Open source · GPL v3

A modern TN5250 terminal emulator for IBM i and AS/400.

Full RFC 1205 protocol, 13 EBCDIC code pages, a dedicated scripting language, and an embedded Model Context Protocol server — built with Qt 6 and C++17, native on Linux, macOS, and Windows.

Download latest release → Source on GitHub

TN5250 / RFC 1205
IBM i · AS/400
MCP server
Qt 6 · C++17
Linux · macOS · Windows
GPL v3

Binaries for Linux, macOS, and Windows are attached to each GitHub release. Or build from source — see install.

5250ng displaying an IBM i sign-on screen

At a glance

Three things that make 5250ng different.

Most TN5250 emulators solve the protocol. 5250ng also solves what comes next: automation that outlives keystroke recordings, and access for the AI agents that are starting to replace screen-scraping pipelines.

Protocol-complete

Full TN5250 per RFC 1205, 24×80 and 27×132 screen modes, TLS/SSL, and 13 EBCDIC code pages for global operations.

Scriptable

A purpose-built DSL with variables, control flow, functions, error handlers, and EXPECT/EXTRACT — not a macro recorder.

Agent-ready

An embedded Model Context Protocol server exposes 24 tools over JSON-RPC 2.0. Any MCP-capable client can drive a session.

Terminal emulation

Full TN5250, the way IBM specifies it.

Connect to any AS/400 or IBM i system with the complete 5250 command and display-order set, Telnet option negotiation, and TLS/SSL on the standard port 992 or any configured port.

RFC 1205 with Telnet option negotiation
24×80 (standard) and 27×132 (extended) screen modes
Keyboard state machine per IBM SA21-9247-6
PF1–PF24, Field Exit, Erase EOF, SysReq, Attn
Field attributes: protected, MDT, highlight, blink, reverse, underline
Multi-tab sessions with JSON profiles, auto-connect, and per-session settings

Code pages

Code page	Region
`CP037`	US / Canada (default)
`CP273`	Germany / Austria
`CP277`	Denmark / Norway
`CP278`	Finland / Sweden
`CP280`	Italy
`CP284`	Spain / Latin America
`CP285`	United Kingdom
`CP297`	France
`CP500`	International
`CP870`	Central Europe (Latin-2)
`CP420`	Arabic
`CP424`	Hebrew
`CP838`	Thai

5250Script

A small DSL for screen automation.

5250Script is a line-based scripting language inspired by Unix expect. It treats screens, fields, and the keyboard state as first-class objects, with its own lexer, parser, and AST executor.

TYPE, PRESS, MOVE CURSOR, EXPECT, EXTRACT
IF/WHILE/REPEAT, LABEL/GOTO
DEF/CALL/RETURN with parameters
ON TIMEOUT GOTO, ON ERROR GOTO, ABORT
Variable interpolation and session-scoped variables
Interactive INPUT prompts mid-script

Full reference → docs on GitHub

// login.5250script

# Login to AS/400 and capture the main menu title
EXPECT TEXT "Sign On" AT 1 23
MOVE CURSOR AT INPUTFIELD 1
TYPE "$SESSION_USERNAME"
MOVE CURSOR AT INPUTFIELD 2
TYPE "$SESSION_PASSWORD"
PRESS ENTER

EXPECT TEXT "MAIN MENU"
EXTRACT ROW 1 INTO $HEADER
LOG "Connected: $HEADER"

Macro recordings can be converted to editable 5250Script with one click — the bridge from "I did it once" to "it's in git."

AI & MCP integration

Expose a session to any MCP-capable agent.

5250ng embeds a Model Context Protocol server over HTTP + JSON-RPC 2.0. An MCP client can create sessions, read screens, send keys, execute scripts, and capture screenshots — all as first-class tools with validated schemas. A side-panel AI assistant (Anthropic or OpenAI, API key or OAuth) uses the same surface to drive sessions from natural language.

AI client

Claude Desktop · IDE agents · custom

MCP server

JSON-RPC 2.0 over HTTP · port 9250

5250ng

Session manager · screen buffer

IBM i / AS/400

TN5250 · RFC 1205 · TLS

Session lifecycle

Tool	Description
`create_session`	Open a new TN5250 session to a host
`close_session`	Disconnect and dispose a session
`list_sessions`	Enumerate active sessions
`screenshot`	Capture the current screen as PNG

Screen inspection

Tool	Description
`read_screen`	Return the full 24×80 (or 27×132) buffer
`read_line`	Return a single row
`read_region`	Return a rectangular region
`find_text`	Locate a substring on the screen
`wait_for_text`	Block until text appears (with timeout)
`get_cursor_position`	Return current row/column
`get_screen_size`	Return current screen dimensions
`get_field_at`	Return the field under a given cell

Actions

Tool	Description
`send_keys`	Send a sequence of keystrokes
`press_key`	Send a single AID or navigation key
`press_keys`	Send multiple AID/navigation keys
`type_text`	Type a string with EBCDIC encoding
`set_cursor_position`	Move the cursor to an absolute cell
`move_cursor`	Move the cursor relatively or by field
`clear_inputs`	Clear all unprotected fields
`login`	Convenience login helper
`run_5250script`	Execute a 5250Script against this session

Filesystem

Tool	Description
`read_file`	Read a local file (user-approval gated)
`write_file`	Write a local file (user-approval gated)
`list_files`	List a local directory

Two providers

Anthropic (Claude) and OpenAI. Choose model and system prompt per provider.

Two auth modes

API key or OAuth 2.0 (browser-based). Tokens stored locally.

User-approval gate

Agent-initiated file writes surface as pending, user-approvable operations.

The MCP server listens on TCP port 9250 by default and can be enabled with the --enable-mcp-server flag. Each MCP session is isolated by session ID with a per-session busy guard.

Appearance

13 built-in themes, fully customizable.

From authentic amber phosphor to modern Nord and Dracula. Every theme exposes font, 16-color palette, cursor, CRT scanline intensity, phosphor bloom, and background image layout. Themes are plain JSON — share them, fork them, commit them.