diff --git a/Packages/macOS/CmuxControlSocket/Sources/CmuxControlSocket/Coordinator/Workspace/ControlCommandCoordinator+Workspace.swift b/Packages/macOS/CmuxControlSocket/Sources/CmuxControlSocket/Coordinator/Workspace/ControlCommandCoordinator+Workspace.swift
index e6e11ea6d839..cd34ec176c76 100644
--- a/Packages/macOS/CmuxControlSocket/Sources/CmuxControlSocket/Coordinator/Workspace/ControlCommandCoordinator+Workspace.swift
+++ b/Packages/macOS/CmuxControlSocket/Sources/CmuxControlSocket/Coordinator/Workspace/ControlCommandCoordinator+Workspace.swift
@@ -273,8 +273,11 @@ extension ControlCommandCoordinator {
         let afterID = uuid(params, "after_workspace_id")
         let dryRun = bool(params, "dry_run") ?? false
 
-        let targetCount = (index != nil ? 1 : 0) + (beforeID != nil ? 1 : 0) + (afterID != nil ? 1 : 0)
-        if targetCount != 1 {
+        // Count supplied selectors, not resolved identities. An unknown ref
+        // must neither look like a missing target nor hide a conflicting one.
+        let targetCount = ["index", "before_workspace_id", "after_workspace_id"]
+            .filter { hasNonNull(params, $0) }.count
+        if targetCount != 1 || (hasNonNull(params, "index") && index == nil) {
             return .err(
                 code: "invalid_params",
                 message: "Specify exactly one target: index, before_workspace_id, or after_workspace_id",
@@ -282,14 +285,20 @@ extension ControlCommandCoordinator {
             )
         }
 
-        let resolution = context?.controlReorderWorkspace(
-            routing: routingSelectors(params),
-            workspaceID: workspaceID,
-            toIndex: index,
-            beforeWorkspaceID: beforeID,
-            afterWorkspaceID: afterID,
-            dryRun: dryRun
-        ) ?? .notFound
+        let resolution: ControlWorkspaceReorderResolution
+        if (hasNonNull(params, "before_workspace_id") && beforeID == nil)
+            || (hasNonNull(params, "after_workspace_id") && afterID == nil) {
+            resolution = .notFound
+        } else {
+            resolution = context?.controlReorderWorkspace(
+                routing: routingSelectors(params),
+                workspaceID: workspaceID,
+                toIndex: index,
+                beforeWorkspaceID: beforeID,
+                afterWorkspaceID: afterID,
+                dryRun: dryRun
+            ) ?? .notFound
+        }
         switch resolution {
         case .notFound:
             return .err(code: "not_found", message: "Workspace not found", data: .object([
diff --git a/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/ControlWorkspaceReorderTargetTests.swift b/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/ControlWorkspaceReorderTargetTests.swift
new file mode 100644
index 000000000000..4d9bfb4b0d84
--- /dev/null
+++ b/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/ControlWorkspaceReorderTargetTests.swift
@@ -0,0 +1,67 @@
+import Foundation
+import Testing
+@testable import CmuxControlSocket
+
+@MainActor
+struct ControlWorkspaceReorderTargetTests {
+    @Test(arguments: ["before_workspace_id", "after_workspace_id"])
+    func unresolvedRelativeTargetReportsNotFound(key: String) {
+        let context = FakeWorkspaceControlCommandContext()
+        let coordinator = ControlCommandCoordinator(context: context)
+        let result = coordinator.handle(ControlRequest(id: .int(1), method: "workspace.reorder", params: [
+            "workspace_id": .string(UUID().uuidString),
+            key: .string("workspace:999999"),
+            "dry_run": .bool(true)
+        ]))
+        guard case .err(let code, _, _) = result else {
+            Issue.record("An unknown relative target must fail")
+            return
+        }
+        #expect(code == "not_found")
+        #expect(context.reorderCall == nil)
+    }
+
+    @Test(arguments: ["before_workspace_id", "after_workspace_id"])
+    func unresolvedTargetStillConflictsWithIndex(key: String) {
+        let context = FakeWorkspaceControlCommandContext()
+        let coordinator = ControlCommandCoordinator(context: context)
+        let result = coordinator.handle(ControlRequest(id: .int(1), method: "workspace.reorder", params: [
+            "workspace_id": .string(UUID().uuidString),
+            key: .string("workspace:999999"),
+            "index": .int(0)
+        ]))
+        guard case .err(let code, _, _) = result else {
+            Issue.record("Conflicting targets must fail")
+            return
+        }
+        #expect(code == "invalid_params")
+        #expect(context.reorderCall == nil)
+    }
+
+    @Test(arguments: ["before_workspace_id", "after_workspace_id"], [true, false])
+    func knownRelativeTargetReachesPlannerWithoutIndex(key: String, dryRun: Bool) throws {
+        let context = FakeWorkspaceControlCommandContext()
+        let coordinator = ControlCommandCoordinator(context: context)
+        let workspaceID = UUID()
+        let targetID = UUID()
+        let workspaceRef = coordinator.ensureRef(kind: .workspace, uuid: workspaceID)
+        let targetRef = coordinator.ensureRef(kind: .workspace, uuid: targetID)
+        context.reorderResolution = .resolved(
+            windowID: nil,
+            plan: ControlWorkspaceReorderPlanItem(workspaceID: workspaceID, fromIndex: 1, toIndex: 0)
+        )
+        let result = coordinator.handle(ControlRequest(id: .int(1), method: "workspace.reorder", params: [
+            "workspace_id": .string(workspaceRef), key: .string(targetRef), "dry_run": .bool(dryRun)
+        ]))
+        guard case .ok = result else {
+            Issue.record("A known relative target must succeed")
+            return
+        }
+        let call = try #require(context.reorderCall)
+        #expect(call.workspaceID == workspaceID)
+        #expect(call.index == nil)
+        #expect(call.before == (key == "before_workspace_id" ? targetID : nil))
+        #expect(call.after == (key == "after_workspace_id" ? targetID : nil))
+        #expect(call.dryRun == dryRun)
+    }
+}
diff --git a/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/FakeWorkspaceControlCommandContext.swift b/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/FakeWorkspaceControlCommandContext.swift
index 35daf637eafa..2694f4a9a1a3 100644
--- a/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/FakeWorkspaceControlCommandContext.swift
+++ b/Packages/macOS/CmuxControlSocket/Tests/CmuxControlSocketTests/FakeWorkspaceControlCommandContext.swift
@@ -12,6 +12,8 @@ final class FakeWorkspaceControlCommandContext: ControlCommandContext {
     var listResolution: ControlWorkspaceListResolution = .tabManagerUnavailable
     var currentResolution: ControlWorkspaceCurrentResolution = .tabManagerUnavailable
     var closeResolution: ControlWorkspaceCloseResolution = .tabManagerUnavailable
+    var reorderResolution: ControlWorkspaceReorderResolution = .notFound
+    var reorderCall: (workspaceID: UUID, index: Int?, before: UUID?, after: UUID?, dryRun: Bool)?
     var addWorkspaceToGroupResolution: ControlWorkspaceGroupAddResolution = .tabManagerUnavailable
     var addWorkspaceToGroupCall: (
         groupID: UUID,
@@ -144,6 +146,18 @@ final class FakeWorkspaceControlCommandContext: ControlCommandContext {
         return addWorkspaceToGroupResolution
     }
 
+    func controlReorderWorkspace(
+        routing: ControlRoutingSelectors,
+        workspaceID: UUID,
+        toIndex: Int?,
+        beforeWorkspaceID: UUID?,
+        afterWorkspaceID: UUID?,
+        dryRun: Bool
+    ) -> ControlWorkspaceReorderResolution {
+        reorderCall = (workspaceID, toIndex, beforeWorkspaceID, afterWorkspaceID, dryRun)
+        return reorderResolution
+    }
+
     func controlWorkspaceRemoteTerminalSessionEnd(
         workspaceID: UUID, surfaceID: UUID, relayPort: Int?,
         terminalLifecycleID: UUID?, sessionID: String?,