diff --git a/CLI/cmux.swift b/CLI/cmux.swift
index 10073fc6cc7d..4441706781b4 100644
--- a/CLI/cmux.swift
+++ b/CLI/cmux.swift
@@ -7662,6 +7662,17 @@ struct CMUXCLI {
             let surfaceArg = sfArg ?? (wsArg == nil && windowRaw == nil ? ProcessInfo.processInfo.environment["CMUX_SURFACE_ID"] : nil)
             let keyArgs = rem2.first == "--" ? Array(rem2.dropFirst()) : rem2
             guard let key = keyArgs.first else { throw CLIError(message: "send-key requires a key") }
+            if keyArgs.count > 1 {
+                let trailing = keyArgs.dropFirst().joined(separator: " ")
+                throw CLIError(message: String(
+                    format: String(
+                        localized: "cli.readSelection.error.unexpectedArguments",
+                        defaultValue: "%@: unexpected arguments: %@"
+                    ),
+                    "send-key",
+                    trailing
+                ))
+            }
             var params: [String: Any] = ["key": key]
             let winId = try normalizeWindowHandle(windowRaw, client: client)
             if let winId { params["window_id"] = winId }
@@ -7706,6 +7717,17 @@ struct CMUXCLI {
             let skpArgs = rem2.first == "--" ? Array(rem2.dropFirst()) : rem2
             let key = skpArgs.first ?? ""
             guard !key.isEmpty else { throw CLIError(message: "send-key-panel requires a key") }
+            if skpArgs.count > 1 {
+                let trailing = skpArgs.dropFirst().joined(separator: " ")
+                throw CLIError(message: String(
+                    format: String(
+                        localized: "cli.readSelection.error.unexpectedArguments",
+                        defaultValue: "%@: unexpected arguments: %@"
+                    ),
+                    "send-key-panel",
+                    trailing
+                ))
+            }
             var params: [String: Any] = ["key": key]
             let winId = try normalizeWindowHandle(windowRaw, client: client)
             if let winId { params["window_id"] = winId }
diff --git a/cmuxCLITests/CLIExplicitSurfaceRoutingTests.swift b/cmuxCLITests/CLIExplicitSurfaceRoutingTests.swift
index dda1fbd188f1..ac6fa5bae265 100644
--- a/cmuxCLITests/CLIExplicitSurfaceRoutingTests.swift
+++ b/cmuxCLITests/CLIExplicitSurfaceRoutingTests.swift
@@ -43,6 +43,34 @@ struct CLIExplicitSurfaceRoutingTests {
         )
     }
 
+    @Test func sendKeyCommandsRejectExtraArgumentsWithoutSocketRequest() throws {
+        let cases: [[String]] = [
+            ["send-key", "--surface", Self.targetSurfaceRef, "ctrl+c", "enter"],
+            ["send-key-panel", "--panel", Self.targetSurfaceRef, "ctrl+c", "enter"],
+        ]
+
+        for (index, arguments) in cases.enumerated() {
+            let execution = try runMockCommand(
+                arguments: arguments,
+                socketName: "key-arity-\(index)"
+            ) { line in
+                Self.malformedRequestResponse(raw: line)
+            }
+
+            let requests = try execution.state.requestObjects()
+            #expect(requests.isEmpty, Comment(rawValue: String(describing: requests)))
+            #expect(!execution.result.timedOut, Comment(rawValue: execution.result.stderr))
+            #expect(
+                execution.result.status != 0,
+                Comment(rawValue: execution.result.stderr + execution.result.stdout)
+            )
+            #expect(
+                execution.result.stderr.contains("unexpected arguments"),
+                Comment(rawValue: execution.result.stderr)
+            )
+        }
+    }
+
     @Test func numericSurfaceHandleStillInheritsCallerWorkspaceForIndexResolution() throws {
         let socketPath = Self.makeSocketPath("numeric")
         let listenerFD = try Self.bindUnixSocket(at: socketPath)