diff --git a/TODO.md b/TODO.md
index 49525b6..7004d95 100644
--- a/TODO.md
+++ b/TODO.md
@@ -45,6 +45,13 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: A method the wallet does not implement is refused with EIP-1193
+  code `4200` ([#279](https://git.eeqj.de/sneak/AutistMask/issues/279)). The
+  background's `Unsupported method: <method>` error carried no code, so a site
+  probing for an optional method could not tell "not implemented" from "the call
+  failed". The message is unchanged; the background's other errors with no code
+  are untouched.
+
 - 2026-10-04: Settings lists the sites connected without "Remember", and
   removing a site there disconnects it
   ([#406](https://git.eeqj.de/sneak/AutistMask/issues/406)). Such a connection
diff --git a/src/background/index.js b/src/background/index.js
index 42b5c27..5172cba 100644
--- a/src/background/index.js
+++ b/src/background/index.js
@@ -932,7 +932,9 @@ async function handleRpc(method, params, origin) {
         }
     }
 
-    return { error: { message: "Unsupported method: " + method } };
+    // EIP-1193 4200 lets a site tell "this wallet does not implement that"
+    // from "that call failed", and fall back.
+    return { error: { code: 4200, message: "Unsupported method: " + method } };
 }
 
 // The body of eth_sendTransaction, from the connection check through to the
diff --git a/src/content/inpage.js b/src/content/inpage.js
index 604e5e4..2a9da49 100644
--- a/src/content/inpage.js
+++ b/src/content/inpage.js
@@ -31,7 +31,7 @@
     // an error instead of accepting the refusal.
     //
     // Whatever code arrived is passed through verbatim rather than being
-    // matched against a list: the extension emits 4001, 4100 and 4902 today,
+    // matched against a list: the extension emits 4001, 4100, 4200 and 4902,
     // and a code this file has never heard of is still the truth about what
     // happened. An error reported with no code at all stays a plain Error —
     // a ProviderRpcError whose `code` is undefined would advertise a
diff --git a/tests/inpageErrors.test.js b/tests/inpageErrors.test.js
index c45446b..7498f74 100644
--- a/tests/inpageErrors.test.js
+++ b/tests/inpageErrors.test.js
@@ -54,6 +54,7 @@ class StubCustomEvent extends StubEvent {
 // through whatever arrived — but the cases below are the real ones.
 const REJECTED = 4001; // user rejected the request
 const UNAUTHORIZED = 4100; // site not connected / wrong address
+const UNSUPPORTED_METHOD = 4200; // a method the wallet does not implement
 const UNRECOGNIZED_CHAIN = 4902; // switch/add to an unsupported chain
 
 // A stub window with the four things inpage.js touches: message listeners,
@@ -115,6 +116,41 @@ async function rejectionFrom(start, response) {
     return outcome.error;
 }
 
+// The reply the real background worker (src/background/index.js) sends for
+// `method`, loaded against just enough of the extension API to receive one
+// RPC message. Same shape as tests/coldWorkerChainId.test.js.
+function backgroundReply(method) {
+    jest.resetModules();
+    jest.doMock("../src/shared/alarms", () => ({
+        BALANCE_REFRESH_ALARM: "balance",
+        BALANCE_REFRESH_PERIOD_MINUTES: 1,
+        ensureRecurringAlarms: async () => {},
+        registerAlarmHandlers: () => {},
+    }));
+
+    let messageListener = null;
+    global.chrome = {
+        runtime: {
+            onMessage: {
+                addListener: (fn) => {
+                    messageListener = fn;
+                },
+            },
+            onConnect: { addListener: () => {} },
+        },
+    };
+
+    require("../src/background/index");
+
+    return new Promise((resolve) => {
+        messageListener(
+            { type: "AUTISTMASK_RPC", method, params: [] },
+            { origin: "https://dapp.example" },
+            resolve,
+        );
+    });
+}
+
 describe("an EIP-1193 code reaches the page", () => {
     test("a user rejection arrives as code 4001", async () => {
         const err = await rejectionFrom(
@@ -203,6 +239,26 @@ describe("an EIP-1193 code reaches the page", () => {
     });
 });
 
+// The reply here is the background's own, not one written in this file: it
+// used to carry no code for a method the wallet does not implement
+// (https://git.eeqj.de/sneak/AutistMask/issues/279), so a site probing for an
+// optional method could not tell "not implemented" from "the call failed".
+describe("a method the wallet does not implement", () => {
+    afterEach(() => {
+        delete global.chrome;
+    });
+
+    test("reaches the page as code 4200", async () => {
+        const method = "wallet_noSuchMethod";
+        const err = await rejectionFrom(
+            (p) => p.request({ method }),
+            await backgroundReply(method),
+        );
+        expect(err.code).toBe(UNSUPPORTED_METHOD);
+        expect(err.message).toBe("Unsupported method: " + method);
+    });
+});
+
 describe("the message is untouched", () => {
     test("a coded error keeps the message byte for byte", async () => {
         const message =