LangSmith Gateway-backed Slack MCP agent

This project exposes a Deep Agent graph through the LangSmith Deployment MCP endpoint. The agent uses claude-sonnet-4-6 through LangSmith Gateway and can list the private channels visible to the authenticated Slack identity, along with all visible public channels.

The agent has seven Slack tools:

• list_channels: all visible public and accessible private channels.
• list_eval_channels: every visible public and private eval- channel.
• list_customer_channels: every visible public and private customer- channel.
• list_public: all active public channels visible to the Slack identity.
• list_private: all active private channels accessible to the Slack identity.
• refresh_channels: replace the caller's durable snapshot from Slack.
• match_accounts_to_channels: deterministically reconcile up to 200 Salesforce account names with cached Slack channels and membership.

Local setup

Create a Slack app with the channels:read and groups:read scopes. Each caller provides their own Slack user OAuth token at request time, so results reflect that person's Slack access.

Store only the model Gateway credential in an untracked .env file:

LANGSMITH_API_GATEWAY_TOKEN=<gateway-user-service-key>

The Gateway key must use the gateway-user role. LANGSMITH_API_GATEWAY_TOKEN is the only supported Gateway credential name.

Invoke the agent locally:

set -a && source .env && set +a
# Invoke through the LangGraph server with x-slack-user-token; direct Python
# invocation does not run deployment authentication.

LangSmith Deployment

Deploy the repository and configure LANGSMITH_API_GATEWAY_TOKEN as a deployment secret. Do not store Slack user tokens in the deployment environment. The Anthropic provider key remains in the LangSmith workspace's Gateway configuration and is not needed by this deployment.

Fleet must send both headers on every MCP request:

x-api-key: <LangSmith deployment key>
x-slack-user-token: <caller's Slack user OAuth token>

Custom authentication validates the LangSmith key against the LangSmith API and the Slack token against Slack, then makes the Slack token available only as request metadata. That authenticated Slack identity selects a private durable filesystem namespace; Liam and Morgan therefore read different /channels/directory.jsonl files without putting either name in a caller- controlled tool argument. Channel results reflect the caller's own visibility. The Slack token is never written to the filesystem or placed in graph state, model messages, tool results, or application logs.

Add the server in Arcade

1. In Arcade, go to Servers and select Add Server.
2. Choose Remote MCP.
3. Give the server a user-specific name, such as MCP-Slack-Channels-Liam.
4. Enter this exact URL, with no trailing slash:

   https://slack-private-channel-read-ffbdd8bf080155209516781f051ac271.us.langgraph.app/mcp

5. Add these two custom headers:

   Header	Value
   x-api-key	The user's valid LangSmith API key
   x-slack-user-token	The user's Slack user OAuth token

6. Save the server. Arcade sends both headers automatically on every MCP request.
7. Request IT to add to Fleet. The MCP will appear in "Integrations" tab.

Fleet custom MCP setup

Each Fleet user must configure the MCP with their own Slack user token. Do not share one Fleet MCP credential profile between users: Liam's profile must contain Liam's Slack token, Morgan's must contain Morgan's, and so on.

Prerequisites

Obtain both credentials before starting:

1. A valid LangSmith API key authorized to access the deployment.
2. The current user's Slack user OAuth token with channels:read and groups:read scopes.

Keep both values in an approved secret manager. Do not paste them into source control, chat messages, logs, or shared documentation.

Add the server in Fleet

1. Open Fleet's MCP settings and select Add custom MCP.

2. Enter a user-specific name, such as MCP-Slack-Channels-Liam.

3. Enter this exact URL, with no trailing slash:

   https://slack-private-channel-read-ffbdd8bf080155209516781f051ac271.us.langgraph.app/mcp
   

4. Select Static Headers. Do not select either OAuth option.

5. Add these two headers:

   Header	Value
   x-api-key	The user's valid LangSmith API key
   x-slack-user-token	The user's Slack user OAuth token

6. Save the server. Fleet sends both headers automatically on every MCP request.

If Fleet says the URL already exists, edit the existing MCP entry for that user instead of creating a duplicate. Replace its authentication mode and headers with the values above.

Initialize and use the channel directory

The first list request does not perform a slow Slack scan automatically. Ask the agent to call refresh_channels once to create the authenticated user's durable /channels/directory.jsonl file. The initial refresh can take approximately 15–20 seconds for a large workspace.

After that refresh, normal tools read the durable file and typically return in under a second:

• list_eval_channels
• list_customer_channels
• list_public
• list_private
• list_channels
• match_accounts_to_channels

Run refresh_channels again whenever the user wants the saved directory updated. Refreshing one user never changes another user's file.

For Salesforce reconciliation, pass the relevant account names directly to match_accounts_to_channels instead of loading the entire Slack directory. The tool searches every cached channel type—including customer-, eval-, internal-, ext-, external-, langchain-, and future prefixes—and returns compact structured results:

• matched: one deterministic exact normalized match.
• ambiguous: multiple exact matches or bounded possible matches requiring review.
• unmatched: no candidate channel.

Every match includes channel_id, channel_name, channel_type, visibility, and the definitive is_member value for the authenticated Slack user. The tool does not use an LLM for fuzzy matching and accepts at most 200 accounts per call.

Fleet should make one match_accounts_to_channels call for the Salesforce account set. It should not run separate customer/eval/private listing passes or copy the full directory into memory.

Suggested verification requests:

Refresh my Slack channel directory.
List every eval- channel visible to me.
List my private Slack channels.

Troubleshooting

Error	Resolution
MCP OAuth discovery failed or dynamic client registration error	Reconfigure the server to use Static Headers, not OAuth.
Authentication failed	Verify both headers, confirm the LangSmith key is active, and confirm the Slack token is a valid user OAuth token.
No saved channel directory exists yet	Call refresh_channels once, then retry the list request.
Fleet reports that the URL already exists	Edit the user's existing MCP entry instead of adding another copy.
A list request returns old channels	Call refresh_channels to replace that user's snapshot.
A refresh takes several seconds	This is expected: refresh queries Slack. Normal list tools read the stored file and should be much faster.

Because this is a model-backed agent, Slack tool results are sent to Claude through LangSmith Gateway so the model can compose its response.

Every directory operation is also exposed as a direct, argument-free MCP tool:

• list_channels
• list_public
• list_private
• list_eval_channels
• list_customer_channels
• refresh_channels
• match_accounts_to_channels (accepts accounts; all other direct tools are argument-free)

These tools bypass Claude. List operations read a durable, user-isolated Deep Agents virtual file backed by LangGraph Store, which survives deployment restarts and is shared across replicas. The conversational agent can read and search this file but cannot modify it. If the file is missing, list tools return immediately with an instruction to run refresh_channels; they never silently fall through to a slow Slack scan. refresh_channels creates or replaces the snapshot. The store contains channel metadata only—never either credential. The file uses JSONL with one minimized channel record per line, so Deep Agent grep calls return individual channels instead of a multi-megabyte single line. A lightweight five-minute process cache still collapses concurrent cold misses, with a one-hour stale fallback during temporary Slack rate limits.