sgl-project/sglang · error · ValueError

prompt has {num_placeholders} image placeholder token(s) but

Error message

prompt has {num_placeholders} image placeholder token(s) but {len(counts)} image(s) were provided

What it means

_expand_input_ids verifies that the number of placeholder token ids in the already-tokenized prompt exactly equals the number of expansion 'counts' (per-image expanded token counts). A mismatch means the prompt text and the media list disagree, so retokenize-avoidance expansion cannot proceed safely.

Source

Thrown at python/sglang/srt/multimodal/processors/base_processor.py:1631

        counts: List[int],
        placeholder_token_id: Optional[int],
    ) -> List[int]:
        """Rebuild final input_ids for a pre-tokenized (list[int]) prompt.

        Keep the user's ORIGINAL tokens verbatim and expand the i-th image
        placeholder into ``counts[i]`` copies of ``placeholder_token_id``. The HF
        processor's re-tokenization is discarded, so non-media tokens cannot
        drift.

        """
        if placeholder_token_id is None:
            raise ValueError("placeholder_token_id is not set for this processor")

        num_placeholders = sum(
            1 for token_id in original_ids if token_id == placeholder_token_id
        )
        if num_placeholders != len(counts):
            raise ValueError(
                f"prompt has {num_placeholders} image placeholder token(s) but "
                f"{len(counts)} image(s) were provided"
            )

        rebuilt: List[int] = []
        next_image_idx = 0
        for token_id in original_ids:
            if token_id == placeholder_token_id:
                rebuilt.extend([placeholder_token_id] * counts[next_image_idx])
                next_image_idx += 1
            else:
                rebuilt.append(token_id)
        return rebuilt

    def process_and_combine_mm_data(
        self,
        base_output: BaseMultiModalProcessorOutput,
        mm_tokens: MultimodalSpecialTokens,

View on GitHub (pinned to 0132848349)

Solutions

1. Regenerate the prompt with the model's chat template so placeholders match len(images) exactly
2. Ensure the tokenized ids correspond to the same prompt/media pairing used to compute counts
3. Avoid manually inserting or deleting placeholder tokens in the token ids

Example fix

// before
input_ids = tokenizer('<image> <image>')  # 2 placeholders
counts = [c for c in per_img]  # 3 images
// after
input_ids = tokenizer('<image>' * len(images))
counts = [c for c in per_img]  # len == len(images) == placeholders

num_ph = sum(1 for t in input_ids if t == placeholder_token_id)
assert num_ph == len(counts), f'{num_ph} placeholders vs {len(counts)} images'

When it happens

Trigger: Calling process_and_combine_mm_data where the tokenized prompt contains N placeholder tokens but the resolved per-image counts list has a different length — e.g. prompt with 2 <image> tags but 3 images provided, or custom text embedding extra placeholder tokens.

Common situations: Chat templates that add/remove placeholder tokens conditionally; offline tokenized prompts reused with different image counts; duplicated placeholder tokens from string interpolation.

Related errors

• processor image placeholder count mismatch: processor={proce
• InklingMultimodalProcessor: {n_img_ph} image placeholder tok
• Unsupported image type: {type(image)}
• When using multiple prompts with multiple input images, prov
• {key}.position_ids is required

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/3d15c37e92e962a3. Report an issue: GitHub.