fix: report the fresh context's usage when ExitPlanMode clears context (#1484)

Choosing one of `ExitPlanMode`'s "clear context" options restarts
Claude's conversation with only the plan. The ACP session ID and the
turn stay the same, and the adapter resets its context count to 0, but
it didn't tell the client. Clients kept showing the old context's usage
until the fresh context first reported its own, so nothing on the client
side showed that the context had been cleared.

`publishSessionState` now sends a `usage_update` for the empty fresh
context (`used: 0`, the fresh session's context window as `size`) right
after the `current_mode_update` and `config_option_update` it already
sends. `used: 0` slightly understates the fresh context, which already
holds the system prompt and the plan, until Claude's next usage report.

Testing:
- `npm run build`, `npm run test:run`, and `npm run check` pass.
- `clear-context-coordinator.test.ts` checks the update's order and
values.

src/clear-context-coordinator.ts

@@ -45,6 +45,7 @@ export type ClearContextSession<Turn extends ClearContextTurn = ClearContextTurn
   turnQueue?: Turn[];
   pendingExitPlanContextReset?: ClearContextReset;
   contextUsedTokens?: number;
+  contextWindowSize: number;
   sentUserUuids?: Set<string>;
   input: { push(message: SDKUserMessage): unknown };
 };
@@ -61,10 +62,13 @@ export type ClearContextCoordinatorHost<
   ): Promise<Session>;
   applyFastMode(session: Session, enabled: boolean): Promise<void>;
   applyUltracode(session: Session, enabled: boolean): Promise<void>;
+  /** Publishes the fresh context's mode, config options, and usage: the
+   *  context starts empty in a window of `contextWindowSize` tokens. */
   publishSessionState(
     sessionId: string,
     mode: PermissionMode,
     configOptions: SessionConfigOption[],
+    contextWindowSize: number,
   ): Promise<void>;
   continuationMessage(sessionId: string, plan: string, promptUuid: string): SDKUserMessage;
   ensureConsumer(session: Session, sessionId: string): void;
@@ -165,7 +169,12 @@ export async function continuePlanInFreshContext<
   freshSession.contextUsedTokens = 0;
 
   try {
-    await host.publishSessionState(sessionId, reset.mode, freshSession.configOptions);
+    await host.publishSessionState(
+      sessionId,
+      reset.mode,
+      freshSession.configOptions,
+      freshSession.contextWindowSize,
+    );
   } catch (error) {
     host.logError("Failed to publish clear-context session state:", error);
   }

src/exit-plan.ts

@@ -213,7 +213,7 @@ export class ExitPlanCoordinator<
     try {
       const clearContextHost: ClearContextCoordinatorHost<Session, Turn> = {
         ...this.host,
-        publishSessionState: async (id, mode, configOptions) => {
+        publishSessionState: async (id, mode, configOptions, contextWindowSize) => {
           await this.host.sessionUpdate({
             sessionId: id,
             update: { sessionUpdate: "current_mode_update", currentModeId: mode },
@@ -222,6 +222,12 @@ export class ExitPlanCoordinator<
             sessionId: id,
             update: { sessionUpdate: "config_option_update", configOptions },
           });
+          // Otherwise the client shows the old context's usage until the fresh
+          // context first reports its own.
+          await this.host.sessionUpdate({
+            sessionId: id,
+            update: { sessionUpdate: "usage_update", used: 0, size: contextWindowSize },
+          });
           // The approved plan becomes the session's plan once the fresh
           // context that carries it out exists. Its ExitPlanMode ended with
           // an error result, so toAcpNotifications did not publish it.

src/tests/clear-context-coordinator.test.ts

@@ -28,6 +28,7 @@ function testSession(overrides: Partial<TestSession> = {}): TestSession {
       },
     },
     models: { currentModelId: "default" },
+    contextWindowSize: 200_000,
     configOptions: [],
     currentAgent: "default",
     fastModeEnabled: false,
@@ -464,6 +465,7 @@ describe("ExitPlanCoordinator", () => {
     expect(order).toEqual([
       "current_mode_update",
       "config_option_update",
+      "usage_update",
       ...(supportsPlanOperations
         ? [
             "plan_update " +
@@ -476,6 +478,10 @@ describe("ExitPlanCoordinator", () => {
         : []),
       "continuation",
     ]);
+    expect(host.sessionUpdate).toHaveBeenCalledWith({
+      sessionId: "public-session",
+      update: { sessionUpdate: "usage_update", used: 0, size: 200_000 },
+    });
     if (airClient) {
       expect(host.sessionUpdate).toHaveBeenCalledWith({
         sessionId: "public-session",