Show HN: Amla Sandbox – WASM bash shell sandbox for AI agents
amla-sandbox
Every popular agent framework runs LLM-generated code via subprocess or exec(). That’s arbitrary code execution on your host. One prompt injection and you’re done.
| Framework | Execution Method | Source |
|---|---|---|
| LangChain | exec(command, globals, locals) |
CVE-2025-68664, GitHub #5294 |
| AutoGen | subprocess.run() |
Code Executors docs |
| SWE-Agent | subprocess.run(["bash", ...]) |
SWE-ReX |
Some frameworks offer Docker isolation (OpenHands, AutoGen), but that requires running a Docker daemon and managing container infrastructure.
amla-sandbox is a WASM sandbox with capability enforcement. Agents can only call tools you explicitly provide, with constraints you define. Sandboxed virtual filesystem. No network. No shell escape.
uv pip install "git+https://github.com/amlalabs/amla-sandbox"
No Docker. No VM. One binary, works everywhere.
console.log(disputed[0]);
”’, language=”javascript”)
# Or with shell pipelines
result = sandbox.run(”’
tool stripe.listTransactions –customer cus_123 | jq ‘[.[] | select(.disputed)] | .[0]’
”’, language=”shell”)”>
from amla_sandbox import create_sandbox_tool sandbox = create_sandbox_tool(tools=[stripe_api, database]) # Agent writes one script instead of 10 tool calls (JavaScript) result = sandbox.run(''' const txns = await stripe.listTransactions({customer: "cus_123"}); const disputed = txns.filter(t => t.disputed); console.log(disputed[0]); ''', language="javascript") # Or with shell pipelines result = sandbox.run(''' tool stripe.listTransactions --customer cus_123 | jq '[.[] | select(.disputed)] | .[0]' ''', language="shell")
Why this matters
Tool-calling is expensive. Every MCP call is a round trip through the model:
LLM → tool → LLM → tool → LLM → tool → ...
Ten tool calls = ten LLM invocations. Code mode collapses this:
LLM → script that does all 10 things → result
But you can’t just eval whatever the model spits out. So people either pay the token tax or run unsafe code. This gives you both: code-mode efficiency with actual isolation.
Security model
The sandbox runs inside WebAssembly with WASI for a minimal syscall interface. WASM provides memory isolation by design—linear memory is bounds-checked, and there’s no way to escape to the host address space. The wasmtime runtime we use is built with defense-in-depth and has been formally verified for memory safety.
On top of WASM isolation, every tool call goes through capability validation:
from amla_sandbox import Sandbox, MethodCapability, ConstraintSet, Param sandbox = Sandbox( capabilities=[ MethodCapability( method_pattern="stripe/charges/*", constraints=ConstraintSet([ Param("amount") <= 10000, Param("currency").is_in(["USD", "EUR"]), ]), max_calls=100, ), ], tool_handler=my_handler, ) # This works sandbox.execute('await stripe.charges.create({amount: 500, currency: "USD"})') # This fails - amount exceeds capability sandbox.execute('await stripe.charges.create({amount: 50000, currency: "USD"})')
The design draws from capability-based security as implemented in systems like seL4—access is explicitly granted, not implicitly available. Agents don’t get ambient authority just because they’re running in your process. This matters because prompt injection is a fundamental unsolved problem; defense in depth through capability restriction limits the blast radius.
Quick start
# Shell
sandbox.run(“echo ‘hello’ | tr ‘a-z’ ‘A-Z'”, language=”shell”) # -> “HELLO”
# With tools
def get_weather(city: str) -> dict:
return {“city”: city, “temp”: 72}
sandbox = create_sandbox_tool(tools=[get_weather])
sandbox.run(“const w = await get_weather({city: ‘SF’}); console.log(w);”, language=”javascript”)”>
from amla_sandbox import create_sandbox_tool sandbox = create_sandbox_tool() # JavaScript sandbox.run("console.log('hello'.toUpperCase())", language="javascript") # -> "HELLO" # Shell sandbox.run("echo 'hello' | tr 'a-z' 'A-Z'", language="shell") # -> "HELLO" # With tools def get_weather(city: str) -> dict: return {"city": city, "temp": 72} sandbox = create_sandbox_tool(tools=[get_weather]) sandbox.run("const w = await get_weather({city: 'SF'}); console.log(w);", language="javascript")
With constraints:
sandbox = create_sandbox_tool( tools=[transfer_money], constraints={ "transfer_money": { "amount": "<=1000", "currency": ["USD", "EUR"], }, }, max_calls={"transfer_money": 10}, )
JavaScript API Notes
Tools require object syntax:
// WORKS - tools always take an object argument await get_weather({city: "SF"}); await transfer({to: "alice", amount: 500}); // FAILS - positional arguments don't work await get_weather("SF"); // Error: argument after ** must be a mapping
Use return or console.log() for output:
return {a: 1, b: 2}; // -> {“a”:1,”b”:2}
return “hello”; // -> hello (strings not double-quoted)
// console.log also works
console.log(JSON.stringify({a: 1})); // -> {“a”:1}
// No return = no output
const x = 42; // -> (no output)”>
// Return value is captured and output return await get_weather({city: "SF"}); // -> {"city":"SF","temp":72} return {a: 1, b: 2}; // -> {"a":1,"b":2} return "hello"; // -> hello (strings not double-quoted) // console.log also works console.log(JSON.stringify({a: 1})); // -> {"a":1} // No return = no output const x = 42; // -> (no output)
VFS is writable only under /workspace and /tmp:
// WORKS - /workspace and /tmp are ReadWrite await fs.writeFile('/workspace/data.json', '{}'); await fs.mkdir('/tmp/cache'); // FAILS - root is read-only await fs.mkdir('/mydir'); // EACCES: Permission denied
LangGraph
For LangGraph integration:
from langgraph.prebuilt import create_react_agent from langchain_anthropic import ChatAnthropic from amla_sandbox import create_sandbox_tool sandbox = create_sandbox_tool(tools=[get_weather, search_db]) agent = create_react_agent( ChatAnthropic(model="claude-sonnet-4-20250514"), [sandbox.as_langchain_tool()] # LLM writes JS/shell that calls your tools )
For fine-grained capability control:
from amla_sandbox import SandboxTool, MethodCapability, ConstraintSet, Param caps = [ MethodCapability( method_pattern="mcp:search_db", constraints=ConstraintSet([Param("query").starts_with("SELECT")]), max_calls=5, ) ] sandbox_tool = SandboxTool.from_functions([search_db], capabilities=caps) agent = create_react_agent(model, [sandbox_tool.as_langchain_tool()])
Architecture
┌────────────────────────────────────────────────┐
│ WASM Sandbox │
│ ┌──────────────────────────────────────────┐ │
│ │ Async Scheduler │ │
│ │ tasks waiting/running/ready │ │
│ └──────────────────────────────────────────┘ │
│ ┌────────────┐ ┌──────────┐ ┌──────────────┐ │
│ │ VFS │ │ Shell │ │ Capabilities │ │
│ │ /workspace │ │ builtins │ │ validation │ │
│ └────────────┘ └──────────┘ └──────────────┘ │
│ ↓ yield │
└════════════════════════════════════════════════┘
│
▼
┌─────────────────────────────────────────────┐
│ Python Host │
│ │
│ while sandbox.has_work(): │
│ req = sandbox.step() # tool call │
│ sandbox.resume(execute(req)) │
│ │
└─────────────────────────────────────────────┘
The sandbox yields on tool calls. Host executes them (after capability checks) and resumes. QuickJS runs inside WASM for the JS runtime.
Precompilation
First run compiles the WASM module (~300ms). Cache it:
Subsequent loads: ~0.5ms.
Constraint DSL
Param(“amount”) <= 10000, Param("currency").is_in(["USD", "EUR"]), Param("path").starts_with("/api/"), ])">
from amla_sandbox import Param, ConstraintSet constraints = ConstraintSet([ Param("amount") >= 100, Param("amount") <= 10000, Param("currency").is_in(["USD", "EUR"]), Param("path").starts_with("/api/"), ])
Pattern matching for method names:
stripe/charges/create— exact matchstripe/charges/*— single path segmentstripe/**— zero or more segments
Tradeoffs
What you get: Isolation without infrastructure. Capability enforcement. Token efficiency.
What you don’t get: Full Linux environment. Native module support. GPU access. Infinite loop protection (a while(true){} will hang – the step limit only counts WASM yields, not JS instructions).
If you need a real VM with persistent state and arbitrary dependencies, use e2b or Modal. amla-sandbox is for the common case: agents running generated code with controlled tool access.
License
Python code is MIT. The WASM binary is proprietary—you can use it with this package but can’t extract or redistribute it separately.
