Add previous session ID templating and CLI support

docs/cli.md

@@ -30,6 +30,7 @@ For config semantics, see [docs/config.md](./config.md). For operator lifecycle
 - `--config <path>`: optional explicit `pipeline.yml` path.
 - `--session <path>`: optional explicit `session.yml` path.
 - `--session-id <value>`: session template variable value.
+- `--previous-session-id <value>`: previous-session template variable value.
 - `--force`: force stage execution.
 - `--artifacts <names>`: analyze artifact keys to execute (repeatable or comma-separated).
 
@@ -38,6 +39,7 @@ For config semantics, see [docs/config.md](./config.md). For operator lifecycle
 - `--config <path>`
 - `--session <path>`
 - `--session-id <value>`
+- `--previous-session-id <value>`
 - `--force`
 
 ### `resume`
@@ -45,6 +47,7 @@ For config semantics, see [docs/config.md](./config.md). For operator lifecycle
 - `--config <path>`
 - `--session <path>`
 - `--session-id <value>`
+- `--previous-session-id <value>`
 - `--force`
 - `--artifacts <names>`: analyze artifact keys to execute (repeatable or comma-separated).
 
@@ -53,6 +56,7 @@ For config semantics, see [docs/config.md](./config.md). For operator lifecycle
 - `--config <path>`
 - `--session <path>`
 - `--session-id <value>`
+- `--previous-session-id <value>`
 - `--force`
 - `--artifacts <names>`: analyze artifact keys to execute (repeatable or comma-separated).
 - positional `<stage>`: required stage name.
@@ -74,6 +78,7 @@ Valid stage names:
 - `--config <path>`
 - `--session <path>`
 - `--session-id <value>`
+- `--previous-session-id <value>`
 - `--dry-run`: plan restore actions without writing local files.
 - `--force`: overwrite local conflicting files with remote archive files.
 - `--include-audio`: include durable archived `audio/**` files in restore scope.
@@ -92,7 +97,7 @@ Purpose:
 Syntax:
 
 ```bash
-narratio run [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--force] [--artifacts <name[,name...]>]
+narratio run [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--previous-session-id <id>] [--force] [--artifacts <name[,name...]>]
 ```
 
 Success output:
@@ -112,7 +117,7 @@ Purpose:
 Syntax:
 
 ```bash
-narratio plan [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--force]
+narratio plan [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--previous-session-id <id>] [--force]
 ```
 
 Success output includes:
@@ -132,7 +137,7 @@ Purpose:
 Syntax:
 
 ```bash
-narratio resume [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--force] [--artifacts <name[,name...]>]
+narratio resume [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--previous-session-id <id>] [--force] [--artifacts <name[,name...]>]
 ```
 
 Success output:
@@ -172,7 +177,7 @@ Purpose:
 Syntax:
 
 ```bash
-narratio run-stage [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--force] [--artifacts <name[,name...]>] <stage>
+narratio run-stage [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--previous-session-id <id>] [--force] [--artifacts <name[,name...]>] <stage>
 ```
 
 Success output:
@@ -196,7 +201,7 @@ Purpose:
 Syntax:
 
 ```bash
-narratio restore [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--dry-run] [--force] [--include-audio]
+narratio restore [--config <pipeline.yml>] [--session <session.yml>] [--session-id <id>] [--previous-session-id <id>] [--dry-run] [--force] [--include-audio]
 ```
 
 Success output (dry-run):

docs/config.md

@@ -48,9 +48,13 @@ Template behavior:
 - supported placeholders:
   - `{{session_id}}`
   - `{{ session_id }}`
+  - `{{previous_session_id}}`
+  - `{{ previous_session_id }}`
 - `--session-id <value>` supplies the placeholder value.
+- `--previous-session-id <value>` supplies the previous-session placeholder value.
 - unresolved placeholders fail load.
 - if rendered `session_id` mismatches `--session-id`, load fails.
+- if rendered `previous_session_id` mismatches `--previous-session-id`, load fails.
 
 ## 4. Minimal pipeline config
 
@@ -69,6 +73,7 @@ Why this is sufficient:
 
 ```yaml
 session_id: "{{ session_id }}"
+previous_session_id: "{{ previous_session_id }}"
 campaign: sample-campaign
 inputs:
   audio_dir: ./audio
@@ -279,6 +284,7 @@ Restore-related implications:
 | Path | Type | Required | Default |
 | --- | --- | --- | --- |
 | `session.session_id` | string | Yes | none |
+| `session.previous_session_id` | string | No | empty |
 | `session.campaign` | string | Yes | none |
 | `session.date` | string | No | empty |
 | `session.title` | string | No | empty |
@@ -297,6 +303,10 @@ Audio-source rule:
   - `audio_s3.prefix`
 - `audio_s3` cannot be combined with local audio fields.
 
+Previous-session rule:
+
+- if `session.previous_session_id` is set, it must not equal `session.session_id`.
+
 ## 9. Secrets
 
 Narratio supports filesystem-based secret injection via `pipeline.secrets.env_dir`.

internal/app/plan.go

@@ -22,10 +22,12 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var sessionID string
+	var previousSessionID string
 	var force bool
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
+	fs.StringVar(&previousSessionID, "previous-session-id", "", "previous session identifier for session.yml templates")
 	fs.BoolVar(&force, "force", false, "force stage execution (reserved for future behavior)")
 
 	if err := fs.Parse(args); err != nil {
@@ -44,7 +46,8 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 	}
 
 	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
-		SessionID: sessionID,
+		SessionID:         sessionID,
+		PreviousSessionID: previousSessionID,
 	})
 	if err != nil {
 		return fmt.Errorf("plan: %w", err)

internal/app/restore.go

@@ -28,17 +28,19 @@ func Restore(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var sessionID string
+	var previousSessionID string
 	var dryRun bool
 	var force bool
 	var includeAudio bool
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
+	fs.StringVar(&previousSessionID, "previous-session-id", "", "previous session identifier for session.yml templates")
 	fs.BoolVar(&dryRun, "dry-run", false, "plan restore actions without writing local files")
 	fs.BoolVar(&force, "force", false, "overwrite local conflicts with remote state")
 	fs.BoolVar(&includeAudio, "include-audio", false, "include archived session-level audio objects")
 	fs.Usage = func() {
-		_, _ = fmt.Fprintln(out, "Usage: narratio restore [--config <path>] [--session <path>] [--session-id <value>] [--dry-run] [--force] [--include-audio]")
+		_, _ = fmt.Fprintln(out, "Usage: narratio restore [--config <path>] [--session <path>] [--session-id <value>] [--previous-session-id <value>] [--dry-run] [--force] [--include-audio]")
 		_, _ = fmt.Fprintln(out)
 		_, _ = fmt.Fprintln(out, "Flags:")
 		fs.PrintDefaults()
@@ -63,7 +65,8 @@ func Restore(ctx context.Context, args []string, out io.Writer) error {
 	}
 
 	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
-		SessionID: sessionID,
+		SessionID:         sessionID,
+		PreviousSessionID: previousSessionID,
 	})
 	if err != nil {
 		return fmt.Errorf("restore: %w", err)

internal/app/resume.go

@@ -19,11 +19,13 @@ func Resume(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var sessionID string
+	var previousSessionID string
 	var force bool
 	var selectedArtifacts artifactSelectionFlag
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
+	fs.StringVar(&previousSessionID, "previous-session-id", "", "previous session identifier for session.yml templates")
 	fs.BoolVar(&force, "force", false, "force stage execution")
 	fs.Var(&selectedArtifacts, "artifacts", "artifact names to execute during analyze (comma-separated or repeatable)")
 
@@ -43,7 +45,8 @@ func Resume(ctx context.Context, args []string, out io.Writer) error {
 	}
 
 	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
-		SessionID: sessionID,
+		SessionID:         sessionID,
+		PreviousSessionID: previousSessionID,
 	})
 	if err != nil {
 		return fmt.Errorf("resume: %w", err)

internal/app/run.go

@@ -17,11 +17,13 @@ func Run(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var sessionID string
+	var previousSessionID string
 	var force bool
 	var selectedArtifacts artifactSelectionFlag
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
+	fs.StringVar(&previousSessionID, "previous-session-id", "", "previous session identifier for session.yml templates")
 	fs.BoolVar(&force, "force", false, "force stage execution (reserved for future behavior)")
 	fs.Var(&selectedArtifacts, "artifacts", "artifact names to execute during analyze (comma-separated or repeatable)")
 
@@ -41,7 +43,8 @@ func Run(ctx context.Context, args []string, out io.Writer) error {
 	}
 
 	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
-		SessionID: sessionID,
+		SessionID:         sessionID,
+		PreviousSessionID: previousSessionID,
 	})
 	if err != nil {
 		return fmt.Errorf("run: %w", err)

internal/app/run_stage.go

@@ -17,11 +17,13 @@ func RunStage(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var sessionID string
+	var previousSessionID string
 	var force bool
 	var selectedArtifacts artifactSelectionFlag
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
+	fs.StringVar(&previousSessionID, "previous-session-id", "", "previous session identifier for session.yml templates")
 	fs.BoolVar(&force, "force", false, "force stage execution (reserved for future behavior)")
 	fs.Var(&selectedArtifacts, "artifacts", "artifact names to execute during analyze (comma-separated or repeatable)")
 
@@ -54,7 +56,8 @@ func RunStage(ctx context.Context, args []string, out io.Writer) error {
 	}
 
 	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
-		SessionID: sessionID,
+		SessionID:         sessionID,
+		PreviousSessionID: previousSessionID,
 	})
 	if err != nil {
 		return fmt.Errorf("run-stage: %w", err)

internal/app/session_cli_test.go

@@ -9,11 +9,12 @@ import (
 	"testing"
 )
 
-func TestPlanUsesDiscoveredSessionTemplateWithSessionID(t *testing.T) {
+func TestPlanUsesDiscoveredSessionTemplateWithSessionIDs(t *testing.T) {
 	workspaceRoot := t.TempDir()
 	pipelinePath, sessionPath := writeValidConfigFiles(t, workspaceRoot)
 
 	sessionTemplate := `session_id: "{{ session_id }}"
+previous_session_id: "{{ previous_session_id }}"
 campaign: sample-campaign
 inputs:
   audio_dir: ./audio
@@ -36,7 +37,11 @@ inputs:
 	t.Cleanup(func() { _ = os.Chdir(originalWD) })
 
 	var out bytes.Buffer
-	if err := Plan(context.Background(), []string{"--config", pipelinePath, "--session-id", "2026-04-04"}, &out); err != nil {
+	if err := Plan(context.Background(), []string{
+		"--config", pipelinePath,
+		"--session-id", "2026-04-04",
+		"--previous-session-id", "2026-03-28",
+	}, &out); err != nil {
 		t.Fatalf("Plan() error = %v", err)
 	}
 	if !strings.Contains(out.String(), "narratio plan: workdir prepared") {
@@ -58,6 +63,38 @@ func TestPlanFailsWhenSessionIDMismatchesConcreteSession(t *testing.T) {
 	}
 }
 
+func TestPlanFailsWhenPreviousSessionIDMismatchesConcreteSession(t *testing.T) {
+	workspaceRoot := t.TempDir()
+	pipelinePath, sessionPath := writeValidConfigFiles(t, workspaceRoot)
+
+	sessionYAML := `session_id: 2026-05-03
+previous_session_id: 2026-04-26
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	var out bytes.Buffer
+	err := Plan(context.Background(), []string{
+		"--config", pipelinePath,
+		"--session", sessionPath,
+		"--session-id", "2026-05-03",
+		"--previous-session-id", "2026-04-25",
+	}, &out)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "previous_session_id mismatch") {
+		t.Fatalf("error = %q, want mismatch context", err.Error())
+	}
+}
+
 func TestRunStageAcceptsSessionIDFlagAndParsesStageName(t *testing.T) {
 	workspaceRoot := t.TempDir()
 	pipelinePath, sessionPath := writeValidConfigFiles(t, workspaceRoot)

internal/config/config.go

@@ -27,11 +27,12 @@ type PipelineConfig struct {
 
 // SessionConfig contains per-session inputs and metadata.
 type SessionConfig struct {
-	SessionID string              `yaml:"session_id"`
+	SessionID         string              `yaml:"session_id"`
-	Campaign  string              `yaml:"campaign"`
+	PreviousSessionID string              `yaml:"previous_session_id"`
-	Date      string              `yaml:"date"`
+	Campaign          string              `yaml:"campaign"`
-	Title     string              `yaml:"title"`
+	Date              string              `yaml:"date"`
-	Inputs    SessionInputsConfig `yaml:"inputs"`
+	Title             string              `yaml:"title"`
+	Inputs            SessionInputsConfig `yaml:"inputs"`
 }
 
 // WorkspaceConfig configures local workspace behavior.

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 = replaceTemplateVariable(rendered, "session_id", sessionID)
-		rendered = strings.ReplaceAll(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) {

internal/config/load_validate_test.go

@@ -187,6 +187,43 @@ inputs:
 `,
 			wantValidate: "session config \"session.yml\" invalid: session.session_id is required",
 		},
+		{
+			name: "valid previous_session_id passes",
+			pipelineYAML: `workspace:
+  root: /tmp/narratio
+whisperx:
+  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
+seriatim:
+  binary: seriatim
+`,
+			sessionYAML: `session_id: 2026-05-03
+previous_session_id: 2026-04-26
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`,
+		},
+		{
+			name: "previous_session_id equal to session_id fails",
+			pipelineYAML: `workspace:
+  root: /tmp/narratio
+whisperx:
+  transcribe_url: https://transcription.ai.rakestrawhome.com/transcribe
+seriatim:
+  binary: seriatim
+`,
+			sessionYAML: `session_id: 2026-05-03
+previous_session_id: 2026-05-03
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`,
+			wantValidate: "session config \"session.yml\" invalid: session.previous_session_id must not equal session.session_id",
+		},
 		{
 			name: "missing transcribe_url fails",
 			pipelineYAML: `workspace:

internal/config/session_template_test.go

@@ -55,6 +55,34 @@ inputs:
 	}
 }
 
+func TestLoadSessionWithOptionsRendersPreviousSessionPlaceholder(t *testing.T) {
+	dir := t.TempDir()
+	sessionPath := filepath.Join(dir, "session.yml")
+	sessionYAML := `session_id: "{{ session_id }}"
+previous_session_id: "{{ previous_session_id }}"
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	cfg, err := LoadSessionWithOptions(sessionPath, SessionLoadOptions{
+		SessionID:         "2026-04-04",
+		PreviousSessionID: "2026-03-28",
+	})
+	if err != nil {
+		t.Fatalf("LoadSessionWithOptions() error = %v", err)
+	}
+	if cfg.PreviousSessionID != "2026-03-28" {
+		t.Fatalf("PreviousSessionID = %q, want 2026-03-28", cfg.PreviousSessionID)
+	}
+}
+
 func TestLoadSessionWithOptionsUnresolvedPlaceholderFails(t *testing.T) {
 	dir := t.TempDir()
 	sessionPath := filepath.Join(dir, "session.yml")
@@ -82,6 +110,37 @@ inputs:
 	}
 }
 
+func TestLoadSessionWithOptionsUnresolvedPreviousSessionPlaceholderFails(t *testing.T) {
+	dir := t.TempDir()
+	sessionPath := filepath.Join(dir, "session.yml")
+	sessionYAML := `session_id: 2026-05-03
+previous_session_id: "{{ previous_session_id }}"
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	_, err := LoadSessionWithOptions(sessionPath, SessionLoadOptions{})
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "unresolved template variable") {
+		t.Fatalf("error = %q, want unresolved-variable context", err.Error())
+	}
+	if !strings.Contains(err.Error(), "previous_session_id") {
+		t.Fatalf("error = %q, want previous_session_id variable", err.Error())
+	}
+	if !strings.Contains(err.Error(), "--previous-session-id") {
+		t.Fatalf("error = %q, want previous-session-id guidance", err.Error())
+	}
+}
+
 func TestLoadSessionWithOptionsMismatchFails(t *testing.T) {
 	dir := t.TempDir()
 	sessionPath := filepath.Join(dir, "session.yml")
@@ -106,6 +165,34 @@ inputs:
 	}
 }
 
+func TestLoadSessionWithOptionsPreviousSessionMismatchFails(t *testing.T) {
+	dir := t.TempDir()
+	sessionPath := filepath.Join(dir, "session.yml")
+	sessionYAML := `session_id: 2026-05-03
+previous_session_id: 2026-04-26
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+  speakers_file: ./speakers.yml
+  autocorrect_file: ./autocorrect.yml
+  glossary_file: ./glossary.yml
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	_, err := LoadSessionWithOptions(sessionPath, SessionLoadOptions{
+		SessionID:         "2026-05-03",
+		PreviousSessionID: "2026-04-25",
+	})
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "previous_session_id mismatch") {
+		t.Fatalf("error = %q, want mismatch context", err.Error())
+	}
+}
+
 func TestLoadSessionWithOptionsUnknownFieldStillRejectedAfterRendering(t *testing.T) {
 	dir := t.TempDir()
 	sessionPath := filepath.Join(dir, "session.yml")

internal/config/validate.go

@@ -487,8 +487,14 @@ func validateScriptorium(cfg *ScriptoriumConfig) error {
 }
 
 func validateSession(cfg *SessionConfig) error {
-	if strings.TrimSpace(cfg.SessionID) == "" {
+	if err := validateSessionIdentifier("session.session_id", cfg.SessionID, true); err != nil {
-		return fmt.Errorf("session.session_id is required")
+		return err
+	}
+	if err := validateSessionIdentifier("session.previous_session_id", cfg.PreviousSessionID, false); err != nil {
+		return err
+	}
+	if strings.TrimSpace(cfg.PreviousSessionID) != "" && strings.TrimSpace(cfg.PreviousSessionID) == strings.TrimSpace(cfg.SessionID) {
+		return fmt.Errorf("session.previous_session_id must not equal session.session_id")
 	}
 	if strings.TrimSpace(cfg.Campaign) == "" {
 		return fmt.Errorf("session.campaign is required")
@@ -525,6 +531,16 @@ func validateSession(cfg *SessionConfig) error {
 	return nil
 }
 
+func validateSessionIdentifier(fieldName, value string, required bool) error {
+	if strings.TrimSpace(value) == "" {
+		if required {
+			return fmt.Errorf("%s is required", fieldName)
+		}
+		return nil
+	}
+	return nil
+}
+
 func validateCrossConfig(pipeline *PipelineConfig, session *SessionConfig) error {
 	if pipeline == nil || session == nil {
 		return nil