diff --git a/packages/core/src/agents/runtime/agent-core.ts b/packages/core/src/agents/runtime/agent-core.ts
index d0c7704ce1..30cb199ca2 100644
--- a/packages/core/src/agents/runtime/agent-core.ts
+++ b/packages/core/src/agents/runtime/agent-core.ts
@@ -1349,6 +1349,28 @@ export class AgentCore {
     return this.messages;
   }
 
+  /**
+   * Prune oldest messages from the message history when `heapUsed` exceeds
+   * 1.5 GB. Returns the number of messages pruned (0 if no pruning was
+   * needed). Call this after each reasoning round to prevent unbounded
+   * memory growth in long-lived interactive sessions.
+   *
+   * Messages are pruned in chunks — removing ~20% of the oldest messages
+   * each time, which naturally scales with memory pressure rather than
+   * a hard count cap. This catches the actual root cause (heap usage)
+   * regardless of how the memory ballooned (many small entries vs few huge).
+   */
+  pruneMessages(): number {
+    const threshold = 1.5 * 1024 * 1024 * 1024; // 1.5 GB
+    if (process.memoryUsage().heapUsed < threshold) {
+      return 0;
+    }
+    // Remove ~20% of the oldest messages to relieve heap pressure.
+    const toPrune = Math.max(1, Math.ceil(this.messages.length * 0.2));
+    this.messages.splice(0, toPrune);
+    return toPrune;
+  }
+
   /**
    * Tool calls currently awaiting user approval. Mutated by
    * AgentInteractive's TOOL_WAITING_APPROVAL handler; headless agents
diff --git a/packages/core/src/agents/runtime/agent-interactive.ts b/packages/core/src/agents/runtime/agent-interactive.ts
index b7fbba1df0..0b99dc4799 100644
--- a/packages/core/src/agents/runtime/agent-interactive.ts
+++ b/packages/core/src/agents/runtime/agent-interactive.ts
@@ -123,6 +123,10 @@ export class AgentInteractive {
       while (message !== null && !this.masterAbortController.signal.aborted) {
         this.addMessage('user', message);
         await this.runOneRound(message);
+        // Prune old messages to prevent unbounded memory growth in
+        // long-lived interactive sessions (81+ minute sessions with
+        // hundreds of rounds can hit 4 GB without pruning).
+        this.core.pruneMessages();
         message = this.queue.dequeue();
       }
 
diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index 4f06666c40..790909cf94 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -388,6 +388,19 @@ export class GeminiChat {
   // model.
   private sendPromise: Promise<void> = Promise.resolve();
 
+  /**
+   * Heap memory threshold (2 GB). When `heapUsed` exceeds this, chat
+   * compression is forced regardless of `hasFailedCompressionAttempt`.
+   * This is a memory safety net independent of the 70% token compaction
+   * threshold — catching the actual root cause (heap pressure) rather than
+   * a proxy (entry count). Protects against both many small entries AND
+   * few huge entries (large file reads, shell outputs).
+   *
+   * Note: set below Node's default 4 GB heap limit so there is headroom
+   * for one more GC cycle before the process is killed.
+   */
+  private static readonly HEAP_MEMORY_THRESHOLD = 2 * 1024 * 1024 * 1024; // 2 GB
+
   /**
    * Per-chat last-prompt-token-count, populated from `usageMetadata` on each
    * model response. Used by the compaction threshold check so that subagents
@@ -464,6 +477,18 @@ export class GeminiChat {
     signal?: AbortSignal,
     options?: TryCompressOptions,
   ): Promise<ChatCompressionInfo> {
+    // Force compression when heapUsed exceeds the memory threshold,
+    // regardless of `hasFailedCompressionAttempt`. This is a memory
+    // safety net — catches the actual root cause (heap pressure) rather
+    // than a proxy (entry count). Protects against both many small entries
+    // AND few huge entries (large file reads, shell outputs).
+    if (
+      !force &&
+      process.memoryUsage().heapUsed > GeminiChat.HEAP_MEMORY_THRESHOLD
+    ) {
+      force = true;
+    }
+
     const service = new ChatCompressionService();
     const { newHistory, info } = await service.compress(this, {
       promptId,