diff --git a/canvas/next.config.ts b/canvas/next.config.ts
index 079e21c2..351fd84a 100644
--- a/canvas/next.config.ts
+++ b/canvas/next.config.ts
@@ -17,6 +17,24 @@ import { dirname, join } from "node:path";
 // update one heuristic. Production is unaffected: `output: "standalone"`
 // bakes resolved env into the build, and the marker file isn't shipped.
 loadMonorepoEnv();
+// Boot-time matched-pair guard for ADMIN_TOKEN / NEXT_PUBLIC_ADMIN_TOKEN.
+// When ADMIN_TOKEN is set on the workspace-server (server-side bearer
+// gate, wsauth_middleware.go ~L245), the canvas MUST send the matching
+// NEXT_PUBLIC_ADMIN_TOKEN as `Authorization: Bearer ...` on every API
+// call. If only one is set, every workspace API call 401s silently —
+// the canvas hydrates with empty data and the user sees a broken page
+// with no console hint about the auth-config mismatch.
+//
+// Pre-fix the matched-pair contract was descriptive only (a comment in
+// .env): future devs/agents could re-misconfigure with one of the two
+// unset and silently 401. Closes the post-PR-#174 self-review gap.
+//
+// Warn-only (not exit) — production canvas Docker images bake these
+// vars into the build at image-build time, and a missed pair there
+// would still emit the warning at runtime via the standalone server's
+// startup. Killing the process on misconfiguration would turn a
+// recoverable auth issue into a hard crashloop.
+checkAdminTokenPair();
 
 const nextConfig: NextConfig = {
   output: "standalone",
@@ -57,6 +75,43 @@ function loadMonorepoEnv() {
   );
 }
 
+// Boot-time matched-pair guard. Runs after .env has been loaded so the
+// check sees the post-load state. The two env vars must be set or
+// unset together; one-without-the-other is the silent-401 footgun.
+//
+// Treats empty string ("") as unset. An explicitly-empty `KEY=` in
+// .env counts as set-to-empty in `process.env`, but for auth purposes
+// an empty bearer token is equivalent to no token — so both
+// `ADMIN_TOKEN=` and an unset ADMIN_TOKEN are equivalent relative to
+// the matched-pair invariant.
+//
+// Returns void; side effect is the console.error warning. Kept as a
+// separate function (exported) so a future test can reset env, call
+// this, and assert on captured stderr.
+export function checkAdminTokenPair(): void {
+  const serverSet = !!process.env.ADMIN_TOKEN;
+  const clientSet = !!process.env.NEXT_PUBLIC_ADMIN_TOKEN;
+  if (serverSet === clientSet) return;
+  // Distinct messages so the operator can tell which half is missing
+  // — the fix is symmetric (set the other one) but the diagnostic
+  // mentions which side is currently set so they don't have to grep.
+  if (serverSet && !clientSet) {
+    // eslint-disable-next-line no-console
+    console.error(
+      "[next.config] ADMIN_TOKEN is set but NEXT_PUBLIC_ADMIN_TOKEN is not — " +
+        "canvas will 401 against workspace-server because the bearer header " +
+        "is never attached. Set both to the same value, or unset both.",
+    );
+  } else {
+    // eslint-disable-next-line no-console
+    console.error(
+      "[next.config] NEXT_PUBLIC_ADMIN_TOKEN is set but ADMIN_TOKEN is not — " +
+        "workspace-server will reject the bearer because no AdminAuth gate " +
+        "is configured. Set both to the same value, or unset both.",
+    );
+  }
+}
+
 function findMonorepoRoot(start: string): string | null {
   let dir = start;
   for (let i = 0; i < 6; i++) {
diff --git a/canvas/src/lib/__tests__/admin-token-pair.test.ts b/canvas/src/lib/__tests__/admin-token-pair.test.ts
new file mode 100644
index 00000000..e19777b0
--- /dev/null
+++ b/canvas/src/lib/__tests__/admin-token-pair.test.ts
@@ -0,0 +1,130 @@
+// @vitest-environment node
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+
+// Tests for the boot-time matched-pair guard added to next.config.ts.
+//
+// Why this lives in src/lib/__tests__ even though the function is in
+// canvas/next.config.ts:
+//   - next.config.ts runs as ESM-but-also-CJS depending on which
+//     consumer loads it (Next.js dev server vs Next.js build); we
+//     want the test to be a plain ESM module Vitest already handles.
+//   - Importing from "../../../next.config" pulls in the rest of the
+//     file (loadMonorepoEnv, the default export, etc.) which has
+//     side effects on module load (it runs loadMonorepoEnv()
+//     immediately). To keep the test hermetic we don't import — we
+//     duplicate the function under test.
+//
+// Sourcing the function from a shared module would be cleaner, but
+// next.config.ts is required to be a single self-contained file by
+// Next.js's loader on some host configurations. Pin invariant: the
+// duplicated function below MUST stay byte-identical to the one in
+// next.config.ts. If you change one, change the other and bump this
+// comment.
+
+function checkAdminTokenPair(): void {
+  const serverSet = !!process.env.ADMIN_TOKEN;
+  const clientSet = !!process.env.NEXT_PUBLIC_ADMIN_TOKEN;
+  if (serverSet === clientSet) return;
+  if (serverSet && !clientSet) {
+    // eslint-disable-next-line no-console
+    console.error(
+      "[next.config] ADMIN_TOKEN is set but NEXT_PUBLIC_ADMIN_TOKEN is not — " +
+        "canvas will 401 against workspace-server because the bearer header " +
+        "is never attached. Set both to the same value, or unset both.",
+    );
+  } else {
+    // eslint-disable-next-line no-console
+    console.error(
+      "[next.config] NEXT_PUBLIC_ADMIN_TOKEN is set but ADMIN_TOKEN is not — " +
+        "workspace-server will reject the bearer because no AdminAuth gate " +
+        "is configured. Set both to the same value, or unset both.",
+    );
+  }
+}
+
+describe("checkAdminTokenPair", () => {
+  // Snapshot env so individual tests can stomp on it without leaking.
+  // Rebuild from snapshot in afterEach so the next test sees a known
+  // baseline regardless of mutation pattern.
+  let originalEnv: Record<string, string | undefined>;
+  let errorSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    originalEnv = {
+      ADMIN_TOKEN: process.env.ADMIN_TOKEN,
+      NEXT_PUBLIC_ADMIN_TOKEN: process.env.NEXT_PUBLIC_ADMIN_TOKEN,
+    };
+    delete process.env.ADMIN_TOKEN;
+    delete process.env.NEXT_PUBLIC_ADMIN_TOKEN;
+    errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    if (originalEnv.ADMIN_TOKEN === undefined) delete process.env.ADMIN_TOKEN;
+    else process.env.ADMIN_TOKEN = originalEnv.ADMIN_TOKEN;
+    if (originalEnv.NEXT_PUBLIC_ADMIN_TOKEN === undefined) delete process.env.NEXT_PUBLIC_ADMIN_TOKEN;
+    else process.env.NEXT_PUBLIC_ADMIN_TOKEN = originalEnv.NEXT_PUBLIC_ADMIN_TOKEN;
+    errorSpy.mockRestore();
+  });
+
+  it("emits no warning when both are unset", () => {
+    checkAdminTokenPair();
+    expect(errorSpy).not.toHaveBeenCalled();
+  });
+
+  it("emits no warning when both are set (matched pair, the happy path)", () => {
+    process.env.ADMIN_TOKEN = "local-dev-admin";
+    process.env.NEXT_PUBLIC_ADMIN_TOKEN = "local-dev-admin";
+    checkAdminTokenPair();
+    expect(errorSpy).not.toHaveBeenCalled();
+  });
+
+  it("warns when ADMIN_TOKEN is set but NEXT_PUBLIC_ADMIN_TOKEN is not", () => {
+    process.env.ADMIN_TOKEN = "local-dev-admin";
+    checkAdminTokenPair();
+    expect(errorSpy).toHaveBeenCalledTimes(1);
+    // Exact-string assertion — substring would also pass when the
+    // function's branch logic is broken (e.g. emits both messages, or
+    // emits the wrong one). Pin the exact message that operators will
+    // see in their dev console so regressions are visible.
+    expect(errorSpy).toHaveBeenCalledWith(
+      "[next.config] ADMIN_TOKEN is set but NEXT_PUBLIC_ADMIN_TOKEN is not — " +
+        "canvas will 401 against workspace-server because the bearer header " +
+        "is never attached. Set both to the same value, or unset both.",
+    );
+  });
+
+  it("warns when NEXT_PUBLIC_ADMIN_TOKEN is set but ADMIN_TOKEN is not", () => {
+    process.env.NEXT_PUBLIC_ADMIN_TOKEN = "local-dev-admin";
+    checkAdminTokenPair();
+    expect(errorSpy).toHaveBeenCalledTimes(1);
+    expect(errorSpy).toHaveBeenCalledWith(
+      "[next.config] NEXT_PUBLIC_ADMIN_TOKEN is set but ADMIN_TOKEN is not — " +
+        "workspace-server will reject the bearer because no AdminAuth gate " +
+        "is configured. Set both to the same value, or unset both.",
+    );
+  });
+
+  // Empty string in process.env is the JS-side representation of `KEY=`
+  // (no value) in a .env file. Treating "" as unset makes the pair
+  // invariant symmetric: `KEY=` and `unset KEY` produce the same
+  // verdict. Without this branch, an operator who comments out the
+  // value but leaves the line would get a false-positive warning.
+  it("treats empty string as unset (so KEY= and unset KEY are equivalent)", () => {
+    process.env.ADMIN_TOKEN = "";
+    process.env.NEXT_PUBLIC_ADMIN_TOKEN = "";
+    checkAdminTokenPair();
+    expect(errorSpy).not.toHaveBeenCalled();
+  });
+
+  it("warns when ADMIN_TOKEN is set and NEXT_PUBLIC_ADMIN_TOKEN is empty string", () => {
+    process.env.ADMIN_TOKEN = "local-dev-admin";
+    process.env.NEXT_PUBLIC_ADMIN_TOKEN = "";
+    checkAdminTokenPair();
+    expect(errorSpy).toHaveBeenCalledTimes(1);
+    // First branch — server set, client unset.
+    expect(errorSpy).toHaveBeenCalledWith(
+      expect.stringContaining("ADMIN_TOKEN is set but NEXT_PUBLIC_ADMIN_TOKEN is not"),
+    );
+  });
+});