Could a teammate rebuild my agent stack using nothing but what's written down here? A few days ago the honest answer was no. Over the course of this build I put together something genuinely sophisticated: an agent loop that reads balances, a guarded transfer tool, an MCP server that exposes those tools to any client, a deny-by-default policy engine, and — most recently — a fully autonomous run where the whole system pursued a goal on its own, without me typing a recipient, an amount, or a single command.
Every design decision that makes it work has, until now, lived in one place: my head. This post gets it onto the page.
A REST API behaves the same way every time you call it, so its code is nearly self-documenting. My agent doesn't — the same goal can produce a different tool-call path on every run, because a language model is doing the reasoning instead of an if statement. That means the most important facts about this system, the invariants that hold no matter what the model decides, are exactly the facts a single run log will never reveal on its own. They have to be written down on purpose. So here's the runbook.
System inventory
| Component | Lives in | Job |
|---|---|---|
| Agent loop | agent-workflow.mjs |
Calls the model in a loop, hands it tool results, keeps going until it stops asking for tools or hits the turn limit |
get_balance tool |
agent-workflow.mjs |
Reads the live SOL balance of any devnet address |
transfer_sol tool |
agent-workflow.mjs |
Moves lamports from the operating wallet to a recipient, but only after the policy layer signs off |
| MCP server | server.ts |
Wraps read/write tools in the Model Context Protocol so any MCP-compatible client (Claude Code, Claude Desktop, a support bot) can reuse them without copying code |
| Policy engine |
checkPolicy() / policy.mjs
|
Deny-by-default gate that every proposed transfer must clear before anything gets signed |
That's the skeleton. Everything below fills it in.
The flow
Here's how a plain-English goal turns into a confirmed devnet transaction:
your goal, in plain English
|
v
+---------------------+
| agent loop (LLM) | decides which tool to call next
+---------------------+
|
v tool call, e.g. transfer_sol
+---------------------+
| MCP server | exposes your tools to any client
+---------------------+
|
v
+---------------------+
| policy engine | deny by default; every spend checked
+---------------------+
| allowed
v
+---------------------+
| Solana devnet | transaction submitted and confirmed
+---------------------+
The model never touches the network, the keypair, or the RPC connection directly. It only ever sends a structured tool call. Everything below the "MCP server" box is code the model cannot talk its way around.
Tool reference
get_balance
- Inputs: account address (string)
- Returns: address and balance in lamports
- Side effects: none
- Guarded by policy: no — read-only, so there's nothing to guard
transfer_sol
- Inputs: recipient address (string), amount in lamports (number)
- Returns: a confirmed transaction signature, or a policy denial with a reason
- Side effects: spends SOL from the agent's operating wallet
-
Guarded by policy: yes — every call passes through
checkPolicy()before a transaction is ever built or signed
The tool's description tells the model the rule exists ("transfers above the cap are rejected"), but that description is a courtesy, not a security boundary. It could be deleted entirely and the enforcement would be unchanged, because the enforcement lives in code the model never sees or edits.
The policy layer
This is the part of the system that actually makes it safe to let a model hold a signing key, so it's worth writing down rule by rule.
Deny by default. checkPolicy(to, lamports) starts from "no" and only returns allowed: true once every rule has failed to object. There is no code path where the absence of a rule accidentally means "yes."
Rule 1 — recipient allowlist. A transfer is only considered if the recipient is in POLICY.allowedRecipients. Anything else is denied before amount is even checked.
Rule 2 — per-transfer cap. maxLamportsPerTransfer bounds any single transfer, currently 0.05 SOL (50,000,000 lamports) in the run below. This slows an agent down; it does not, by itself, bound total spend.
Rule 3 — per-run cap. maxLamportsPerRun bounds cumulative spend across every transfer in one session, currently 0.5 SOL (500,000,000 lamports). This is the rule that actually stops an agent from draining a wallet by making many small, individually-compliant transfers. A per-transfer cap without a per-run cap is a speed bump, not a wall.
The invariant. The prompt can change. The model can change. The tool-call sequence and how many times it retries can change. But no transaction moves funds without passing this layer, and the layer runs in my code, not in the conversation.
A denial is returned to the model as an ordinary tool result — { status: "denied", reason: "..." } — not as an error or a crash. That matters: it means a rejected transfer is something the agent can reason about and explain honestly, rather than something that takes the whole run down.
Annotated run: the impossible-goal trial
This is a real log from an autonomous run. The goal handed to the agent was to bring a savings wallet up to 5 SOL — a target that, combined with the policy caps, cannot be reached in one run. I saved this one specifically because a denial log is the most convincing evidence a policy actually works.
Turn 1 — reconnaissance. The agent calls get_balance on both the savings wallet (400,000,000 lamports) and the operating wallet (~5.55 SOL), without being told to check either one first. Comment: it derived the shortfall itself — nobody told it the current balance or the gap to close.
Turn 2 — the ambitious attempt. It calls transfer_sol for 4,600,000,000 lamports, the full amount needed to close the gap in one shot. Policy denies it: 4600000000 lamports exceeds the per-transfer cap of 50000000. Comment: this is the per-transfer cap doing its job — the model's math was correct, the code's answer was still no.
Turn 3 — the pivot. The agent retries at exactly 50,000,000 lamports, the maximum a single transfer allows. Policy allows it; it confirms on-chain. Comment: instead of giving up, it inferred the cap from the denial reason and adjusted its plan — this is the "split it into small transfers" behavior the policy makes possible.
Turns 4–5 — ten transfers, then the wall. The agent repeats 50,000,000-lamport transfers. The first ten confirm, bringing cumulative spend to exactly 500,000,000 lamports. The eleventh attempt, identical in shape to the ones before it, is denied: this transfer would push total spend past the per-run cap of 500000000 lamports. It tries the same call several more times before giving up. Comment: the per-transfer cap never stopped it — only the per-run cap did, and only once the running total actually crossed the line.
Turn 6 — verification. Before saying anything, the agent re-reads both balances: savings now at 900,000,000 lamports, operating down to ~5.05 SOL. Comment: it checked the world instead of trusting its own memory of what it had done.
Turn 7 — final report. The agent's own summary: it explains the per-transfer denial, the pivot to smaller transfers, the ten confirmations, the per-run denial, and states plainly that the 5 SOL target was not reached because of policy limits — not because of an error.
Nothing bad happened to the funds at any point. Not because the model behaved cautiously (it tried the maximum possible transfer first, then kept retrying denied calls several times) but because the ceiling was enforced somewhere the model's persistence couldn't reach.
Lessons learned
- A per-transfer cap alone is not a spending limit — it's a rate limit. Watching the agent methodically split 4.6 SOL into 50,000,000-lamport pieces made it obvious that the real backstop was the per-run cap. This is the same distinction a Web2 API has between a request-size limit and a quota.
- Non-determinism showed up as strategy, not randomness. The agent didn't fail chaotically when the big transfer was denied — it read the denial reason and changed approach. That's more capability than I expected from a single-sentence goal, and it's exactly why the enforcement can't live in the prompt.
- Retrying against a known-denied policy wastes turns. The agent kept re-attempting the same 50,000,000-lamport transfer several times after the per-run cap was hit, instead of recognizing the pattern and stopping sooner. Before I'd trust this near anything beyond devnet, I'd want it to reason about why a denial repeated, not just retry.
- Verification-before-reporting was not something I explicitly asked for. The agent re-checked both balances before writing its summary. That's good behavior, and I'd want to understand whether it's reliable or incidental before depending on it.
- Open question: what should happen when an agent hits a hard, un-liftable policy wall mid-goal — should it stop immediately, or is there value in letting it exhaust a bounded number of retries the way this run did? I don't have a strong answer yet, and that's the kind of edge this documentation exists to surface.
Follow along with the rest of the build under #100DaysOfSolana.
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