test(browser): reject unmodeled CDP commands in protocol fixtures

docs/testing.md

@@ -30,6 +30,10 @@ one golden malformed-input case and a real filesystem lifecycle that proves
 configure, update, readback, and removal; an add-only journey does not cover the
 whole lifecycle.
 
+Protocol fixtures must model the commands they support and their producer reply
+shapes. Reject unmodeled commands instead of returning a generic success; an
+empty result is appropriate only for a supported command with no result fields.
+
 Keep transport-only in-memory MCP sessions in the MCP boundary lane and direct
 filesystem workflows in the filesystem boundary lane. Real process-tree capture
 belongs in the serial process boundary lane, including captures invoked through

tests/boundary/browser/cdpConnection.test.ts

@@ -62,6 +62,28 @@ describe("CDP connection", () => {
       await connection.close();
     }
   });
+
+  it("surfaces an unmodeled fake command as a CDP method rejection", async () => {
+    const browser = await startFakeCdpBrowser();
+    browsers.push(browser);
+    const connection = await CdpConnection.connect(
+      browser.browserWebSocketUrl,
+      "observe_web_session",
+    );
+    try {
+      await expect(
+        connection.send("Fixture.unmodeledMethod"),
+      ).rejects.toMatchObject({
+        _tag: "BrowserObservationError",
+        command: "Fixture.unmodeledMethod",
+        code: -32_601,
+        reportedMessage: "Method not found: Fixture.unmodeledMethod",
+      });
+    } finally {
+      await connection.close();
+    }
+  });
+
   it("preserves the selected payload limit reason for pending and subsequent commands", async () => {
     const browser = await startFakeCdpBrowser({
       commandResult: () => ({ oversized: "x".repeat(2_048) }),

tests/fixtures/fakeCdpBrowser.ts

@@ -178,15 +178,28 @@ const replyToCommand = (
   options: FakeOptions,
   frameTreeReads: number,
 ): void => {
+  const result =
+    options.commandResult?.(
+      command,
+      `http://127.0.0.1:${String(port)}`,
+      frameTreeReads,
+    ) ?? resultFor(command, port, options, frameTreeReads);
+  if (result === undefined) {
+    socket.send(
+      JSON.stringify({
+        id: command.id,
+        error: {
+          code: -32_601,
+          message: `Method not found: ${command.method}`,
+        },
+      }),
+    );
+    return;
+  }
   socket.send(
     JSON.stringify({
       id: command.id,
-      result:
-        options.commandResult?.(
-          command,
-          `http://127.0.0.1:${String(port)}`,
-          frameTreeReads,
-        ) ?? resultFor(command, port, options, frameTreeReads),
+      result,
     }),
   );
   if (options.malformedEventOnMethod === command.method)

tests/fixtures/fakeCdpBrowserResults.ts

@@ -6,7 +6,7 @@ export const resultFor = (
   port: number,
   options: FakeOptions,
   frameTreeReads: number,
-): Readonly<Record<string, unknown>> => {
+): Readonly<Record<string, unknown>> | undefined => {
   switch (command.method) {
     case "Target.attachToTarget":
       return attachToTargetResult(options);
@@ -44,8 +44,25 @@ export const resultFor = (
       return cacheEntriesResult(port);
     case "CacheStorage.requestCachedResponse":
       return cachedResponseResult(options);
-    default:
+    case "Page.enable":
+    case "Page.disable":
+    case "Page.setLifecycleEventsEnabled":
+    case "Network.enable":
+    case "Network.disable":
+    case "Debugger.disable":
+    case "Runtime.enable":
+    case "Runtime.disable":
+    case "Runtime.releaseObjectGroup":
+    case "DOM.enable":
+    case "DOM.disable":
       return {};
+    case "Debugger.enable":
+      return { debuggerId: "fake-debugger" };
+    case "Target.detachFromTarget":
+    case "WebMCP.enable":
+      return {};
+    default:
+      return undefined;
   }
 };
 

tests/fixtures/webRuntime.ts

@@ -21,6 +21,13 @@ const runtimeResult = (
   origin: string,
 ): Readonly<Record<string, unknown>> | undefined => {
   switch (command.method) {
+    case "Profiler.enable":
+    case "Profiler.disable":
+      return {};
+    case "Profiler.startPreciseCoverage":
+      return { timestamp: 12 };
+    case "Profiler.stopPreciseCoverage":
+      return {};
     case "Page.getFrameTree":
       return {
         frameTree: {
@@ -31,8 +38,6 @@ const runtimeResult = (
           },
         },
       };
-    case "Profiler.startPreciseCoverage":
-      return { timestamp: 12 };
     case "Profiler.takePreciseCoverage":
       return {
         timestamp: 13,