diff --git a/src/bot/PlannerBot.ts b/src/bot/PlannerBot.ts
index 5b3c076..4c97ae7 100644
--- a/src/bot/PlannerBot.ts
+++ b/src/bot/PlannerBot.ts
@@ -170,6 +170,11 @@ export class PlannerBot extends TeamsActivityHandler {
       return;
     }
 
+    // listPlansWithBuckets already drops "Deprecated" plans; mirror that for tasks
+    // so the LLM can't try to update a task in a hidden plan.
+    const validPlanIds = new Set(plans.map((p) => p.planId));
+    recentTasks = recentTasks.filter((t) => validPlanIds.has(t.planId));
+
     const action = await this.classifier.classify({
       utterance,
       plans,
diff --git a/src/graph/plannerClient.ts b/src/graph/plannerClient.ts
index e48ee0b..2a629a9 100644
--- a/src/graph/plannerClient.ts
+++ b/src/graph/plannerClient.ts
@@ -11,12 +11,17 @@ import { PlanContext, Progress, RecentTaskContext } from "../llm/types";
 export class PlannerClient {
   constructor(private readonly graph: Client) {}
 
-  /** All plans the signed-in user can see, with their buckets pre-fetched. */
+  /** All plans the signed-in user can see, with their buckets pre-fetched.
+   *  Plans whose title contains "deprecated" (case-insensitive) are filtered out
+   *  so old/archived plans don't pollute the LLM context.
+   */
   async listPlansWithBuckets(): Promise<PlanContext[]> {
     // Plans this user can access via the groups they belong to.
     // Graph exposes /me/planner/plans which returns plans for groups the user is a member of.
     const plansResp = await this.graph.api("/me/planner/plans").get();
-    const plans: Array<{ id: string; title: string }> = plansResp.value ?? [];
+    const plans: Array<{ id: string; title: string }> = (plansResp.value ?? []).filter(
+      (p: { title?: string }) => !/deprecated/i.test(p.title ?? ""),
+    );
 
     const result: PlanContext[] = [];
     for (const p of plans) {