diff --git a/agent/reasoning_timeouts.py b/agent/reasoning_timeouts.py
index 8af5ab799f48..9c5fc2020152 100644
--- a/agent/reasoning_timeouts.py
+++ b/agent/reasoning_timeouts.py
@@ -106,6 +106,14 @@ _REASONING_STALE_TIMEOUT_FLOORS: tuple[tuple[str, int], ...] = (
     ("claude-sonnet-5", 180),
     ("claude-sonnet-4.5", 180),
     ("claude-sonnet-4.6", 180),
+    # Anthropic Mythos-class named reasoning models (claude-fable-5, …).
+    # 1M context + 128K output — heavier thinking phase than the
+    # numbered Claude line, so the floor is in the deep-reasoning tier
+    # alongside o1 / deepseek-r1 / nemotron-3-ultra.  Without this
+    # entry the stale-stream detector kills fable-5's thinking phase
+    # at the default 180s (300s with context scaling), tripping the
+    # cross-turn circuit breaker after 5 consecutive stale kills.
+    ("claude-fable", 600),
     # xAI Grok reasoning variants.  Explicit reasoning-only keys
     # plus one for the ``non-reasoning`` variant so users picking
     # the fast variant don't get the 300s floor.  Bare ``grok-3``,
@@ -207,6 +215,8 @@ def get_reasoning_stale_timeout_floor(model: object) -> Optional[float]:
     300.0
     >>> get_reasoning_stale_timeout_floor("anthropic/claude-opus-4-6")
     240.0
+    >>> get_reasoning_stale_timeout_floor("anthropic/claude-fable-5")
+    600.0
     >>> get_reasoning_stale_timeout_floor("gpt-4o") is None
     True
     >>> get_reasoning_stale_timeout_floor("olmo-1") is None
diff --git a/contributors/emails/reinbeumer@gmail.com b/contributors/emails/reinbeumer@gmail.com
new file mode 100644
index 000000000000..26c0b64b9500
--- /dev/null
+++ b/contributors/emails/reinbeumer@gmail.com
@@ -0,0 +1 @@
+reinbeumer
diff --git a/tests/agent/test_reasoning_stale_timeout_floor.py b/tests/agent/test_reasoning_stale_timeout_floor.py
index ec05cb54dd60..608c83d10408 100644
--- a/tests/agent/test_reasoning_stale_timeout_floor.py
+++ b/tests/agent/test_reasoning_stale_timeout_floor.py
@@ -74,6 +74,10 @@ import pytest
     ("anthropic/claude-opus-4-20250514", 240.0),
     ("anthropic/claude-sonnet-4.5", 180.0),
     ("anthropic/claude-sonnet-4.6", 180.0),
+    # Anthropic Mythos-class named reasoning models — deep-reasoning tier.
+    ("anthropic/claude-fable-5", 600.0),
+    ("claude-fable-5", 600.0),
+    ("claude-fable", 600.0),
     # xAI Grok reasoning variants — explicit, not bare `grok`.
     ("x-ai/grok-4-fast-reasoning", 300.0),
     ("x-ai/grok-4.20-reasoning", 300.0),