Apr 24, 2026

To prevent a connection's Authorization header from being forwarded to an unintended host, the proxy execute endpoint (POST /api/v3/tools/execute/proxy) now requires that the outbound endpoint URL share the same scheme and registrable domain (eTLD+1) as the connection's resolved base_url.

Cross-subdomain requests on the same registrable domain continue to work — for example, a Gmail connection with base https://gmail.googleapis.com can still call https://www.googleapis.com/.... Relative endpoints (/users/me/messages) are resolved against the connection's base_url as before and are unaffected.

Migration

If you currently pass an absolute URL that points at a different domain than your connection's base_url, switch to a relative endpoint (resolved against base_url) or an absolute URL under the same registrable domain.

Before — absolute URL on a different domain (will be rejected):

{
  "endpoint": "https://api.someservice.com/v1/items",
  "method": "GET",
  "connected_account_id": "ca_..."
}

After — relative endpoint (recommended):

{
  "endpoint": "/v1/items",
  "method": "GET",
  "connected_account_id": "ca_..."
}

Or — absolute URL on the same registrable domain as the connection's base_url:

{
  "endpoint": "https://uploads.someservice.com/v1/items",
  "method": "GET",
  "connected_account_id": "ca_..."
}

If your integration legitimately needs to call a different registrable domain for the same connection, reach out so we can add the additional host to the toolkit allowlist.

What changed

• Absolute endpoint URLs on a different registrable domain than the connection's base_url are rejected with HTTP 400 OriginMismatch. The upstream request is never made.
• Proxy calls that provide neither connected_account_id nor custom_connection_data now return HTTP 400 MissingAuthContext instead of being forwarded without auth.

Examples

Assume a connected account whose toolkit base_url is https://api.linear.app.

Allowed — relative endpoint:

curl -X POST https://backend.composio.dev/api/v3/tools/execute/proxy \
  -H 'x-api-key: <YOUR_API_KEY>' \
  -H 'Content-Type: application/json' \
  -d '{
    "endpoint": "/graphql",
    "method": "POST",
    "connected_account_id": "ca_..."
  }'

Allowed — same registrable domain (different subdomain):

{ "endpoint": "https://uploads.linear.app/...", "connected_account_id": "ca_..." }

Rejected — different registrable domain:

{ "endpoint": "https://attacker.example/leak", "connected_account_id": "ca_..." }

Response:

{
  "error": {
    "code": "OriginMismatch",
    "message": "Endpoint host does not match the connection's base_url host."
  }
}

What to do

• If you call proxy execute with absolute URLs, make sure the host matches (or is a subdomain of) the connection's base_url.
• Prefer relative endpoints (/path) — they are resolved against base_url and are not affected by this change.
• If your integration legitimately needs to span multiple registrable domains for the same connection, reach out to us so we can add the additional host to the toolkit allowlist.