.2.0...react@17.0.0",
+            );
+        });
+    });
+
+    it("navigates to new diff when version is selected (side b)", async () => {
+        const mockVersions = [
+            { version: "18.2.0", time: "2023-01-01" },
+            { version: "17.0.0", time: "2022-11-01" },
+        ];
+
+        mockFetch.mockResolvedValueOnce({
+            ok: true,
+            json: async () => mockVersions,
+        } as Response);
+
+        render(
+            <VersionSelector
+                currentSpec={{ name: "react", version: "17.0.0" }}
+                otherSpec={{ name: "react", version: "18.0.0" }}
+                side="b"
+            />,
+        );
+
+        await waitFor(() => {
+            expect(screen.getByRole("combobox")).toBeInTheDocument();
+        });
+
+        const select = screen.getByRole("combobox") as HTMLSelectElement;
+        fireEvent.change(select, { target: { value: "18.2.0" } });
+
+        await waitFor(() => {
+            expect(mockPush).toHaveBeenCalledWith(
+                "/react@18.0.0...react@18.2.0",
+            );
+        });
+    });
+
+    it("displays version tags when available", async () => {
+        const mockVersions = [
+            {
+                version: "18.2.0",
+                time: "2023-01-01",
+                tags: ["latest", "next"],
+            },
+            { version: "18.0.0", time: "2022-11-01" },
+        ];
+
+        mockFetch.mockResolvedValueOnce({
+            ok: true,
+            json: async () => mockVersions,
+        } as Response);
+
+        render(
+            <VersionSelector
+                currentSpec={{ name: "react", version: "18.0.0" }}
+                otherSpec={{ name: "react", version: "17.0.0" }}
+                side="a"
+            />,
+        );
+
+        await waitFor(() => {
+            expect(screen.getByRole("combobox")).toBeInTheDocument();
+        });
+
+        // Check if option with tags exists
+        const option = screen.getByText("18.2.0 (latest, next)");
+        expect(option).toBeInTheDocument();
+    });
+});
diff --git a/src/app/[...parts]/_page/DiffIntro/VersionSelector.tsx b/src/app/[...parts]/_page/DiffIntro/VersionSelector.tsx
new file mode 100644
index 00000000..16404734
--- /dev/null
+++ b/src/app/[...parts]/_page/DiffIntro/VersionSelector.tsx
@@ -0,0 +1,112 @@
+"use client";
+
+import { useRouter } from "next/navigation";
+import { useEffect, useState } from "react";
+import { type Version } from "^/app/api/-/versions/types";
+import type SimplePackageSpec from "^/lib/SimplePackageSpec";
+
+interface VersionSelectorProps {
+    /** The current package spec */
+    currentSpec: SimplePackageSpec;
+    /** The other package spec (to keep in the diff) */
+    otherSpec: SimplePackageSpec;
+    /** Whether this is the 'a' (left/from) or 'b' (right/to) side */
+    side: "a" | "b";
+    /** Class name for the select element */
+    className?: string;
+}
+
+export default function VersionSelector({
+    currentSpec,
+    otherSpec,
+    side,
+    className,
+}: VersionSelectorProps) {
+    const router = useRouter();
+    const [versions, setVersions] = useState<Version[]>([]);
+    const [isLoading, setIsLoading] = useState(true);
+    const [hasError, setHasError] = useState(false);
+
+    useEffect(() => {
+        const fetchVersions = async () => {
+            try {
+                setIsLoading(true);
+                setHasError(false);
+                const response = await fetch(
+                    `/api/-/versions?package=${encodeURIComponent(currentSpec.name)}`,
+                );
+                if (response.ok) {
+                    const data: Version[] = await response.json();
+                    setVersions(data);
+                } else {
+                    console.error(
+                        `Failed to fetch versions: ${response.status} ${response.statusText}`,
+                    );
+                    setHasError(true);
+                }
+            } catch (error) {
+                console.error("Failed to fetch versions:", error);
+                setHasError(true);
+            } finally {
+                setIsLoading(false);
+            }
+        };
+
+        fetchVersions();
+    }, [currentSpec.name]);
+
+    const handleVersionChange = (newVersion: string) => {
+        if (newVersion === currentSpec.version) {
+            return; // No change
+        }
+
+        // Build the new URL based on which side we're updating
+        const newA =
+            side === "a"
+                ? `${currentSpec.name}@${newVersion}`
+                : `${otherSpec.name}@${otherSpec.version}`;
+        const newB =
+            side === "b"
+                ? `${currentSpec.name}@${newVersion}`
+                : `${otherSpec.name}@${otherSpec.version}`;
+
+        // Navigate to the new diff URL
+        router.push(`/${newA}...${newB}`);
+    };
+
+    if (isLoading) {
+        return (
+            <span className={className} aria-busy="true" aria-live="polite">
+                {currentSpec.version}
+            </span>
+        );
+    }
+
+    if (hasError || versions.length === 0) {
+        // Fallback to plain text if we can't fetch versions
+        return <span className={className}>{currentSpec.version}</span>;
+    }
+
+    // Sort versions in reverse chronological order (newest first)
+    const sortedVersions = [...versions].sort((a, b) => {
+        return new Date(b.time).getTime() - new Date(a.time).getTime();
+    });
+
+    return (
+        <select
+            value={currentSpec.version}
+            onChange={(e) => handleVersionChange(e.target.value)}
+            className={className}
+            aria-label={`Select version for ${currentSpec.name}`}
+        >
+            {sortedVersions.map((version) => (
+                <option key={version.version} value={version.version}>
+                    {version.version}
+                    {version.tags && version.tags.length > 0
+                        ? ` (${version.tags.join(", ")})`
+                        : ""}
+                </option>
+            ))}
+        </select>
+    );
+}
diff --git a/src/components/ui/PkgWithVersionSelector.tsx b/src/components/ui/PkgWithVersionSelector.tsx
new file mode 100644
index 00000000..a2a1861e
--- /dev/null
+++ b/src/components/ui/PkgWithVersionSelector.tsx
@@ -0,0 +1,43 @@
+"use client";
+
+import { type ElementRef, forwardRef } from "react";
+import VersionSelector from "^/app/[...parts]/_page/DiffIntro/VersionSelector";
+import type SimplePackageSpec from "^/lib/SimplePackageSpec";
+import Code, { type CodeProps } from "./Code";
+
+export interface PkgWithVersionSelectorProps extends CodeProps {
+    pkg: SimplePackageSpec;
+    otherPkg: SimplePackageSpec;
+    side: "a" | "b";
+}
+
+const PkgWithVersionSelector = forwardRef<
+    ElementRef<typeof Code>,
+    PkgWithVersionSelectorProps
+>(
+    (
+        {
+            pkg,
+            otherPkg,
+            side,
+            className,
+            ...props
+        }: PkgWithVersionSelectorProps,
+        ref,
+    ) => {
+        return (
+            <Code {...props} className={className} ref={ref}>
+                {pkg.name}@
+                <VersionSelector
+                    currentSpec={pkg}
+                    otherSpec={otherPkg}
+                    side={side}
+                    className="cursor-pointer border-none bg-transparent underline decoration-dotted outline-none hover:decoration-solid focus:decoration-solid"
+                />
+            </Code>
+        );
+    },
+);
+PkgWithVersionSelector.displayName = "PkgWithVersionSelector";
+
+export default PkgWithVersionSelector;