Bug: Missing f-prefix in error message in _normalize_token() makes stop_tokens debugging impossible

[prince-shakyaa]

Summary

The error message in _normalize_token() inside gemma/gm/text/_sampler.py is missing the f prefix, so {token!r} is printed literally instead of interpolating the actual token value. This makes debugging stop_tokens / forbidden_tokens misconfigurations nearly impossible.

Affected Code

File: gemma/gm/text/_sampler.py, lines 579–581

# Current (broken — NOT an f-string):
raise ValueError(
    'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
    ' map to single token ids in the vocab.'
)

Root Cause

The string literal uses {token!r} but there is no f prefix, so Python treats it as a plain string. The user sees the unhelpful message:

ValueError: Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must map to single token ids in the vocab.

Instead of the intended:

ValueError: Invalid token: 'hello world'. `stop_token`s and `forbidden_token`s must map to single token ids in the vocab.

Reproduction

from gemma.gm.text._sampler import _normalize_token

class FakeTokenizer:
    def encode(self, text):
        return [1, 2]  # multi-token, will trigger the error

_normalize_token(FakeTokenizer(), 'hello world')
# => ValueError: Invalid token: {token!r}. ...   <-- literal, not interpolated

Fix

# Before:
raise ValueError(
    'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
    ' map to single token ids in the vocab.'
)

# After:
raise ValueError(
    f'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
    ' map to single token ids in the vocab.'
)

A one-character fix — just add f before the opening quote.

Impact

• Severity: Low (no correctness impact — the error is still raised), but High UX impact — users who misconfigure stop_tokens or forbidden_tokens receive a completely useless error message with no information about which token was invalid.
• All platforms affected.

Environment

Python	3.12.13
Platform	macOS Apple Silicon (arm64)
gemma	4.0.1 (editable install from HEAD)