emcli/specifications/SPEC.md

# emcli — Specification

**Status:** Draft for implementation
**Source:** `specifications/PRD.md`
**Date:** 2026-06-21

## 1. Summary

`emcli` is a single cross-platform Go binary that mediates all email access for an AI
agent. The agent never holds email credentials and never connects to IMAP/SMTP directly —
every read and send passes through `emcli`, which enforces user-configured restrictions.
The goal is to contain the blast radius of agent hallucination when operating on live
email: even with faulty instructions, the agent cannot read mail it is not permitted to
see or send mail to recipients it is not permitted to contact.

## 2. Goals & non-goals

### Goals
- Single static binary, no runtime dependencies, cross-compiled per OS/arch.
- Email credentials and OAuth tokens are encrypted at rest and never exposed to the agent.
- Per-account enforcement of read-only/read-write mode, inbound/outbound whitelists, and
  subject filtering.
- Machine-readable JSON for agent commands; human-readable/interactive output for admin.
- Auditable history of agent actions with configurable retention.

### Non-goals
- Not a full email client (no threading UI, no flags/labels management beyond what is
  needed, no draft management).
- No graphical UI. TUI is used only for init and (re)configuration.
- No multi-user/server mode — `emcli` is a local utility invoked per-process.

## 3. Technology choices

| Concern | Choice |
| --- | --- |
| Language/runtime | Go (no CGO) |
| IMAP | `github.com/emersion/go-imap` |
| SMTP | `github.com/emersion/go-smtp` |
| MIME parsing/building | `github.com/emersion/go-message` |
| SASL / XOAUTH2 | `github.com/emersion/go-sasl` |
| SQLite | `modernc.org/sqlite` (pure Go) |
| TUI | `github.com/charmbracelet/bubbletea` (+ `lipgloss`) |
| Crypto | Go stdlib `crypto/aes` + `crypto/cipher` (AES-256-GCM) |

Rationale: the emersion email libraries are mature and designed to be used together,
including XOAUTH2 for Gmail. Pure-Go SQLite avoids CGO so the binary is genuinely static
and trivially cross-compiled.

## 4. Architecture

The binary is organized into independently testable packages:

- **`store`** — encrypted SQLite config and state. Owns the schema, migrations, and
  field-level encryption of secret columns.
- **`policy`** — pure enforcement functions: mode (RO/RW), whitelist-in, whitelist-out,
  subject regex, folder access. No I/O. The single gate every agent action passes through.
- **`mail`** — IMAP read and SMTP send, including SASL XOAUTH2 and password auth.
- **`oauth`** — loopback-redirect consent flow, refresh-token storage, automatic
  access-token refresh.
- **`audit`** — append-only action log with retention-based purge.
- **`cli`** — command dispatch and the two output surfaces (agent JSON, admin
  human-readable/TUI).

### Trust boundary
- The agent invokes only the **agent commands** (Section 7.1).
- `EMCLI_KEY` is supplied by the environment/orchestrator that launches `emcli`, never as
  an argument the agent constructs. The agent has no command that reveals secret values.
- All policy decisions happen inside `emcli`; the agent cannot bypass them because it has
  no other path to the mail servers.

## 5. Configuration & secrets

- **Encryption key:** `EMCLI_KEY` env var, a base64-encoded 32-byte key (AES-256). If
  absent or malformed, every command that touches the DB fails closed with an error
  envelope; no plaintext fallback.
- **Database path:** `EMCLI_DB` env var; default `~/.config/emcli/emcli.db`
  (`%AppData%\emcli\emcli.db` on Windows).
- **Field-level encryption:** secret columns are stored as AES-256-GCM ciphertext with a
  random 96-bit nonce per value, prefixed to the ciphertext. Non-secret config remains
  plaintext for debuggability. Decryption with the wrong key fails (GCM auth tag) and is
  surfaced as an error, never silently ignored.

Secret columns: account password, OAuth client secret, OAuth refresh token.

## 6. Data model (SQLite)

```
accounts
  id                   INTEGER PK
  name                 TEXT UNIQUE          -- agent-facing identifier
  mode                 TEXT                 -- 'RO' | 'RW'
  imap_host            TEXT
  imap_port            INTEGER
  imap_security        TEXT                 -- 'tls' | 'starttls'
  smtp_host            TEXT                 -- nullable for RO accounts
  smtp_port            INTEGER
  smtp_security        TEXT                 -- 'tls' | 'starttls'
  auth_type            TEXT                 -- 'password' | 'oauth2'
  username             TEXT
  enc_password         BLOB                 -- encrypted (password auth)
  enc_oauth_client_id  BLOB                 -- encrypted (oauth2)
  enc_oauth_client_secret BLOB              -- encrypted (oauth2)
  enc_oauth_refresh_token BLOB              -- encrypted (oauth2)
  whitelist_in_enabled  INTEGER             -- 0 | 1
  whitelist_out_enabled INTEGER             -- 0 | 1
  subject_regex        TEXT                 -- nullable; blank/null = no subject filter

whitelist_in
  account_id           INTEGER FK
  address              TEXT                 -- exact addr or '@domain.com'

whitelist_out
  account_id           INTEGER FK
  address              TEXT

read_pointers
  account_id           INTEGER FK
  folder               TEXT
  uidvalidity          INTEGER
  last_uid             INTEGER
  PRIMARY KEY (account_id, folder)

audit_log
  id                   INTEGER PK
  ts                   TEXT                 -- RFC3339 UTC
  account              TEXT
  action               TEXT                 -- 'list' | 'get' | 'send'
  target               TEXT                 -- folder or recipient set
  result               TEXT                 -- 'allowed' | 'blocked'
  reason               TEXT                 -- nullable; populated on block

settings
  key                  TEXT PK
  value                TEXT
  -- includes: audit_retention_days, schema_version
```

Notes:
- Folders are agent-specified; there is no folder whitelist. Read state is tracked per
  `(account, folder)`.
- `read_pointers` stores `uidvalidity`; if the server reports a different `UIDVALIDITY`
  for a folder than the stored value, the pointer is reset (treated as `last_uid = 0`) and
  the new `uidvalidity` recorded.

## 7. Command surface

### 7.1 Agent commands (JSON output only)

All agent commands emit a single JSON object (Section 8) and nothing else on stdout.

**`emcli list --account <name> --folder <folder> [--new] [--limit N]`**
- Returns message headers only: `uid`, `from`, `to`, `subject`, `date`, `message_id`,
  `has_attachments`.
- `--new` returns only messages with `uid` greater than the stored pointer for
  `(account, folder)`, then advances the pointer to the highest UID returned.
- Without `--new`, the pointer is not advanced.
- `--limit` caps the number of messages returned (default applied if omitted; see 7.3).
- Whitelist-in and subject-regex filtering are applied before results are returned
  (Section 9).

**`emcli get --account <name> --folder <folder> --uid <uid>`**
- Returns full message: headers, decoded plain-text body, and attachments as
  `{name, size, mime, content_b64}`.
- If the message is filtered by whitelist-in or subject-regex, returns an error envelope
  (not-found) — the agent cannot retrieve filtered mail.

**`emcli send --account <name> --to <addr>… [--cc <addr>…] [--bcc <addr>…] --subject <s> --body <text> [--attach <path>]… [--reply-to <uid>]`**
- Sends a plain-text message via the account's SMTP endpoint.
- `--reply-to <uid>` fetches the source message's `Message-ID` and `References` and sets
  `In-Reply-To`/`References` headers so the reply threads correctly. The referenced UID is
  read from the same account (subject to inbound filtering — a filtered source UID cannot
  be replied to).
- Enforcement: RO accounts are rejected; whitelist-out (if enabled) must pass for every
  recipient across to/cc/bcc or the entire send is blocked (Section 9).

### 7.2 Admin commands (human-readable / TUI)

- **`emcli init`** — TUI flow: creates the DB (generating schema), adds the first account,
  and runs OAuth consent if the account is OAuth2.
- **`emcli account add | edit | remove | list`** — interactive add/edit; `list` prints a
  table (never secrets).
- **`emcli whitelist in|out add|remove|list --account <name>`** — manage whitelist entries.
- **`emcli config set|get`** — global settings (e.g. `audit_retention_days`).
- **`emcli audit list [--account <name>] [--limit N]`** — view recent audit entries.
- **`emcli doctor`** — verifies `EMCLI_KEY` is present and valid, the DB opens, and each
  account's IMAP/SMTP connectivity and auth succeed. Human-readable diagnostics.

### 7.3 Defaults & limits
- `list --limit` default: 50; maximum: 500.
- Attachment handling in `get`: full base64 contents are returned. (No size cap in v1; the
  caller is responsible for limits. Revisit if payloads prove unwieldy.)

## 8. JSON output envelope

Every agent command prints exactly one object:

```json
{
  "error": false,
  "error_detail": {},
  "data": {}
}
```

- `error` — boolean.
- `error_detail` — object; empty `{}` on success, otherwise `{ "code": "...", "message": "..." }`.
  Never contains secret values.
- `data` — command-specific payload; `{}` or `[]` when not applicable.
- Process exit code mirrors `error` (0 on success, non-zero on error) for scripting, but
  the JSON is authoritative.

## 9. Enforcement semantics

Enforcement lives entirely in the `policy` package and is exercised on every agent action.

### Inbound (read: `list`, `get`)
- If `whitelist_in_enabled`, the message sender must match a `whitelist_in` entry.
- If `subject_regex` is set (non-empty), the subject must match the regex.
- A message that fails either check is **invisible**: excluded from `list` results and
  not retrievable via `get` (returns not-found). The agent has no way to learn that the
  message exists.

### Outbound (send)
- If account `mode` is `RO`, `send` is rejected.
- If `whitelist_out_enabled`, every recipient (to + cc + bcc) must match a `whitelist_out`
  entry. If any recipient fails, the **entire** send is blocked — no partial send.

### Address matching (both directions)
- Case-insensitive.
- An entry of the form `@domain.com` matches any address at that domain.
- Any other entry matches a full address exactly.

### Audit
- Every action (`list`, `get`, `send`), allowed or blocked, writes one `audit_log` row.
- Blocked actions record a `reason` (e.g. `ro_mode`, `whitelist_out`, `filtered`).
- On each run that opens the DB, audit rows older than `audit_retention_days` are purged.

## 10. OAuth2 (Gmail and compatible)

- The user supplies their own OAuth client ID/secret (registered in their Google Cloud
  project) during account configuration.
- Consent uses the **loopback redirect** flow: `emcli` starts a temporary listener on
  `127.0.0.1:<ephemeral-port>`, opens the consent URL, captures the authorization code on
  redirect, exchanges it for tokens, and stores the **refresh token** (encrypted).
- Access tokens are obtained/refreshed automatically before IMAP/SMTP use and held only in
  memory.
- IMAP and SMTP authenticate via SASL XOAUTH2 using the current access token.

## 11. Error handling

- All agent-command failures return the JSON error envelope; they never crash with an
  uncaught panic or emit partial non-JSON output on stdout.
- Categories: configuration/key errors, DB errors, network/connection errors, auth
  errors, policy blocks, and not-found. Each maps to a stable `error_detail.code`.
- Secrets never appear in output, error details, or the audit log.

## 12. Testing

- **`policy`** — table-driven unit tests covering the matrix of mode × whitelist-in ×
  whitelist-out × subject-regex, including domain-match and case-insensitivity, and the
  "any recipient fails ⇒ whole send blocked" rule.
- **`store`** — encryption round-trip; decryption with the wrong key fails closed; schema
  migration; pointer reset on `UIDVALIDITY` change.
- **`mail`** — integration tests against a containerized IMAP/SMTP server (e.g. GreenMail
  or Dovecot) for list/get/send and threading headers.
- **`oauth`** — token exchange/refresh against a mocked authorization server; loopback
  capture logic unit-tested.
- **CLI** — golden-file tests of the JSON envelope for representative success and error
  cases; assert no secret ever appears in output.

## 13. Open items / future work

- Optional attachment size cap on `get` if payloads prove unwieldy.
- Additional auth mechanisms (e.g. OAuth for non-Google providers) follow the same model.
- Whitelist semantics are currently per-account only; global defaults with overrides are
  explicitly out of scope for v1.
```