Ostra in Claude, step by step

2.1What this sets up#

Claude gets the Ostra tools through a custom connector, served by Ostra's gateway at mcp.ostraagent.xyz. The vault stays the owner's: it is made and kept in the Ostra web app, in the owner's browser. Claude works through an agent key of that vault, kept by Ostra's gateway: a key of its own that the vault accepts only within a policy, which says what actions it may take, the most one spend may move, a budget for each day, the only recipients it may pay, and the day the key ends. The owner sets all of it in the web app and can stop the key at any time.

Claude lists the tools of the MCP server an agent runs on its own machine (agents and the MCP server), 96 of its 98: installing an agent key happens only through the connect page below. What a tool may do is what the policy allows: with the first connection's policy the agent pays through channels and sends USDG to allowlisted recipients, and swaps, lending or credit wait until the owner adds them, while the owner's own settings (guardians, recovery, the withdrawal lock, enabling venues, registering the identity) never run under an agent key. Every spend is checked before it is signed, and paid in USDG through the Ostra relayer, so the vault needs no ETH.

2.2What is needed#

2.31. Add the connector#

1. In Claude, go to Customize > Connectors and choose Add custom connector.
2. Name it Ostra Agent and enter this URL:

https://mcp.ostraagent.xyz/mcp

3. Claude reads the sign-in settings from the server and marks the ones it found. Keep them: under Authentication, Sign in now; under OAuth client, Register automatically. No client ID, secret or header is needed. The other OAuth client options do not work with Ostra.
4. Choose Add.

A connector added on claude.ai is in the Claude desktop and phone apps of the same account too.

2.42. Connect it to a vault#

If the sign-in did not start by itself, open the connector under Customize > Connectors and choose Connect. The browser opens a page of the Ostra web app, app.ostraagent.xyz/connect, that names the app asking and where it sends the browser back.

• Go on only after pressing Connect in Claude oneself, and only when Sends you back to shows claude.ai or claude.com. The app names itself and Ostra does not check that name; a connect link someone else sent would give that person the agent key.
• The request belongs to the browser Claude opened: the vault has to be in that browser. The same link pasted into another browser is refused.
• With no vault in this browser yet, the page offers to create one, or to restore one from its backup file. A new vault asks for a passphrase and opens only once its backup file is downloaded and kept off the device; then the page goes on with the connection.

2.53. Fund, set the limits, sign#

An empty vault asks for USDG first, with its address to send to. Send a few USDG on Robinhood Chain (chain 4663) from a wallet or an exchange; the page sees them arrive by itself. Check the network before sending: USDG sent on another network does not arrive. Then set what the agent may do:

Sign and connect has the vault sign, one after the other: the policy, the relayer's channel on the allowlist (so the agent pays its fees in USDG), and the agent key. The Ostra relayer sends each one, paid in USDG from the vault. If one cannot be paid, it is not signed; what was done stays done, and pressing again goes on from there. The browser then goes back to Claude, and the connector shows as connected.

Connecting again later, from another Claude account or after the key ends, installs a new agent key. By default the apps connected before stop; the box Keep the apps connected to this vault before keeps them.

2.64. Use it in a chat#

In a chat, choose +, then Connectors, and turn on Ostra Agent. Then ask in plain words. For example:

With Ostra Agent, show what my vault holds and what
the agent key may still spend today.

With Ostra Agent, call v5_status and explain the agent
key, its limits and the allowlist in plain words.

Claude asks before each tool call until a tool is set to Always allow. Each tool's permission is set under Customize > Connectors, Ostra Agent; a tool not wanted can be set to Blocked. A tool that would spend more than the policy allows is refused before anything is signed.

2.75. Changes only the owner makes#

Three tools change what the agent may do: policy_set (the limits and actions), allow_set (the allowlist) and agent_key_revoke (stop the key). Through the connector they are never signed by the agent key. Claude answers with a link instead remote-auth.ts:64-66:

With Ostra Agent, raise the agent key's limit to
10 USDG a spend and 30 USDG a day.

1. Open the link in the browser that keeps the vault, within 15 minutes. The page shows the change asked for, field by field.
2. Approve and sign signs it with the vault's own key, sent by the relayer and paid in USDG; Decline drops it.

Approve only a change the owner asked for, and check every address on the page in full. Text Claude reads, a web page or a message, could try to make it ask for a recipient that is not the owner's.

A change that only tightens is in force at once. One that lets the agent do more waits the withdrawal lock's delay, when the vault has a lock OstraVaultV5.sol:12-15. The same settings are in the web app without Claude: the vault page's Agent tab shows the key, its end, the limits, what is left today and the allowlist, adds and removes recipients, changes the policy, and has Stop the agent key now, which stops every app connected with it at once.

2.8An agent that already has a vault#

An agent that runs the MCP server on its own machine can move its vault to the web app and use it from Claude. Every vault runs version 5, the one with agent keys, so no new vault is needed. A seed signs in one place at a time, and a restored vault starts signing as soon as it opens, so the order matters:

1. Stop the server on the machine.
2. Restore the seed in the web app with the backup file the server made (backup export; restoring in the web app).
3. Connect as above. Never run the server with that seed again while the vault is open in a browser.

2.9Good to know#

• The connector is free. The vault pays what it always pays: the relayer's fee in USDG for each transaction, and the fees of what the agent uses.
• The owner's seed stays in the owner's browser. The agent key's seed is kept by Ostra's gateway, which runs one server for each connected vault, in a sandbox of its own remote.ts:2-57. What that key can spend is what the policy and the allowlist allow, until it ends or the owner stops it. The gateway sees every tool call and its answer; the security page says so in full.
• For now the connector is for Claude. Other MCP clients use the server on their own machine (getting started).
• Keep some USDG in the vault: each transaction is paid from it.