diff --git a/services/companion/src/server.test.ts b/services/companion/src/server.test.ts
index bdce8ad..02ae01e 100644
--- a/services/companion/src/server.test.ts
+++ b/services/companion/src/server.test.ts
@@ -83,6 +83,55 @@ describe("createCompanionApp", () => {
     await app.close();
   });
 
+  it("rejects an empty x-webchain-token header", async () => {
+    const { app } = await createCompanionApp({
+      runtime: mockRuntime(),
+      logger: false,
+      localToken: "secret",
+    });
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/sessions",
+      headers: { "x-webchain-token": "" },
+    });
+
+    expect(res.statusCode).toBe(401);
+    await app.close();
+  });
+
+  it("refuses to boot when localToken is empty or whitespace", async () => {
+    await expect(
+      createCompanionApp({
+        runtime: mockRuntime(),
+        logger: false,
+        localToken: "",
+      }),
+    ).rejects.toThrow(/WEBCHAIN_LOCAL_TOKEN must be a non-empty string/);
+
+    await expect(
+      createCompanionApp({
+        runtime: mockRuntime(),
+        logger: false,
+        localToken: "   ",
+      }),
+    ).rejects.toThrow(/WEBCHAIN_LOCAL_TOKEN must be a non-empty string/);
+  });
+
+  it("refuses to boot when WEBCHAIN_LOCAL_TOKEN is empty", async () => {
+    vi.stubEnv("WEBCHAIN_LOCAL_TOKEN", "");
+    try {
+      await expect(
+        createCompanionApp({
+          runtime: mockRuntime(),
+          logger: false,
+        }),
+      ).rejects.toThrow(/WEBCHAIN_LOCAL_TOKEN must be a non-empty string/);
+    } finally {
+      vi.unstubAllEnvs();
+    }
+  });
+
   it("creates a session with valid token", async () => {
     const runtime = mockRuntime();
     const { app } = await createCompanionApp({
diff --git a/services/companion/src/server.ts b/services/companion/src/server.ts
index c79ee84..5326f2a 100644
--- a/services/companion/src/server.ts
+++ b/services/companion/src/server.ts
@@ -111,13 +111,43 @@ export type CreateCompanionAppOptions = {
   logger?: boolean;
 };
 
+const DEFAULT_LOCAL_DEV_TOKEN = "change-me-in-local-dev";
+
+/**
+ * `??` keeps empty strings from env/options, which previously matched an empty
+ * `x-webchain-token` header and authenticated as a valid local token.
+ *
+ * @example
+ * resolveCompanionLocalToken("secret")
+ */
+function resolveCompanionLocalToken(explicitToken: string | undefined): string {
+  const raw =
+    explicitToken ??
+    process.env.WEBCHAIN_LOCAL_TOKEN ??
+    DEFAULT_LOCAL_DEV_TOKEN;
+  const token = raw.trim();
+  if (token.length === 0) {
+    throw new Error(
+      `WEBCHAIN_LOCAL_TOKEN must be a non-empty string; received ${JSON.stringify(raw)}, expected a non-empty token`,
+    );
+  }
+  return token;
+}
+
+function presentedCompanionToken(
+  header: string | string[] | undefined,
+): string | undefined {
+  if (typeof header !== "string") {
+    return undefined;
+  }
+  const token = header.trim();
+  return token.length > 0 ? token : undefined;
+}
+
 export async function createCompanionApp(
   options: CreateCompanionAppOptions,
 ): Promise<{ app: FastifyInstance; localToken: string }> {
-  const localToken =
-    options.localToken ??
-    process.env.WEBCHAIN_LOCAL_TOKEN ??
-    "change-me-in-local-dev";
+  const localToken = resolveCompanionLocalToken(options.localToken);
 
   const app = Fastify({ logger: options.logger ?? true });
 
@@ -141,7 +171,7 @@ export async function createCompanionApp(
       return;
     }
 
-    const token = request.headers["x-webchain-token"];
+    const token = presentedCompanionToken(request.headers["x-webchain-token"]);
 
     if (token !== localToken) {
       return reply.code(401).send({