ralph

Open source by Santander AI Lab. A dependency-free Bash / PowerShell developer tool that runs an AI coding CLI / LLM agent in a loop, starting a fresh agent session on every iteration for long unattended runs.

Part of Santander AI Open Source — open source AI projects from Banco Santander (santander.com).

ralph runs an AI coding CLI in a loop, starting a fresh session on every iteration and feeding it the same prompt. It is a thin, dependency-free Bash wrapper (ralph-loop.sh) around the CLIs you already have installed — Codex, Claude Code, Gemini CLI, and Devin CLI — so you can drive long, unattended "keep working on this until it's done" runs.

The name comes from the "Ralph Wiggum" technique: run the same prompt against a clean agent over and over, letting the work accumulate in the repository between runs.

How it works

Each iteration the script:

1. Checks for a stop signal (stop.md) and exits cleanly if found.
2. Reloads configuration (see Live reload).
3. Launches the selected CLI as a brand-new session with your prompt piped to it, running in the directory where you invoked the script.
4. Writes a timestamped log to .ralph/logs/ and rotates old logs.

Because every iteration is a fresh session, all continuity has to live in the workspace itself: the files the agent edits, a plan, notes, etc. The prompt should tell the agent to read that state and make incremental progress.

Requirements

• Bash.
• At least one of the supported CLIs on your PATH: codex, claude, gemini, or devin.
• Optionally just for the install recipe.

Installation

The quickest way — a dependency-free installer (needs only curl and tar) that copies ralph-loop.sh to ~/.local/bin and installs the bundled skills. Re-run it any time to update:

curl -fsSL https://raw.githubusercontent.com/SantanderAI/ralph/main/install.sh | sh

It accepts --no-skills, --ref <branch|tag>, --repo <owner/name> and --install-dir <dir> (also available as the RALPH_SKIP_SKILLS, RALPH_REF, RALPH_REPO and RALPH_INSTALL_DIR environment variables), e.g.:

curl -fsSL https://raw.githubusercontent.com/SantanderAI/ralph/main/install.sh | sh -s -- --no-skills

On Windows / PowerShell (pwsh 6+), the equivalent installs ralph-loop.ps1:

powershell -c "irm https://raw.githubusercontent.com/SantanderAI/ralph/main/install.ps1 | iex"

The PowerShell installer is configured through the same RALPH_SKIP_SKILLS, RALPH_REF, RALPH_REPO and RALPH_INSTALL_DIR environment variables.

Alternatively, from a clone with just:

just install

Or just copy ralph-loop.sh somewhere on your PATH and make it executable.

Usage

ralph-loop.sh MAX_ITERATIONS PROMPT_FILE

• MAX_ITERATIONS — positive integer; the maximum number of loops to run.
• PROMPT_FILE — a file whose contents are sent to the CLI as the prompt.

Example — run Claude with a high-capability model up to 25 times. Configuration lives in .ralph/.env (created with defaults on first run), so edit it and then launch:

# .ralph/.env
RALPH_TOOL=claude
RALPH_MODEL_CAPABILITY=high

ralph-loop.sh 25 prompt.md

The directory you invoke the script from is treated as the workspace. Run ralph-loop.sh with the wrong number of arguments to print full inline help.

Configuration

Behavior is controlled entirely through .ralph/.env in your workspace, created with these defaults the first time you run the loop. It is the only configuration source — the shell environment is ignored. Quote values that contain spaces. The most common keys:

Each tool also has overrides for its command, flags, and the model used for each capability tier, e.g. RALPH_CLAUDE_COMMAND, RALPH_CLAUDE_FLAGS, RALPH_CLAUDE_MODEL_HIGH, and the equivalents for CODEX, GEMINI, and DEVIN. The capability tier (low/med/high) is translated into the right per-tool model name and reasoning effort / thinking budget automatically.

Devin is special in two ways: it has no reasoning/thinking knob (so RALPH_THINKING is ignored for it and the tier only selects the model), and it reads the prompt from --prompt-file rather than stdin. Its RALPH_DEVIN_MODEL_* defaults point at Devin's Claude models (claude-haiku-4.5, claude-sonnet-4.6, claude-opus-4.8); run devin --model x to print the full list of valid identifiers. On the Devin Free tier, override all three tiers with swe-1.6-slow, the only model that plan can access.

See the inline help in ralph-loop.sh for the complete list.

Live reload of configuration

.ralph/.env is sourced before every iteration, so the loop can be reconfigured while it is running — no restart needed. Edit the file mid-run and the change takes effect on the next iteration.

# .ralph/.env
RALPH_TOOL=claude
RALPH_MODEL_CAPABILITY=high
RALPH_THINKING=true

Notes:

• Plain KEY=value lines work; quote values that contain spaces.
• An invalid value is reported and the previous good configuration is kept, so a typo will not abort the loop.
• The file is sourced as plain shell variables (not exported), so the shell environment is never a configuration channel. RALPH_LOCAL_DIR (the script-managed .ralph directory) is the only exported variable and cannot be moved by the file.

Stopping the loop

Create a stop.md file to stop before the next iteration:

• stop.md in the invocation directory, or
• any stop.md anywhere inside the plan/ subtree.

The script checks for it before each iteration and exits cleanly without deleting the file. If a matching stop.md already exists at startup, the loop exits immediately. This lets the agent itself signal "I'm done" by creating the file as part of its work.

Auto-switching agent on token exhaustion

When an iteration exits non-zero, the loop can detect that the current agent ran out of tokens (usage/quota/credits or rate limit) and switch to a different agent automatically. This is on by default (RALPH_SWITCH_ON_EXHAUSTION=true).

Agent rotation

Switching follows a fixed cycle, where each agent's successor is both the detector consulted on failure and the agent switched to:

codex → claude → gemini → devin → codex

The successor is always a different agent from the one that failed. Because each exhaustion advances one step, repeated token exhaustion walks the full cycle (e.g. codex exhausted → claude; if claude later exhausts → gemini, and so on, wrapping back to codex).

How it works

1. After a non-zero iteration, the next agent in the rotation is launched as a cheap, low-capability "detector". It reads the tail of the failed iteration's log.
2. The detector answers a single structured line — TOKENS_EXHAUSTED=true or TOKENS_EXHAUSTED=false.
3. If true, RALPH_TOOL is rewritten in .ralph/.env to that next agent, so the next iteration runs with it (the failed iteration is not retried). If false, nothing changes.

If the next agent's CLI is not installed, the check is skipped and no switch happens. The whole detection (detector agent, its output, and the decision) is appended to the iteration's log. Set RALPH_SWITCH_ON_EXHAUSTION=false to disable.

Hard RAM limit

Agents can leak or balloon in memory over long autonomous runs. RALPH_MEMORY_MAX (default 8G) caps the RAM available to the agent process. The limit is enforced by the Linux kernel: each iteration runs the agent inside a transient systemd user scope created with

systemd-run --user --scope -p MemoryMax=<RALPH_MEMORY_MAX> -p MemorySwapMax=0 -- <agent>

MemorySwapMax=0 keeps the cap on real RAM rather than letting it spill to swap. If the agent exceeds the limit it is OOM-killed, the iteration exits non-zero (and is then subject to the usual exhaustion check / agent switch). The value accepts any systemd memory format (8G, 512M, raw bytes, or a percentage of total RAM); set it empty to disable the limit.

If `systemd