Restrict Anthropic Sign-Ins to One Org

Overview

The Restrict Anthropic sign-ins recipe creates this network rule and policy for you. See Recipes.

Anthropic Tenant Restrictions let a proxy allowlist organization IDs. Your proxy injects anthropic-allowed-org-ids on Claude traffic. Anthropic then allows only those orgs for sign-in and API use. Formal adds that header with a request-stage rewrite . A network rule selects the Anthropic traffic that the policy runs on. Users on the Formal Endpoint can sign in only with your Anthropic org. Personal Claude accounts and other orgs receive a 403 from Anthropic. This guide uses the Formal Endpoint on macOS.

Find your Anthropic organization ID

You need the UUID Anthropic shows for your org.

Open Anthropic settings

For Enterprise, go to Settings → Account, or Organization settings → Organization. For Console orgs, go to Settings → Organization.

Copy the Organization ID

Copy the UUID. It looks like 000a0000-a00a-00a0-a000-000000000000.

See Anthropic’s Tenant Restrictions article for where the ID appears in each plan.

Route Anthropic traffic through Formal

The Formal Endpoint must intercept Anthropic requests so the rewrite can run. A network rule selects that traffic.

Enable Transparent Mode

On macOS, install and start Transparent Mode:

formal transparent-proxy install
formal transparent-proxy enable
formal transparent-proxy status

Verify: the status command reports that Transparent Mode is running.

Create the network rule

Intercept Anthropic hostnames and classify them as LLM traffic.

Control Plane
Terraform

Navigate to Network Rules
Create a rule named anthropic-llm
Paste the CEL below
Leave Forward to Connector unset
Save the rule and set it to Active

{
  "condition": {
    "pre_tcp": true,
    "pre_tls": hostname == "api.anthropic.com" || hostname.endsWith(".anthropic.com"),
    "post_tls": technology == "llm"
  }
}

resource "formal_network_rule" "anthropic_llm" {
  name        = "anthropic-llm"
  description = "Intercept Anthropic LLM traffic on the Formal Endpoint"
  status      = "active"

  cel_expression = <<-EOT
    {
      "condition": {
        "pre_tcp": true,
        "pre_tls": hostname == "api.anthropic.com" || hostname.endsWith(".anthropic.com"),
        "post_tls": technology == "llm"
      }
    }
  EOT
}

Verify: send an Anthropic request and confirm it appears in Logs.

Apps can keep using api.anthropic.com. The Endpoint intercepts that hostname and applies policies.

Create the rewrite policy

The policy merges anthropic-allowed-org-ids into outgoing LLM request headers. object.union puts your value second, so it overwrites a client-supplied header. Replace <YOUR_ORG_ID> with the UUID you copied.

Control Plane
Terraform

Navigate to Policies
Click Create Policy
In Choose a Template, select Restrict Anthropic Sign-Ins To One Org
Replace <YOUR_ORG_ID> with the UUID you copied
Click Create Policy to save

You can also paste this Rego if you skip the template:

package formal.v2

import future.keywords.if

rewritten_headers := object.union(input.http.headers, {
  "anthropic-allowed-org-ids": ["<YOUR_ORG_ID>"],
})

request := {
  "action": "rewrite",
  "headers": rewritten_headers,
} if {
  input.resource.technology == "llm"
}

resource "formal_policy" "restrict_anthropic_org" {
  name        = "restrict-anthropic-org"
  description = "Allow Anthropic sign-ins only for the approved organization ID"
  status      = "active"

  module = <<-EOT
    package formal.v2

    import future.keywords.if

    rewritten_headers := object.union(input.http.headers, {
      "anthropic-allowed-org-ids": ["<YOUR_ORG_ID>"],
    })

    request := {
      "action": "rewrite",
      "headers": rewritten_headers,
    } if {
      input.resource.technology == "llm"
    }
  EOT
}

Header values must be arrays of strings. For more than one org, use a comma-separated list in a single value. Do not put spaces between IDs:

"anthropic-allowed-org-ids": ["<ORG_ID_1>,<ORG_ID_2>"],

Anthropic rejects duplicate anthropic-allowed-org-ids headers. Keep one header value. Do not list each org as a separate array element.

This policy matches every LLM request. If you also intercept other providers, scope the rule to Anthropic hostnames:

request := {
  "action": "rewrite",
  "headers": rewritten_headers,
} if {
  input.resource.technology == "llm"
  input.http.hostname == "api.anthropic.com"
}

Create the policy in Draft or Dry-run first. Then set it to Active.

Verify

Verify an allowed org:

Sign in to Claude with an account in your Anthropic org.
Send a Messages API request, or use Claude Code as usual.
Open Logs and confirm the rewrite ran.

Look for triggered_policies.type:rewrite on LLM requests. Sent headers should include anthropic-allowed-org-ids with your UUID. Verify a blocked org: Sign in with a personal Claude account, or call the API with a key from another org. Anthropic should return HTTP 403:

{
  "type": "error",
  "error": {
    "type": "permission_error",
    "message": "Access restricted by network policy. Contact IT Administrator.",
    "error_code": "tenant_restriction_violation"
  }
}

You can also probe the API with your org key after the header is in place:

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{"model":"claude-sonnet-4-6","max_tokens":32,"messages":[{"role":"user","content":"Hello"}]}'

Expected: HTTP 200 when the key belongs to <YOUR_ORG_ID>. HTTP 403 tenant_restriction_violation otherwise.

Troubleshooting

Possible causes:

Transparent Mode is not running
The network rule or the policy is Draft or Dry-run, not Active
Traffic uses a host the network rule does not match (claude.ai vs api.anthropic.com)

Fix:

Run formal transparent-proxy status
Set the network rule and the policy to Active
Add the Anthropic hostname the app uses to the network rule

Requests return 403 tenant_restriction_violation

Possible causes:

The UUID is not the org that owns the account or API key
Extra spaces or quotes in the header value
You are testing with a personal account (this is expected)

Fix:

Re-copy the Organization ID from Anthropic settings
Keep a single UUID string in the header array
Sign in with an account in that org

Requests return 400 for the header

Possible causes:

More than one anthropic-allowed-org-ids header on the request
Multiple array elements instead of a comma-separated string

Fix:

Use object.union so Formal overwrites the header
Put every org ID in one comma-separated value

Next Steps

LLM Resources

Proxy Anthropic and other LLM APIs

Rewrite Action

Header rewrite reference

LLM Policy Inputs

Request-stage LLM fields and examples

Formal Endpoint

Enable Transparent Mode

Network Rules

Select which Endpoint traffic Formal intercepts

Getting Started

Example Use Cases

AI Governance

Configuring Formal

Integrations

Configuration

Restrict Anthropic Sign-Ins to One Org

Overview

Find your Anthropic organization ID

Route Anthropic traffic through Formal

Create the rewrite policy

Verify

Troubleshooting

Next Steps

LLM Resources

Rewrite Action

LLM Policy Inputs

Formal Endpoint

Network Rules

​Overview

​Find your Anthropic organization ID

​Route Anthropic traffic through Formal

​Create the rewrite policy

​Verify

​Troubleshooting

​Next Steps

LLM Resources

Rewrite Action

LLM Policy Inputs

Formal Endpoint

Network Rules

Overview

Find your Anthropic organization ID

Route Anthropic traffic through Formal

Create the rewrite policy

Verify

Troubleshooting

Next Steps