feat: skip already-indexed revisions, add --force flag

When indexing a revision that already exists, the indexer now returns
early with information about the existing revision instead of re-indexing.
Use the --force flag to re-index an existing revision.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

TODO.md

@@ -1,9 +1,5 @@
 # TODO - Future Improvements
 
-## Quick Wins
-
-- [ ] Check if revision exists before indexing (skip or require `--force`)
-
 ## Usability
 
 - [ ] Progress reporting during indexing ("Fetching nixpkgs... Parsing options... Indexing files...")

cmd/nixos-options/main.go

@@ -50,12 +50,17 @@ func main() {
 						Name:  "no-files",
 						Usage: "Skip indexing file contents (faster, disables get_file tool)",
 					},
+					&cli.BoolFlag{
+						Name:    "force",
+						Aliases: []string{"f"},
+						Usage:   "Force re-indexing even if revision already exists",
+					},
 				},
 				Action: func(c *cli.Context) error {
 					if c.NArg() < 1 {
 						return fmt.Errorf("revision argument required")
 					}
-					return runIndex(c, c.Args().First(), !c.Bool("no-files"))
+					return runIndex(c, c.Args().First(), !c.Bool("no-files"), c.Bool("force"))
 				},
 			},
 			{
@@ -162,7 +167,7 @@ func runServe(c *cli.Context) error {
 	return server.Run(ctx, os.Stdin, os.Stdout)
 }
 
-func runIndex(c *cli.Context, revision string, indexFiles bool) error {
+func runIndex(c *cli.Context, revision string, indexFiles bool, force bool) error {
 	ctx := context.Background()
 
 	store, err := openStore(c.String("database"))
@@ -178,11 +183,22 @@ func runIndex(c *cli.Context, revision string, indexFiles bool) error {
 	indexer := nixos.NewIndexer(store)
 
 	fmt.Printf("Indexing revision: %s\n", revision)
-	result, err := indexer.IndexRevision(ctx, revision)
+
+	var result *nixos.IndexResult
+	if force {
+		result, err = indexer.ReindexRevision(ctx, revision)
+	} else {
+		result, err = indexer.IndexRevision(ctx, revision)
+	}
 	if err != nil {
 		return fmt.Errorf("indexing failed: %w", err)
 	}
 
+	if result.AlreadyIndexed {
+		fmt.Printf("Revision already indexed (%d options). Use --force to re-index.\n", result.OptionCount)
+		return nil
+	}
+
 	fmt.Printf("Indexed %d options in %s\n", result.OptionCount, result.Duration)
 	fmt.Printf("Git hash: %s\n", result.Revision.GitHash)
 	if result.Revision.ChannelName != "" {

internal/mcp/handlers.go

@@ -217,6 +217,20 @@ func (s *Server) makeIndexHandler(indexer *nixos.Indexer) ToolHandler {
 			return ErrorContent(fmt.Errorf("indexing failed: %w", err)), nil
 		}
 
+		// If already indexed, return early with info
+		if result.AlreadyIndexed {
+			var sb strings.Builder
+			sb.WriteString(fmt.Sprintf("Revision already indexed: %s\n", result.Revision.GitHash))
+			if result.Revision.ChannelName != "" {
+				sb.WriteString(fmt.Sprintf("Channel: %s\n", result.Revision.ChannelName))
+			}
+			sb.WriteString(fmt.Sprintf("Options: %d\n", result.OptionCount))
+			sb.WriteString(fmt.Sprintf("Indexed at: %s\n", result.Revision.IndexedAt.Format("2006-01-02 15:04")))
+			return CallToolResult{
+				Content: []Content{TextContent(sb.String())},
+			}, nil
+		}
+
 		// Index files by default
 		fileCount, err := indexer.IndexFiles(ctx, result.Revision.ID, result.Revision.GitHash)
 		if err != nil {

internal/nixos/indexer.go

@@ -39,6 +39,7 @@ type IndexResult struct {
 	OptionCount    int
 	FileCount      int
 	Duration       time.Duration
+	AlreadyIndexed bool // True if revision was already indexed (skipped)
 }
 
 // IndexRevision indexes a nixpkgs revision by git hash or channel name.
@@ -58,6 +59,7 @@ func (idx *Indexer) IndexRevision(ctx context.Context, revision string) (*IndexR
 			Revision:       existing,
 			OptionCount:    existing.OptionCount,
 			Duration:       time.Since(start),
+			AlreadyIndexed: true,
 		}, nil
 	}
 
@@ -112,6 +114,25 @@ func (idx *Indexer) IndexRevision(ctx context.Context, revision string) (*IndexR
 	}, nil
 }
 
+// ReindexRevision forces re-indexing of a revision, deleting existing data first.
+func (idx *Indexer) ReindexRevision(ctx context.Context, revision string) (*IndexResult, error) {
+	ref := resolveRevision(revision)
+
+	// Delete existing revision if present
+	existing, err := idx.store.GetRevision(ctx, ref)
+	if err != nil {
+		return nil, fmt.Errorf("failed to check existing revision: %w", err)
+	}
+	if existing != nil {
+		if err := idx.store.DeleteRevision(ctx, existing.ID); err != nil {
+			return nil, fmt.Errorf("failed to delete existing revision: %w", err)
+		}
+	}
+
+	// Now index fresh
+	return idx.IndexRevision(ctx, revision)
+}
+
 // buildOptions builds options.json for a nixpkgs revision.
 func (idx *Indexer) buildOptions(ctx context.Context, ref string) (string, func(), error) {
 	// Create temp directory