diff --git a/site/base.mjs b/site/base.mjs
index a7ca4ed8..8b0e29d0 100644
--- a/site/base.mjs
+++ b/site/base.mjs
@@ -5,20 +5,60 @@
  * base-dependent output. It lives in its own module so none of those has to
  * import the full Astro config just to learn one string.
  *
- * This file exists because the literal was previously copied into four places
- * — astro.config.mjs, link-audit/lib.ts, and three test files — each with a
- * comment saying it had to match the others. Moving the site to the
- * codearbiter.dev apex domain desynced all four at once: four test failures and
- * 19,940 link-audit failures from a one-line change.
- *
- * MUST be "" for an apex domain, never "/". rehypeBaseLinks prefixes every
- * root-absolute href/src with this value, so "/" would rewrite
- * "/diagrams/x.svg" into "//diagrams/x.svg" — a protocol-relative URL that
- * resolves against a different host and fails silently rather than 404-ing.
- * Empty string makes the prefixing a correct no-op. Consumers that need a real
- * path (Astro's own `base`) use `BASE || "/"`.
+ * This file exists because the literal was previously copied into six places —
+ * astro.config.mjs, link-audit/lib.ts, and four test files — each with a comment
+ * saying it had to match the others. Moving the site to the codearbiter.dev apex
+ * domain desynced all six at once: four test failures and 19,940 link-audit
+ * failures from a one-line change.
  *
  * For a project subpath the value is "/<repo>" with no trailing slash, e.g.
- * "/codeArbiter" when served from arbiterforge.github.io/codeArbiter/.
+ * "/codeArbiter" when served from arbiterforge.github.io/codeArbiter/. For an
+ * apex domain it is the empty string.
+ */
+
+/** The configured value, before validation. */
+const RAW_BASE = "";
+
+/** Reject any base that would corrupt links, loudly and at import time.
+ *
+ * The dangerous value is "/". rehypeBaseLinks prefixes every root-absolute
+ * href/src with the base, so a base of "/" rewrites "/diagrams/x.svg" into
+ * "//diagrams/x.svg" — a protocol-relative URL that resolves against a
+ * DIFFERENT HOST. It does not 404. The build succeeds, the link audit passes
+ * (the target still parses), and the page quietly fetches from somewhere else.
+ * That failure mode is why this is a thrown error and not a comment: prose
+ * cannot stop anyone, and "/" is the obvious thing to reach for when moving a
+ * site to a domain root.
+ *
+ * Exported for the tests; callers should import BASE.
+ *
+ * @param {string} value
+ * @returns {string} the validated base
  */
-export const BASE = "";
+export function validateBase(value) {
+  if (typeof value !== "string") {
+    throw new TypeError(`site base must be a string, got ${typeof value}`);
+  }
+  // The apex-domain form. Makes rehypeBaseLinks a correct no-op, because a
+  // root-absolute target already starts with `${BASE}/`.
+  if (value === "") return value;
+  if (!value.startsWith("/")) {
+    throw new Error(
+      `site base ${JSON.stringify(value)} must be "" or start with "/" ` +
+        `(a base is a root-absolute path, e.g. "/codeArbiter")`,
+    );
+  }
+  // Catches "/" as well as "/codeArbiter/". A trailing slash doubles the
+  // separator when the base is prefixed onto a root-absolute target.
+  if (value.endsWith("/")) {
+    const hint =
+      value === "/"
+        ? ` Use "" for a domain root: a base of "/" turns "/diagrams/x.svg" into ` +
+          `"//diagrams/x.svg", a protocol-relative URL pointing at another host.`
+        : ` Drop the trailing slash.`;
+    throw new Error(`site base ${JSON.stringify(value)} must not end with "/".${hint}`);
+  }
+  return value;
+}
+
+export const BASE = validateBase(RAW_BASE);
diff --git a/site/test/base.test.ts b/site/test/base.test.ts
new file mode 100644
index 00000000..6bf54fbd
--- /dev/null
+++ b/site/test/base.test.ts
@@ -0,0 +1,79 @@
+/** base.test.ts — the site base path is the one value that silently corrupts
+ * every internal link when it is wrong, so its invariants are gated rather than
+ * documented.
+ *
+ * The motivating failure: a base of "/" makes rehypeBaseLinks rewrite
+ * "/diagrams/x.svg" into "//diagrams/x.svg", a protocol-relative URL that
+ * resolves against a DIFFERENT HOST. Nothing 404s. The build succeeds and the
+ * link audit passes, because the target still parses as a valid reference — the
+ * page just quietly fetches from somewhere else. A comment saying "never use
+ * '/'" stops nobody; "/" is the obvious thing to reach for when moving a site to
+ * a domain root.
+ */
+import { describe, it, expect } from "vitest";
+// @ts-expect-error -- untyped .mjs module
+import { BASE, validateBase } from "../base.mjs";
+
+/** The link-prefixing rule from scripts/rehype-base-links.ts, reproduced so this
+ * file can demonstrate the corruption a bad base causes without depending on the
+ * plugin's internals. */
+function prefix(value: string, base: string): string {
+  if (value === "" || value.startsWith("#") || value.startsWith("//") || !value.startsWith("/")) {
+    return value;
+  }
+  if (value === base || value.startsWith(`${base}/`)) return value;
+  return `${base}${value}`;
+}
+
+describe("validateBase", () => {
+  it("accepts the apex-domain form", () => {
+    expect(validateBase("")).toBe("");
+  });
+
+  it("accepts a project subpath", () => {
+    expect(validateBase("/codeArbiter")).toBe("/codeArbiter");
+  });
+
+  it("rejects '/' — the value that produces protocol-relative URLs", () => {
+    expect(() => validateBase("/")).toThrow(/protocol-relative/);
+  });
+
+  it("rejects a trailing slash", () => {
+    expect(() => validateBase("/codeArbiter/")).toThrow(/must not end with/);
+  });
+
+  it("rejects a base that is not root-absolute", () => {
+    expect(() => validateBase("codeArbiter")).toThrow(/must be "" or start with/);
+  });
+
+  it("rejects a non-string", () => {
+    expect(() => validateBase(42 as unknown as string)).toThrow(TypeError);
+  });
+});
+
+describe("why '/' is rejected", () => {
+  it("would rewrite a root-absolute asset into a protocol-relative URL", () => {
+    // This is the corruption the guard exists to prevent. Pinned as an
+    // executable statement of the hazard, not a comment about it.
+    expect(prefix("/diagrams/x.svg", "/")).toBe("//diagrams/x.svg");
+    expect(prefix("/diagrams/x.svg", "/").startsWith("//")).toBe(true);
+  });
+
+  it("is a correct no-op under the sanctioned apex form", () => {
+    expect(prefix("/diagrams/x.svg", "")).toBe("/diagrams/x.svg");
+  });
+
+  it("prefixes correctly under a sanctioned subpath", () => {
+    expect(prefix("/diagrams/x.svg", "/codeArbiter")).toBe("/codeArbiter/diagrams/x.svg");
+  });
+});
+
+describe("the configured BASE", () => {
+  it("is itself valid — the guard runs at import time, so this cannot regress", () => {
+    expect(() => validateBase(BASE)).not.toThrow();
+  });
+
+  it("never ends with a slash", () => {
+    expect(BASE.endsWith("/")).toBe(false);
+  });
+});