diff --git a/packages/cli/src/commands/push.ts b/packages/cli/src/commands/push.ts
index ed374be..6c6ae6c 100644
--- a/packages/cli/src/commands/push.ts
+++ b/packages/cli/src/commands/push.ts
@@ -40,10 +40,16 @@ function toTuiSessions(sessions: LocalSession[]): TuiSession[] {
   }));
 }
 
-function findSession(sessions: LocalSession[], ref: string) {
-  const index = Number(ref);
-  if (Number.isInteger(index) && index >= 1 && index <= sessions.length) return sessions[index - 1];
-  return sessions.find((s) => s.key === ref || s.id === ref || s.id.startsWith(ref)) ?? null;
+export function findSession(sessions: LocalSession[], ref: string) {
+  const wanted = ref.trim();
+  if (!wanted) return null;
+  const exact = sessions.filter((s) => s.key === wanted || s.id === wanted);
+  if (exact.length > 1) throw new Error(`Session "${ref}" is ambiguous; use a full harness:id key.`);
+  if (exact.length === 1) return exact[0];
+  if (/^[1-9]\d*$/.test(wanted)) return sessions[Number(wanted) - 1] ?? null;
+  const matches = sessions.filter((s) => s.id.startsWith(wanted));
+  if (matches.length > 1) throw new Error(`Session "${ref}" is ambiguous; use a full harness:id key.`);
+  return matches[0] ?? null;
 }
 
 /**
diff --git a/packages/cli/test/session-selection.test.ts b/packages/cli/test/session-selection.test.ts
new file mode 100644
index 0000000..62c8e9c
--- /dev/null
+++ b/packages/cli/test/session-selection.test.ts
@@ -0,0 +1,22 @@
+import assert from 'node:assert/strict';
+import { test } from 'node:test';
+import { findSession } from '../src/commands/push';
+import type { LocalSession } from '../src/harnesses';
+
+const session = (id: string, harness = 'claude') => ({id, harness, key: `${harness}:${id}`} as LocalSession);
+test('prefixes must identify exactly one session and exact IDs take precedence', () => {
+  const sessions = [session('abc-1'), session('abc-2'), session('abc')];
+  assert.throws(() => findSession(sessions, 'ab'), /ambiguous/);
+  assert.equal(findSession(sessions, 'abc'), sessions[2]);
+  assert.equal(findSession(sessions, ' claude:abc-1 '), sessions[0]);
+  assert.equal(findSession(sessions, ''), null);
+  assert.equal(findSession(sessions, 'missing'), null);
+  assert.equal(findSession(sessions, '2'), sessions[1]);
+  assert.equal(findSession(sessions, '999'), null);
+  assert.equal(findSession(sessions, '1e0'), null);
+  const sameId = [session('same'), session('same', 'pi')];
+  assert.throws(() => findSession(sameId, 'same'), /ambiguous/);
+  assert.equal(findSession(sameId, 'pi:same'), sameId[1]);
+  const numeric = [session('other'), session('1')];
+  assert.equal(findSession(numeric, '1'), numeric[1]);
+});