feat: add transcript parser for last user/agent messages

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/transcript/testdata/long.jsonl

@@ -0,0 +1,2 @@
+{"role": "user", "content": "q"}
+{"role": "assistant", "content": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"}

internal/transcript/testdata/sample.jsonl

@@ -0,0 +1,4 @@
+{"role":"user","content":"hello"}
+{"role":"assistant","content":"world"}
+{"role":"user","content":"second question"}
+{"role":"assistant","content":"final answer"}

internal/transcript/transcript.go

@@ -0,0 +1,93 @@
+package transcript
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+	"os"
+	"strings"
+	"unicode/utf8"
+)
+
+const tailWindow = 256 * 1024
+
+// LastMessages reads a JSONL transcript and returns the last user and assistant
+// messages, each truncated to maxLen runes.
+func LastMessages(path string, maxLen int) (lastUser, lastAgent string, err error) {
+	f, err := os.Open(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", "", err
+		}
+		return "", "", err
+	}
+	defer f.Close()
+
+	info, err := f.Stat()
+	if err != nil {
+		return "", "", err
+	}
+	size := info.Size()
+
+	var r io.Reader = f
+	if size > tailWindow {
+		start := size - tailWindow
+		if _, err := f.Seek(start, io.SeekStart); err != nil {
+			return "", "", err
+		}
+		tail := make([]byte, tailWindow)
+		n, err := io.ReadFull(f, tail)
+		if err != nil && !errors.Is(err, io.ErrUnexpectedEOF) {
+			return "", "", err
+		}
+		tail = tail[:n]
+		if i := bytes.IndexByte(tail, '\n'); i >= 0 {
+			tail = tail[i+1:]
+		}
+		r = bytes.NewReader(tail)
+	}
+
+	sc := bufio.NewScanner(r)
+	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if line == "" {
+			continue
+		}
+		var row struct {
+			Role    string `json:"role"`
+			Type    string `json:"type"`
+			Content string `json:"content"`
+		}
+		if err := json.Unmarshal([]byte(line), &row); err != nil {
+			continue
+		}
+		role := row.Role
+		if role == "" {
+			role = row.Type
+		}
+		switch role {
+		case "user":
+			lastUser = row.Content
+		case "assistant":
+			lastAgent = row.Content
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return "", "", err
+	}
+
+	lastUser = truncateRunes(lastUser, maxLen)
+	lastAgent = truncateRunes(lastAgent, maxLen)
+	return lastUser, lastAgent, nil
+}
+
+func truncateRunes(s string, maxLen int) string {
+	if maxLen <= 0 || utf8.RuneCountInString(s) <= maxLen {
+		return s
+	}
+	runes := []rune(s)
+	return string(runes[:maxLen])
+}

internal/transcript/transcript_test.go

@@ -0,0 +1,21 @@
+package transcript
+
+import "testing"
+
+func TestLastMessages(t *testing.T) {
+	u, a, err := LastMessages("testdata/sample.jsonl", 2000)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if u != "second question" || a != "final answer" {
+		t.Fatalf("got user=%q agent=%q", u, a)
+	}
+}
+
+func TestTruncate2000(t *testing.T) {
+	u, a, err := LastMessages("testdata/long.jsonl", 2000)
+	if err != nil || len(a) != 2000 {
+		t.Fatalf("len=%d err=%v", len(a), err)
+	}
+	_ = u
+}