🤖 fix: prevent compaction infinite loop and stale usage display (#886)

Two related bugs were causing compaction to loop forever:

## Bug 1: Message queue overwrote compaction metadata

When a user typed a follow-up message while a compaction was queued, the
queue would overwrite `latestOptions`, losing the `muxMetadata` that
marks the message as a `compaction-request`. The compaction would then
be sent as a normal message, never triggering actual history
replacement.

**Fix**: Follow-up messages added after a compaction request are now
merged into the compaction's `continueMessage` field rather than
overwriting options. This preserves compaction metadata and sends
follow-ups after compaction completes.

## Bug 2: Usage display showed pre-compaction context

After compaction, the summary message's usage field contained the
pre-compaction context size (since that's what was sent to generate the
summary). This inflated the context window display, making it appear
usage was still high and triggering auto-compaction again.

**Fix**: Skip compacted messages when computing `lastContextUsage`. The
summary's usage reflects historical cost, not current context.

---

_Generated with `mux`_

Author: ammar-agent

src/browser/stores/WorkspaceStore.ts

@@ -476,10 +476,16 @@ export class WorkspaceStore {
 
       // Get last message's context usage for context window display
       // Uses contextUsage (last step) if available, falls back to usage for old messages
+      // Skips compacted messages - their usage reflects pre-compaction context, not current
       const lastContextUsage = (() => {
         for (let i = messages.length - 1; i >= 0; i--) {
           const msg = messages[i];
           if (msg.role === "assistant") {
+            // Skip compacted messages - their usage is from pre-compaction context
+            // and doesn't reflect current context window size
+            if (msg.metadata?.compacted) {
+              continue;
+            }
             const rawUsage = msg.metadata?.contextUsage ?? msg.metadata?.usage;
             const providerMeta =
               msg.metadata?.contextProviderMetadata ?? msg.metadata?.providerMetadata;

src/node/services/messageQueue.test.ts

@@ -35,7 +35,7 @@ describe("MessageQueue", () => {
       expect(queue.getDisplayText()).toBe("/compact -t 3000");
     });
 
-    it("should show actual messages when compaction is added after normal message", () => {
+    it("should throw when adding compaction after normal message", () => {
       queue.add("First message");
 
       const metadata: MuxFrontendMetadata = {
@@ -49,11 +49,11 @@ describe("MessageQueue", () => {
         muxMetadata: metadata,
       };
 
-      queue.add("Summarize this conversation...", options);
-
-      // When multiple messages are queued, compaction metadata is lost when sent,
-      // so display shows actual messages (not rawCommand) to match what will be sent
-      expect(queue.getDisplayText()).toBe("First message\nSummarize this conversation...");
+      // Compaction requests cannot be mixed with other messages to prevent
+      // silent failures where compaction metadata would be lost
+      expect(() => queue.add("Summarize this conversation...", options)).toThrow(
+        /Cannot queue compaction request/
+      );
     });
 
     it("should return joined messages when metadata type is not compaction-request", () => {
@@ -117,6 +117,48 @@ describe("MessageQueue", () => {
     });
   });
 
+  describe("hasCompactionRequest", () => {
+    it("should return false for empty queue", () => {
+      expect(queue.hasCompactionRequest()).toBe(false);
+    });
+
+    it("should return false for normal messages", () => {
+      queue.add("Regular message", { model: "gpt-4" });
+      expect(queue.hasCompactionRequest()).toBe(false);
+    });
+
+    it("should return true when compaction request is queued", () => {
+      const metadata: MuxFrontendMetadata = {
+        type: "compaction-request",
+        rawCommand: "/compact",
+        parsed: {},
+      };
+
+      queue.add("Summarize...", {
+        model: "claude-3-5-sonnet-20241022",
+        muxMetadata: metadata,
+      });
+
+      expect(queue.hasCompactionRequest()).toBe(true);
+    });
+
+    it("should return false after clearing", () => {
+      const metadata: MuxFrontendMetadata = {
+        type: "compaction-request",
+        rawCommand: "/compact",
+        parsed: {},
+      };
+
+      queue.add("Summarize...", {
+        model: "claude-3-5-sonnet-20241022",
+        muxMetadata: metadata,
+      });
+      queue.clear();
+
+      expect(queue.hasCompactionRequest()).toBe(false);
+    });
+  });
+
   describe("multi-message batching", () => {
     it("should batch multiple follow-up messages", () => {
       queue.add("First message");
@@ -127,7 +169,7 @@ describe("MessageQueue", () => {
       expect(queue.getDisplayText()).toBe("First message\nSecond message\nThird message");
     });
 
-    it("should batch follow-up message after compaction", () => {
+    it("should preserve compaction metadata when follow-up is added", () => {
       const metadata: MuxFrontendMetadata = {
         type: "compaction-request",
         rawCommand: "/compact",
@@ -140,11 +182,20 @@ describe("MessageQueue", () => {
       });
       queue.add("And then do this follow-up task");
 
-      // When a follow-up is added, compaction metadata is lost (latestOptions overwritten),
-      // so display shows actual messages to match what will be sent
+      // Display shows all messages (multiple messages = not just compaction)
       expect(queue.getDisplayText()).toBe("Summarize...\nAnd then do this follow-up task");
-      // Raw messages have the actual prompt
+
+      // getMessages includes both
       expect(queue.getMessages()).toEqual(["Summarize...", "And then do this follow-up task"]);
+
+      // produceMessage preserves compaction metadata from first message
+      const { message, options } = queue.produceMessage();
+      expect(message).toBe("Summarize...\nAnd then do this follow-up task");
+      const muxMeta = options?.muxMetadata as MuxFrontendMetadata;
+      expect(muxMeta.type).toBe("compaction-request");
+      if (muxMeta.type === "compaction-request") {
+        expect(muxMeta.rawCommand).toBe("/compact");
+      }
     });
 
     it("should produce combined message for API call", () => {

src/node/services/messageQueue.ts

@@ -17,22 +17,37 @@ function isCompactionMetadata(meta: unknown): meta is CompactionMetadata {
  *
  * Stores:
  * - Message texts (accumulated)
- * - Latest options (model, thinking level, etc. - overwrites on each add)
+ * - First muxMetadata (preserved - never overwritten by subsequent adds)
+ * - Latest options (model, etc. - updated on each add)
  * - Image parts (accumulated across all messages)
  *
+ * IMPORTANT: muxMetadata from the first message is preserved even when
+ * subsequent messages are added. This prevents compaction requests from
+ * losing their metadata when follow-up messages are queued.
+ *
  * Display logic:
  * - Single compaction request → shows rawCommand (/compact)
- * - Multiple messages → shows all actual message texts (since compaction metadata is lost anyway)
+ * - Multiple messages → shows all actual message texts
  */
 export class MessageQueue {
   private messages: string[] = [];
+  private firstMuxMetadata?: unknown;
   private latestOptions?: SendMessageOptions;
   private accumulatedImages: ImagePart[] = [];
 
+  /**
+   * Check if the queue currently contains a compaction request.
+   */
+  hasCompactionRequest(): boolean {
+    return isCompactionMetadata(this.firstMuxMetadata);
+  }
+
   /**
    * Add a message to the queue.
-   * Updates to latest options, accumulates image parts.
-   * Allows image-only messages (empty text with images).
+   * Preserves muxMetadata from first message, updates other options.
+   * Accumulates image parts.
+   *
+   * @throws Error if trying to add a compaction request when queue already has messages
    */
   add(message: string, options?: SendMessageOptions & { imageParts?: ImagePart[] }): void {
     const trimmedMessage = message.trim();
@@ -43,13 +58,30 @@ export class MessageQueue {
       return;
     }
 
+    const incomingIsCompaction = isCompactionMetadata(options?.muxMetadata);
+    const queueHasMessages = !this.isEmpty();
+
+    // Cannot add compaction to a queue that already has messages
+    // (user should wait for those messages to send first)
+    if (incomingIsCompaction && queueHasMessages) {
+      throw new Error(
+        "Cannot queue compaction request: queue already has messages. " +
+          "Wait for current stream to complete before compacting."
+      );
+    }
+
     // Add text message if non-empty
     if (trimmedMessage.length > 0) {
       this.messages.push(trimmedMessage);
     }
 
     if (options) {
       const { imageParts, ...restOptions } = options;
+
+      // Preserve first muxMetadata (see class docblock for rationale)
+      if (options.muxMetadata !== undefined && this.firstMuxMetadata === undefined) {
+        this.firstMuxMetadata = options.muxMetadata;
+      }
       this.latestOptions = restOptions;
 
       if (imageParts && imageParts.length > 0) {
@@ -68,14 +100,12 @@ export class MessageQueue {
   /**
    * Get display text for queued messages.
    * - Single compaction request shows rawCommand (/compact)
-   * - Multiple messages or non-compaction show actual message texts
+   * - Multiple messages show all actual message texts
    */
   getDisplayText(): string {
     // Only show rawCommand for single compaction request
-    // (compaction metadata is only preserved when no follow-up messages are added)
-    const muxMetadata = this.latestOptions?.muxMetadata as unknown;
-    if (this.messages.length === 1 && isCompactionMetadata(muxMetadata)) {
-      return muxMetadata.rawCommand;
+    if (this.messages.length === 1 && isCompactionMetadata(this.firstMuxMetadata)) {
+      return this.firstMuxMetadata.rawCommand;
     }
 
     return this.messages.join("\n");
@@ -90,17 +120,21 @@ export class MessageQueue {
 
   /**
    * Get combined message and options for sending.
-   * Returns joined messages with latest options + accumulated images.
    */
   produceMessage(): {
     message: string;
     options?: SendMessageOptions & { imageParts?: ImagePart[] };
   } {
     const joinedMessages = this.messages.join("\n");
-
+    // First metadata takes precedence (preserves compaction requests)
+    const muxMetadata =
+      this.firstMuxMetadata !== undefined
+        ? this.firstMuxMetadata
+        : (this.latestOptions?.muxMetadata as unknown);
     const options = this.latestOptions
       ? {
           ...this.latestOptions,
+          muxMetadata,
           imageParts: this.accumulatedImages.length > 0 ? this.accumulatedImages : undefined,
         }
       : undefined;
@@ -113,6 +147,7 @@ export class MessageQueue {
    */
   clear(): void {
     this.messages = [];
+    this.firstMuxMetadata = undefined;
     this.latestOptions = undefined;
     this.accumulatedImages = [];
   }