Sendmux token-efficient usage

Use this skill to choose the lowest-cost Sendmux route that still answers the task correctly.

Boundaries

• Do not ask the user to paste an API key.
• For API-key authentication, use send-capable smx_mbx_* keys or owner-approved Sending-resource smx_agent_* tokens for Sending calls, and smx_mbx_* keys for normal Mailbox calls.
• For a self-registered agent, reuse one durable CLI profile for reads. Sending stays blocked until owner approval, then sending:* commands exchange and cache a one-hour delegated token automatically.
• Its inbox is capped at 500 MiB before approval. Owner-approved sending raises it to at least 5 GiB first. Revoking sending does not itself change the current inbox storage allocation.
• For API-key authentication, use smx_root_* for Management calls. REST OAuth profiles can use their approved surfaces, scopes and mailboxes; route login and refresh to sendmux-cli.
• Do not default to MCP for every task. MCP is best when the required tool is curated; CLI and SDK cover broader surfaces.
• Do not pipe real attachments through model context as base64. Route attachment transfer to sendmux-attachments; prefer file_path, presigned URLs, CLI --attach, or SDK file helpers. Mailbox uploads cap each attachment at 7,500,000 bytes; Sending uploads cap each file at 18 MiB; MCP inline base64 caps at 32 KiB decoded.
• Do not read full mailbox bodies, every message, or every log row unless the user asks for full content and narrower calls cannot answer.

Validate credentials with the selected surface's get-connection CLI operation or MCP mailbox_get_connection, management_get_connection, or sending_get_connection. These checks need no mailbox selector and send no email; public OpenAPI discovery does not validate credentials.

Surface choice

Situation	Use	Why
Connected agent and curated tool exists	MCP tool	Small schema and no SDK boilerplate.
One-off terminal task	sendmux CLI with --json	Direct, scriptable, exposes the full generated operation set.
Application code or repeated workflow	SDK for the project already in use	Reuses client setup, pagination, headers, and retry helpers.
MCP lacks the needed operation	CLI for terminal work, SDK for code	Do not invent uncurated MCP tools.
No package/tooling available	Direct HTTP	Keep request bodies and headers aligned to OpenAPI.

Cheapest-call map

Task	Cheapest correct default
Send one outbound email	sending_send_email, CLI sending:send, SDK sendingSendEmail; include Idempotency-Key.
Send multiple outbound emails	sending_send_email_batch, CLI sending:send:batch, SDK sendingSendEmailBatch; do not loop single sends.
Send or read attachments	sendmux-attachments; use file_path, presigned upload/download URLs, CLI --attach, SDK file helpers, blob_id for mailbox sends, and attachment_id for Sending sends instead of inline base64.
Count matching mailbox messages	mailbox_count_messages, CLI mailbox:count-messages, SDK mailboxCountMessages.
Search mailbox text	mailbox_search_message_snippets, CLI mailbox:search-message-snippets, SDK mailboxSearchMessageSnippets; then fetch selected IDs.
Read several known messages	mailbox_batch_get_messages, CLI mailbox:batch-get-messages, SDK mailboxBatchGetMessages.
Update/delete several messages	Batch update/delete after explicit confirmation.
Resume broad mailbox sync	mailbox_get_changes, CLI mailbox:get-changes, SDK mailboxGetChanges.
Resume filtered mailbox sync	CLI/SDK mailbox:query-message-changes / mailboxQueryMessageChanges; MCP does not curate it yet.
Watch live mailbox events	CLI/SDK mailbox:stream-events / mailboxStreamEvents; MCP does not curate it yet.
Scan threads	List threads, then fetch one thread or its messages.
Manage domains/mailboxes/keys	Management MCP for curated create/list/get/update/suspend/resume/key tools; CLI/SDK for uncovered lifecycle work.
Manage sending accounts	CLI/SDK; MCP does not curate provider tools yet.
Manage webhooks	MCP for list/create/test; CLI/SDK for get/update/delete/rotate/delivery payloads.
Inspect spend, logs, metrics	Summary/metrics first; filter log lists with small limit, then fetch one selected row.

For an agent with no key, avoid manual protocol calls and token copying:

sendmux agent:register my-agent --default --json
sendmux mailbox:me:get --profile my-agent --json
sendmux agent:invite-owner owner@example.com --profile my-agent --json

Register once, then reuse the durable profile across processes. After owner acceptance and sending approval, use the same profile with sending:*; the CLI handles the one-hour delegated token exchange and cache.

Read less

For mailbox questions, reduce the result set before reading content:

1. Count when the user asks "how many" or when the query may be broad.
2. Search snippets with a small limit when the user needs examples.
3. Batch-get only selected message IDs.
4. Request clean body/content only when message text affects the answer.

CLI:

sendmux mailbox:count-messages \
  --query q=invoice \
  --query is_unread=true \
  --json

sendmux mailbox:search-message-snippets \
  --query q=invoice \
  --query is_unread=true \
  --query limit=10 \
  --json

sendmux mailbox:batch-get-messages \
  --body '{
    "ids": ["eml_abc", "eml_def"],
    "body_mode": "clean_json",
    "max_body_chars": 4000,
    "strip_quotes": true,
    "strip_signature": true,
    "include_attachments": "metadata"
  }' \
  --json

Write fewer requests

Batch when there is more than one target.

sendmux sending:send:batch \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --body-file ./messages.json \
  --json

sendmux mailbox:batch-update-messages \
  --body '{
    "ids": ["eml_abc", "eml_def"],
    "seen": true,
    "if_in_state": "state_from_prior_read"
  }' \
  --json

For batch sends, inspect every per-message result before reporting success. Batch can contain mixed outcomes.

Sync by delta

Use sync endpoints instead of re-listing stable data.

Broad mailbox sync:

sendmux mailbox:get-changes \
  --query messages_since_state="$MESSAGES_STATE" \
  --query folders_since_state="$FOLDERS_STATE" \
  --query threads_since_state="$THREADS_STATE" \
  --query limit=100 \
  --json

Filtered message sync:

sendmux mailbox:query-message-changes \
  --query since_query_state="$QUERY_STATE" \
  --query q=invoice \
  --query is_unread=true \
  --query limit=100 \
  --json

Store the returned state token. Continue with the same filters only while has_more is true and the next page is needed.

Transfer less

• Use small limit values on list calls.
• Follow pagination.next_cursor only until enough evidence has been gathered.
• Prefer summary or metrics endpoints before log lists.
• Use If-None-Match for repeated detail reads that previously returned an ETag.
• Use If-Match for updates when the prior read returned an ETag.
• For inbound attachments, fetch metadata and use the short-lived download_url; if it expires, re-fetch metadata instead of building URLs manually.
• For outbound attachments, a file path or presigned URL is usually under 100 tokens, while base64 can burn thousands of tokens and corrupt large files.

CLI conditional examples:

sendmux management:get-email-log \
  --path public_id=dlog_abc \
  --if-none-match "$ETAG" \
  --json

sendmux management:update-mailbox \
  --path public_id=mbx_abc \
  --if-match "$ETAG" \
  --body '{"display_name":"Agent Inbox"}' \
  --json

SDK helpers:

import {
  conditionalHeaders,
  idempotencyHeaders,
  paginate,
  responseEtag,
} from "@sendmux/core";

const headers = conditionalHeaders({ ifNoneMatch: priorEtag });
const writeHeaders = {
  ...conditionalHeaders({ etag: priorEtag }),
  ...idempotencyHeaders(operationKey),
};

Retry safely

Use Idempotency-Key on supported mutations so retrying does not create duplicate work.

Good candidates:

• sending:send and sending:send:batch.
• mailbox:send-message.
• Management creates, mailbox key creation, suspend/resume, provider mutations, webhook create/rotate/test.

When retrying application code, prefer SDK retry helpers only for safe reads or idempotent writes. Non-idempotent writes should fail rather than risk duplicate side effects.

Routing

• Setup, key scopes, first call: sendmux-getting-started.
• Email send bodies and SMTP-vs-HTTP choice: sendmux-send-email.
• Attachment upload/download mechanics: sendmux-attachments.
• Mailbox read/search/sync/triage/reply details: sendmux-mailbox-agent.
• Management domains, mailboxes, webhooks, billing, logs: sendmux-management.
• CLI syntax and profiles: sendmux-cli.
• MCP installation and client config: sendmux-mcp-setup.