Add the conversation fingerprint and the v1 config keys

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/fingerprint/fingerprint.go

@@ -0,0 +1,88 @@
+// Package fingerprint identifies a chat-completions conversation without a
+// session id: the system prompt and the first user message never change from
+// turn to turn, so hashing them pins the conversation.
+package fingerprint
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"strings"
+)
+
+// maxPart is the number of bytes of each input string that contributes to the
+// key: 4 KiB keeps a huge first message from slowing every turn.
+const maxPart = 4096
+
+// Of returns the lowercase hex SHA-256 of the system prompt and the first user
+// message of a chat-completions body (first 4 KiB of each, joined with "\n"),
+// or "" when the body is not a JSON object with a "messages" array containing a
+// user message.
+func Of(body []byte) string {
+	var doc struct {
+		Messages []json.RawMessage `json:"messages"`
+	}
+	if err := json.Unmarshal(body, &doc); err != nil || doc.Messages == nil {
+		return ""
+	}
+
+	var system, user string
+	seenSystem, seenUser := false, false
+	for _, raw := range doc.Messages {
+		var msg struct {
+			Role    string          `json:"role"`
+			Content json.RawMessage `json:"content"`
+		}
+		if err := json.Unmarshal(raw, &msg); err != nil {
+			return ""
+		}
+		switch msg.Role {
+		case "system":
+			if !seenSystem {
+				seenSystem = true
+				system = contentText(msg.Content)
+			}
+		case "user":
+			if !seenUser {
+				seenUser = true
+				user = contentText(msg.Content)
+			}
+		}
+	}
+	if !seenUser {
+		return ""
+	}
+
+	if len(system) > maxPart {
+		system = system[:maxPart]
+	}
+	if len(user) > maxPart {
+		user = user[:maxPart]
+	}
+	sum := sha256.Sum256([]byte(system + "\n" + user))
+	return hex.EncodeToString(sum[:])
+}
+
+// contentText renders a message content value: a JSON string is returned as-is,
+// an array of parts is the concatenation of its text parts (other types
+// ignored), and anything else is "".
+func contentText(raw json.RawMessage) string {
+	var text string
+	if err := json.Unmarshal(raw, &text); err == nil {
+		return text
+	}
+	var parts []struct {
+		Type string `json:"type"`
+		Text string `json:"text"`
+	}
+	if err := json.Unmarshal(raw, &parts); err != nil {
+		return ""
+	}
+	var b strings.Builder
+	for _, p := range parts {
+		if p.Type == "text" {
+			b.WriteString(p.Text)
+		}
+	}
+	return b.String()
+}