diff --git a/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.test.ts b/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.test.ts
index def493b9e8..f93318b721 100644
--- a/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.test.ts
+++ b/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.test.ts
@@ -2818,6 +2818,14 @@ describe("CodexAppServerAgent", () => {
       _meta: {},
     } as unknown as NewSessionRequest);
 
+    // The last usage reading before compaction feeds the boundary's preTokens/contextSize.
+    stub.emit("thread/tokenUsage/updated", {
+      tokenUsage: {
+        last: { inputTokens: 180000, outputTokens: 5000, totalTokens: 185000 },
+        modelContextWindow: 200000,
+      },
+    });
+
     // The compaction item brackets it: started → in progress, completed → boundary.
     stub.emit("item/started", {
       item: { type: "contextCompaction", id: "c1" },
@@ -2830,7 +2838,12 @@ describe("CodexAppServerAgent", () => {
     expect(
       extNotifications.find((n) => n.method === "_posthog/compact_boundary")
         ?.params,
-    ).toMatchObject({ sessionId: "t" });
+    ).toMatchObject({
+      sessionId: "t",
+      trigger: "auto",
+      preTokens: 185000,
+      contextSize: 200000,
+    });
     // ...and a user-visible marker lands in the transcript.
     expect(sessionUpdates).toContainEqual({
       sessionId: "t",
diff --git a/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.ts b/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.ts
index 74a7c15390..6f4b21e311 100644
--- a/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.ts
+++ b/packages/agent/src/adapters/codex-app-server/codex-app-server-agent.ts
@@ -1747,6 +1747,10 @@ export class CodexAppServerAgent extends BaseAcpAgent {
     void this.client
       .extNotification(POSTHOG_NOTIFICATIONS.COMPACT_BOUNDARY, {
         sessionId: this.sessionId,
+        // codex only auto-compacts; there's no manual /compact.
+        trigger: "auto",
+        preTokens: this.usage.contextTokens(),
+        contextSize: this.usage.contextSize(),
       })
       .catch(() => undefined);
     void this.client
diff --git a/packages/agent/src/adapters/codex-app-server/usage-tracker.ts b/packages/agent/src/adapters/codex-app-server/usage-tracker.ts
index fd107a7c13..42c8ef7927 100644
--- a/packages/agent/src/adapters/codex-app-server/usage-tracker.ts
+++ b/packages/agent/src/adapters/codex-app-server/usage-tracker.ts
@@ -27,6 +27,8 @@ export class UsageTracker {
   private baseline: ContextBreakdownBaseline = emptyBaseline();
   private lastTurn?: Usage;
   private contextUsed?: number;
+  // Model context window is a constant, so it survives resetForTurn.
+  private contextWindow?: number;
 
   setBaseline(baseline: ContextBreakdownBaseline): void {
     this.baseline = baseline;
@@ -48,6 +50,7 @@ export class UsageTracker {
     const { context, used, size } = reading;
     // Drives the per-source breakdown's "conversation" bucket on turn complete.
     this.contextUsed = used;
+    if (size != null) this.contextWindow = size;
     const inputTokens = context.inputTokens ?? 0;
     const outputTokens = context.outputTokens ?? 0;
     const cachedReadTokens = context.cachedInputTokens ?? 0;
@@ -81,4 +84,9 @@ export class UsageTracker {
   contextTokens(): number | undefined {
     return this.contextUsed;
   }
+
+  /** Model context window last reported by codex, or undefined pre-usage. */
+  contextSize(): number | undefined {
+    return this.contextWindow;
+  }
 }