🤖 fix: stabilize post-compaction context

src/browser/components/RightSidebar/CostsTab.tsx

@@ -22,8 +22,6 @@ import { useAutoCompactionSettings } from "@/browser/hooks/useAutoCompactionSett
 import { Tooltip, TooltipTrigger, TooltipContent } from "../ui/tooltip";
 import { PostCompactionSection } from "./PostCompactionSection";
 import { usePostCompactionState } from "@/browser/hooks/usePostCompactionState";
-import { useExperimentValue } from "@/browser/contexts/ExperimentsContext";
-import { EXPERIMENT_IDS } from "@/common/constants/experiments";
 import { useOptionalWorkspaceContext } from "@/browser/contexts/WorkspaceContext";
 
 /**
@@ -59,8 +57,7 @@ const CostsTabComponent: React.FC<CostsTabProps> = ({ workspaceId }) => {
   const { options } = useProviderOptions();
   const use1M = options.anthropic?.use1MContext ?? false;
 
-  // Post-compaction context state for UI display (gated by experiment)
-  const postCompactionEnabled = useExperimentValue(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+  // Post-compaction context state for UI display
   const postCompactionState = usePostCompactionState(workspaceId);
 
   // Get runtimeConfig for SSH-aware editor opening
@@ -136,16 +133,14 @@ const CostsTabComponent: React.FC<CostsTabProps> = ({ workspaceId }) => {
               );
             })()}
           </div>
-          {postCompactionEnabled && (
-            <PostCompactionSection
-              workspaceId={workspaceId}
-              planPath={postCompactionState.planPath}
-              trackedFilePaths={postCompactionState.trackedFilePaths}
-              excludedItems={postCompactionState.excludedItems}
-              onToggleExclusion={postCompactionState.toggleExclusion}
-              runtimeConfig={runtimeConfig}
-            />
-          )}
+          <PostCompactionSection
+            workspaceId={workspaceId}
+            planPath={postCompactionState.planPath}
+            trackedFilePaths={postCompactionState.trackedFilePaths}
+            excludedItems={postCompactionState.excludedItems}
+            onToggleExclusion={postCompactionState.toggleExclusion}
+            runtimeConfig={runtimeConfig}
+          />
         </div>
       )}
 

src/browser/components/Settings/sections/ExperimentsSection.tsx

@@ -23,7 +23,6 @@ import type { ApiServerStatus } from "@/common/orpc/types";
 import { Input } from "@/browser/components/ui/input";
 import { useAPI } from "@/browser/contexts/API";
 import { useFeatureFlags } from "@/browser/contexts/FeatureFlagsContext";
-import { useWorkspaceContext } from "@/browser/contexts/WorkspaceContext";
 import { useTelemetry } from "@/browser/hooks/useTelemetry";
 
 interface ExperimentRowProps {
@@ -485,7 +484,6 @@ function StatsTabRow() {
 
 export function ExperimentsSection() {
   const allExperiments = getExperimentList();
-  const { refreshWorkspaceMetadata } = useWorkspaceContext();
   const { api } = useAPI();
 
   // Only show user-overridable experiments (non-overridable ones are hidden since users can't change them)
@@ -495,13 +493,6 @@ export function ExperimentsSection() {
     [allExperiments]
   );
 
-  // When post-compaction experiment is toggled, refresh metadata to fetch/clear bundled state
-  const handlePostCompactionToggle = useCallback(() => {
-    refreshWorkspaceMetadata().catch(() => {
-      // ignore
-    });
-  }, [refreshWorkspaceMetadata]);
-
   const handleConfigurableBindUrlToggle = useCallback(
     (enabled: boolean) => {
       if (enabled) {
@@ -531,11 +522,9 @@ export function ExperimentsSection() {
               name={exp.name}
               description={exp.description}
               onToggle={
-                exp.id === EXPERIMENT_IDS.POST_COMPACTION_CONTEXT
-                  ? handlePostCompactionToggle
-                  : exp.id === EXPERIMENT_IDS.CONFIGURABLE_BIND_URL
-                    ? handleConfigurableBindUrlToggle
-                    : undefined
+                exp.id === EXPERIMENT_IDS.CONFIGURABLE_BIND_URL
+                  ? handleConfigurableBindUrlToggle
+                  : undefined
               }
             />
             {exp.id === EXPERIMENT_IDS.CONFIGURABLE_BIND_URL && <ConfigurableBindUrlControls />}

src/browser/contexts/ExperimentsContext.test.tsx

@@ -39,12 +39,12 @@ describe("ExperimentsProvider", () => {
 
       if (callCount === 1) {
         return Promise.resolve({
-          [EXPERIMENT_IDS.POST_COMPACTION_CONTEXT]: { value: null, source: "cache" },
+          [EXPERIMENT_IDS.SYSTEM_1]: { value: null, source: "cache" },
         } satisfies Record<string, ExperimentValue>);
       }
 
       return Promise.resolve({
-        [EXPERIMENT_IDS.POST_COMPACTION_CONTEXT]: { value: "test", source: "posthog" },
+        [EXPERIMENT_IDS.SYSTEM_1]: { value: "test", source: "posthog" },
       } satisfies Record<string, ExperimentValue>);
     });
 
@@ -56,7 +56,7 @@ describe("ExperimentsProvider", () => {
     };
 
     function Observer() {
-      const enabled = useExperimentValue(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+      const enabled = useExperimentValue(EXPERIMENT_IDS.SYSTEM_1);
       return <div data-testid="enabled">{String(enabled)}</div>;
     }
 

src/browser/hooks/useExperiments.test.ts

@@ -24,30 +24,30 @@ describe("isExperimentEnabled", () => {
   });
 
   test("returns undefined when no local override exists for a user-overridable experiment", () => {
-    expect(isExperimentEnabled(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT)).toBeUndefined();
+    expect(isExperimentEnabled(EXPERIMENT_IDS.SYSTEM_1)).toBeUndefined();
   });
 
   test("returns boolean when local override exists", () => {
-    const key = getExperimentKey(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+    const key = getExperimentKey(EXPERIMENT_IDS.SYSTEM_1);
 
     globalThis.window.localStorage.setItem(key, JSON.stringify(true));
-    expect(isExperimentEnabled(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT)).toBe(true);
+    expect(isExperimentEnabled(EXPERIMENT_IDS.SYSTEM_1)).toBe(true);
 
     globalThis.window.localStorage.setItem(key, JSON.stringify(false));
-    expect(isExperimentEnabled(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT)).toBe(false);
+    expect(isExperimentEnabled(EXPERIMENT_IDS.SYSTEM_1)).toBe(false);
   });
 
   test('treats literal "undefined" as no override', () => {
-    const key = getExperimentKey(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+    const key = getExperimentKey(EXPERIMENT_IDS.SYSTEM_1);
 
     globalThis.window.localStorage.setItem(key, "undefined");
-    expect(isExperimentEnabled(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT)).toBeUndefined();
+    expect(isExperimentEnabled(EXPERIMENT_IDS.SYSTEM_1)).toBeUndefined();
   });
 
   test("treats non-boolean stored value as no override", () => {
-    const key = getExperimentKey(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+    const key = getExperimentKey(EXPERIMENT_IDS.SYSTEM_1);
 
     globalThis.window.localStorage.setItem(key, JSON.stringify("test"));
-    expect(isExperimentEnabled(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT)).toBeUndefined();
+    expect(isExperimentEnabled(EXPERIMENT_IDS.SYSTEM_1)).toBeUndefined();
   });
 });

src/browser/hooks/usePostCompactionState.ts

@@ -2,8 +2,6 @@ import { useEffect, useState, useCallback, useRef } from "react";
 import { useAPI } from "@/browser/contexts/API";
 import { readPersistedState, updatePersistedState } from "@/browser/hooks/usePersistedState";
 import { getPostCompactionStateKey } from "@/common/constants/storage";
-import { useExperimentValue } from "@/browser/hooks/useExperiments";
-import { EXPERIMENT_IDS } from "@/common/constants/experiments";
 
 interface PostCompactionState {
   planPath: string | null;
@@ -35,12 +33,11 @@ function loadFromCache(wsId: string) {
  * Hook to get post-compaction context state for a workspace.
  * Fetches lazily from the backend API and caches in localStorage.
  * This avoids the expensive runtime.stat calls during workspace.list().
- * Only fetches when the POST_COMPACTION_CONTEXT experiment is enabled.
+ *
+ * Always enabled: post-compaction context is a stable feature (not an experiment).
  */
 export function usePostCompactionState(workspaceId: string): PostCompactionState {
   const { api } = useAPI();
-  const experimentEnabled = useExperimentValue(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
-
   const [state, setState] = useState(() => loadFromCache(workspaceId));
 
   // Track which workspaceId the current state belongs to.
@@ -51,9 +48,9 @@ export function usePostCompactionState(workspaceId: string): PostCompactionState
     setState(loadFromCache(workspaceId));
   }
 
-  // Fetch fresh data when workspaceId changes (only if experiment enabled)
+  // Fetch fresh data when workspaceId changes
   useEffect(() => {
-    if (!api || !experimentEnabled) return;
+    if (!api) return;
 
     let cancelled = false;
     const fetchState = async () => {
@@ -84,11 +81,11 @@ export function usePostCompactionState(workspaceId: string): PostCompactionState
     return () => {
       cancelled = true;
     };
-  }, [api, workspaceId, experimentEnabled]);
+  }, [api, workspaceId]);
 
   const toggleExclusion = useCallback(
     async (itemId: string) => {
-      if (!api || !experimentEnabled) return;
+      if (!api) return;
       const isCurrentlyExcluded = state.excludedItems.has(itemId);
       const result = await api.workspace.setPostCompactionExclusion({
         workspaceId,
@@ -117,7 +114,7 @@ export function usePostCompactionState(workspaceId: string): PostCompactionState
         });
       }
     },
-    [api, workspaceId, state.excludedItems, experimentEnabled]
+    [api, workspaceId, state.excludedItems]
   );
 
   return { ...state, toggleExclusion };

src/browser/hooks/useSendMessageOptions.ts

@@ -36,7 +36,6 @@ function applyGatewayTransform(modelId: string, gateway: GatewayState): string {
 }
 
 interface ExperimentValues {
-  postCompactionContext: boolean | undefined;
   programmaticToolCalling: boolean | undefined;
   programmaticToolCallingExclusive: boolean | undefined;
   system1: boolean | undefined;
@@ -93,7 +92,6 @@ function constructSendMessageOptions(
     // toolPolicy is computed by backend from agent definitions (resolveToolPolicyForAgent)
     providerOptions,
     experiments: {
-      postCompactionContext: experimentValues.postCompactionContext,
       programmaticToolCalling: experimentValues.programmaticToolCalling,
       programmaticToolCallingExclusive: experimentValues.programmaticToolCallingExclusive,
       system1: experimentValues.system1,
@@ -139,7 +137,6 @@ export function useSendMessageOptions(workspaceId: string): SendMessageOptionsWi
 
   // Subscribe to local override state so toggles apply immediately.
   // If undefined, the backend will apply the PostHog assignment.
-  const postCompactionContext = useExperimentOverrideValue(EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
   const programmaticToolCalling = useExperimentOverrideValue(
     EXPERIMENT_IDS.PROGRAMMATIC_TOOL_CALLING
   );
@@ -177,7 +174,7 @@ export function useSendMessageOptions(workspaceId: string): SendMessageOptionsWi
     providerOptions,
     defaultModel,
     gateway,
-    { postCompactionContext, programmaticToolCalling, programmaticToolCallingExclusive, system1 },
+    { programmaticToolCalling, programmaticToolCallingExclusive, system1 },
     system1Model,
     system1ThinkingLevel
   );

src/browser/stories/App.settings.stories.tsx

@@ -20,7 +20,11 @@ import { createWorkspace, groupWorkspacesByProject } from "./mockFactory";
 import { selectWorkspace } from "./storyHelpers";
 import { createMockORPCClient } from "@/browser/stories/mocks/orpc";
 import { within, userEvent, waitFor } from "@storybook/test";
-import { getExperimentKey, EXPERIMENT_IDS } from "@/common/constants/experiments";
+import {
+  getExperimentKey,
+  EXPERIMENT_IDS,
+  type ExperimentId,
+} from "@/common/constants/experiments";
 import type { AgentAiDefaults } from "@/common/types/agentAiDefaults";
 import type { TaskSettings } from "@/common/types/tasks";
 import type { LayoutPresetsConfig } from "@/common/types/uiLayouts";
@@ -54,7 +58,7 @@ function setupSettingsStory(options: {
   // Pre-set experiment states if provided
   if (options.experiments) {
     for (const [experimentId, enabled] of Object.entries(options.experiments)) {
-      const key = getExperimentKey(experimentId as typeof EXPERIMENT_IDS.POST_COMPACTION_CONTEXT);
+      const key = getExperimentKey(experimentId as ExperimentId);
       window.localStorage.setItem(key, JSON.stringify(enabled));
     }
   }
@@ -483,7 +487,7 @@ export const ExperimentsToggleOn: AppStory = {
     <AppWithMocks
       setup={() =>
         setupSettingsStory({
-          experiments: { [EXPERIMENT_IDS.POST_COMPACTION_CONTEXT]: true },
+          experiments: { [EXPERIMENT_IDS.SYSTEM_1]: true },
         })
       }
     />

src/browser/utils/messages/attachmentRenderer.test.ts

@@ -1,6 +1,13 @@
 import { describe, it, expect } from "@jest/globals";
-import { renderAttachmentToContent } from "./attachmentRenderer";
-import type { TodoListAttachment } from "@/common/types/attachment";
+import {
+  renderAttachmentToContent,
+  renderAttachmentsToContentWithBudget,
+} from "./attachmentRenderer";
+import type {
+  TodoListAttachment,
+  PlanFileReferenceAttachment,
+  EditedFilesReferenceAttachment,
+} from "@/common/types/attachment";
 
 describe("attachmentRenderer", () => {
   it("renders todo list inline and mentions todo_read", () => {
@@ -27,4 +34,35 @@ describe("attachmentRenderer", () => {
     expect(content).not.toContain("todos.json");
     expect(content).not.toContain("~/.mux");
   });
+
+  it("respects a maxChars budget and truncates oversized plan content", () => {
+    const attachment: PlanFileReferenceAttachment = {
+      type: "plan_file_reference",
+      planFilePath: "~/.mux/plans/cmux/ws.md",
+      planContent: "a".repeat(10_000),
+    };
+
+    const content = renderAttachmentsToContentWithBudget([attachment], { maxChars: 400 });
+
+    expect(content.length).toBeLessThanOrEqual(400);
+    expect(content).toContain("Plan contents");
+    expect(content).toContain("...(truncated)");
+    expect(content).toContain("<system-update>");
+  });
+
+  it("emits an omitted-file-diffs note when edited file diffs do not fit", () => {
+    const attachment: EditedFilesReferenceAttachment = {
+      type: "edited_files_reference",
+      files: [
+        { path: "src/a.ts", diff: "a".repeat(2000), truncated: false },
+        { path: "src/b.ts", diff: "b".repeat(2000), truncated: false },
+      ],
+    };
+
+    const content = renderAttachmentsToContentWithBudget([attachment], { maxChars: 120 });
+
+    expect(content.length).toBeLessThanOrEqual(120);
+    expect(content).toContain("omitted 2 file diffs");
+    expect(content).toContain("<system-update>");
+  });
 });