diff --git a/src/commands/sessions-tail.test.ts b/src/commands/sessions-tail.test.ts
index b609b4b8e162..e238aa991268 100644
--- a/src/commands/sessions-tail.test.ts
+++ b/src/commands/sessions-tail.test.ts
@@ -188,6 +188,18 @@ describe("sessionsTailCommand", () => {
     expect(output).toContain("tool.result");
   });
 
+  it("rejects tail counts that exceed JavaScript safe integer precision", async () => {
+    const runtime = makeRuntime();
+
+    await sessionsTailCommand({ store: storePath, sessionKey, tail: "9007199254740992" }, runtime);
+
+    expect(runtime.error).toHaveBeenCalledWith(
+      "--tail must be a non-negative integer, for example --tail 25.",
+    );
+    expect(runtime.exit).toHaveBeenCalledWith(1);
+    expect(runtime.log).not.toHaveBeenCalled();
+  });
+
   it("uses a session trajectory pointer for relocated runtime files", async () => {
     const runtime = makeRuntime();
     const relocatedDir = path.join(tmpDir, "relocated-trajectories");
diff --git a/src/commands/sessions-tail.ts b/src/commands/sessions-tail.ts
index f72ad038e955..9fa909a55ba9 100644
--- a/src/commands/sessions-tail.ts
+++ b/src/commands/sessions-tail.ts
@@ -14,6 +14,7 @@ import { listSessionEntries } from "../config/sessions/session-accessor.js";
 import type { SessionEntry } from "../config/sessions/types.js";
 import { resolveStoredSessionKeyForAgentStore } from "../gateway/session-store-key.js";
 import { formatErrorMessage } from "../infra/errors.js";
+import { parseStrictNonNegativeInteger } from "../infra/parse-finite-number.js";
 import { resolveAgentIdFromSessionKey } from "../routing/session-key.js";
 import type { RuntimeEnv } from "../runtime.js";
 import {
@@ -90,14 +91,7 @@ function parseTailCount(value: string | number | undefined): number | null {
   if (value === undefined) {
     return DEFAULT_TAIL_COUNT;
   }
-  if (typeof value === "number") {
-    return Number.isInteger(value) && value >= 0 ? value : null;
-  }
-  const trimmed = value.trim();
-  if (!/^\d+$/.test(trimmed)) {
-    return null;
-  }
-  return Number.parseInt(trimmed, 10);
+  return parseStrictNonNegativeInteger(value) ?? null;
 }
 
 function toOptionalString(value: unknown): string | undefined {