Add previous session ID templating and CLI support

internal/config/load.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strings"
 
 	"gopkg.in/yaml.v3"
@@ -28,7 +29,8 @@ func LoadSession(path string) (*SessionConfig, error) {
 
 // SessionLoadOptions configures session template rendering behavior.
 type SessionLoadOptions struct {
-	SessionID string
+	SessionID         string
+	PreviousSessionID string
 }
 
 // LoadSessionWithOptions loads session configuration from a YAML file with
@@ -56,6 +58,16 @@ func LoadSessionWithOptions(path string, opts SessionLoadOptions) (*SessionConfi
 			strings.TrimSpace(cfg.SessionID),
 		)
 	}
+	if strings.TrimSpace(opts.PreviousSessionID) != "" &&
+		strings.TrimSpace(cfg.PreviousSessionID) != "" &&
+		strings.TrimSpace(cfg.PreviousSessionID) != strings.TrimSpace(opts.PreviousSessionID) {
+		return nil, fmt.Errorf(
+			"load session config: session file %q: previous_session_id mismatch: --previous-session-id %q does not match rendered previous_session_id %q",
+			path,
+			strings.TrimSpace(opts.PreviousSessionID),
+			strings.TrimSpace(cfg.PreviousSessionID),
+		)
+	}
 	return &cfg, nil
 }
 
@@ -114,24 +126,36 @@ var sessionTemplatePattern = regexp.MustCompile(`\{\{\s*([a-zA-Z_][a-zA-Z0-9_]*)
 
 func renderSessionTemplate(content string, opts SessionLoadOptions) (string, error) {
 	sessionID := strings.TrimSpace(opts.SessionID)
+	previousSessionID := strings.TrimSpace(opts.PreviousSessionID)
 	rendered := content
 	if sessionID != "" {
-		rendered = strings.ReplaceAll(rendered, "{{session_id}}", sessionID)
-		rendered = strings.ReplaceAll(rendered, "{{ session_id }}", sessionID)
+		rendered = replaceTemplateVariable(rendered, "session_id", sessionID)
+	}
+	if previousSessionID != "" {
+		rendered = replaceTemplateVariable(rendered, "previous_session_id", previousSessionID)
 	}
 
 	unresolved := sessionTemplatePattern.FindAllStringSubmatch(rendered, -1)
 	if len(unresolved) > 0 {
+		seenVars := map[string]struct{}{}
 		vars := make([]string, 0, len(unresolved))
 		for _, m := range unresolved {
 			if len(m) > 1 {
-				vars = append(vars, m[1])
+				name := m[1]
+				if _, ok := seenVars[name]; ok {
+					continue
+				}
+				seenVars[name] = struct{}{}
+				vars = append(vars, name)
 			}
 		}
+		sort.Strings(vars)
 		if len(vars) > 0 {
+			hints := unresolvedTemplateHints(vars)
 			return "", fmt.Errorf(
-				"session file template rendering failed: unresolved template variable(s): %s; pass --session-id when using {{ session_id }}",
+				"session file template rendering failed: unresolved template variable(s): %s%s",
 				strings.Join(vars, ", "),
+				hints,
 			)
 		}
 		return "", fmt.Errorf("session file template rendering failed: unresolved template placeholders remain")
@@ -140,6 +164,35 @@ func renderSessionTemplate(content string, opts SessionLoadOptions) (string, err
 	return rendered, nil
 }
 
+func replaceTemplateVariable(content, name, value string) string {
+	rendered := strings.ReplaceAll(content, "{{"+name+"}}", value)
+	rendered = strings.ReplaceAll(rendered, "{{ "+name+" }}", value)
+	return rendered
+}
+
+func unresolvedTemplateHints(vars []string) string {
+	seen := map[string]struct{}{}
+	flags := make([]string, 0, 2)
+	for _, name := range vars {
+		switch name {
+		case "session_id":
+			if _, ok := seen["--session-id"]; !ok {
+				seen["--session-id"] = struct{}{}
+				flags = append(flags, "--session-id")
+			}
+		case "previous_session_id":
+			if _, ok := seen["--previous-session-id"]; !ok {
+				seen["--previous-session-id"] = struct{}{}
+				flags = append(flags, "--previous-session-id")
+			}
+		}
+	}
+	if len(flags) == 0 {
+		return ""
+	}
+	return "; pass " + strings.Join(flags, " and ") + " when using those template variable(s)"
+}
+
 func shortName(path, fallback string) string {
 	base := filepath.Base(path)
 	if base == "." || base == string(filepath.Separator) {