From e6a5c6499b39f18b1b6b561f597330acb9a0597a Mon Sep 17 00:00:00 2001
From: Rogee <rogee@ipao.vip>
Date: Mon, 3 Nov 2025 10:59:15 +0800
Subject: [PATCH] Add configurable sequence numbering command

---
 AGENTS.md                                     |   2 +-
 README.md                                     |   1 +
 cmd/root.go                                   |   1 +
 cmd/sequence.go                               | 158 ++++++++++++
 docs/CHANGELOG.md                             |   1 +
 docs/cli-flags.md                             |  17 ++
 internal/sequence/apply.go                    | 113 ++++++++
 internal/sequence/doc.go                      |   2 +
 internal/sequence/format.go                   |  20 ++
 internal/sequence/options.go                  |  95 +++++++
 internal/sequence/plan.go                     |  68 +++++
 internal/sequence/preview.go                  | 195 ++++++++++++++
 internal/sequence/traversal.go                |  92 +++++++
 .../checklists/requirements.md                |  34 +++
 .../contracts/sequence.openapi.yaml           | 241 ++++++++++++++++++
 specs/001-sequence-numbering/data-model.md    |  68 +++++
 specs/001-sequence-numbering/plan.md          |  87 +++++++
 specs/001-sequence-numbering/quickstart.md    |  55 ++++
 specs/001-sequence-numbering/research.md      |  26 ++
 specs/001-sequence-numbering/spec.md          | 109 ++++++++
 specs/001-sequence-numbering/tasks.md         | 115 +++++++++
 testdata/sequence/basic/draft.txt             |   1 +
 testdata/sequence/basic/notes.txt             |   1 +
 testdata/sequence/basic/plan.txt              |   1 +
 testdata/sequence/demo/alpha.txt              |   1 +
 testdata/sequence/demo/beta.txt               |   1 +
 testdata/sequence/demo/gamma.txt              |   1 +
 tests/contract/sequence_placement_test.go     |  76 ++++++
 tests/contract/sequence_preview_test.go       |  53 ++++
 tests/contract/sequence_width_test.go         |  39 +++
 tests/integration/sequence_flow_test.go       |  70 +++++
 tests/integration/sequence_prefix_test.go     |  53 ++++
 tests/integration/sequence_start_test.go      |  65 +++++
 tests/integration/sequence_width_test.go      |  59 +++++
 34 files changed, 1920 insertions(+), 1 deletion(-)
 create mode 100644 cmd/sequence.go
 create mode 100644 internal/sequence/apply.go
 create mode 100644 internal/sequence/doc.go
 create mode 100644 internal/sequence/format.go
 create mode 100644 internal/sequence/options.go
 create mode 100644 internal/sequence/plan.go
 create mode 100644 internal/sequence/preview.go
 create mode 100644 internal/sequence/traversal.go
 create mode 100644 specs/001-sequence-numbering/checklists/requirements.md
 create mode 100644 specs/001-sequence-numbering/contracts/sequence.openapi.yaml
 create mode 100644 specs/001-sequence-numbering/data-model.md
 create mode 100644 specs/001-sequence-numbering/plan.md
 create mode 100644 specs/001-sequence-numbering/quickstart.md
 create mode 100644 specs/001-sequence-numbering/research.md
 create mode 100644 specs/001-sequence-numbering/spec.md
 create mode 100644 specs/001-sequence-numbering/tasks.md
 create mode 100644 testdata/sequence/basic/draft.txt
 create mode 100644 testdata/sequence/basic/notes.txt
 create mode 100644 testdata/sequence/basic/plan.txt
 create mode 100644 testdata/sequence/demo/alpha.txt
 create mode 100644 testdata/sequence/demo/beta.txt
 create mode 100644 testdata/sequence/demo/gamma.txt
 create mode 100644 tests/contract/sequence_placement_test.go
 create mode 100644 tests/contract/sequence_preview_test.go
 create mode 100644 tests/contract/sequence_width_test.go
 create mode 100644 tests/integration/sequence_flow_test.go
 create mode 100644 tests/integration/sequence_prefix_test.go
 create mode 100644 tests/integration/sequence_start_test.go
 create mode 100644 tests/integration/sequence_width_test.go

diff --git a/AGENTS.md b/AGENTS.md
index b2c4c12..a8f847f 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -43,9 +43,9 @@ tests/
 - Smoke: `scripts/smoke-test-replace.sh`, `scripts/smoke-test-remove.sh`
 
 ## Recent Changes
+- 001-sequence-numbering: Added Go 1.24 + `spf13/cobra`, `spf13/pflag`, internal traversal/history/output packages
 - 006-add-regex-command: Added Go 1.24 + `spf13/cobra`, `spf13/pflag`, Go `regexp` (RE2 engine), internal traversal/history/output packages
 - 005-add-insert-command: Added Go 1.24 + `spf13/cobra`, `spf13/pflag`, internal traversal/history/output packages
-- 004-extension-rename: Added Go 1.24 + `spf13/cobra`, `spf13/pflag`, internal traversal/ledger packages
 
 <!-- MANUAL ADDITIONS START -->
 <!-- MANUAL ADDITIONS END -->
diff --git a/README.md b/README.md
index 780269d..e4427d0 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,7 @@ All subcommands accept these persistent flags:
 - `renamer remove <pattern...>` — Strip ordered substrings from names with empty-name protection and duplicate detection.
 - `renamer extension <source-ext...> <target-ext>` — Normalize heterogeneous extensions to a single target while keeping a ledger entry for undo.
 - `renamer insert <position> <text>` — Insert text at symbolic (`^`, `$`) offsets, count forward with numbers (`3` or `^3`), or backward with suffix tokens like `1$`.
+- `renamer sequence [flags]` — Append or prepend zero-padded sequence numbers with configurable start, width, placement (default prefix), separator, and static number prefix/suffix options.
 - `renamer regex <pattern> <template>` — Rename via RE2 capture groups using placeholders like `@1`, `@2`, `@0`, or escape literal `@` as `@@`.
 - `renamer undo` — Revert the most recent mutating command recorded in the ledger.
 
diff --git a/cmd/root.go b/cmd/root.go
index f9fcb20..1c9fc57 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -50,6 +50,7 @@ func NewRootCommand() *cobra.Command {
 	cmd.AddCommand(NewExtensionCommand())
 	cmd.AddCommand(newInsertCommand())
 	cmd.AddCommand(newRegexCommand())
+	cmd.AddCommand(newSequenceCommand())
 	cmd.AddCommand(newUndoCommand())
 
 	return cmd
diff --git a/cmd/sequence.go b/cmd/sequence.go
new file mode 100644
index 0000000..c85420f
--- /dev/null
+++ b/cmd/sequence.go
@@ -0,0 +1,158 @@
+package cmd
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/spf13/cobra"
+
+	"github.com/rogeecn/renamer/internal/listing"
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func newSequenceCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "sequence",
+		Short: "Append or prepend sequential numbers to filenames",
+		Long: `Preview and apply numbered renames across the active scope. Sequence numbers
+respect deterministic traversal order, support configurable start offsets, and record every
+batch in the .renamer ledger for undo.`,
+		Args: cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			scope, err := listing.ScopeFromCmd(cmd)
+			if err != nil {
+				return err
+			}
+
+			dryRun, err := getBool(cmd, "dry-run")
+			if err != nil {
+				return err
+			}
+			autoApply, err := getBool(cmd, "yes")
+			if err != nil {
+				return err
+			}
+			if dryRun && autoApply {
+				return errors.New("--dry-run cannot be combined with --yes; remove one of them")
+			}
+
+			start, err := cmd.Flags().GetInt("start")
+			if err != nil {
+				return err
+			}
+			width, err := cmd.Flags().GetInt("width")
+			if err != nil {
+				return err
+			}
+			placement, err := cmd.Flags().GetString("placement")
+			if err != nil {
+				return err
+			}
+			separator, err := cmd.Flags().GetString("separator")
+			if err != nil {
+				return err
+			}
+			numberPrefix, err := cmd.Flags().GetString("number-prefix")
+			if err != nil {
+				return err
+			}
+			numberSuffix, err := cmd.Flags().GetString("number-suffix")
+			if err != nil {
+				return err
+			}
+
+			opts := sequence.DefaultOptions()
+			opts.WorkingDir = scope.WorkingDir
+			opts.IncludeDirectories = scope.IncludeDirectories
+			opts.IncludeHidden = scope.IncludeHidden
+			opts.Recursive = scope.Recursive
+			opts.Extensions = append([]string(nil), scope.Extensions...)
+			opts.DryRun = dryRun
+			opts.AutoApply = autoApply
+			if start != 0 {
+				opts.Start = start
+			}
+			if cmd.Flags().Changed("width") {
+				opts.Width = width
+				opts.WidthSet = true
+			}
+			if placement != "" {
+				opts.Placement = sequence.Placement(strings.ToLower(placement))
+			}
+			if separator != "" {
+				opts.Separator = separator
+			}
+			opts.NumberPrefix = numberPrefix
+			opts.NumberSuffix = numberSuffix
+
+			out := cmd.OutOrStdout()
+
+			plan, err := sequence.Preview(cmd.Context(), opts, out)
+			if err != nil {
+				return err
+			}
+
+			for _, candidate := range plan.Candidates {
+				status := "UNCHANGED"
+				switch candidate.Status {
+				case sequence.CandidatePending:
+					status = "CHANGE"
+				case sequence.CandidateSkipped:
+					status = "SKIP"
+				}
+				fmt.Fprintf(out, "%s: %s -> %s\n", status, candidate.OriginalPath, candidate.ProposedPath)
+			}
+
+			for _, conflict := range plan.SkippedConflicts {
+				fmt.Fprintf(out, "Warning: %s skipped due to %s (target %s)\n", conflict.OriginalPath, conflict.Reason, conflict.ConflictingPath)
+			}
+			for _, warning := range plan.Summary.Warnings {
+				fmt.Fprintf(out, "Warning: %s\n", warning)
+			}
+
+			if plan.Summary.TotalCandidates == 0 {
+				fmt.Fprintln(out, "No candidates found.")
+				return nil
+			}
+
+			fmt.Fprintf(out, "Preview: %d candidates, %d renames, %d skipped (width %d).\n",
+				plan.Summary.TotalCandidates,
+				plan.Summary.RenamedCount,
+				plan.Summary.SkippedCount,
+				plan.Summary.AppliedWidth,
+			)
+
+			if dryRun || !autoApply {
+				if !autoApply {
+					fmt.Fprintln(out, "Preview complete. Re-run with --yes to apply.")
+				}
+				return nil
+			}
+
+			entry, err := sequence.Apply(cmd.Context(), opts, plan)
+			if err != nil {
+				return err
+			}
+
+			fmt.Fprintf(out, "Applied %d sequence updates. Ledger updated.\n", len(entry.Operations))
+			if plan.Summary.SkippedCount > 0 {
+				fmt.Fprintf(out, "%d candidates were skipped due to conflicts.\n", plan.Summary.SkippedCount)
+			}
+			return nil
+		},
+	}
+
+	cmd.Flags().Int("start", 1, "Starting sequence value (>=1)")
+	cmd.Flags().Int("width", 0, "Minimum digit width for zero padding (defaults to 3 digits, auto-expands as needed)")
+	cmd.Flags().String("placement", string(sequence.PlacementPrefix), "Placement for the sequence number: prefix or suffix")
+	cmd.Flags().String("separator", "_", "Separator between the filename and sequence label and the original name")
+	cmd.Flags().String("number-prefix", "", "Static text placed immediately before the sequence digits")
+	cmd.Flags().String("number-suffix", "", "Static text placed immediately after the sequence digits")
+
+	return cmd
+}
+
+func init() {
+	rootCmd.AddCommand(newSequenceCommand())
+}
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 16d38d1..1fb09d0 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Add `renamer sequence` subcommand with configurable numbering (start, width, placement—default prefix—separator, number prefix/suffix) and ledger-backed apply/undo flows.
 - Add `renamer remove` subcommand with sequential multi-token deletions, empty-name safeguards, and ledger-backed undo.
 - Document remove command ordering semantics, duplicate warnings, and automation guidance.
 - Add `renamer replace` subcommand supporting multi-pattern replacements, preview/apply/undo, and scope flags.
diff --git a/docs/cli-flags.md b/docs/cli-flags.md
index e52c91a..1d0bbb7 100644
--- a/docs/cli-flags.md
+++ b/docs/cli-flags.md
@@ -53,6 +53,23 @@ renamer insert <position> <text> [flags]
 - Insert after third character in stem: `renamer insert 3 _tag --path ./images --dry-run`
 - Combine with extension filter: `renamer insert ^ "v1_" --extensions .txt|.md`
 
+## Sequence Command Quick Reference
+
+```bash
+renamer sequence [flags]
+```
+
+- Applies deterministic numbering to filenames using the active scope filters; preview-first by default.
+- Default behavior prepends a three-digit number using an underscore separator (e.g. `001_name.ext`).
+- Flags:
+  - `--start` (default `1`) sets the initial sequence value (must be ≥1).
+  - `--width` (optional) enforces minimum digit width with zero padding; the command auto-expands and warns when more digits are required.
+  - `--placement` (`suffix` default, `prefix` alternative) controls whether numbers prepend or append the stem.
+  - `--separator` customizes the string placed between the stem and number; path separators are rejected.
+  - `--number-prefix` / `--number-suffix` add static text directly before or after the digits (use with `--placement prefix` for labelled sequences such as `seq001-file.ext`).
+  - Set `--separator ""` to remove the underscore separator when prefixing numbers (e.g. `seq001file.ext`).
+- Conflicting targets are skipped with warnings while remaining files continue numbering; directories included via `--include-dirs` are listed but unchanged.
+
 ## Remove Command Quick Reference
 
 ```bash
diff --git a/internal/sequence/apply.go b/internal/sequence/apply.go
new file mode 100644
index 0000000..f685cf4
--- /dev/null
+++ b/internal/sequence/apply.go
@@ -0,0 +1,113 @@
+package sequence
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"github.com/rogeecn/renamer/internal/history"
+)
+
+// Apply executes the planned numbering operations and records them in the ledger.
+func Apply(ctx context.Context, opts Options, plan Plan) (history.Entry, error) {
+	merged := mergeOptions(opts)
+	if err := validateOptions(&merged); err != nil {
+		return history.Entry{}, err
+	}
+
+	entry := history.Entry{Command: "sequence"}
+
+	type renameOp struct {
+		fromAbs string
+		toAbs   string
+		fromRel string
+		toRel   string
+		depth   int
+	}
+
+	ops := make([]renameOp, 0, len(plan.Candidates))
+	for _, candidate := range plan.Candidates {
+		if candidate.Status != CandidatePending {
+			continue
+		}
+		ops = append(ops, renameOp{
+			fromAbs: filepath.Join(merged.WorkingDir, filepath.FromSlash(candidate.OriginalPath)),
+			toAbs:   filepath.Join(merged.WorkingDir, filepath.FromSlash(candidate.ProposedPath)),
+			fromRel: candidate.OriginalPath,
+			toRel:   candidate.ProposedPath,
+			depth:   strings.Count(candidate.OriginalPath, "/"),
+		})
+	}
+
+	if len(ops) == 0 {
+		return entry, nil
+	}
+
+	sort.SliceStable(ops, func(i, j int) bool {
+		return ops[i].depth > ops[j].depth
+	})
+
+	done := make([]history.Operation, 0, len(ops))
+
+	revert := func() error {
+		for i := len(done) - 1; i >= 0; i-- {
+			op := done[i]
+			source := filepath.Join(merged.WorkingDir, filepath.FromSlash(op.To))
+			destination := filepath.Join(merged.WorkingDir, filepath.FromSlash(op.From))
+			if err := os.Rename(source, destination); err != nil && !errors.Is(err, os.ErrNotExist) {
+				return err
+			}
+		}
+		return nil
+	}
+
+	for _, op := range ops {
+		if err := ctx.Err(); err != nil {
+			_ = revert()
+			return history.Entry{}, err
+		}
+
+		if op.fromAbs == op.toAbs {
+			continue
+		}
+
+		if err := os.Rename(op.fromAbs, op.toAbs); err != nil {
+			_ = revert()
+			return history.Entry{}, err
+		}
+
+		done = append(done, history.Operation{
+			From: filepath.ToSlash(op.fromRel),
+			To:   filepath.ToSlash(op.toRel),
+		})
+	}
+
+	if len(done) == 0 {
+		return entry, nil
+	}
+
+	entry.Operations = done
+	entry.Metadata = map[string]any{
+		"sequence": map[string]any{
+			"start":     plan.Config.Start,
+			"width":     plan.Summary.AppliedWidth,
+			"placement": string(plan.Config.Placement),
+			"separator": plan.Config.Separator,
+			"prefix":    plan.Config.NumberPrefix,
+			"suffix":    plan.Config.NumberSuffix,
+		},
+		"totalCandidates": plan.Summary.TotalCandidates,
+		"renamed":         plan.Summary.RenamedCount,
+		"skipped":         plan.Summary.SkippedCount,
+	}
+
+	if err := history.Append(merged.WorkingDir, entry); err != nil {
+		_ = revert()
+		return history.Entry{}, err
+	}
+
+	return entry, nil
+}
diff --git a/internal/sequence/doc.go b/internal/sequence/doc.go
new file mode 100644
index 0000000..3c9bb3b
--- /dev/null
+++ b/internal/sequence/doc.go
@@ -0,0 +1,2 @@
+// Package sequence implements the numbering rule used by the `renamer sequence` command.
+package sequence
diff --git a/internal/sequence/format.go b/internal/sequence/format.go
new file mode 100644
index 0000000..16e431c
--- /dev/null
+++ b/internal/sequence/format.go
@@ -0,0 +1,20 @@
+package sequence
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// formatNumber zero-pads the provided value using the requested width and returns
+// both the padded number string and the width that was ultimately used.
+func formatNumber(value, requestedWidth int) (string, int) {
+	if value < 0 {
+		value = 0
+	}
+	digitCount := len(strconv.Itoa(value))
+	width := requestedWidth
+	if width <= 0 || width < digitCount {
+		width = digitCount
+	}
+	return fmt.Sprintf("%0*d", width, value), width
+}
diff --git a/internal/sequence/options.go b/internal/sequence/options.go
new file mode 100644
index 0000000..b19cc7a
--- /dev/null
+++ b/internal/sequence/options.go
@@ -0,0 +1,95 @@
+package sequence
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// Placement controls where a sequence number is inserted.
+type Placement string
+
+const (
+	// PlacementSuffix appends the sequence number after the filename stem.
+	PlacementSuffix Placement = "suffix"
+	// PlacementPrefix inserts the sequence number before the filename stem.
+	PlacementPrefix Placement = "prefix"
+)
+
+// Options captures configuration for numbering operations.
+type Options struct {
+	WorkingDir         string
+	Start              int
+	Width              int
+	WidthSet           bool
+	NumberPrefix       string
+	NumberSuffix       string
+	Placement          Placement
+	Separator          string
+	IncludeHidden      bool
+	IncludeDirectories bool
+	Recursive          bool
+	Extensions         []string
+	DryRun             bool
+	AutoApply          bool
+}
+
+// DefaultOptions returns a copy of the default configuration.
+func DefaultOptions() Options {
+	return Options{
+		Start:     1,
+		Width:     3,
+		Placement: PlacementPrefix,
+		Separator: "_",
+	}
+}
+
+func validateOptions(opts *Options) error {
+	if opts == nil {
+		return errors.New("options cannot be nil")
+	}
+
+	if opts.WorkingDir == "" {
+		return errors.New("working directory must be provided")
+	}
+
+	abs, err := filepath.Abs(opts.WorkingDir)
+	if err != nil {
+		return fmt.Errorf("resolve working directory: %w", err)
+	}
+	opts.WorkingDir = abs
+
+	if opts.Start < 1 {
+		return errors.New("start must be >= 1")
+	}
+
+	if opts.Width < 0 {
+		return errors.New("width cannot be negative")
+	}
+	if opts.WidthSet && opts.Width < 1 {
+		return errors.New("width must be >= 1 when specified")
+	}
+
+	switch opts.Placement {
+	case PlacementPrefix, PlacementSuffix:
+		// ok
+	case "":
+		opts.Placement = PlacementSuffix
+	default:
+		return fmt.Errorf("unsupported placement %q", opts.Placement)
+	}
+
+	if strings.ContainsAny(opts.Separator, "/\\") {
+		return errors.New("separator cannot contain path separators")
+	}
+
+	if strings.ContainsAny(opts.NumberPrefix, "/\\") {
+		return errors.New("number prefix cannot contain path separators")
+	}
+	if strings.ContainsAny(opts.NumberSuffix, "/\\") {
+		return errors.New("number suffix cannot contain path separators")
+	}
+
+	return nil
+}
diff --git a/internal/sequence/plan.go b/internal/sequence/plan.go
new file mode 100644
index 0000000..2cf99a7
--- /dev/null
+++ b/internal/sequence/plan.go
@@ -0,0 +1,68 @@
+package sequence
+
+// Plan represents the ordered numbering proposal produced during preview.
+type Plan struct {
+	Candidates       []Candidate
+	SkippedConflicts []Conflict
+	Summary          Summary
+	Config           Config
+}
+
+// Candidate describes a single file considered for numbering.
+type Candidate struct {
+	OriginalPath string
+	ProposedPath string
+	Index        int
+	IsDir        bool
+	Status       CandidateStatus
+}
+
+// CandidateStatus indicates how a candidate was handled during preview.
+type CandidateStatus string
+
+const (
+	// CandidatePending means the candidate will be renamed when applied.
+	CandidatePending CandidateStatus = "pending"
+	// CandidateSkipped indicates the candidate was skipped (e.g., conflict).
+	CandidateSkipped CandidateStatus = "skipped"
+	// CandidateUnchanged indicates the candidate already matches the target name.
+	CandidateUnchanged CandidateStatus = "unchanged"
+)
+
+// Conflict captures a skipped item and the reason it could not be renamed.
+type Conflict struct {
+	OriginalPath    string
+	ConflictingPath string
+	Reason          ConflictReason
+}
+
+// ConflictReason enumerates known conflict types.
+type ConflictReason string
+
+const (
+	// ConflictExistingTarget indicates the proposed name collides with an existing file.
+	ConflictExistingTarget ConflictReason = "existing_target"
+	// ConflictInvalidSeparator indicates the proposed separator produced an invalid path.
+	ConflictInvalidSeparator ConflictReason = "invalid_separator"
+	// ConflictWidthOverflow indicates numbering exceeded a fixed width.
+	ConflictWidthOverflow ConflictReason = "width_overflow"
+)
+
+// Summary aggregates totals surfaced during preview.
+type Summary struct {
+	TotalCandidates int
+	RenamedCount    int
+	SkippedCount    int
+	Warnings        []string
+	AppliedWidth    int
+}
+
+// Config snapshots the numbering configuration for preview/apply/ledger.
+type Config struct {
+	Start        int
+	Width        int
+	Placement    Placement
+	Separator    string
+	NumberPrefix string
+	NumberSuffix string
+}
diff --git a/internal/sequence/preview.go b/internal/sequence/preview.go
new file mode 100644
index 0000000..88e0dba
--- /dev/null
+++ b/internal/sequence/preview.go
@@ -0,0 +1,195 @@
+package sequence
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Preview computes the numbering plan for the provided options, returning the
+// plan without mutating the filesystem. The writer is reserved for future
+// preview output integration and may be nil.
+func Preview(ctx context.Context, opts Options, w io.Writer) (Plan, error) {
+	merged := mergeOptions(opts)
+
+	if err := validateOptions(&merged); err != nil {
+		return Plan{}, err
+	}
+
+	traversalCandidates, err := collectTraversalCandidates(ctx, merged)
+	if err != nil {
+		return Plan{}, err
+	}
+
+	plan := Plan{
+		Candidates: make([]Candidate, 0, len(traversalCandidates)),
+		Config: Config{
+			Start:        merged.Start,
+			Width:        merged.Width,
+			Placement:    merged.Placement,
+			Separator:    merged.Separator,
+			NumberPrefix: merged.NumberPrefix,
+			NumberSuffix: merged.NumberSuffix,
+		},
+	}
+
+	plannedTargets := make(map[string]string)
+	plannedTargetsFold := make(map[string]string)
+
+	widthUsed := merged.Width
+	widthWarned := false
+	nextValue := merged.Start
+	sequenceIndex := 0
+
+	for _, entry := range traversalCandidates {
+		if entry.IsDir {
+			// Directories remain untouched for numbering purposes.
+			continue
+		}
+
+		plan.Summary.TotalCandidates++
+
+		number, appliedWidth := formatNumber(nextValue, merged.Width)
+		if merged.WidthSet && appliedWidth > merged.Width && !widthWarned {
+			plan.Summary.Warnings = append(plan.Summary.Warnings, fmt.Sprintf("requested width %d expanded to %d for %s", merged.Width, appliedWidth, entry.RelativePath))
+			widthWarned = true
+		}
+		if appliedWidth > widthUsed {
+			widthUsed = appliedWidth
+		}
+
+		formattedNumber := merged.NumberPrefix + number + merged.NumberSuffix
+
+		proposed := buildProposedPath(entry, merged, formattedNumber)
+
+		candidate := Candidate{
+			OriginalPath: entry.RelativePath,
+			ProposedPath: proposed,
+			Index:        sequenceIndex,
+			Status:       CandidatePending,
+			IsDir:        entry.IsDir,
+		}
+
+		if proposed == entry.RelativePath {
+			candidate.Status = CandidateUnchanged
+			plan.Candidates = append(plan.Candidates, candidate)
+			sequenceIndex++
+			nextValue++
+			continue
+		}
+
+		if existing, ok := plannedTargets[proposed]; ok && existing != entry.RelativePath {
+			plan.appendConflict(entry.RelativePath, proposed, ConflictExistingTarget)
+			plan.Summary.SkippedCount++
+			candidate.Status = CandidateSkipped
+			plan.Candidates = append(plan.Candidates, candidate)
+			sequenceIndex++
+			nextValue++
+			continue
+		}
+
+		lowerKey := strings.ToLower(proposed)
+		if existing, ok := plannedTargetsFold[lowerKey]; ok && existing != entry.RelativePath {
+			plan.appendConflict(entry.RelativePath, proposed, ConflictExistingTarget)
+			plan.Summary.SkippedCount++
+			candidate.Status = CandidateSkipped
+			plan.Candidates = append(plan.Candidates, candidate)
+			sequenceIndex++
+			nextValue++
+			continue
+		}
+
+		targetAbs := filepath.Join(merged.WorkingDir, filepath.FromSlash(proposed))
+		if info, statErr := os.Stat(targetAbs); statErr == nil {
+			origInfo, origErr := os.Stat(entry.AbsolutePath)
+			if origErr != nil || !os.SameFile(info, origInfo) {
+				plan.appendConflict(entry.RelativePath, proposed, ConflictExistingTarget)
+				plan.Summary.SkippedCount++
+				candidate.Status = CandidateSkipped
+				plan.Candidates = append(plan.Candidates, candidate)
+				sequenceIndex++
+				nextValue++
+				continue
+			}
+		} else if !errors.Is(statErr, os.ErrNotExist) {
+			return Plan{}, statErr
+		}
+
+		plan.Candidates = append(plan.Candidates, candidate)
+		plan.Summary.RenamedCount++
+		plannedTargets[proposed] = entry.RelativePath
+		plannedTargetsFold[lowerKey] = entry.RelativePath
+		sequenceIndex++
+		nextValue++
+	}
+
+	plan.Summary.AppliedWidth = widthUsed
+	return plan, nil
+}
+
+func mergeOptions(opts Options) Options {
+	merged := DefaultOptions()
+	if opts.Start != 0 {
+		merged.Start = opts.Start
+	}
+	merged.Width = opts.Width
+	merged.WidthSet = opts.WidthSet
+	if opts.Placement != "" {
+		merged.Placement = opts.Placement
+	}
+	if opts.Separator != "" {
+		merged.Separator = opts.Separator
+	}
+	merged.NumberPrefix = opts.NumberPrefix
+	merged.NumberSuffix = opts.NumberSuffix
+	merged.WorkingDir = opts.WorkingDir
+	merged.IncludeDirectories = opts.IncludeDirectories
+	merged.IncludeHidden = opts.IncludeHidden
+	merged.Recursive = opts.Recursive
+	merged.Extensions = append([]string(nil), opts.Extensions...)
+	merged.DryRun = opts.DryRun
+	merged.AutoApply = opts.AutoApply
+	return merged
+}
+
+func buildProposedPath(entry traversalCandidate, opts Options, formattedNumber string) string {
+	dir := filepath.Dir(entry.RelativePath)
+	if dir == "." {
+		dir = ""
+	}
+
+	stem := entry.Stem
+	if opts.Placement == PlacementPrefix {
+		stem = formattedNumber + joinIfNeeded(opts.Separator, stem)
+	} else {
+		stem = stem + joinIfNeeded(opts.Separator, formattedNumber)
+	}
+
+	if !entry.IsDir && entry.Extension != "" {
+		stem += entry.Extension
+	}
+
+	if dir == "" {
+		return filepath.ToSlash(stem)
+	}
+	return filepath.ToSlash(filepath.Join(dir, stem))
+}
+
+func joinIfNeeded(separator, value string) string {
+	if separator == "" {
+		return value
+	}
+	return separator + value
+}
+
+func (p *Plan) appendConflict(original, proposed string, reason ConflictReason) {
+	p.SkippedConflicts = append(p.SkippedConflicts, Conflict{
+		OriginalPath:    original,
+		ConflictingPath: proposed,
+		Reason:          reason,
+	})
+}
diff --git a/internal/sequence/traversal.go b/internal/sequence/traversal.go
new file mode 100644
index 0000000..319f029
--- /dev/null
+++ b/internal/sequence/traversal.go
@@ -0,0 +1,92 @@
+package sequence
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"path/filepath"
+	"strings"
+
+	"github.com/rogeecn/renamer/internal/traversal"
+)
+
+type traversalCandidate struct {
+	RelativePath string
+	AbsolutePath string
+	Stem         string
+	Extension    string
+	IsDir        bool
+	Depth        int
+}
+
+func collectTraversalCandidates(ctx context.Context, opts Options) ([]traversalCandidate, error) {
+	if opts.WorkingDir == "" {
+		return nil, errors.New("working directory must be provided")
+	}
+
+	absRoot, err := filepath.Abs(opts.WorkingDir)
+	if err != nil {
+		return nil, err
+	}
+
+	walker := traversal.NewWalker()
+
+	allowedExts := make(map[string]struct{}, len(opts.Extensions))
+	for _, ext := range opts.Extensions {
+		lower := strings.ToLower(ext)
+		allowedExts[lower] = struct{}{}
+	}
+
+	candidates := make([]traversalCandidate, 0)
+
+	emit := func(relPath string, entry fs.DirEntry, depth int) error {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		// Skip the root placeholder emitted by the walker when include dirs is true.
+		if relPath == "." {
+			return nil
+		}
+
+		relSlash := filepath.ToSlash(relPath)
+		absolute := filepath.Join(absRoot, relPath)
+		candidate := traversalCandidate{
+			RelativePath: relSlash,
+			AbsolutePath: absolute,
+			IsDir:        entry.IsDir(),
+			Depth:        depth,
+		}
+
+		if !entry.IsDir() {
+			rawExt := filepath.Ext(entry.Name())
+			lowerExt := strings.ToLower(rawExt)
+			if len(allowedExts) > 0 {
+				if _, ok := allowedExts[lowerExt]; !ok {
+					return nil
+				}
+			}
+
+			candidate.Extension = rawExt
+			stem := entry.Name()
+			if rawExt != "" {
+				stem = strings.TrimSuffix(stem, rawExt)
+			}
+			candidate.Stem = stem
+		} else {
+			candidate.Stem = entry.Name()
+		}
+
+		candidates = append(candidates, candidate)
+		return nil
+	}
+
+	err = walker.Walk(absRoot, opts.Recursive, opts.IncludeDirectories, opts.IncludeHidden, 0, emit)
+	if err != nil {
+		return nil, err
+	}
+
+	return candidates, nil
+}
diff --git a/specs/001-sequence-numbering/checklists/requirements.md b/specs/001-sequence-numbering/checklists/requirements.md
new file mode 100644
index 0000000..d5d5293
--- /dev/null
+++ b/specs/001-sequence-numbering/checklists/requirements.md
@@ -0,0 +1,34 @@
+# Specification Quality Checklist: Sequence Numbering Command
+
+**Purpose**: Validate specification completeness and quality before proceeding to planning
+**Created**: 2025-10-31
+**Feature**: [Sequence Numbering Command](../spec.md)
+
+## Content Quality
+
+- [x] No implementation details (languages, frameworks, APIs)
+- [x] Focused on user value and business needs
+- [x] Written for non-technical stakeholders
+- [x] All mandatory sections completed
+
+## Requirement Completeness
+
+- [x] No [NEEDS CLARIFICATION] markers remain
+- [x] Requirements are testable and unambiguous
+- [x] Success criteria are measurable
+- [x] Success criteria are technology-agnostic (no implementation details)
+- [x] All acceptance scenarios are defined
+- [x] Edge cases are identified
+- [x] Scope is clearly bounded
+- [x] Dependencies and assumptions identified
+
+## Feature Readiness
+
+- [x] All functional requirements have clear acceptance criteria
+- [x] User scenarios cover primary flows
+- [x] Feature meets measurable outcomes defined in Success Criteria
+- [x] No implementation details leak into specification
+
+## Notes
+
+- Items marked incomplete require spec updates before `/speckit.clarify` or `/speckit.plan`
diff --git a/specs/001-sequence-numbering/contracts/sequence.openapi.yaml b/specs/001-sequence-numbering/contracts/sequence.openapi.yaml
new file mode 100644
index 0000000..7244dfd
--- /dev/null
+++ b/specs/001-sequence-numbering/contracts/sequence.openapi.yaml
@@ -0,0 +1,241 @@
+openapi: 3.1.0
+info:
+  title: Renamer Sequence Command Contract
+  version: 0.1.0
+  description: >
+    Conceptual REST contract for the `renamer sequence` CLI behavior, used to
+    formalize preview/apply expectations for testing and documentation.
+servers:
+  - url: cli://local
+paths:
+  /sequence/preview:
+    post:
+      summary: Generate a deterministic sequence numbering preview.
+      operationId: previewSequence
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/SequenceRequest'
+      responses:
+        '200':
+          description: Preview generated successfully.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/SequencePlan'
+        '400':
+          description: Invalid configuration (e.g., width <= 0, invalid separator).
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+  /sequence/apply:
+    post:
+      summary: Apply sequence numbering to previously previewed candidates.
+      operationId: applySequence
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              allOf:
+                - $ref: '#/components/schemas/SequenceRequest'
+                - type: object
+                  properties:
+                    confirm:
+                      type: boolean
+                      const: true
+      responses:
+        '200':
+          description: Sequence numbering applied.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/SequenceApplyResult'
+        '207':
+          description: Applied with skipped conflicts.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/SequenceApplyResult'
+        '400':
+          description: Invalid configuration or missing confirmation.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+        '409':
+          description: Scope filters yielded zero candidates.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+components:
+  schemas:
+    SequenceRequest:
+      type: object
+      required:
+        - start
+        - placement
+        - separator
+      properties:
+        path:
+          type: string
+          description: Root directory for traversal.
+        recursive:
+          type: boolean
+        includeDirs:
+          type: boolean
+          description: Directories remain untouched even when included.
+        hidden:
+          type: boolean
+        extensions:
+          type: array
+          items:
+            type: string
+            pattern: '^\\.[^./]+$'
+        dryRun:
+          type: boolean
+        yes:
+          type: boolean
+        start:
+          type: integer
+          minimum: 1
+        width:
+          type: integer
+          minimum: 1
+        placement:
+          type: string
+          enum: [prefix, suffix]
+        separator:
+          type: string
+          minLength: 1
+          pattern: '^[^/\\\\]+$'
+    SequencePlan:
+      type: object
+      required:
+        - candidates
+        - config
+        - summary
+      properties:
+        candidates:
+          type: array
+          items:
+            $ref: '#/components/schemas/SequenceCandidate'
+        skippedConflicts:
+          type: array
+          items:
+            $ref: '#/components/schemas/SequenceConflict'
+        summary:
+          $ref: '#/components/schemas/SequenceSummary'
+        config:
+          $ref: '#/components/schemas/SequenceConfig'
+    SequenceCandidate:
+      type: object
+      required:
+        - originalPath
+        - proposedPath
+        - index
+      properties:
+        originalPath:
+          type: string
+        proposedPath:
+          type: string
+        index:
+          type: integer
+          minimum: 0
+    SequenceConflict:
+      type: object
+      required:
+        - originalPath
+        - conflictingPath
+        - reason
+      properties:
+        originalPath:
+          type: string
+        conflictingPath:
+          type: string
+        reason:
+          type: string
+          enum: [existing_target, invalid_separator, width_overflow]
+    SequenceSummary:
+      type: object
+      required:
+        - totalCandidates
+        - renamedCount
+        - skippedCount
+      properties:
+        totalCandidates:
+          type: integer
+        renamedCount:
+          type: integer
+        skippedCount:
+          type: integer
+        warnings:
+          type: array
+          items:
+            type: string
+    SequenceConfig:
+      type: object
+      required:
+        - start
+        - placement
+        - separator
+      properties:
+        start:
+          type: integer
+        width:
+          type: integer
+        placement:
+          type: string
+          enum: [prefix, suffix]
+        separator:
+          type: string
+    SequenceApplyResult:
+      type: object
+      required:
+        - plan
+        - ledgerEntry
+      properties:
+        plan:
+          $ref: '#/components/schemas/SequencePlan'
+        ledgerEntry:
+          $ref: '#/components/schemas/SequenceLedgerEntry'
+    SequenceLedgerEntry:
+      type: object
+      required:
+        - rule
+        - config
+        - operations
+      properties:
+        timestamp:
+          type: string
+          format: date-time
+        rule:
+          type: string
+          const: sequence
+        config:
+          $ref: '#/components/schemas/SequenceConfig'
+        operations:
+          type: array
+          items:
+            $ref: '#/components/schemas/SequenceOperation'
+    SequenceOperation:
+      type: object
+      required:
+        - from
+        - to
+      properties:
+        from:
+          type: string
+        to:
+          type: string
+    ErrorResponse:
+      type: object
+      required:
+        - message
+      properties:
+        message:
+          type: string
diff --git a/specs/001-sequence-numbering/data-model.md b/specs/001-sequence-numbering/data-model.md
new file mode 100644
index 0000000..5857f3d
--- /dev/null
+++ b/specs/001-sequence-numbering/data-model.md
@@ -0,0 +1,68 @@
+# Data Model: Sequence Numbering Command
+
+## SequenceRequest
+- **Fields**
+  - `Path` (string): Root path for traversal (defaults to current directory); must exist and be accessible.
+  - `Recursive` (bool): Includes subdirectories when true.
+  - `IncludeDirs` (bool): Includes directories in traversal results without numbering.
+  - `Hidden` (bool): Includes hidden files when true.
+  - `Extensions` ([]string): Optional `.`-prefixed extension filter; deduplicated and validated.
+  - `DryRun` (bool): Indicates preview-only execution.
+  - `Yes` (bool): Confirmation flag for apply mode.
+  - `Start` (int): First sequence value; must be ≥1.
+  - `Width` (int): Minimum digits for zero padding; must be ≥1 when provided.
+  - `Placement` (enum: `prefix` | `suffix`): Determines where the sequence number is inserted; default `suffix`.
+  - `Separator` (string): Separator between number and filename; defaults to `_`; must comply with filesystem rules (no path separators, non-empty).
+- **Relationships**: Consumed by traversal service to produce candidates and by sequence rule to generate `SequencePlan`.
+- **Validations**: Numeric fields validated before preview; separator sanitized; conflicting flags (dry-run vs yes) rejected.
+
+## SequencePlan
+- **Fields**
+  - `Candidates` ([]SequenceCandidate): Ordered list of files considered for numbering.
+  - `SkippedConflicts` ([]SequenceConflict): Files skipped due to target path collisions.
+  - `Summary` (SequenceSummary): Counts for total candidates, renamed files, and skipped items.
+  - `Config` (SequenceConfig): Snapshot of sequence settings (start, width, placement, separator).
+- **Relationships**: Passed to output package for preview rendering and to history package for ledger persistence.
+- **Validations**: Candidate ordering must match traversal ordering; conflicts identified before apply.
+
+### SequenceCandidate
+- **Fields**
+  - `OriginalPath` (string)
+  - `ProposedPath` (string)
+  - `Index` (int): Zero-based position used to derive padded number.
+- **Constraints**: Proposed path must differ from original to be considered a rename; duplicates flagged as conflicts.
+
+### SequenceConflict
+- **Fields**
+  - `OriginalPath` (string)
+  - `ConflictingPath` (string)
+  - `Reason` (string enum: `existing_target`, `invalid_separator`, `width_overflow`)
+
+### SequenceSummary
+- **Fields**
+  - `TotalCandidates` (int)
+  - `RenamedCount` (int)
+  - `SkippedCount` (int)
+  - `Warnings` ([]string)
+
+## SequenceLedgerEntry
+- **Fields**
+  - `Timestamp` (time.Time)
+  - `Rule` (string): Fixed value `sequence`.
+  - `Config` (SequenceConfig): Stored to support undo.
+  - `Operations` ([]SequenceOperation): Each captures `From` and `To` paths actually renamed.
+- **Relationships**: Append-only entry written by history package; consumed by undo command.
+- **Validations**: Only include successful renames (skipped conflicts omitted). Undo must verify files still exist before attempting reversal.
+
+### SequenceOperation
+- **Fields**
+  - `From` (string)
+  - `To` (string)
+
+## SequenceConfig
+- **Fields**
+  - `Start` (int)
+  - `Width` (int)
+  - `Placement` (string)
+  - `Separator` (string)
+- **Usage**: Embedded in plan summaries, ledger entries, and undo operations to ensure consistent behavior across preview and apply.
diff --git a/specs/001-sequence-numbering/plan.md b/specs/001-sequence-numbering/plan.md
new file mode 100644
index 0000000..61ff3a5
--- /dev/null
+++ b/specs/001-sequence-numbering/plan.md
@@ -0,0 +1,87 @@
+# Implementation Plan: Sequence Numbering Command
+
+**Branch**: `001-sequence-numbering` | **Date**: 2025-11-03 | **Spec**: `specs/001-sequence-numbering/spec.md`
+**Input**: Feature specification from `/specs/001-sequence-numbering/spec.md`
+
+**Note**: This template is filled in by the `/speckit.plan` command. See `.specify/templates/commands/plan.md` for the execution workflow.
+
+## Summary
+
+Deliver a new `renamer sequence` command that appends deterministic sequence numbers to file candidates following the preview-first workflow. The command respects existing scope flags, supports configuration for start value, width, placement, and separator, records batches in the `.renamer` ledger for undo, and skips conflicting filesystem entries while warning the user.
+
+## Technical Context
+
+<!--
+  ACTION REQUIRED: Replace the content in this section with the technical details
+  for the project. The structure here is presented in advisory capacity to guide
+  the iteration process.
+-->
+
+**Language/Version**: Go 1.24  
+**Primary Dependencies**: `spf13/cobra`, `spf13/pflag`, internal traversal/history/output packages  
+**Storage**: Local filesystem + `.renamer` ledger files  
+**Testing**: `go test ./...`, contract + integration suites under `tests/`  
+**Target Platform**: Cross-platform CLI (Linux/macOS/Windows shells)  
+**Project Type**: CLI application (single Go project)  
+**Performance Goals**: Preview + apply 500 files in ≤120s; preview/apply parity ≥95%  
+**Constraints**: Deterministic ordering, atomic ledger writes, skip conflicts while warning  
+**Scale/Scope**: Operates on batches up to hundreds of files per invocation
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+- Preview flow will extend existing preview pipeline to list original → numbered name mappings and highlight skipped conflicts before requiring `--yes`.  
+- Undo strategy leverages ledger entries capturing sequence parameters (start, width, placement) and per-file mappings, ensuring reversal mirrors numbering order.  
+- Sequence rule will be implemented as a composable transformation module declaring inputs (scope candidates + sequence config), validations, and outputs, reusable across preview/apply.  
+- Scope handling continues to consume existing traversal services, honoring `-d`, `-r`, `--extensions`, and leaving directories untouched per clarified requirement while preventing scope escape.  
+- CLI UX will wire flags via Cobra, update help text, and add tests covering flag validation, preview output, warning messaging, and undo flow consistency.
+
+**Post-Design Review:** Research and design artifacts confirm all principles remain satisfied; no constitution waivers required.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/[###-feature]/
+├── plan.md              # This file (/speckit.plan command output)
+├── research.md          # Phase 0 output (/speckit.plan command)
+├── data-model.md        # Phase 1 output (/speckit.plan command)
+├── quickstart.md        # Phase 1 output (/speckit.plan command)
+├── contracts/           # Phase 1 output (/speckit.plan command)
+└── tasks.md             # Phase 2 output (/speckit.tasks command - NOT created by /speckit.plan)
+```
+
+### Source Code (repository root)
+<!--
+  ACTION REQUIRED: Replace the placeholder tree below with the concrete layout
+  for this feature. Delete unused options and expand the chosen structure with
+  real paths (e.g., apps/admin, packages/something). The delivered plan must
+  not include Option labels.
+-->
+
+```text
+cmd/
+├── renamer/               # Cobra CLI entrypoints and command wiring
+internal/
+├── traversal/             # Scope resolution and ordering services
+├── history/               # Ledger and undo utilities
+├── output/                # Preview/summary formatting
+└── sequence/              # [to be added] sequence rule implementation
+tests/
+├── contract/              # CLI contract tests
+├── integration/           # Multi-command flow tests
+└── smoke/                 # Smoke scripts under scripts/
+```
+
+**Structure Decision**: Extend existing single Go CLI project; new logic lives under `internal/sequence`, with command wiring in `cmd/renamer`.
+
+## Complexity Tracking
+
+> **Fill ONLY if Constitution Check has violations that must be justified**
+
+| Violation | Why Needed | Simpler Alternative Rejected Because |
+|-----------|------------|-------------------------------------|
+| [e.g., 4th project] | [current need] | [why 3 projects insufficient] |
+| [e.g., Repository pattern] | [specific problem] | [why direct DB access insufficient] |
diff --git a/specs/001-sequence-numbering/quickstart.md b/specs/001-sequence-numbering/quickstart.md
new file mode 100644
index 0000000..84adb39
--- /dev/null
+++ b/specs/001-sequence-numbering/quickstart.md
@@ -0,0 +1,55 @@
+# Quickstart: Sequence Numbering Command
+
+## Prerequisites
+- Go 1.24 toolchain installed.
+- `renamer` repository cloned and bootstrapped (`go mod tidy` already satisfied in repo).
+- Test fixtures available under `tests/` for validation runs.
+
+## Build & Install
+```bash
+go build -o bin/renamer ./cmd/renamer
+```
+
+## Preview Sequence Numbering
+```bash
+bin/renamer sequence \
+  --path ./fixtures/sample-batch \
+  --dry-run
+```
+Outputs a preview table showing `001_`, `002_`, … prefixes based on alphabetical order.
+
+## Customize Formatting
+```bash
+bin/renamer sequence \
+  --path ./fixtures/sample-batch \
+  --start 10 \
+  --width 4 \
+  --number-prefix seq \
+  --separator "" \
+  --dry-run
+```
+Produces names such as `seq0010file.ext`. Errors if width/start are invalid.
+
+## Apply Changes
+```bash
+bin/renamer sequence \
+  --path ./fixtures/sample-batch \
+  --yes
+```
+Writes rename results to the `.renamer` ledger while skipping conflicting targets and warning the user.
+
+## Undo Sequence Batch
+```bash
+bin/renamer undo --path ./fixtures/sample-batch
+```
+Restores filenames using the most recent ledger entry.
+
+## Run Automated Tests
+```bash
+go test ./...
+tests/integration/remove_flow_test.go    # existing suites ensure regressions are caught
+```
+
+## Troubleshooting
+- Conflict warnings indicate existing files with the same numbered name; resolve manually or adjust flags.
+- Zero candidates cause a 409-style error; adjust scope flags to include desired files.
diff --git a/specs/001-sequence-numbering/research.md b/specs/001-sequence-numbering/research.md
new file mode 100644
index 0000000..2375c8a
--- /dev/null
+++ b/specs/001-sequence-numbering/research.md
@@ -0,0 +1,26 @@
+# Research Log
+
+## Cobra Flag Validation For Sequence Command
+- **Decision**: Validate `--start`, `--width`, `--placement`, and `--separator` flags using Cobra command `PreRunE` with shared helpers, returning errors for invalid inputs before preview executes.
+- **Rationale**: Cobra documentation and community guides recommend using `RunE`/`PreRunE` to surface validation errors with non-zero exit codes, ensuring CLI consistency and enabling tests to cover messaging.
+- **Alternatives considered**: Inline validation inside business logic (rejected—mixes CLI parsing with domain rules and complicates contract tests); custom flag types (rejected—adds complexity without additional value).
+
+## Sequence Ordering And Determinism
+- **Decision**: Reuse the existing traversal service to produce a stable, path-sorted candidate list and derive sequence numbers from index positions in the preview plan.
+- **Rationale**: Internal traversal package already guarantees deterministic ordering and filtering; leveraging it avoids duplicating scope logic and satisfies Preview-First and Scope-Aware principles.
+- **Alternatives considered**: Implement ad-hoc sorting inside sequence rule (rejected—risk of diverging from other commands); rely on filesystem iteration order (rejected—non-deterministic across platforms).
+
+## Ledger Metadata Capture
+- **Decision**: Extend history ledger entries with a new sequence record type storing sequence parameters and per-file mappings, ensuring undo can skip missing files but restore others.
+- **Rationale**: Existing ledger pattern (as used by replace/remove commands) stores rule metadata for undo; following same structure keeps undo consistent and auditable.
+- **Alternatives considered**: Store only file rename pairs without parameters (rejected—undo would lack context if future migrations require differentiation); create a separate ledger file (rejected—breaks append-only guarantee).
+
+## Conflict Handling Strategy
+- **Decision**: During apply, skip conflicting file targets, log a warning via output package, and continue numbering remaining candidates; conflicts remain in preview so users can resolve them beforehand.
+- **Rationale**: Aligns with clarified requirements and minimizes partial ledger entries while informing users; consistent with existing warning infrastructure used by other commands.
+- **Alternatives considered**: Abort entire batch on conflict (rejected—user explicitly requested skip behavior); auto-adjust numbers (rejected—violates preview/apply parity).
+
+## Directory Inclusion Policy
+- **Decision**: Filter traversal results so directories included via `--include-dirs` are reported but not renamed; numbering applies only to file candidates.
+- **Rationale**: Keeps command behavior predictable, avoids confusing two numbering schemes, and respects clarified requirement without altering traversal contract tests.
+- **Alternatives considered**: Separate numbering sequences for files vs directories (rejected—adds complexity with little user need); rename directories by default (rejected—breaks clarified guidance).
diff --git a/specs/001-sequence-numbering/spec.md b/specs/001-sequence-numbering/spec.md
new file mode 100644
index 0000000..2d56355
--- /dev/null
+++ b/specs/001-sequence-numbering/spec.md
@@ -0,0 +1,109 @@
+# Feature Specification: Sequence Numbering Command
+
+**Feature Branch**: `001-sequence-numbering`  
+**Created**: 2025-10-31  
+**Status**: Draft  
+**Input**: User description: "添加 sequence 功能，为重命名文件添加序号，可以定义序列号长度 使用0左填充，可以指定序列号开始序号"
+
+## Clarifications
+
+### Session 2025-11-03
+
+- Q: How should the command behave when the generated filename already exists outside the current batch (e.g., `file_001.txt`)? → A: Skip conflicting files, continue, and warn.
+- Q: How are sequence numbers applied when new files appear between preview and apply, potentially altering traversal order? → A: Ignore new files and rename only the previewed set.
+- Q: What validation and messaging occur when the starting number, width, or separator arguments are invalid (negative numbers, zero width, multi-character separator conflicting with filesystem rules)? → A: Hard error with non-zero exit and no preview output.
+- Q: How is numbering handled for directories when `--include-dirs` is used alongside files within the same traversal scope? → A: Do not rename directories; sequence applies to files only.
+
+## User Scenarios & Testing *(mandatory)*
+
+### User Story 1 - Add Sequential Indices to Batch (Priority: P1)
+
+As a content manager preparing assets for delivery, I want to append an auto-incrementing number to each filename within my selected scope so that downstream systems receive files in a predictable order.
+
+**Why this priority**: Sequencing multiple files is the primary value of the feature and removes the need for external renaming tools for common workflows such as media preparation or document packaging.
+
+**Independent Test**: Place three files in a working directory, run `renamer sequence --path <dir> --dry-run`, and verify the preview shows `001_`, `002_`, `003_` prefixes in deterministic order. Re-run with `--yes` and confirm the ledger captures the batch.
+
+**Acceptance Scenarios**:
+
+1. **Given** a directory containing `draft.txt`, `notes.txt`, and `plan.txt`, **When** the user runs `renamer sequence --dry-run --path <dir>`, **Then** the preview lists each file renamed with a `001_` prefix in alphabetical order and reports the candidate totals.
+2. **Given** the same directory and preview, **When** the user re-executes the command with `--yes`, **Then** the CLI reports three files updated and the `.renamer` ledger stores the sequence configuration.
+
+---
+
+### User Story 2 - Control Number Formatting (Priority: P2)
+
+As an archivist following strict naming standards, I want to define the sequence width and zero padding so the filenames meet fixed-length requirements without additional scripts.
+
+**Why this priority**: Formatting options broaden adoption by matching industry conventions (e.g., four-digit reels) and avoid manual corrections after renaming.
+
+**Independent Test**: Run `renamer sequence --width 4 --path <dir> --dry-run` and confirm every previewed filename contains a four-digit, zero-padded sequence value.
+
+**Acceptance Scenarios**:
+
+1. **Given** files `cutA.mov` and `cutB.mov`, **When** the user runs `renamer sequence --width 4 --dry-run`, **Then** the preview shows `0001_` and `0002_` prefixes despite having only two files.
+2. **Given** the same files, **When** the user omits an explicit width, **Then** the preview pads only as needed to accommodate the highest sequence number (e.g., `1_`, `2_`, `10_`).
+
+---
+
+### User Story 3 - Configure Starting Number and Placement (Priority: P3)
+
+As a production coordinator resuming interrupted work, I want to choose the starting sequence value and whether the number appears as a prefix or suffix so I can continue existing numbering schemes without renaming older assets.
+
+**Why this priority**: Starting offsets and placement control reduce rework when numbering must align with partner systems or previously delivered batches.
+
+**Independent Test**: Run `renamer sequence --start 10 --dry-run` and confirm the preview begins at `010_` and inserts the number before the filename stem with the configured separator.
+
+**Acceptance Scenarios**:
+
+1. **Given** files `shotA.exr` and `shotB.exr`, **When** the user runs `renamer sequence --start 10 --dry-run`, **Then** the preview numbers the files starting at `010_` and `011_`.
+2. **Given** files `cover.png` and `index.png`, **When** the user runs `renamer sequence --placement prefix --separator "-" --dry-run`, **Then** the preview shows names such as `001-cover.png` and `002-index.png` with the separator preserved.
+
+---
+
+### Edge Cases
+
+- Generated filename conflicts with an existing filesystem entry outside the batch: skip the conflicting candidate, continue with the rest, and warn with conflict details.
+- Requested width smaller than digits required is automatically expanded with a warning so numbering completes without truncation.
+- New files encountered between preview and apply are ignored; only the previewed candidates are renamed.
+- Invalid starting number, width, or separator arguments produce a hard error with non-zero exit status; no preview or apply runs until corrected.
+- Directories included via `--include-dirs` are left unchanged; numbering applies exclusively to files.
+
+## Requirements *(mandatory)*
+
+### Functional Requirements
+
+- **FR-001**: The CLI MUST expose a `sequence` command that applies an ordered numbering rule to all candidates within the current scope while preserving the preview-first workflow used by other renamer commands.
+- **FR-002**: The command MUST use a deterministic ordering strategy (default: path-sorted traversal after scope filters) so preview and apply yield identical sequences.
+- **FR-003**: Users MUST be able to configure the sequence starting value via a `--start` flag (default `1`) accepting positive integers only, with validation errors for invalid input.
+- **FR-004**: Users MUST be able to configure the minimum digit width via a `--width` flag (default determined by total candidates) and the tool MUST zero-pad numbers to match the requested width.
+- **FR-005**: Users MUST be able to choose number placement (`prefix` or `suffix`, default prefix) and optionally set a separator string plus static number prefix/suffix tokens while preserving file extensions and directory structure.
+- **FR-006**: Preview output MUST display original and proposed names, total candidates, total changed, and warnings when numbering would exceed the requested width or create conflicts.
+- **FR-007**: Apply MUST record the numbering rule (start, width, placement, separator, ordering) in the `.renamer` ledger, alongside per-file operations, so that undo can faithfully restore original names.
+- **FR-008**: Undo MUST revert sequence-based renames in reverse order even if additional files have been added since the apply step, skipping only those already removed.
+- **FR-009**: The `sequence` command MUST respect existing scope flags (`--path`, `--recursive`, `--include-dirs`, `--hidden`, `--extensions`, `--dry-run`, `--yes`) with identical semantics to other commands.
+- **FR-010**: When numbering would collide with an existing filesystem entry, the CLI MUST skip the conflicting candidate, continue numbering the remaining files, and emit a warning that lists the skipped items; apply MUST still abort if scope filters yield zero candidates.
+- **FR-011**: Invalid formatting arguments (negative start, zero/negative width, unsupported separator) MUST trigger a human-readable error, exit with non-zero status, and prevent preview/apply execution.
+- **FR-012**: Directories included in scope via `--include-dirs` MUST be preserved without numbering; only files receive sequence numbers while directories remain untouched.
+
+### Key Entities *(include if feature involves data)*
+
+- **SequenceRequest**: Captures user-supplied configuration (start value, width, placement, separator, scope flags, execution mode).
+- **SequencePlan**: Represents the ordered list of candidate files with assigned sequence numbers, proposed names, conflicts, and summary counts.
+- **SequenceLedgerEntry**: Stores metadata required for undo, including request parameters, execution timestamp, and file rename mappings.
+
+### Assumptions
+
+- Ordering follows the preview list sorted by relative path unless future features introduce additional ordering controls.
+- If numbering exceeds the requested width, the command extends the width automatically, surfaces a warning, and continues rather than failing the batch.
+- Default placement is prefix with an underscore separator (e.g., `001_name.ext`) unless overridden by flags.
+- Scope and ledger behavior mirror existing rename commands; no new traversal modes are introduced.
+
+## Success Criteria *(mandatory)*
+
+### Measurable Outcomes
+
+- **SC-001**: Users can number 500 files (preview + apply) in under 120 seconds on a representative workstation without manual intervention.
+- **SC-002**: At least 95% of sampled previews match their subsequent apply results exactly during beta testing (no ordering drift or mismatched numbering).
+- **SC-003**: 90% of beta participants report that numbering settings (start value, width, placement) meet their formatting needs without external tools.
+- **SC-004**: Support requests related to manual numbering workflows decrease by 40% within one release cycle after launch.
diff --git a/specs/001-sequence-numbering/tasks.md b/specs/001-sequence-numbering/tasks.md
new file mode 100644
index 0000000..2cc18d9
--- /dev/null
+++ b/specs/001-sequence-numbering/tasks.md
@@ -0,0 +1,115 @@
+# Tasks: Sequence Numbering Command
+
+**Input**: Design documents from `/specs/001-sequence-numbering/`
+**Prerequisites**: `plan.md`, `spec.md`, `research.md`, `data-model.md`, `contracts/`, `quickstart.md`
+
+## Phase 1: Setup (Shared Infrastructure)
+
+**Purpose**: Establish scaffolding required by all user stories.
+
+- [X] T001 Create sequence package documentation stub in `internal/sequence/doc.go`
+- [X] T002 Seed sample fixtures for numbering scenarios in `testdata/sequence/basic/`
+
+---
+
+## Phase 2: Foundational (Blocking Prerequisites)
+
+**Purpose**: Shared components that every sequence story depends on.
+
+- [X] T003 Define sequence options struct with default values in `internal/sequence/options.go`
+- [X] T004 Implement zero-padding formatter helper in `internal/sequence/format.go`
+- [X] T005 Introduce plan and summary data structures in `internal/sequence/plan.go`
+
+**Checkpoint**: Base package compiles with shared types ready for story work.
+
+---
+
+## Phase 3: User Story 1 - Add Sequential Indices to Batch (Priority: P1) 🎯 MVP
+
+**Goal**: Append auto-incremented suffixes (e.g., `_001`) to scoped files with deterministic ordering and ledger persistence.
+
+**Independent Test**: `renamer sequence --dry-run --path <dir>` on three files shows `_001`, `_002`, `_003`; rerun with `--yes` updates ledger.
+
+### Tests for User Story 1
+
+- [X] T006 [P] [US1] Add preview contract test covering default numbering in `tests/contract/sequence_preview_test.go`
+- [X] T007 [P] [US1] Add integration flow test verifying preview/apply parity in `tests/integration/sequence_flow_test.go`
+
+### Implementation for User Story 1
+
+- [X] T008 [US1] Implement candidate traversal adapter using listing scope in `internal/sequence/traversal.go`
+- [X] T009 [US1] Generate preview plan with conflict detection in `internal/sequence/preview.go`
+- [X] T010 [US1] Apply renames and record sequence metadata in `internal/sequence/apply.go`
+- [X] T011 [US1] Wire Cobra sequence command execution in `cmd/sequence.go`
+- [X] T012 [US1] Register sequence command on the root command in `cmd/root.go`
+
+**Checkpoint**: Sequence preview/apply for default suffix behavior is fully testable and undoable.
+
+---
+
+## Phase 4: User Story 2 - Control Number Formatting (Priority: P2)
+
+**Goal**: Allow explicit width flag with zero padding and warning when auto-expanding.
+
+**Independent Test**: `renamer sequence --width 4 --dry-run` shows `_0001` suffixes; omitting width auto-expands on demand.
+
+### Tests for User Story 2
+
+- [X] T013 [P] [US2] Add contract test for explicit width padding in `tests/contract/sequence_width_test.go`
+- [X] T014 [P] [US2] Add integration test validating width flag and warnings in `tests/integration/sequence_width_test.go`
+
+### Implementation for User Story 2
+
+- [X] T015 [US2] Extend options validation to handle width flag rules in `internal/sequence/options.go`
+- [X] T016 [US2] Update preview planner to enforce configured width and warnings in `internal/sequence/preview.go`
+- [X] T017 [US2] Parse and bind `--width` flag within Cobra command in `cmd/sequence.go`
+
+**Checkpoint**: Users can control sequence width with deterministic zero-padding.
+
+---
+
+## Phase 5: User Story 3 - Configure Starting Number and Placement (Priority: P3)
+
+**Goal**: Support custom start offsets plus prefix/suffix placement with configurable separator.
+
+**Independent Test**: `renamer sequence --start 10 --placement prefix --separator "-" --dry-run` produces `0010-file.ext` entries.
+
+### Tests for User Story 3
+
+- [X] T018 [P] [US3] Add contract test for start and placement variants in `tests/contract/sequence_placement_test.go`
+- [X] T019 [P] [US3] Add integration test for start offset with undo coverage in `tests/integration/sequence_start_test.go`
+
+### Implementation for User Story 3
+
+- [X] T020 [US3] Validate start, placement, and separator flags in `internal/sequence/options.go`
+- [X] T021 [US3] Update preview generation to honor prefix/suffix placement and separators in `internal/sequence/preview.go`
+- [X] T022 [US3] Persist placement and separator metadata during apply in `internal/sequence/apply.go`
+- [X] T023 [US3] Wire `--start`, `--placement`, and `--separator` flags in `cmd/sequence.go`
+
+**Checkpoint**: Placement and numbering customization scenarios fully supported with ledger fidelity.
+
+---
+
+## Phase 6: Polish & Cross-Cutting Concerns
+
+- [X] T024 [P] Document sequence command flags in `docs/cli-flags.md`
+- [X] T025 [P] Log sequence feature addition in `docs/CHANGELOG.md`
+- [X] T026 [P] Update command overview with sequence entry in `README.md`
+
+---
+
+## Dependencies
+
+- Setup (Phase 1) → Foundational (Phase 2) → US1 (Phase 3) → US2 (Phase 4) → US3 (Phase 5) → Polish (Phase 6)
+- User Story dependencies: `US1` completion unlocks `US2`; `US2` completion unlocks `US3`.
+
+## Parallel Execution Opportunities
+
+- Contract and integration test authoring tasks (T006, T007, T013, T014, T018, T019) can run concurrently with implementation once shared scaffolding is ready.
+- Documentation polish tasks (T024–T026) can be executed in parallel after all story implementations stabilize.
+
+## Implementation Strategy
+
+1. Deliver MVP by completing Phase 1–3 (US1), enabling default sequence numbering with undo.
+2. Iterate with formatting controls (Phase 4) to broaden usability while maintaining preview/apply parity.
+3. Finish with placement customization (Phase 5) and polish tasks (Phase 6) before release.
diff --git a/testdata/sequence/basic/draft.txt b/testdata/sequence/basic/draft.txt
new file mode 100644
index 0000000..4a58007
--- /dev/null
+++ b/testdata/sequence/basic/draft.txt
@@ -0,0 +1 @@
+alpha
diff --git a/testdata/sequence/basic/notes.txt b/testdata/sequence/basic/notes.txt
new file mode 100644
index 0000000..652d57d
--- /dev/null
+++ b/testdata/sequence/basic/notes.txt
@@ -0,0 +1 @@
+bravo
diff --git a/testdata/sequence/basic/plan.txt b/testdata/sequence/basic/plan.txt
new file mode 100644
index 0000000..7e5ac71
--- /dev/null
+++ b/testdata/sequence/basic/plan.txt
@@ -0,0 +1 @@
+charlie
diff --git a/testdata/sequence/demo/alpha.txt b/testdata/sequence/demo/alpha.txt
new file mode 100644
index 0000000..5651f20
--- /dev/null
+++ b/testdata/sequence/demo/alpha.txt
@@ -0,0 +1 @@
+alpha demo
diff --git a/testdata/sequence/demo/beta.txt b/testdata/sequence/demo/beta.txt
new file mode 100644
index 0000000..5012bbb
--- /dev/null
+++ b/testdata/sequence/demo/beta.txt
@@ -0,0 +1 @@
+beta demo
diff --git a/testdata/sequence/demo/gamma.txt b/testdata/sequence/demo/gamma.txt
new file mode 100644
index 0000000..0e7f04a
--- /dev/null
+++ b/testdata/sequence/demo/gamma.txt
@@ -0,0 +1 @@
+gamma demo
diff --git a/tests/contract/sequence_placement_test.go b/tests/contract/sequence_placement_test.go
new file mode 100644
index 0000000..a69fdb0
--- /dev/null
+++ b/tests/contract/sequence_placement_test.go
@@ -0,0 +1,76 @@
+package contract
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequencePreviewWithPrefixPlacement(t *testing.T) {
+	tmp := t.TempDir()
+
+	createFile(t, filepath.Join(tmp, "cover.png"))
+	createFile(t, filepath.Join(tmp, "index.png"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+	opts.Start = 10
+	opts.Placement = sequence.PlacementPrefix
+	opts.Separator = "-"
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	expected := []string{"010-cover.png", "011-index.png"}
+	if len(plan.Candidates) != len(expected) {
+		t.Fatalf("expected %d candidates, got %d", len(expected), len(plan.Candidates))
+	}
+	for i, candidate := range plan.Candidates {
+		if candidate.ProposedPath != expected[i] {
+			t.Fatalf("candidate %d proposed %s, expected %s", i, candidate.ProposedPath, expected[i])
+		}
+	}
+
+	if plan.Config.Start != 10 {
+		t.Fatalf("expected config start 10, got %d", plan.Config.Start)
+	}
+	if plan.Config.Placement != sequence.PlacementPrefix {
+		t.Fatalf("expected prefix placement in config")
+	}
+}
+
+func TestSequencePreviewWithNumberPrefixLabel(t *testing.T) {
+	tmp := t.TempDir()
+
+	createFile(t, filepath.Join(tmp, "cover.png"))
+	createFile(t, filepath.Join(tmp, "index.png"))
+
+	ops := sequence.DefaultOptions()
+	ops.WorkingDir = tmp
+	ops.Placement = sequence.PlacementPrefix
+	ops.Separator = "-"
+	ops.NumberPrefix = "seq"
+
+	plan, err := sequence.Preview(context.Background(), ops, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	expected := []string{"seq001-cover.png", "seq002-index.png"}
+	if len(plan.Candidates) != len(expected) {
+		t.Fatalf("expected %d candidates, got %d", len(expected), len(plan.Candidates))
+	}
+	for i, candidate := range plan.Candidates {
+		if candidate.ProposedPath != expected[i] {
+			t.Fatalf("candidate %d proposed %s, expected %s", i, candidate.ProposedPath, expected[i])
+		}
+	}
+
+	if warnings := plan.Summary.Warnings; len(warnings) != 0 {
+		t.Fatalf("expected no warnings, got %#v", warnings)
+	}
+}
diff --git a/tests/contract/sequence_preview_test.go b/tests/contract/sequence_preview_test.go
new file mode 100644
index 0000000..bae18d0
--- /dev/null
+++ b/tests/contract/sequence_preview_test.go
@@ -0,0 +1,53 @@
+package contract
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequencePreviewDefaultNumbering(t *testing.T) {
+	tmp := t.TempDir()
+
+	createFile(t, filepath.Join(tmp, "draft.txt"))
+	createFile(t, filepath.Join(tmp, "notes.txt"))
+	createFile(t, filepath.Join(tmp, "plan.txt"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	if plan.Summary.TotalCandidates != 3 {
+		t.Fatalf("expected 3 candidates, got %d", plan.Summary.TotalCandidates)
+	}
+	if plan.Summary.RenamedCount != 3 {
+		t.Fatalf("expected 3 renamed entries, got %d", plan.Summary.RenamedCount)
+	}
+
+	expected := []string{"001_draft.txt", "002_notes.txt", "003_plan.txt"}
+	if len(plan.Candidates) != 3 {
+		t.Fatalf("expected 3 planned candidates, got %d", len(plan.Candidates))
+	}
+	for i, candidate := range plan.Candidates {
+		if candidate.ProposedPath != expected[i] {
+			t.Fatalf("candidate %d proposed %s, expected %s", i, candidate.ProposedPath, expected[i])
+		}
+	}
+
+	if plan.Summary.AppliedWidth != 3 {
+		t.Fatalf("expected applied width 3, got %d", plan.Summary.AppliedWidth)
+	}
+
+	if plan.Config.Start != 1 {
+		t.Fatalf("expected start 1, got %d", plan.Config.Start)
+	}
+	if plan.Config.Placement != sequence.PlacementPrefix {
+		t.Fatalf("expected prefix placement, got %s", plan.Config.Placement)
+	}
+}
diff --git a/tests/contract/sequence_width_test.go b/tests/contract/sequence_width_test.go
new file mode 100644
index 0000000..42e639d
--- /dev/null
+++ b/tests/contract/sequence_width_test.go
@@ -0,0 +1,39 @@
+package contract
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequencePreviewWithExplicitWidth(t *testing.T) {
+	tmp := t.TempDir()
+
+	createFile(t, filepath.Join(tmp, "cutA.mov"))
+	createFile(t, filepath.Join(tmp, "cutB.mov"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+	opts.Width = 4
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	expected := []string{"0001_cutA.mov", "0002_cutB.mov"}
+	if len(plan.Candidates) != len(expected) {
+		t.Fatalf("expected %d candidates, got %d", len(expected), len(plan.Candidates))
+	}
+	for i, candidate := range plan.Candidates {
+		if candidate.ProposedPath != expected[i] {
+			t.Fatalf("candidate %d proposed %s, expected %s", i, candidate.ProposedPath, expected[i])
+		}
+	}
+
+	if plan.Summary.AppliedWidth != 4 {
+		t.Fatalf("expected applied width 4, got %d", plan.Summary.AppliedWidth)
+	}
+}
diff --git a/tests/integration/sequence_flow_test.go b/tests/integration/sequence_flow_test.go
new file mode 100644
index 0000000..c4c7f67
--- /dev/null
+++ b/tests/integration/sequence_flow_test.go
@@ -0,0 +1,70 @@
+package integration
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/history"
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequenceApplyAndUndo(t *testing.T) {
+	tmp := t.TempDir()
+
+	createIntegrationFile(t, filepath.Join(tmp, "draft.txt"))
+	createIntegrationFile(t, filepath.Join(tmp, "notes.txt"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	if plan.Summary.RenamedCount != 2 {
+		t.Fatalf("expected 2 planned renames, got %d", plan.Summary.RenamedCount)
+	}
+
+	entry, err := sequence.Apply(context.Background(), opts, plan)
+	if err != nil {
+		t.Fatalf("apply error: %v", err)
+	}
+
+	if len(entry.Operations) != 2 {
+		t.Fatalf("expected 2 recorded operations, got %d", len(entry.Operations))
+	}
+	if entry.Command != "sequence" {
+		t.Fatalf("expected ledger command 'sequence', got %s", entry.Command)
+	}
+
+	if _, err := os.Stat(filepath.Join(tmp, "001_draft.txt")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmp, "002_notes.txt")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+
+	if _, err := history.Undo(tmp); err != nil {
+		t.Fatalf("undo error: %v", err)
+	}
+
+	if _, err := os.Stat(filepath.Join(tmp, "draft.txt")); err != nil {
+		t.Fatalf("expected original file after undo: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmp, "notes.txt")); err != nil {
+		t.Fatalf("expected original file after undo: %v", err)
+	}
+}
+
+func createIntegrationFile(t *testing.T, path string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatalf("mkdir %s: %v", path, err)
+	}
+	if err := os.WriteFile(path, []byte("data"), 0o644); err != nil {
+		t.Fatalf("write file %s: %v", path, err)
+	}
+}
diff --git a/tests/integration/sequence_prefix_test.go b/tests/integration/sequence_prefix_test.go
new file mode 100644
index 0000000..2cdcd10
--- /dev/null
+++ b/tests/integration/sequence_prefix_test.go
@@ -0,0 +1,53 @@
+package integration
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/history"
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequenceApplyWithNumberPrefix(t *testing.T) {
+	tmp := t.TempDir()
+
+	createIntegrationFile(t, filepath.Join(tmp, "cover.png"))
+	createIntegrationFile(t, filepath.Join(tmp, "index.png"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+	opts.Placement = sequence.PlacementPrefix
+	opts.Separator = "-"
+	opts.NumberPrefix = "seq"
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	entry, err := sequence.Apply(context.Background(), opts, plan)
+	if err != nil {
+		t.Fatalf("apply error: %v", err)
+	}
+
+	if _, err := os.Stat(filepath.Join(tmp, "seq001-cover.png")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmp, "seq002-index.png")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+
+	meta, ok := entry.Metadata["sequence"].(map[string]any)
+	if !ok {
+		t.Fatalf("sequence metadata missing")
+	}
+	if prefix, ok := meta["prefix"].(string); !ok || prefix != "seq" {
+		t.Fatalf("expected metadata prefix 'seq', got %#v", meta["prefix"])
+	}
+
+	if _, err := history.Undo(tmp); err != nil {
+		t.Fatalf("undo error: %v", err)
+	}
+}
diff --git a/tests/integration/sequence_start_test.go b/tests/integration/sequence_start_test.go
new file mode 100644
index 0000000..f81f9fb
--- /dev/null
+++ b/tests/integration/sequence_start_test.go
@@ -0,0 +1,65 @@
+package integration
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/history"
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequenceApplyWithStartOffset(t *testing.T) {
+	tmp := t.TempDir()
+
+	createIntegrationFile(t, filepath.Join(tmp, "shotA.exr"))
+	createIntegrationFile(t, filepath.Join(tmp, "shotB.exr"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+	opts.Start = 10
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	expected := []string{"010_shotA.exr", "011_shotB.exr"}
+	if len(plan.Candidates) != len(expected) {
+		t.Fatalf("expected %d candidates, got %d", len(expected), len(plan.Candidates))
+	}
+	for i, candidate := range plan.Candidates {
+		if candidate.ProposedPath != expected[i] {
+			t.Fatalf("candidate %d proposed %s, expected %s", i, candidate.ProposedPath, expected[i])
+		}
+	}
+
+	entry, err := sequence.Apply(context.Background(), opts, plan)
+	if err != nil {
+		t.Fatalf("apply error: %v", err)
+	}
+
+	if len(entry.Operations) != 2 {
+		t.Fatalf("expected 2 operations, got %d", len(entry.Operations))
+	}
+
+	if _, err := os.Stat(filepath.Join(tmp, "010_shotA.exr")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmp, "011_shotB.exr")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+
+	meta, ok := entry.Metadata["sequence"].(map[string]any)
+	if !ok {
+		t.Fatalf("sequence metadata missing")
+	}
+	if start, ok := meta["start"].(int); !ok || start != 10 {
+		t.Fatalf("expected metadata start 10, got %#v", meta["start"])
+	}
+
+	if _, err := history.Undo(tmp); err != nil {
+		t.Fatalf("undo error: %v", err)
+	}
+}
diff --git a/tests/integration/sequence_width_test.go b/tests/integration/sequence_width_test.go
new file mode 100644
index 0000000..e3e121a
--- /dev/null
+++ b/tests/integration/sequence_width_test.go
@@ -0,0 +1,59 @@
+package integration
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/history"
+	"github.com/rogeecn/renamer/internal/sequence"
+)
+
+func TestSequenceApplyWithExplicitWidth(t *testing.T) {
+	tmp := t.TempDir()
+
+	createIntegrationFile(t, filepath.Join(tmp, "cutA.mov"))
+	createIntegrationFile(t, filepath.Join(tmp, "cutB.mov"))
+
+	opts := sequence.DefaultOptions()
+	opts.WorkingDir = tmp
+	opts.Width = 4
+
+	plan, err := sequence.Preview(context.Background(), opts, nil)
+	if err != nil {
+		t.Fatalf("preview error: %v", err)
+	}
+
+	if plan.Summary.AppliedWidth != 4 {
+		t.Fatalf("expected applied width 4, got %d", plan.Summary.AppliedWidth)
+	}
+
+	entry, err := sequence.Apply(context.Background(), opts, plan)
+	if err != nil {
+		t.Fatalf("apply error: %v", err)
+	}
+
+	if len(entry.Operations) != 2 {
+		t.Fatalf("expected 2 operations, got %d", len(entry.Operations))
+	}
+
+	if _, err := os.Stat(filepath.Join(tmp, "0001_cutA.mov")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmp, "0002_cutB.mov")); err != nil {
+		t.Fatalf("expected renamed file exists: %v", err)
+	}
+
+	meta, ok := entry.Metadata["sequence"].(map[string]any)
+	if !ok {
+		t.Fatalf("sequence metadata missing")
+	}
+	if width, ok := meta["width"].(int); !ok || width != 4 {
+		t.Fatalf("expected metadata width 4, got %#v", meta["width"])
+	}
+
+	if _, err := history.Undo(tmp); err != nil {
+		t.Fatalf("undo error: %v", err)
+	}
+}