Slack

CommunicationPublisher: Slack Technologies

Route Slack MCP traffic through Formal to control and log what AI agents do in Slack:

Block risky tools, such as send_message.
Mask sensitive data in tool results, such as emails in get_users responses.
Keep credentials at the Connector, so users and agents never see Slack tokens.
Log every tool call with the user, the MCP client and its parameters.

0. Prepare your Connector

The Connector needs an MCP listener and a hostname with wildcard DNS and TLS. Set them up once by following Deploy the Connector. Every MCP server reuses them.

1. Add Slack MCP server to your Resources

UI
Terraform
Pulumi
API

Open the Admin Catalog and click Add MCP.
Select the Slack template.
Enter a DNS-label Name, such as slack.
Choose the Space containing your Connector, such as default.
Configure upstream authentication as described below, then save.

If no Connector is in the selected Space, the Configuration tab shows MCP clients can’t connect directly yet. The reason reads: No Connector is in this server's Space.

Set space_id to the ID of the Space containing your Connector.

resource "formal_resource" "slack" {
  name                    = "slack"
  technology              = "mcp"
  hostname                = "mcp.slack.com"
  space_id                = "<SPACE_ID>"
  native_users_v3_enabled = true
  port                    = 443
  tags = {
    "mcp-endpoint" = "/mcp"
  }
}

resource "formal_resource_oauth_settings" "slack" {
  resource_id = formal_resource.slack.id
}

Set spaceId to the ID of the Space containing your Connector.

import * as formal from "@formalco/pulumi";

const resource = new formal.Resource("mcp", {
  name: "slack",
  technology: "mcp",
  hostname: "mcp.slack.com",
  port: 443,
  spaceId: "<SPACE_ID>",
  tags: { "mcp-endpoint": "/mcp" },
  nativeUsersV3Enabled: true,
});

new formal.ResourceOauthSettings("mcpOAuth", {
  resourceId: resource.id,
});

Create a Resource with the MCP endpoint and its Space ID.

export FORMAL_API_KEY="<YOUR_API_KEY>"
RESOURCE_RESPONSE=$(curl --fail --silent --show-error \
  --request POST \
  https://api.formal.ai/core.v1.ResourceService/CreateResource \
  --header "x-api-key: ${FORMAL_API_KEY}" \
  --header "Content-Type: application/json" \
  --data-binary @- <<JSON
  {
    "name": "slack",
    "hostname": "mcp.slack.com",
    "port": 443,
    "technology": "mcp",
    "spaceId": "<SPACE_ID>",
    "tags": [
      {
        "key": "mcp-endpoint",
        "value": "/mcp"
      }
    ]
  }
JSON
)
RESOURCE_ID=$(printf '%s' "${RESOURCE_RESPONSE}" | jq -r '.resource.id')
curl --fail --silent --show-error \
  --request POST \
  https://api.formal.ai/core.v1.ResourceService/CreateResourceOAuthSettings \
  --header "x-api-key: ${FORMAL_API_KEY}" \
  --header "Content-Type: application/json" \
  --data-binary @- <<JSON
  {
    "resourceId": "${RESOURCE_ID}"
  }
JSON

2. Setup Authentication

Linked Accounts (Recommended)
Shared Credentials
Pass Through

Each user links their own Slack account once in the Employee Catalog, and the Connector sends that user’s token to Slack.

Linked Accounts selected under How does Formal authenticate to this resource?

Under OAuth app, choose one of the two options.

1. Formal’s App (default)

Formal has a registered OAuth app with Slack, so you register nothing. Users see Formal on Slack’s consent screen.

2. Your OAuth App

Users see your app’s name on Slack’s consent screen.

Set up your own Slack OAuth app

See Slack OAuth documentation and register an app.

Create the app

Open Slack app settings, select From a manifest, enter a manifest, and click Next, then Create.

Enable MCP

Open Agents and toggle Slack Model Context Protocol (MCP) Server On.

Choose scopes

Open OAuth & Permissions > Scopes, and add only the User Token Scopes needed by your MCP tools.

Set the callback

Under Redirect URLs, click Add New Redirect URL, paste Formal’s Redirect URI, then click Add and Save URLs.

Get credentials

Open Basic Information > App Credentials, and copy the Client ID and Client Secret. You can skip the client secret. Formal then authenticates with PKCE alone.

Enter the app in Formal

UI
Terraform
Pulumi
API

Back in the Admin Catalog, enter the Client ID and Client secret, then save. Users then link their accounts on the Slack server’s page in the Employee Catalog.

Your OAuth App selected in the Admin Catalog, with the Redirect URI, Client ID and Client secret fields

Use these OAuth settings instead of the ones from step 1.

variable "slack_oauth_client_secret" {
  type      = string
  sensitive = true
}

resource "formal_resource_oauth_settings" "slack" {
  resource_id              = formal_resource.slack.id
  upstream_oauth_mode      = "pre_registered"
  client_id                = "<YOUR_CLIENT_ID>"
  client_secret_wo         = var.slack_oauth_client_secret
  client_secret_wo_version = 1
}

Use these OAuth settings instead of the ones from step 1. Store the secret with pulumi config set --secret slackOauthClientSecret.

import * as pulumi from "@pulumi/pulumi";

const config = new pulumi.Config();

new formal.ResourceOauthSettings("mcpOAuth", {
  resourceId: resource.id,
  upstreamOauthMode: "pre_registered",
  clientId: "<YOUR_CLIENT_ID>",
  clientSecret: config.requireSecret("slackOauthClientSecret"),
});

Use this call instead of the CreateResourceOAuthSettings call from step 1. The OAuth app is fixed at creation.

curl --fail --silent --show-error \
  --request POST \
  https://api.formal.ai/core.v1.ResourceService/CreateResourceOAuthSettings \
  --header "x-api-key: ${FORMAL_API_KEY}" \
  --header "Content-Type: application/json" \
  --data-binary @- <<JSON
  {
    "resourceId": "${RESOURCE_ID}",
    "upstreamOauthMode": "pre_registered",
    "clientId": "<YOUR_CLIENT_ID>",
    "clientSecret": "<YOUR_CLIENT_SECRET>"
  }
JSON

Switching between Formal’s App and your OAuth app unlinks every account. Users must link again.

3. Route Endpoint traffic

Auth is managed at the Connector level when you use Linked Accounts or Shared Credentials, so the Endpoint needs to forward the MCP traffic to it. Laptops and VMs with the Formal Endpoint keep using https://mcp.slack.com/mcp, and a network rule sends that traffic to the Connector.

UI
Terraform
Pulumi
API

Go to Network Rules and create a rule named slack-mcp.
Paste this expression, set the rule to Active, then save:

{
  "condition": {
    "pre_tcp": process.is_agent,
    // Every MCP server added to Formal: process.is_agent && resource.technology == "mcp"
    "pre_tls": process.is_agent && hostname == "mcp.slack.com",
    "post_tls": true
  },
  "outputs": {
    "forward_to_connector": true
  }
}

resource "formal_network_rule" "slack_mcp" {
  name        = "slack-mcp"
  description = "Forward Slack MCP traffic from agents to the Connector"
  status      = "active"

  # Every MCP server added to Formal: set pre_tls to process.is_agent && resource.technology == "mcp"
  cel_expression = <<-EOT
    {
      "condition": {
        "pre_tcp": process.is_agent,
        "pre_tls": process.is_agent && hostname == "mcp.slack.com",
        "post_tls": true
      },
      "outputs": {
        "forward_to_connector": true
      }
    }
  EOT
}

import * as formal from "@formalco/pulumi";

new formal.NetworkRule("slackMcp", {
  name: "slack-mcp",
  description: "Forward Slack MCP traffic from agents to the Connector",
  status: "active",
  // Every MCP server added to Formal: set pre_tls to process.is_agent && resource.technology == "mcp"
  celExpression: `
    {
      "condition": {
        "pre_tcp": process.is_agent,
        "pre_tls": process.is_agent && hostname == "mcp.slack.com",
        "post_tls": true
      },
      "outputs": {
        "forward_to_connector": true
      }
    }
  `,
});

export FORMAL_API_KEY="<YOUR_API_KEY>"
# Every MCP server added to Formal: set pre_tls to process.is_agent && resource.technology == "mcp"
CEL_EXPRESSION=$(cat <<'CEL'
{
  "condition": {
    "pre_tcp": process.is_agent,
    "pre_tls": process.is_agent && hostname == "mcp.slack.com",
    "post_tls": true
  },
  "outputs": {
    "forward_to_connector": true
  }
}
CEL
)
jq -n --arg cel "${CEL_EXPRESSION}" '{
  name: "slack-mcp",
  description: "Forward Slack MCP traffic from agents to the Connector",
  status: "active",
  celExpression: $cel
}' | curl --fail --silent --show-error \
  --request POST \
  https://api.formal.ai/core.v1.DesktopService/CreateDesktopRoutingRule \
  --header "x-api-key: ${FORMAL_API_KEY}" \
  --header "Content-Type: application/json" \
  --data-binary @-

The rule matches known agents only. See Identity headers.Cloud agents such as Claude, ChatGPT and Cursor Cloud Agents don’t run the Endpoint. Point them at this server’s Connector address, https://slack.<connector-hostname>/mcp. Find it in the Admin Catalog: open Slack, then copy Connector address on the Configuration tab.

4. Create policies

Tools

The Admin Catalog lists these tools for Slack:

Tool	Description
`search`	Search Slack messages and files.
`fetch`	Retrieve a Slack message thread or canvas.
`send_message`	Send a message to a channel or direct message.
`reply_to_thread`	Reply in a message thread.
`add_reaction`	Add an emoji reaction to a message.
`list_channels`	List accessible Slack channels.
`get_channel_history`	Retrieve recent messages from a channel.
`get_thread_replies`	Retrieve replies in a message thread.
`get_users`	List users in the workspace.
`get_user_profile`	Retrieve a Slack user profile.

Policies examples

Block a tool
Mask a tool's output

This policy blocks the tools that change data, so agents can only read Slack.

package formal.v2

import future.keywords.if
import future.keywords.in

write_tools := {
  "send_message",
  "reply_to_thread",
  "add_reaction",
}

request := {
  "action": "block",
  "type": "block_with_custom_message",
  "message": "Writing to Slack is not allowed.",
} if {
  input.resource.name == "slack"
  input.mcp.method == "tools/call"
  input.mcp.tool_name in write_tools
}

This policy replaces email addresses in results returned by get_users, get_user_profile, and search.

package formal.v2

import future.keywords.if
import future.keywords.in

email_pattern := `[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}`

response := {
  "action": "rewrite",
  "body": regex.replace(input.http.body, email_pattern, "[REDACTED-EMAIL]"),
  "reason": "Mask emails in Slack results",
} if {
  input.resource.name == "slack"
  input.mcp.tool_name in {"get_users", "get_user_profile", "search"}
  regex.match(email_pattern, input.http.body)
}

See MCP resources for the input.mcp fields and Rewrite Action for body rewrites.

5. Check the logs

Open Logs and search for resource.name:slack. Each Slack tool call is one request log with the user, the MCP client and the tool. Open a log to see its tool parameters and the policies it triggered. A blocked write shows the block policy and its message. A masked search shows the rewrite policy.See Logs for search syntax and filters.

Troubleshooting

The agent says Slack rejected your credentials

The agent relays: slack rejected your credentials. Link or re-link your slack account in Formal…Cause: Slack no longer accepts the linked token, for example after the user revoked access in Slack.Fix: Link Slack again from the Employee Catalog.

Laptop traffic doesn't appear in Logs

Cause: The Endpoint isn’t running, or the network rule doesn’t match the client.Fix: Check the Endpoint is connected. The rule above only matches known agents. Other clients, such as a browser, reach Slack directly unless the rule also matches them. See Network rules.

MCP clients can't connect directly yet

Cause: The Configuration tab lists the reason, most often No Connector is in this server's Space.Fix: Edit the server and choose the Space containing its Connector.

Getting Started

Example Use Cases

AI Governance

Configuring Formal

Integrations

Configuration

0. Prepare your Connector

1. Add Slack MCP server to your Resources

2. Setup Authentication

1. Formal’s App (default)

2. Your OAuth App

3. Route Endpoint traffic

4. Create policies

Tools

Policies examples

5. Check the logs

Troubleshooting

Next Steps

MCP Gateway

All MCP servers

​0. Prepare your Connector

​1. Add Slack MCP server to your Resources

​2. Setup Authentication

​1. Formal’s App (default)

​2. Your OAuth App

​3. Route Endpoint traffic

​4. Create policies

​Tools

​Policies examples

​5. Check the logs

​Troubleshooting

​Next Steps

MCP Gateway

All MCP servers

0. Prepare your Connector

1. Add Slack MCP server to your Resources

2. Setup Authentication

1. Formal’s App (default)

2. Your OAuth App

3. Route Endpoint traffic

4. Create policies

Tools

Policies examples

5. Check the logs

Troubleshooting

Next Steps