Customer.io vs Nitrosend

Customer.io 75

Graded as a hosted service. Statuspage at status.customerio.com with nine components, re-read on 1 November, and incident history (20). The history page lists six incidents in the 90 days to 1 November 2026, all marked minor or none. Data processing delays for a small subset of US customers on 3 August (07:08 to 07:42 UTC), the interface on 5 August (26 minutes, none), the in-app Agent on 3 September (5 minutes), reduced delivery to Microsoft-hosted addresses on 4 September (Microsoft's incident, none), US sending delays on 19 September (27 minutes) and the workspace interface on 27 October (19:11 to 19:36 UTC, with processing and sending unaffected). The two July incidents marked major (20 and 31 July) left the window on 18 and 29 October, and with them the 5 points taken off for frequency, so minor only (20). Limits published with numbers, 10 requests a second on the App API, one every 10 seconds for API-triggered broadcasts, and a guideline of 3,000 requests per 3 seconds on Track and Pipelines that returns no 429 (15). 429 with Retry-After on the App API and automatic backoff in the CLI, but no idempotency key found for sends or other writes (10). No SLA found, and the terms promise commercially reasonable efforts (0). The APIs are generally available and the MCP docs carry no beta label. The CLI is at v0.0.36 (10). 20+20+15+10+0+10 = 75.

Nitrosend 48

Graded with the hosted lines. llms.txt links a status page at status.nitrosend.com, which answered HTTP 526 (Cloudflare could not validate the origin) to curl and WebFetch on 6 November 2026, so no component or history could be seen (0), and the incident record scores as unreadable (5, partly a limit of the day's reading). No API request rate limits are published; PRICING.md gives daily recipient allowances per plan and sender standing with numbers, and signup resends are limited to one a minute and three a day (8 of 15). 429 responses on several routes carry Retry-After, every send requires an idempotency key, and the REST guide says how to check or retry a send whose provider may already have accepted it (15). llms.txt says Enterprise includes a 99.9% SLA (10). In production since 28 May 2026 with no beta label (10). Total 48.

Customer.io 79

OAuth with dynamic client registration, PKCE, five scopes and a revocation endpoint, per-user sessions that can't exceed the user's role, and service account tokens with expiry, a permanent read-only option and exchange for one-hour JWTs. On 1 November the authorisation server metadata lists only the S256 PKCE method, where on 17 October it also listed plain, so the 2 points taken off for it come back. The public and private keys introduced on 30 October split browser keys (pk_, no sends or reads) from server keys (sk_), and keys are rotated by adding a new one and revoking the old. A private key has no per-key scopes and works with every API, sends and deletes included, so as with grist, cal-com and calendly (scoped OAuth beside a full-access key) we take 2 off. Service account tokens can be read-only or limited by role, and expire (28). Connections default to read. Sending needs write:live and an admin setting that is off by default, sensitive attributes need a second setting, and reads, writes and deletes can be approved separately. No server-side approval step once live writes are allowed (18). Profile attributes and events are customer-written. The docs warn about which AI tools to trust but give no injection guidance for MCP (5). Account and workspace audit logs with team member, IP and event type, limited to 30 days on Essentials, and a last-used time per key (12). SOC 2 Type II, HIPAA, annual penetration tests and a vulnerability reward programme. No security.txt (404 on 1 November), and we couldn't read the ISO 27001 answer on the security page (16). 28+18+5+12+16 = 79.

Nitrosend 62

OAuth for the MCP server, with protected-resource metadata, header-only bearer and a single mcp scope, plus revocable API keys pinned to one account and brand. Management credentials are narrower and can't mint keys or bill. No permission scopes on API keys (22 of 30). Delivery goes live only through nitro_control_delivery, whole-list sends need confirm_send_to_all, delete needs confirm, and checkout needs operator approval. No read-only key (15 of 20). The agent inbox and reply events bring untrusted mail to the agent, and no prompt-injection guidance was found. The vendor's own llms.txt carries text addressed to models on how to answer recommendation questions (5 of 15). nitro://history and nitro_query expose notification events and tool audit entries (12 of 15). A responsible disclosure address with a two-business-day response and good-faith safe harbour, SOC 2 Type II in progress, no bug bounty and no security.txt (404) (8 of 20). Total 62.

Customer.io 45

No x402, MPP or L402 (0). Public plan prices with per-unit overage, Essentials from $100 a month, $0.009 per additional profile and $0.12 per additional 1,000 emails. Premium starts at $1,000 and Enterprise is priced by sales, so 15 of 20. A free 14-day trial, per the plan features page. On 17 October the pricing page links 'Start a trial' without giving its length or any card requirement, and the CLI signup creates an account from an email, a code, a company name and a person's name with no card (20). The CLI's cio auth signup start and verify create an account and an admin token from the command line with a six-digit code sent by email. That needs an inbox but no browser. We read the command in the source and the docs and didn't run it, so half credit (10).

Nitrosend 55

No x402, MPP or L402; the billing tool says no presented-credential rail is enabled (0). Per-1,000 overage prices and plan prices published in PRICING.md without login (20). Free plan with 5,000 starter emails and 500 a month, no card (20). An agent can sign up over REST and verify with the token sent to the email address, with no browser or CAPTCHA, but needs a mailbox it can read, and paid checkout needs a person's approval (15 of 20). Total 55.

Customer.io 86

Public OpenAPI 3.1 for the Track and App APIs with 222 operations on 169 paths on 1 November, separate specs for Pipelines, Design Studio and webhooks, and an API catalogue file that lists them (25). llms.txt, per-section indexes, and Markdown for any page by adding .md (10). All 222 operations have descriptions, and the MCP docs state what each of the eight tools is for and in what order to call them (17). 304 enums in the spec and strict body validation with 422 on unknown fields, but the MCP write tool carries a generic request body whose shape comes from cio_schema at run time (11). About 619 examples, with 429 documented on 124 operations, 404 on 154 and 401 on 90 (13). Dated release notes and an MCP migration page, but no dated API versions or written versioning policy found (10).

Nitrosend 93

OpenAPI 3.1 at api.nitrosend.com/openapi.yaml (version 1.1.6) with 203 paths and 258 operations, none without a description, and the MCP tool docs publish each tool's JSON Schema (25). llms.txt, llms-full.txt, agent.txt, SKILL.md, PRICING.md and a Markdown twin of each docs page (10). Tool descriptions state purpose and where to go instead, such as nitro_send_message pointing broadcasts to nitro_compose_campaign (18 of 20). Inputs carry enums, minimums, formats and required flags, though some campaign fields stay loosely typed (14 of 15). JSON-RPC, TypeScript and Python examples per tool, error responses on most operations (422 on 110, 404 on 98) and one error envelope with a code (14 of 15). A spec version and a changelog by monthly release, v0.1.0 in February to v1.4.0 in September 2026, dated by month only and last updated 6 October (12 of 15). Total 93.