llm_preview: Preview a tidy LLM call without spending anything

In LLMR: Interface for Large Language Model APIs in R

Preview a tidy LLM call without spending anything

Description

Renders every row exactly as llm_fn() / llm_mutate() would (no API call, no file I/O), then reports a tidy, row-level summary: the rendered text, the roles, character counts, file presence and existence, the batch plan, and a list-column of issues. Problems that would only surface mid-run (a missing file, a "file" role combined with .rows_per_prompt > 1, an embedding config with row batching, .return = "object" with batching, a schema supplied without .structured, a template that references NA values or renders an empty prompt, a file part with no accompanying user text, or a tag name that collides with the batched ⁠<row_N>⁠ protocol) are collected per row so you see all of them at once rather than hitting the first error.

Usage

llm_preview(
  .data,
  prompt = NULL,
  .messages = NULL,
  .system_prompt = NULL,
  .config = NULL,
  .structured = FALSE,
  .schema = NULL,
  .tags = NULL,
  .return = c("columns", "text", "object"),
  .rows_per_prompt = 1L,
  rows = NULL,
  max_chars = 500L
)

Arguments

.data	A data.frame/tibble whose columns feed the glue templates.
prompt	A single glue template string (mutually exclusive with .messages).
.messages	A character vector of glue templates, optionally named by role ("system", "user", "assistant", "file"). Unnamed entries default to "user".
.system_prompt	Optional system string, prepended when a row has no "system" message.
.config	Optional llm_config(). When supplied, preview checks the embedding-vs-row-batching conflict; otherwise config-dependent checks are skipped.
.structured	Logical; if TRUE, the call would request structured JSON output. Used only to validate .schema/.tags combinations.
.schema	Optional JSON schema (list). Flagged if supplied without .structured = TRUE.
.tags	Optional character vector of tag names. Flagged if combined with .structured = TRUE (structured takes precedence).
.return	One of "columns", "text", "object". Only used to flag the unsupported "object" + batching combination.
.rows_per_prompt	Rows per call. 1 (default) means one call per row; ⁠> 1⁠ or Inf packs rows into batched calls.
rows	Optional integer vector selecting which rows to render (default: all rows).
max_chars	Truncate each row's rendered preview to this many characters (default 500). Set higher to see full prompts.

Details

Batched data travels inside numbered ⁠<row_i>...</row_i>⁠ tags; the rowpack_id / rows_per_prompt / rowpack_row columns show how rows would be grouped into calls at the given .rows_per_prompt.

Value

A tibble of class llmr_preview, one row per previewed input row, with columns: row, ok (no issues), roles, rendered_preview, chars, has_file, file_ok, rowpack_id, rows_per_prompt, rowpack_row, and issues (a list-column of character vectors).

See Also

llm_render_messages(), llm_usage(), llm_failures().

Examples

df <- data.frame(text = c("a", "b", "c"), stringsAsFactors = FALSE)
llm_preview(df, prompt = "Classify: {text}", .rows_per_prompt = 2)

LLMR documentation built on July 11, 2026, 1:06 a.m.