Agent Callback Mailbox
REMOTE · CALLBACKS.INVERSOLABS.US · SCANNED OCT 9
Paid replay-safe callback inboxes for asynchronous agent tasks with structured MCP results.
Available components
How this component scores in each security and reliability category. Every signal is checked automatically against the live server, and we only credit what we can confirm. How we score → Why this is hard to score →
Endpoint Security46
- The endpoint's TLS certificate is valid, in date, and uses a strong key. View diagnostics → Pass
- Authorisation not yet verified: we couldn't confirm whether this endpoint requires it. View diagnostics → Unverified
- HTTPS enforcement could not be verified: the plaintext port answered with HTTP 502, which proves neither a plaintext path nor enforcement. View diagnostics → Unverified
- HSTS check failed: the Strict-Transport-Security header is absent. See how to fix → View diagnostics → Fail
- DNSSEC check failed: this domain isn't protected by DNSSEC. See how to fix → View diagnostics → Fail
Transport & Reachability0
- Transport check failed: declared streamable-http, but the endpoint returned HTTP 502. See how to fix → View diagnostics → Fail
Schema Quality & AI Usability0
- Schema not yet verified: we couldn't read the endpoint's schema, or could read only part of its tool list.Unverified
Stability & Change Management0
- Stability not yet verified: not enough scan history yet (needs a 30-day window).Unverified
Tool Coverage0
- Tool coverage not yet verified: we couldn't read the endpoint's tools, or could read only part of the list.Unverified
Tool Safety0
- Tool safety not yet verified: we couldn't read the endpoint's tools, or could read only part of the list.Unverified
Capabilities0
- Capabilities not yet verified: we couldn't read the endpoint's capabilities.Unverified
Unverified: 5 categories
Categories scored 0 because we could not verify them: authentication we do not have, an unreachable endpoint, or not enough scan history. We only credit what we can confirm.
How do I install the Agent Callback Mailbox MCP server?
Agent Callback Mailbox is a hosted endpoint at https://callbacks.inversolabs.us/mcp, so there is nothing to install locally. Ready-made configuration for Claude, Cursor, VS Code, Codex and 5 more is on this page, copied from each client's own documentation.
remote · callbacks.inversolabs.us
claude mcp add --transport http us-inversolabs-agent-callback-mailbox 'https://callbacks.inversolabs.us/mcp'
{
"mcpServers": {
"us-inversolabs-agent-callback-mailbox": {
"url": "https://callbacks.inversolabs.us/mcp"
}
}
} {
"servers": {
"us-inversolabs-agent-callback-mailbox": {
"type": "http",
"url": "https://callbacks.inversolabs.us/mcp"
}
}
} [mcp_servers.us-inversolabs-agent-callback-mailbox] url = "https://callbacks.inversolabs.us/mcp"
{
"$schema": "https://opencode.ai/config.json",
"mcp": {
"us-inversolabs-agent-callback-mailbox": {
"type": "remote",
"url": "https://callbacks.inversolabs.us/mcp",
"enabled": true
}
}
} openclaw mcp add us-inversolabs-agent-callback-mailbox --url 'https://callbacks.inversolabs.us/mcp' --transport streamable-http
mcp_servers:
us-inversolabs-agent-callback-mailbox:
url: "https://callbacks.inversolabs.us/mcp" {
"McpServers": {
"us-inversolabs-agent-callback-mailbox": {
"Transport": "http",
"Url": "https://callbacks.inversolabs.us/mcp"
}
}
} assistant mcp add us-inversolabs-agent-callback-mailbox -t streamable-http -u 'https://callbacks.inversolabs.us/mcp'
{
"mcpServers": {
"us-inversolabs-agent-callback-mailbox": {
"type": "http",
"url": "https://callbacks.inversolabs.us/mcp"
}
}
} The mcpServers block is a cross-client convention. Remote transports vary, so check your client's docs.
Every change we have recorded for this component, newest first. Security-relevant changes are always shown. ▲ marks a change for the better, ▼ a change for the worse; unmarked changes are neutral.
- 28 Sept 26 0
- We updated how we score, so this day's move reflects our rubric, not a change to the server See what changed → functional
- 25 Sept 26 0
- We updated how we score, so this day's move reflects our rubric, not a change to the server See what changed → functional
- 11 Sept 26 −42
- Endpoint reachability: reachable → not serving MCP ▼ security
- Tool safety: pass → unverified ▼ security
- Stability: 0.50 → unverified ▼ security
- Transport: pass → fail ▼ security
- Authorization: Authorisation not fully verified: no authorisation is required to connect, but we couldn't read the whole tool list to see what that exposes. security
- Capabilities: pass → unverified ▼ functional
- Tool coverage: 100 → unverified ▼ functional
- 9 Sept 26 +1
No change was recorded against any check on this day. Stability & Change Management went from 43 to 47. That category is still filling its 30-day observation window: 13 days of observed history at the previous scan, 14 at this one. The score rises as the window fills, whether or not the server changes.
- 8 Sept 26 +41
- Injection markers: unverified → pass ▲ security
- Transport: fail → pass ▲ security
- Authorization: Authorisation not fully verified: no authorisation is required to call this server, and 1 tool(s) never declared a destructiveHint. The MCP spec treats an absent hint as destructive by default, so we cannot call this surface safe. security
- Endpoint reachability: not serving MCP → reachable ▲ functional
- Tool coverage: unverified → 100 ▲ functional
- Stability: unverified → 0.43 ▲ functional
- MCP protocol: unverified → pass ▲ functional
- 4 Sept 26 0
- Endpoint reachability: reachable → not serving MCP ▼ security
- Stability: 0.27 → unverified ▼ security
- Tool safety: pass → unverified ▼ security
- Transport: pass → fail ▼ security
- Authorization: Authorisation not fully verified: no authorisation is required to connect, but we couldn't read the whole tool list to see what that exposes. security
- Capabilities: pass → unverified ▼ functional
- Tool coverage: 100 → unverified ▼ functional
- 2 Sept 26 0
- Injection markers: unverified → pass ▲ security
- Transport: fail → pass ▲ security
- Authorization: Authorisation not fully verified: no authorisation is required to call this server, and 1 tool(s) never declared a destructiveHint. The MCP spec treats an absent hint as destructive by default, so we cannot call this surface safe. security
- Endpoint reachability: not serving MCP → reachable ▲ functional
- Tool coverage: unverified → 100 ▲ functional
- Stability: unverified → 0.23 ▲ functional
- MCP protocol: unverified → pass ▲ functional
- 1 Sept 26 0
- Endpoint reachability: reachable → not serving MCP ▼ security
- Tool safety: pass → unverified ▼ security
- Stability: 0.17 → unverified ▼ security
- Transport: pass → fail ▼ security
- Authorization: Authorisation not fully verified: no authorisation is required to connect, but we couldn't read the whole tool list to see what that exposes. security
- Capabilities: pass → unverified ▼ functional
- Tool coverage: 100 → unverified ▼ functional
- First check of Schema quality: unverified functional
Diagnostic detail from the automated scan of this channel: what the scanner observed at each step, so you can see exactly where a check passed or failed. It is informational only and never changes the trust score.
Captured 9 Oct 2026 · Probed https://callbacks.inversolabs.us/mcp
TLS valid
Negotiated TLS 1.3 with TLS_AES_128_GCM_SHA256 .
| Subject | Issuer | Valid from | Valid until | Key | Signature | Serial |
|---|---|---|---|---|---|---|
| CN=inversolabs.us | CN=WE1,O=Google Trust Services,C=US | 30 Aug 2026 | 28 Nov 2026 | ECDSA 256 | ECDSA-SHA256 | 92d682f4614ee88e13e8cba8a2d22a74 |
| SANs: inversolabs.us, *.inversolabs.us | ||||||
| CN=WE1,O=Google Trust Services,C=US (CA) | CN=GTS Root R4,O=Google Trust Services LLC,C=US | 13 Dec 2023 | 20 Feb 2029 | ECDSA 256 | ECDSA-SHA384 | 7ff31977972c224a76155d13b6d685e3 |
| CN=GTS Root R4,O=Google Trust Services LLC,C=US (CA) | CN=GlobalSign Root CA,OU=Root CA,O=GlobalSign nv-sa,C=BE | 15 Nov 2023 | 28 Jan 2028 | ECDSA 384 | SHA256-RSA | 7fe530bf331343bedd821610493d8a1b |
Background: What to check on a remote MCP endpoint →
DNSSEC insecure
Validation of callbacks.inversolabs.us. — Not signed
| Zone | DS | Keys | Algorithms | Outcome |
|---|---|---|---|---|
| . | trust_anchor | 20326, 38696 | 8, 8 | Verified |
| us. | present | 59017 | 8 | Verified |
| inversolabs.us. | absent | Unsigned (proven) parent-signed NSEC/NSEC3 proves an unsigned delegation |
Authentication Inconclusive
We could not reach the endpoint well enough to judge its authorisation posture.
| Result | Inconclusive |
|---|---|
| HTTP status | 502 |
| Header | Value |
|---|---|
| x-frame-options | SAMEORIGIN |
| referrer-policy | same-origin |
Background: How OAuth 2.1 works in the 2026 MCP spec →
Transports 2 probes
| Transport | URL | Outcome | Status | Location |
|---|---|---|---|---|
| streamable-http | https://callbacks.inversolabs.us/mcp | HTTP error | 502 | |
| http (plaintext) | http://callbacks.inversolabs.us/mcp | Inconclusive | 502 |
The tools this component advertises to a client, with an estimated token cost for each. Expand a tool to see its parameters and schema. The per-tool counts are indicative and are not scored directly; the schema's total context footprint is one signal in Schema Quality & AI Usability. A tool's description is untrusted text the model reads on every call, which is what makes this list a security surface and not just an inventory: how tool poisoning works →
create_a2a_callback_mailbox Temporary Authenticated A2A Callback Inbox ~153
Create a temporary private authenticated webhook inbox for asynchronous A2A task completion. Let an expensive reasoning agent suspend instead of spending model calls polling for completion; a lightweight worker can receive and persist the callback, then resume the agent when useful data exists. One payment includes up to 1,000 ordered callback messages, replay-safe deduplication, cursor retrieval, a configurable 1-minute to 24-hour lifetime, and up to 4 MiB storage.
| Name | Type | Req | Description |
|---|---|---|---|
| expectedTaskId | string | yes | – |
| ingressCredential | string | yes | – |
| maxBytes | integer | – | – |
| maxEvents | integer | – | – |
| retrievalCredential | string | yes | – |
| ttlMinutes | integer | – | – |
| Name | Type | Req | Description |
|---|---|---|---|
| authentication_scheme | string | yes | – |
| callback_url | string | yes | – |
| expires_at | string | yes | – |
| mailbox_id | string | yes | – |
| max_bytes | integer | yes | – |
| max_events | integer | yes | – |
No examples provided.
What is the Agent Callback Mailbox MCP server?
Agent Callback Mailbox is an MCP server listed in the public MCP registry as us.inversolabs/agent-callback-mailbox. Paid replay-safe callback inboxes for asynchronous agent tasks with structured MCP results. This page covers its hosted endpoint (https://callbacks.inversolabs.us/mcp).
Is the Agent Callback Mailbox MCP server safe to use?
Agent Callback Mailbox scores 18 out of 100 on VerifyMCP. That is a record of what we were able to check automatically, not an endorsement. The category breakdown on this page shows every signal behind the number, including the ones we could not confirm.
What tools does the Agent Callback Mailbox MCP server expose?
Agent Callback Mailbox exposes 1 tool: create_a2a_callback_mailbox. Their descriptions and schemas cost roughly 153 tokens of context every time the server is loaded.
Does the Agent Callback Mailbox MCP server require authentication?
Its publisher declares no required credentials for Agent Callback Mailbox. We have not been able to confirm that against the live endpoint, and a server can require authorisation without declaring it here.
Is the Agent Callback Mailbox MCP server still maintained?
Agent Callback Mailbox is still listed as active in the MCP registry. We last reached this channel on 9 October 2026. Those dates come from our own scans of the registry and the channel itself, not from anything the publisher announced.