diff --git a/src/runtime/bootstrap.ts b/src/runtime/bootstrap.ts
index 239b9cf..0362c47 100644
--- a/src/runtime/bootstrap.ts
+++ b/src/runtime/bootstrap.ts
@@ -157,6 +157,7 @@ import type { AgentLoopEvent, RunTurnResult } from "../agent/agent-loop.js";
 import {
   SessionStore,
   createEmptySessionState,
+  pruneSessionTurns,
   type SessionState,
 } from "../session/index.js";
 
@@ -2101,8 +2102,22 @@ export async function createAgentRuntime(
         maxSteps: runOptions.maxSteps ?? config.agent.maxSteps,
         signal: runOptions.signal ?? new AbortController().signal,
       });
-      sessionStore.save(result.session);
-      return result;
+      // Prune old turns to bound memory growth (issue #121). Without
+      // this, every tool_call / tool_result / assistant_reply is kept
+      // forever — large outputs (file reads, web fetches) push a long
+      // session past the V8 heap limit. packConversation already
+      // determines the visible window at prompt-build time; we apply
+      // the same windowing here so the persistent state stays bounded.
+      const prunedTurns = pruneSessionTurns(
+        result.session.turns,
+        config.agent.conversationMaxTokens,
+      );
+      const prunedSession =
+        prunedTurns.length === result.session.turns.length
+          ? result.session
+          : { ...result.session, turns: prunedTurns };
+      sessionStore.save(prunedSession);
+      return { ...result, session: prunedSession };
     });
   };
 
diff --git a/src/session/conversation-turn.test.ts b/src/session/conversation-turn.test.ts
index 3591ff9..f875b81 100644
--- a/src/session/conversation-turn.test.ts
+++ b/src/session/conversation-turn.test.ts
@@ -8,6 +8,7 @@ import {
   renderTurnForPrompt,
   toolResultTurn,
   trimTurnsToTokens,
+  pruneSessionTurns,
   userTurn,
   type ConversationTurn,
 } from "./conversation-turn.js";
@@ -374,3 +375,59 @@ describe("conversation-turn helpers", () => {
     });
   });
 });
+
+describe("pruneSessionTurns", () => {
+  it("returns a copy of turns when within budget", () => {
+    const turns: ConversationTurn[] = [
+      userTurn("hello", 1),
+      assistantReplyTurn("hi", 2),
+    ];
+    const out = pruneSessionTurns(turns, 1000);
+    expect(out).toHaveLength(2);
+    expect(out).toEqual(turns);
+  });
+
+  it("prunes old turns and prepends a summary when over budget", () => {
+    const turns: ConversationTurn[] = [
+      userTurn("old message " .repeat(50), 1),
+      assistantReplyTurn("old reply " .repeat(50), 2),
+      userTurn("new message", 3),
+      assistantReplyTurn("new reply", 4),
+    ];
+    const out = pruneSessionTurns(turns, 50);
+    expect(out.length).toBeLessThan(turns.length);
+    // First turn should be the summary
+    expect(out[0]?.kind).toBe("user");
+    expect(out[0]?.text).toMatch(/^summary: \d+ older turns dropped/);
+    // Last turns should be preserved
+    expect(out.at(-1)?.kind).toBe("assistant_reply");
+  });
+
+  it("returns empty array for empty input", () => {
+    const out = pruneSessionTurns([], 100);
+    expect(out).toEqual([]);
+  });
+
+  it("does not mutate the input array", () => {
+    const turns: ConversationTurn[] = [
+      userTurn("a ".repeat(100), 1),
+      assistantReplyTurn("b ".repeat(100), 2),
+    ];
+    const original = [...turns];
+    pruneSessionTurns(turns, 10);
+    expect(turns).toEqual(original);
+  });
+
+  it("keeps at least the visible turns from packConversation", () => {
+    const turns: ConversationTurn[] = Array.from({ length: 100 }, (_, i) =>
+      i % 2 === 0
+        ? userTurn(`message ${i} ` .repeat(10), i)
+        : assistantReplyTurn(`reply ${i} ` .repeat(10), i),
+    );
+    const out = pruneSessionTurns(turns, 200);
+    expect(out.length).toBeLessThan(100);
+    // Should have summary + visible turns
+    expect(out[0]?.text).toMatch(/^summary:/);
+    expect(out.length).toBeGreaterThan(1);
+  });
+});
diff --git a/src/session/conversation-turn.ts b/src/session/conversation-turn.ts
index 449d12e..3683b05 100644
--- a/src/session/conversation-turn.ts
+++ b/src/session/conversation-turn.ts
@@ -354,3 +354,39 @@ export function appendTurn(
 ): ConversationTurn[] {
   return [...turns, next];
 }
+
+/**
+ * Prune `state.turns` to a bounded size using the same token-based
+ * windowing that `packConversation` applies at prompt-build time.
+ *
+ * This prevents unbounded memory growth in long sessions (issue #121):
+ * every `assistant_tool_call`, `tool_result`, and `assistant_reply` is
+ * appended forever; large tool outputs (file reads, web fetches) can
+ * push a 25-step session past the V8 heap limit.
+ *
+ * The function is **pure** — it returns a new turns array without
+ * mutating the input. Call it before persisting the session state.
+ *
+ * @param turns  The current (potentially unbounded) turn list.
+ * @param maxTokens  Token budget for the conversation section.
+ *                   Typically `config.agent.conversationMaxTokens` (default 32 000).
+ * @returns  A pruned turns array that fits within `maxTokens` when
+ *           rendered by `packConversation`. When the total is already
+ *           within budget the array is returned unchanged (zero copy).
+ */
+export function pruneSessionTurns(
+  turns: readonly ConversationTurn[],
+  maxTokens: number,
+): ConversationTurn[] {
+  const packed = packConversation(turns, maxTokens);
+  if (packed.droppedCount === 0) return [...turns];
+  // Replace the dropped prefix with a single summary turn so the
+  // prompt renderer and future packConversation calls still have a
+  // compact representation of the truncated history.
+  const summaryTurn: ConversationTurn = {
+    kind: "user",
+    text: packed.droppedSummary ?? "",
+    at: Date.now(),
+  };
+  return [summaryTurn, ...packed.visibleTurns];
+}
diff --git a/src/session/index.ts b/src/session/index.ts
index ed288af..fee6599 100644
--- a/src/session/index.ts
+++ b/src/session/index.ts
@@ -32,6 +32,7 @@ export {
   trimTurnsToTokens,
   packConversation,
   appendTurn,
+  pruneSessionTurns,
 } from "./conversation-turn.js";
 export type {
   ConversationTurn,