Quotas, rate limits, and request ceilings

Signal applies three different kinds of limits: billing-plan quotas, endpoint rate limits, and request-shape ceilings. A useful incident report names the specific boundary instead of calling every 429 or validation failure a quota problem.

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Live plan quotas

Open Settings → Usage for the active project’s current billing period, entitlements, usage, and remaining capacity. That page is authoritative for plan-controlled values such as sends, projects, domains, webhooks, API keys, retention, and enabled add-ons.

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Endpoint rate limits

Each API operation documents its own request policy. For the current email operations: A rate-limited call returns 429 Too Many Requests and can include Retry-After. Wait for that interval when present, use bounded backoff with jitter, and preserve the original idempotency key when retrying an email admission request.

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Request-shape ceilings

• One direct fan-out or audience send can produce at most 50 recipient messages.
• One batch request can contain at most 100 independent email items.
• An audience larger than 50 is rejected; Signal does not silently truncate it.
• Attachments must fit the configured decoded-size limit.
• A message can contain at most 500 MIME parts, including body parts and attachments.
• An explicit schedule must be at least 30 seconds and no more than 30 days in the future.

These boundaries are not interchangeable. For example, a valid 100-item batch can still be rejected by a plan quota, and a project with remaining send capacity can still receive a per-key 429.

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Scheduled traffic

Signal admits a scheduled message first and consumes send quota when delivery is due. Capacity planning therefore has to include already scheduled traffic as well as new requests.

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What to capture during an incident

• The operation and HTTP status.
• The problem type, title, detail, and any structured fields.
• Retry-After when returned.
• The request count or recipient count involved.
• The Usage page’s current period and remaining entitlement.
• The Signal email ID or batch item index, without logging credentials or message content.