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Fix ConcretizationTypeError in dot_product_attention with traced masks #21853

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Fix ConcretizationTypeError in dot_product_attention with traced masks #21853

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bc319e2
Disable flash attention for traced masks
Abhinavexists Nov 15, 2025
a6b65a4
Update keras/src/backend/jax/nn.py
Abhinavexists Nov 15, 2025
862c271
Update keras/src/backend/jax/nn.py
Abhinavexists Nov 15, 2025
ffc4bd6
Fix flash attention handling for TPU with masks
Abhinavexists Nov 26, 2025
1dfcaca
removed duplicate flash_attention = 'False'
Abhinavexists Dec 2, 2025
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4 changes: 4 additions & 0 deletions keras/src/backend/jax/nn.py
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Expand Up @@ -1276,6 +1276,10 @@ def dot_product_attention(
# use flash attention
_can_use_flash_attention(query, key, value, bias, raise_error=True)

# On TPU, traced masks cause ConcretizationTypeError with flash attention.
if is_tpu and mask is not None and isinstance(mask, jax.core.Tracer):
flash_attention = False
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@hertschuh hertschuh Nov 26, 2025

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Because on JAX we compile by default, this means that flash_attention is now always disabled. The whole flash attention implementation was for TPU specifically. So I don't think this is the right fix.

Am I misunderstanding something?

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@Abhinavexists Abhinavexists Nov 26, 2025

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@hertschuh You're absolutely right! Thank you for catching that.

My initial fix using isinstance(mask, jax.core.Tracer) was too broad and would have disabled flash attention even during normal JIT compilation, not just inside control flow operations.

I've revised the fix to instead:

  1. Removed the Tracer check
  2. Improved the exception handling to specifically catch jax.errors.ConcretizationTypeError

The fix now correctly handles traced masks without disabling flash attention for normal use cases.


# TPU-specific flash attention path
if is_tpu and flash_attention:
# Get sharding parameters from distribution context
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