Description

Bug Overview

The make_seq2seq_fields function in gemma/gm/data/_functional.py crashes with a confusing NumPy error when an empty prompt array is passed, instead of handling it gracefully or providing a clear error message.

Bug Location

• File: gemma/gm/data/_functional.py
• Line: 142
• Function: make_seq2seq_fields

Root Cause

The function attempts to create a target mask with this code:

target_mask = np.concatenate([
    np.zeros((len(prompt) - 1,), dtype=np.bool_),
    np.ones((len(response),), dtype=np.bool_),
])

When len(prompt) == 0, the expression len(prompt) - 1 evaluates to -1, causing np.zeros((-1,), ...) to raise:

ValueError: negative dimensions are not allowed

Why This Is a Problem

1. No input validation - The function doesn't check for empty prompts before attempting array operations
2. Confusing error message - Users get a cryptic NumPy error instead of understanding what they did wrong
3. Crashes entire pipeline - Instead of handling the edge case gracefully, the entire operation fails
4. Difficult to debug - The error comes from deep within NumPy code, not from the user's code
5. Poor user experience - Users cannot easily identify that the issue is with an empty prompt

How to Reproduce

Scenario 1: Direct API usage

from gemma import gm

result = gm.data.make_seq2seq_fields(
    prompt=[],  # Empty prompt
    response=[20, 21, 1]
)
# Raises: ValueError: negative dimensions are not allowed

Scenario 2: Through AddSeq2SeqFields transform

from gemma import gm

transform = gm.data.AddSeq2SeqFields(
    in_prompt="prompt",
    in_response="response",
    out_input="input",
    out_target="target",
    out_target_mask="target_mask",
)

element = {
    "prompt": [],  # Empty prompt tokens
    "response": [20, 21, 1]
}

result = transform.map(element)
# Raises: ValueError: negative dimensions are not allowed

Error Traceback

Traceback (most recent call last):
  File ".../_functional.py", line 142, in make_seq2seq_fields
    np.zeros((len(prompt) - 1,), dtype=np.bool_),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: negative dimensions are not allowed

Expected Behavior

The function should either:

1. Raise a clear, descriptive error message indicating that empty prompts are not supported, OR
2. Handle empty prompts gracefully with a warning and continue execution

Impact

• Affects users calling make_seq2seq_fields directly with empty prompt arrays
• Affects data pipelines using AddSeq2SeqFields transform with potentially empty prompt tokens
• Can cause entire batch processing pipelines to fail
• Difficult for users to identify and fix the root cause