fix(run): cap retry backoff at max_delay on OverflowError

src/agents/run_internal/model_retry.py

@@ -284,7 +284,21 @@ def _default_retry_delay(
         else DEFAULT_BACKOFF_JITTER
     )
 
-    base = min(initial_delay * (multiplier ** max(attempt - 1, 0)), max_delay)
+    exponent = max(attempt - 1, 0)
+    # Compute the exponential growth defensively. With a large `attempt` or `multiplier`
+    # the intermediate `multiplier ** exponent` can raise OverflowError even though the
+    # result would be immediately capped at max_delay. Short-circuit to max_delay in that
+    # case so a high retry count does not surface a crash from the backoff helper itself.
+    # `initial_delay == 0` is a valid way to request immediate retries; preserve that
+    # rather than substituting `max_delay` on overflow.
+    if initial_delay == 0:
+        scaled = 0.0
+    else:
+        try:
+            scaled = initial_delay * (multiplier**exponent)
+        except OverflowError:
+            scaled = max_delay
+    base = min(scaled, max_delay)
     if not use_jitter:
         return base
     return min(max(base * (0.875 + random.random() * 0.25), 0.0), max_delay)

tests/models/test_model_retry.py

@@ -24,7 +24,11 @@
     RetryPolicyContext,
     retry_policies,
 )
-from agents.run_internal.model_retry import get_response_with_retry, stream_response_with_retry
+from agents.run_internal.model_retry import (
+    _default_retry_delay,
+    get_response_with_retry,
+    stream_response_with_retry,
+)
 from agents.usage import Usage
 from tests.test_responses import get_text_message
 
@@ -2383,3 +2387,32 @@ async def rewind() -> None:
     await outer_stream.aclose()
 
     assert stream.close_calls == 1
+
+
+def test_default_retry_delay_caps_overflow_at_max_delay() -> None:
+    # A pathologically large multiplier would overflow `multiplier ** exponent` even
+    # though the result would be capped at max_delay; the helper must not propagate
+    # OverflowError to the retry loop.
+    backoff = ModelRetryBackoffSettings(
+        initial_delay=1.0, max_delay=5.0, multiplier=100.0, jitter=False
+    )
+    assert _default_retry_delay(200, backoff) == 5.0
+
+
+def test_default_retry_delay_handles_large_attempt_with_default_multiplier() -> None:
+    # With the default 2.0 multiplier, attempts above ~1075 overflow `2.0 ** exponent`.
+    # The helper should still return a finite delay capped at max_delay.
+    backoff = ModelRetryBackoffSettings(
+        initial_delay=0.25, max_delay=2.0, multiplier=2.0, jitter=False
+    )
+    assert _default_retry_delay(2000, backoff) == 2.0
+
+
+def test_default_retry_delay_preserves_zero_initial_delay_on_overflow() -> None:
+    # `initial_delay=0` requests immediate retries; an overflow in
+    # `multiplier ** exponent` must not promote that to max_delay.
+    backoff = ModelRetryBackoffSettings(
+        initial_delay=0.0, max_delay=5.0, multiplier=100.0, jitter=False
+    )
+    assert _default_retry_delay(200, backoff) == 0.0
+    assert _default_retry_delay(1, backoff) == 0.0