Agent: one turn path, tools and history in their own modules; SDK: one parser

agent/src/history.rs

@@ -0,0 +1,81 @@
+//! History window selection and context-size error detection.
+
+use crate::llm::Entry;
+
+pub fn is_too_large(error: &str) -> bool {
+    let Some((status, body)) = error
+        .strip_prefix("HTTP ")
+        .and_then(|error| error.split_once(':'))
+    else {
+        return false;
+    };
+    if !matches!(status.split_whitespace().next(), Some("400" | "413")) {
+        return false;
+    }
+    let body = body.to_ascii_lowercase();
+    body.contains("context_length_exceeded")
+        || body.contains("maximum context length")
+        || body.contains("context length exceeded")
+        || body.contains("too large")
+        || (body.contains("too long")
+            && ["context", "input", "prompt"]
+                .iter()
+                .any(|term| body.contains(term)))
+}
+
+pub fn next_turn_start(history: &[Entry], from: usize, current: usize) -> Option<usize> {
+    let end = current.min(history.len().checked_sub(1)?);
+    (from..=end).find(|&index| matches!(&history[index], Entry::User { .. }))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn detects_context_size_errors_only_for_http_400_or_413() {
+        assert!(is_too_large("HTTP 400: context_length_exceeded"));
+        assert!(is_too_large("HTTP 413: prompt is too long"));
+        assert!(is_too_large("HTTP 400: maximum context length"));
+        assert!(is_too_large("HTTP 400: request is too large"));
+        assert!(!is_too_large("HTTP 500: prompt is too long"));
+        assert!(!is_too_large("HTTP 400: invalid token"));
+    }
+
+    #[test]
+    fn history_skip_advances_by_whole_user_turns() {
+        let history = vec![
+            Entry::User {
+                text: "first".into(),
+            },
+            Entry::ToolCall {
+                id: "a".into(),
+                name: "tool".into(),
+                arguments: "{}".into(),
+            },
+            Entry::ToolResult {
+                id: "a".into(),
+                output: "done".into(),
+            },
+            Entry::User {
+                text: "second".into(),
+            },
+            Entry::ToolCall {
+                id: "b".into(),
+                name: "tool".into(),
+                arguments: "{}".into(),
+            },
+            Entry::ToolResult {
+                id: "b".into(),
+                output: "done".into(),
+            },
+            Entry::User {
+                text: "current".into(),
+            },
+        ];
+        assert_eq!(next_turn_start(&history, 0, 6), Some(0));
+        assert_eq!(next_turn_start(&history, 1, 6), Some(3));
+        assert_eq!(next_turn_start(&history, 4, 6), Some(6));
+        assert_eq!(next_turn_start(&history, usize::MAX, 6).unwrap_or(6), 6);
+    }
+}