diff --git a/src/repl.zig b/src/repl.zig
new file mode 100644
index 0000000..8f33308
--- /dev/null
+++ b/src/repl.zig
@@ -0,0 +1,161 @@
+const std = @import("std");
+const dbmod = @import("db.zig");
+const txnmod = @import("txn.zig");
+const rowmod = @import("row.zig");
+const idx = @import("index.zig");
+const querymod = @import("query.zig");
+const txntablemod = @import("txn_table.zig");
+const collectionsmod = @import("collections.zig");
+const collttlmod = @import("collttl.zig");
+const ttlmod = @import("ttl.zig");
+const exprmod = @import("expr.zig");
+const joinmod = @import("join.zig");
+const aggregatemod = @import("aggregate.zig");
+const windowmod = @import("window.zig");
+
+const Allocator = std.mem.Allocator;
+const Db = dbmod.Db;
+const Transaction = txnmod.Transaction;
+const Table = querymod.Table;
+const TableTxn = txntablemod.TableTxn;
+const Value = rowmod.Value;
+const Field = rowmod.Field;
+const Row = rowmod.Row;
+const CollKind = collectionsmod.Kind;
+
+/// Errors `Repl.dispatch` can raise on top of whatever the underlying `Db`/
+/// `Transaction`/`Table` call it wraps returns (those propagate as-is, e.g.
+/// `error.Conflict` from a failed commit).
+pub const ReplError = error{
+    UnknownCommand,
+    WrongArgCount,
+    NoActiveTransaction,
+    TransactionAlreadyActive,
+    InvalidField,
+    UnknownCollectionKind,
+    UnknownCollectionOp,
+    InvalidArgument,
+    NoSuchKey,
+};
+
+/// Interactive session bound to one already-open `*Db`. The one-shot CLI in
+/// main.zig opens a fresh `Db` per process per command, and its `txn
+/// <script-file>` command runs a whole transaction non-interactively from a
+/// static file. `Repl` instead keeps the store open across many commands and
+/// lets `begin`/`commit`/`rollback` bracket an interactive `Transaction`:
+/// a `get` between `begin` and `commit` can steer what the *next* `put`
+/// does (e.g. "read balance, then decide how much to add"), which a script
+/// file evaluated top to bottom cannot express.
+///
+/// `table put/get/del` follow the active transaction automatically: with no
+/// transaction open they go straight through `query.Table` (each call is
+/// independently durable the moment it returns, exactly like `table put` on
+/// the plain CLI); with a transaction open they route through a `TableTxn`
+/// bound to it instead, so multiple `table put` calls across multiple
+/// tables in the same `begin`/`commit` bracket commit atomically as one
+/// `WriteBatch`, or none of them do on `error.Conflict` - the interactive
+/// equivalent of `txn <script-file>`.
+///
+/// `scan` always reads the live, already-committed `Db` state regardless of
+/// any open transaction: `Transaction` has no scan/range primitive (only
+/// `get`, which resolves the transaction's own staged writes first), so a
+/// staged-but-uncommitted put is invisible to `scan` until `commit` lands it
+/// for real. This is a deliberate, documented limitation, not an oversight.
+/// `table select`/`table join` are the same story one level up: `TableTxn`
+/// only ever brackets single-row put/get/delete, so both always run against
+/// `query.Table.open(self.db, ...)` directly, never through the active
+/// transaction, exactly like `scan`.
+pub const Repl = struct {
+    db: *Db,
+    allocator: Allocator,
+    txn: ?Transaction = null,
+    tableTxns: std.StringHashMap(TableTxn),
+
+    pub fn init(allocator: Allocator, db: *Db) Repl {
+        return .{
+            .db = db,
+            .allocator = allocator,
+            .tableTxns = std.StringHashMap(TableTxn).init(allocator),
+        };
+    }
+
+    pub fn deinit(self: *Repl) void {
+        self.discardTxnState();
+        self.tableTxns.deinit();
+    }
+
+    pub fn inTransaction(self: *const Repl) bool {
+        return self.txn != null;
+    }
+
+    fn discardTxnState(self: *Repl) void {
+        var it = self.tableTxns.iterator();
+        while (it.next()) |entry| {
+            self.allocator.free(entry.key_ptr.*);
+            entry.value_ptr.deinit();
+        }
+        self.tableTxns.clearRetainingCapacity();
+        if (self.txn) |*t| {
+            t.deinit();
+            self.txn = null;
+        }
+    }
+
+    /// Runs one line of input, writing command output or `error: <name>` to
+    /// `writer`. Returns `false` on `exit`/`quit` (caller should stop
+    /// reading), `true` otherwise - including after a reported error, so one
+    /// bad line never kills the session.
+    pub fn runLine(self: *Repl, line: []const u8, writer: anytype) !bool {
+        const trimmed = std.mem.trim(u8, line, " \t\r\n");
+        if (trimmed.len == 0 or trimmed[0] == '#') return true;
+
+        var tokens = std.ArrayList([]const u8).init(self.allocator);
+        defer tokens.deinit();
+        var it = std.mem.tokenizeAny(u8, trimmed, " \t");
+        while (it.next()) |tok| try tokens.append(tok);
+        if (tokens.items.len == 0) return true;
+
+        const cmd = tokens.items[0];
+        if (std.mem.eql(u8, cmd, "exit") or std.mem.eql(u8, cmd, "quit")) return false;
+
+        self.dispatch(cmd, tokens.items[1..], writer) catch |err| {
+            try writer.print("error: {s}\n", .{@errorName(err)});
+        };
+        return true;
+    }
+
+    /// Reads lines from `reader` until EOF, running each through `runLine`.
+    /// Writes a `cobalt>`/`cobalt(txn)>` prompt before each read when
+    /// `prompt` is set (interactive stdin use); script files pass `false`.
+    pub fn runFromReader(self: *Repl, reader: anytype, writer: anytype, prompt: bool) !void {
+        var buf: [8192]u8 = undefined;
+        while (true) {
+            if (prompt) {
+                try writer.writeAll(if (self.inTransaction()) "cobalt(txn)> " else "cobalt> ");
+            }
+            const line = (try reader.readUntilDelimiterOrEof(&buf, '\n')) orelse break;
+            if (!try self.runLine(line, writer)) break;
+        }
+    }
+
+    fn dispatch(self: *Repl, cmd: []const u8, args: []const []const u8, writer: anytype) !void {
+        if (std.mem.eql(u8, cmd, "begin")) return self.cmdBegin(args, writer);
+        if (std.mem.eql(u8, cmd, "commit")) return self.cmdCommit(args, writer);
+        if (std.mem.eql(u8, cmd, "rollback")) return self.cmdRollback(args, writer);
+        if (std.mem.eql(u8, cmd, "put")) return self.cmdPut(args, writer);
+        if (std.mem.eql(u8, cmd, "get")) return self.cmdGet(args, writer);
+        if (std.mem.eql(u8, cmd, "del")) return self.cmdDel(args, writer);
+        if (std.mem.eql(u8, cmd, "scan")) return self.cmdScan(args, writer);
+        if (std.mem.eql(u8, cmd, "stats")) return self.cmdStats(args, writer);
+        if (std.mem.eql(u8, cmd, "flush")) return self.cmdFlush(args, writer);
+        if (std.mem.eql(u8, cmd, "compact")) return self.cmdCompact(args, writer);
+        if (std.mem.eql(u8, cmd, "table")) return self.cmdTable(args, writer);
+        if (std.mem.eql(u8, cmd, "coll")) return self.cmdColl(args, writer);
+        if (std.mem.eql(u8, cmd, "expire")) return self.cmdExpire(args, writer);
+        if (std.mem.eql(u8, cmd, "persist")) return self.cmdPersist(args, writer);
+        if (std.mem.eql(u8, cmd, "ttl")) return self.cmdTtl(args, writer);
+        if (std.mem.eql(u8, cmd, "expire-sweep")) return self.cmdExpireSweep(args, writer);
+        if (std.mem.eql(u8, cmd, "help")) return self.cmdHelp(writer);
+        return ReplError.UnknownCommand;
+    }
+