diff --git a/authentication.mdx b/authentication.mdx
new file mode 100644
index 0000000..427727b
--- /dev/null
+++ b/authentication.mdx
@@ -0,0 +1,140 @@
+---
+title: "Authentication"
+description: "How authentication works in the Recoup API — API keys, access tokens, and organization access control."
+---
+
+## Overview
+
+Every request to the Recoup API must be authenticated using exactly one of two mechanisms:
+
+| Method | Header | Use case |
+|--------|--------|----------|
+| API Key | `x-api-key` | Server-to-server integrations |
+| Access Token | `Authorization: Bearer <token>` | Frontend apps authenticated via Privy |
+
+Providing both headers in the same request will result in a `401` error.
+
+---
+
+## API Keys
+
+API keys are the primary way to authenticate programmatic access to the Recoup API.
+
+### Creating an API Key
+
+1. Navigate to [chat.recoupable.com/keys](https://chat.recoupable.com/keys)
+2. Enter a descriptive name (e.g. `"Production Server"`)
+3. Click **Create API Key**
+
+<Warning>
+Copy your API key immediately — it is only shown once. Keys are stored as a secure HMAC-SHA256 hash and cannot be retrieved after creation.
+</Warning>
+
+### Using an API Key
+
+Pass your key in the `x-api-key` header:
+
+```bash
+curl -X GET "https://api.recoupable.com/api/tasks" \
+  -H "x-api-key: YOUR_API_KEY"
+```
+
+---
+
+## Access Tokens (Privy)
+
+If you're building a frontend application that authenticates users via [Privy](https://privy.io), you can pass the user's Privy JWT as a Bearer token instead of an API key.
+
+```bash
+curl -X GET "https://api.recoupable.com/api/tasks" \
+  -H "Authorization: Bearer YOUR_PRIVY_JWT"
+```
+
+The API validates the token against Privy, extracts the user's email, and resolves it to the corresponding Recoup account. Bearer tokens always authenticate as a personal account — they cannot act on behalf of an organization.
+
+---
+
+## Personal vs. Organization API Keys
+
+API keys inherit the type of account they were created under.
+
+### Personal API Keys
+
+- Created by a standard user account
+- Can only access **your own account's data**
+- `orgId` is `null` on the resolved auth context
+
+### Organization API Keys
+
+- Created by an organization account (an account that has members)
+- Can access data for **any member account** within the organization
+- `orgId` is set to the organization's account ID
+
+<Info>
+An account is recognized as an organization when it has at least one member in the `account_organization_ids` table.
+</Info>
+
+---
+
+## How We Determine Key Type at Creation
+
+When a key is created under an account, the API checks whether that account has any organization members:
+
+```
+Has members in account_organization_ids?
+  ├── Yes → Organization API Key (orgId = accountId)
+  └── No  → Personal API Key    (orgId = null)
+```
+
+This check happens at **authentication time** (not creation time), so key behavior automatically reflects the current state of the account.
+
+---
+
+## How We Verify Access on API Calls
+
+Every authenticated request goes through `validateAuthContext`, which enforces the following access rules:
+
+### Personal API Key or Bearer Token
+
+Can only access their **own account**. Attempting to pass an `account_id` belonging to another account returns `403 Forbidden`.
+
+### Organization API Key
+
+Can access **any account that is a member of the organization**:
+
+```
+Request includes account_id override?
+  ├── Same as key owner → Allowed (self-access)
+  ├── Is a member of the org → Allowed
+  └── Not a member → 403 Forbidden
+```
+
+Membership is verified by querying the `account_organization_ids` table for a record linking the target account to the organization.
+
+<Note>
+The Recoup internal admin organization has universal access to all accounts.
+</Note>
+
+### Organization Access via `organization_id`
+
+Some endpoints accept an `organization_id` parameter. When provided, the API additionally validates that the authenticated account is either:
+
+- A **member** of the organization, or
+- The **organization account itself**
+
+---
+
+## Error Responses
+
+| Status | Cause |
+|--------|-------|
+| `401` | Missing or invalid credentials, or both `x-api-key` and `Authorization` headers provided |
+| `403` | Valid credentials but insufficient access to the requested `account_id` or `organization_id` |
+
+---
+
+## Security Notes
+
+- API keys are **never stored in plaintext** — only an HMAC-SHA256 hash (keyed with your project secret) is persisted in the database
+- **Never include `account_id` in your API key creation request** — the account is always derived from your authenticated credentials
+- Rotate keys immediately if compromised via the [API Keys Management Page](https://chat.recoupable.com/keys)
diff --git a/docs.json b/docs.json
index 3aa26de..7f818b8 100644
--- a/docs.json
+++ b/docs.json
@@ -19,6 +19,8 @@
               "index",
               "quickstart",
               "mcp",
+              "sdk",
+              "authentication",
               "cli",
               "api-reference/sandboxes/create"
             ]