AI spam classification: the model's opinion as one bounded spam signal, and the llm_prompt Sieve function (AI-1 to AI-28)
The classifier sends only the subject and text, between unforgeable markers after the operator's prompt, to an OpenAI-compatible endpoint the operator configured; nothing is preset. Its answer maps to an LLM_ tag whose score is clamped (+5.0, -1.0 by default) and can never discard or reject on its own; X-Spam-LLM is sanitized, encoded and folded, and a planted one is removed. Failures, timeouts past the ceiling, a full slot or a paused model leave mail flowing untagged. llm_prompt answers trusted scripts, and accounts holding interactAi within an hourly limit. Redirects aren't followed and no content or secret is logged. The limits live in inbuxa:AiLimits. Acceptance tests 1 and 3 to 21; test 2 as the re-enabled shared llm case, whose setup no longer waits on a rules file from a developer's own path; test 22 written as the ignored ai_compat.
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/*
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* SPDX-FileCopyrightText: 2026 Coffey Labs
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*
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* SPDX-License-Identifier: AGPL-3.0-only
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*/
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//! From a model's answer to a spam tag (AI-12, AI-13) and the `X-Spam-LLM`
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//! header (AI-15). The answer is attacker-influenced text: it only ever
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//! selects among the operator's configured categories, and its explanation
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//! is sanitized before it reaches a header.
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use base64::{Engine, engine::general_purpose::STANDARD};
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/// How to read an answer: `x:SpamLlm`'s settings.
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#[derive(Debug, Clone)]
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pub struct Rules<'x> {
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pub separator: &'x str,
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pub pos_category: usize,
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pub pos_confidence: Option<usize>,
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pub pos_explanation: Option<usize>,
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pub categories: &'x [String],
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pub confidence: &'x [String],
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}
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/// A classification: its tag and, if any, the model's explanation.
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#[derive(Debug, Clone, PartialEq, Eq)]
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pub struct Classified {
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pub tag: String,
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pub explanation: Option<String>,
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}
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fn clean(field: &str) -> &str {
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field.trim_matches(|c: char| c.is_whitespace() || matches!(c, '"' | '\'' | '*' | '.'))
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}
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fn tag_part(value: &str) -> String {
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value
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.chars()
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.map(|c| {
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let c = c.to_ascii_uppercase();
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if c.is_ascii_uppercase() || c.is_ascii_digit() {
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c
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} else {
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'_'
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}
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})
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.collect()
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}
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/// Parses an answer (AI-12). `None` when it names no configured category
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/// (or no configured confidence, when one is expected): no tag.
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pub fn parse(answer: &str, rules: &Rules<'_>) -> Option<Classified> {
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if rules.separator.is_empty() {
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return None;
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}
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let line = answer.lines().map(str::trim).find(|l| !l.is_empty())?;
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let fields = line.split(rules.separator).collect::<Vec<_>>();
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let pick = |pos: usize, set: &[String]| -> Option<String> {
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let field = clean(fields.get(pos)?);
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set.iter()
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.find(|entry| entry.eq_ignore_ascii_case(field))
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.cloned()
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};
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let category = pick(rules.pos_category, rules.categories)?;
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let mut tag = format!("LLM_{}", tag_part(&category));
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if let Some(pos) = rules.pos_confidence {
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let confidence = pick(pos, rules.confidence)?;
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tag.push('_');
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tag.push_str(&tag_part(&confidence));
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}
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let explanation = rules.pos_explanation.and_then(|pos| {
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let last_used = rules.pos_category.max(rules.pos_confidence.unwrap_or(0));
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let text = if pos > last_used {
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// The last field runs to the end of the line: explanations have commas
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fields.get(pos..).map(|rest| rest.join(rules.separator))
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} else {
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fields.get(pos).map(|f| f.to_string())
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}?;
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let text = text.trim();
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(!text.is_empty()).then(|| text.to_string())
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});
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Some(Classified { tag, explanation })
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}
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/// Whether a tag is the classifier's (AI-13).
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pub fn is_llm_tag(tag: &str) -> bool {
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tag.get(..4).is_some_and(|p| p.eq_ignore_ascii_case("LLM_"))
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}
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/// A tag's score, clamped (AI-13): the model reads attacker-written text,
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/// so it can add at most `max_added` and take off at most `max_subtracted`.
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pub fn clamp(score: f32, max_added: f32, max_subtracted: f32) -> f32 {
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score.clamp(-max_subtracted.abs(), max_added.abs())
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}
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/// The explanation as it may appear in a header: at most 200 characters,
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/// with control characters (CR and LF included) and parentheses removed.
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pub fn sanitize(explanation: &str) -> String {
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explanation
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.chars()
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.filter(|c| !c.is_control() && !matches!(c, '(' | ')'))
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.take(200)
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.collect::<String>()
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.split_whitespace()
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.collect::<Vec<_>>()
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.join(" ")
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}
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/// Encodes non-ASCII text as RFC 2047 encoded words, each at most 75
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/// characters, split on character boundaries.
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fn encoded_words(text: &str) -> Vec<String> {
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let mut words = Vec::new();
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let mut chunk = String::new();
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for c in text.chars() {
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if chunk.len() + c.len_utf8() > 45 {
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words.push(format!("=?UTF-8?B?{}?=", STANDARD.encode(chunk.as_bytes())));
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chunk.clear();
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}
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chunk.push(c);
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}
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if !chunk.is_empty() {
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words.push(format!("=?UTF-8?B?{}?=", STANDARD.encode(chunk.as_bytes())));
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}
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words
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}
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/// The `X-Spam-LLM` header line, CRLF included (AI-15): `TAG` or
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/// `TAG (explanation)`, the explanation sanitized, RFC 2047-encoded when not
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/// ASCII, and folded to RFC 5322's 78-character lines.
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pub fn header(tag: &str, explanation: Option<&str>) -> String {
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let mut tokens = vec![tag.to_string()];
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let explanation = explanation.map(sanitize).filter(|e| !e.is_empty());
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if let Some(explanation) = explanation {
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let mut words = if explanation.is_ascii() {
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explanation.split(' ').map(str::to_string).collect::<Vec<_>>()
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} else {
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encoded_words(&explanation)
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};
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if let Some(first) = words.first_mut() {
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first.insert(0, '(');
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}
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if let Some(last) = words.last_mut() {
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last.push(')');
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}
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tokens.extend(words);
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}
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let mut out = String::from("X-Spam-LLM:");
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let mut line_len = out.len();
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for token in tokens {
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if line_len + 1 + token.len() > 78 && line_len > 1 {
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out.push_str("\r\n");
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line_len = 0;
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}
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out.push(' ');
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out.push_str(&token);
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line_len += 1 + token.len();
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}
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out.push_str("\r\n");
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out
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}
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/// Removes every `X-Spam-LLM` header from a message's header block, so a
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/// sender can't plant one (AI-15). `None` when there's none to remove.
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pub fn strip_header(raw: &[u8]) -> Option<Vec<u8>> {
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let mut out = Vec::with_capacity(raw.len());
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let mut removed = false;
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let mut skipping = false;
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let mut pos = 0;
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while pos < raw.len() {
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let end = raw[pos..]
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.iter()
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.position(|&b| b == b'\n')
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.map_or(raw.len(), |i| pos + i + 1);
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let line = &raw[pos..end];
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if line == b"\r\n" || line == b"\n" {
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// End of the header block: the body is kept as it is
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out.extend_from_slice(&raw[pos..]);
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break;
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}
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let is_continuation = matches!(line.first(), Some(b' ' | b'\t'));
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if !is_continuation {
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skipping = line.len() > 11 && line[..11].eq_ignore_ascii_case(b"x-spam-llm:");
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}
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if skipping {
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removed = true;
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} else {
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out.extend_from_slice(line);
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}
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pos = end;
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}
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removed.then_some(out)
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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fn rules<'x>(categories: &'x [String], confidence: &'x [String]) -> Rules<'x> {
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Rules {
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separator: ",",
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pos_category: 0,
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pos_confidence: Some(1),
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pos_explanation: Some(2),
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categories,
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confidence,
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}
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}
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fn sets() -> (Vec<String>, Vec<String>) {
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(
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["Unsolicited", "Commercial", "Harmful", "Legitimate"]
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.map(String::from)
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.to_vec(),
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["High", "Medium", "Low"].map(String::from).to_vec(),
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)
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}
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#[test]
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fn parses_answers() {
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let (cats, conf) = sets();
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let r = rules(&cats, &conf);
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assert_eq!(
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parse("Unsolicited,High,Test", &r),
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Some(Classified {
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tag: "LLM_UNSOLICITED_HIGH".into(),
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explanation: Some("Test".into())
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})
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);
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// Test 3: spacing, case and commas in the explanation
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assert_eq!(
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parse("\n unsolicited , HIGH , Lots of commas, here\nmore", &r),
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Some(Classified {
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tag: "LLM_UNSOLICITED_HIGH".into(),
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explanation: Some("Lots of commas, here".into())
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})
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);
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assert_eq!(parse("**Harmful**, 'low'.", &r).unwrap().tag, "LLM_HARMFUL_LOW");
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// Test 4: no tag
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for bad in ["Maybe,High,x", "", "no separator here", "Unsolicited,Very"] {
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assert_eq!(parse(bad, &r), None, "{bad}");
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}
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// Test 5: no confidence position
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let r = Rules {
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pos_confidence: None,
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pos_explanation: None,
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..rules(&cats, &conf)
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};
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assert_eq!(parse("Unsolicited,whatever", &r).unwrap().tag, "LLM_UNSOLICITED");
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}
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#[test]
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fn clamps() {
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assert_eq!(clamp(50.0, 5.0, 1.0), 5.0);
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assert_eq!(clamp(-50.0, 5.0, 1.0), -1.0);
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assert_eq!(clamp(2.0, 5.0, 1.0), 2.0);
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}
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#[test]
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fn headers() {
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assert_eq!(header("LLM_X", None), "X-Spam-LLM: LLM_X\r\n");
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assert_eq!(
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header("LLM_X", Some("Looks (very) fine\r\nX-Evil: yes")),
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"X-Spam-LLM: LLM_X (Looks very fineX-Evil: yes)\r\n"
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);
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let h = header("LLM_UNSOLICITED_HIGH", Some(&"word ".repeat(40)));
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assert!(h.lines().all(|l| l.len() <= 78), "{h}");
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assert_eq!(h.matches("\r\n").count(), h.lines().count());
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let h = header("LLM_X", Some("Ünïcödé explanation"));
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assert!(h.contains("=?UTF-8?B?") && h.is_ascii(), "{h}");
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// Folded continuation lines start with a space
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assert!(h.split("\r\n").skip(1).all(|l| l.is_empty() || l.starts_with(' ')));
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}
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#[test]
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fn strips_planted_headers() {
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let raw = b"From: a@b\r\nX-Spam-LLM: LLM_LEGITIMATE_HIGH\r\n (folded)\r\nSubject: x\r\n\r\nX-Spam-LLM: body stays\r\n";
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let out = strip_header(raw).unwrap();
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assert_eq!(
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out,
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b"From: a@b\r\nSubject: x\r\n\r\nX-Spam-LLM: body stays\r\n"
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);
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assert_eq!(strip_header(b"From: a@b\r\n\r\nbody"), None);
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}
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}
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