Synthflow vs. Vapi: enterprise contract versus self-serve infrastructure

Synthflow and Vapi can both power AI phone agents, but they now present opposite buying paths. For a buyer seeking a Synthflow alternative, Vapi offers a self-serve route where Synthflow's public pricing is sales-led and enterprise-only. Vapi's Build plan charges for call hosting while passing speech, model, voice, and telephony costs through separately. Synthflow pricing, accessed 2026-09-08; Vapi pricing, accessed 2026-08-18.

For a buyer comparing Synthflow vs. Vapi, the first decision is not a few cents per minute. It is whether the team wants a scoped enterprise engagement or developer infrastructure assembled from provider accounts. Synthflow does not publish enough pricing detail to compute a workload. Vapi publishes the hosting line but not the complete configured call cost. Both require more diligence than multiplying one advertised rate.

A third shape is pay-as-you-go infrastructure with an all-in engine rate. ThunderPhone, the publisher of this page, is one and is included transparently after the two named vendors. Every third-party fact below comes from the linked first-party page on the stated access date.

Synthflow: a sales-led enterprise engagement

Synthflow's pricing page shows one plan: Enterprise. “Enterprise contracts start at $30,000 annually,” with final pricing scoped around call volume, concurrency, telephony setup, integrations, security needs, and launch support. The page publishes no per-minute rate, monthly plan, included-minute block, overage rate, or free trial. The call to action is Contact Sales. Synthflow pricing, accessed 2026-09-08.

The documentation describes the same model directly: “Synthflow uses sales-led pricing,” and existing customers should not use public documentation as a quote. Enterprise agreements can contain custom rates, allowances, carryover, overage terms, and invoice terms, but those details are not public. Earlier Pro, Growth, and Agency plans were issued with fixed concurrency limits and are no longer available for new subscriptions. Synthflow billing overview, Enterprise docs, and concurrency docs, accessed 2026-09-08.

Synthflow does publish detailed metering rules. Calls are measured per second from the point when the human and AI connect; partial seconds round up and seconds pool across calls. Four calls totaling 125 seconds become two reported usage minutes. A no-answer or hangup_on_voicemail call is recorded as a flat five seconds, while failed and user-canceled calls are not billed. Billing stops measuring AI usage when a transfer completes. Every five AI-generated chat messages count as one usage minute, and simulations can be metered separately. Synthflow usage docs, accessed 2026-09-08.

New-customer concurrency and call-attempts-per-second capacity are scoped with Synthflow rather than published. The docs say excess calls can return a 429 error or be queued briefly. Telephony choices on the plan card include native telephony, SIP trunking, or approved enterprise telephony; international pricing can vary by destination, carrier path, and setup. Synthflow pricing, usage docs, and concurrency docs, accessed 2026-09-08.

Synthflow's public compliance lists differ by page. Its docs name SOC2, GDPR, ISO 27001, and additional reviews where applicable; its homepage names SOC 2, HIPAA, PCI DSS, and GDPR. The reviewed sources do not establish one reconciled list or the agreement terms for a specific deployment. Synthflow homepage and Synthflow Enterprise docs, accessed 2026-09-08.

Vapi: self-serve hosting with provider pass-through

Vapi's Build plan lists call hosting at $0.05 per minute and “60+ minutes included,” without stating whether that allowance is one-time or monthly. Speech-to-text, language-model, and text-to-speech usage bill at provider cost on top. Customers can bring their own API keys, making those components $0 on Vapi's bill while receiving the corresponding provider bills directly. Telephony is separate and no carrier-minute price appears on the reviewed pricing page. Vapi pricing, accessed 2026-08-18.

Build includes 10 concurrent calls, then charges $10 per added line per month. Call history is retained for 14 days. The Scale tier moves to a fixed platform fee, committed volume, volume-based per-minute pricing, and custom terms; it lists SSO, RBAC, SOC 2, PCI, data-residency options, and customizable retention. Vapi pricing, accessed 2026-08-18.

Vapi's public pricing lists HIPAA compliance at $2,000 per month and Zero Data Retention at $1,000 per month. Its HIPAA docs state that HIPAA mode and Zero Data Retention are mutually exclusive. The homepage states SOC 2, HIPAA, and PCI compliance. GDPR was not stated on the reviewed Vapi pages. Vapi pricing, HIPAA docs, and homepage, accessed 2026-08-18.

Vapi provides up to five free phone numbers for U.S. national use and documents importing Twilio numbers. Bring-your-own SIP works through named provider paths or a generic integration using gateway and authentication details for any SIP provider. Vapi warns that IP-based authentication can produce routing issues on shared infrastructure. Vapi phone-calling docs and SIP docs, accessed 2026-08-18.

Vapi does not publish one platform-wide language count. Its multilingual docs show different provider ceilings for transcription and voices, and automatic detection depends on specified multilingual or auto-detect modes. Its Test Suites support chat tests and real phone calls from scripted scenarios, recorded voice results, and LLM evaluation against success criteria. Suites allow up to 50 cases with five runs each; tests are limited to 15 minutes, and voice tests bill like regular calls. Vapi multilingual docs and Test Suites docs, accessed 2026-08-18.

ThunderPhone (this site): all-in pay-as-you-go engines

ThunderPhone publishes this page and sells a third shape: pay as you go with no subscription or seat fee, as an all-in engine rate. Spark is 2¢/minute, Bolt is 5¢/minute, and Storm is 9¢/minute, covering speech recognition, the language model, and the voice in one number. Premium voice or language paths add up to 3¢/minute, and optional add-ons carry published per-minute surcharges. Telephony is setup-dependent.

The catalog spans 47 languages and 123 voices, with automatic mid-call switching when additional languages are configured. Telephony supports SIP trunks and bring-your-own numbers through direct VoIP connections or manual SIP configuration. Testing includes AI-caller simulations, reusable graded scenarios, regression suites with minimum pass-rate gates, CI execution, and live-traffic A/B experiments; simulations are billable real calls, with the charge shown before a run. The public compliance posture is GDPR and HIPAA compliant, and customers remain responsible for configuring their workflows, data handling, recordings, integrations, and notices for their own obligations.

At a glance

The workload row uses 2,000 calls × 5 connected minutes = 10,000 minutes per month, or 120,000 minutes per year, with fewer than 10 simultaneous calls, one number, no optional add-ons, and U.S. telephony where relevant. Arithmetic is illustrative, not a quote.

Choose Synthflow if…

• You want a scoped enterprise agreement. Pricing explicitly accounts for volume, concurrency, telephony, integrations, security, and launch support. Synthflow pricing, accessed 2026-09-08.
• A negotiated allowance and support model fit procurement. Enterprise docs describe custom allowances, carryover, overage, and invoice terms when included in the agreement. Synthflow Enterprise docs, accessed 2026-09-08.
• Per-second pooled metering is useful. Synthflow documents when metering begins, how seconds round and pool, and how unanswered or failed calls are treated. Synthflow usage docs, accessed 2026-09-08.
• You can validate capacity in a quote. Concurrency and call-attempt limits are sales-scoped rather than fixed by a public self-serve tier. Synthflow concurrency docs, accessed 2026-09-08.

Choose Vapi if…

• You need a self-serve Build plan. The public path starts with usage-based hosting instead of a $30,000 annual contract floor. Vapi pricing, accessed 2026-08-18.
• You want direct provider control. Bring-your-own API keys can leave speech, model, and voice billing with the accounts your team already manages. Vapi pricing, accessed 2026-08-18.
• You want executable chat and voice tests. Vapi documents scripted Test Suites with recordings, LLM evaluation, and repeated runs. Vapi Test Suites docs, accessed 2026-08-18.
• You want a generic SIP path before an enterprise negotiation. Vapi documents any-provider gateway and authentication setup, subject to its routing warning. Vapi SIP docs, accessed 2026-08-18.

Choose ThunderPhone if…

• You want a self-serve total from one engine rate rather than an annual quote or a provider-cost stack.
• You need a fixed public language catalog with configured mid-call switching.
• You want direct VoIP or manual SIP configuration and can own the setup.
• Your release process needs graded regression suites, pass-rate gates, CI execution, or live experiments.

How to evaluate

1. Choose the buying model first. Decide whether procurement wants a negotiated annual package or a self-serve platform assembled and operated by the team.
2. Make both vendors price the same workload. Give Synthflow annual minutes, peak concurrency, telephony routes, integrations, and support needs. Add Vapi hosting to actual speech, model, voice, carrier, and compliance bills.
3. Separate prices from arithmetic. Synthflow's $2,500 monthly equivalent and $0.25 effective minute are math on the contract floor, not quoted workload prices. Vapi's $500 is hosting, not the complete call cost.
4. Test the production phone route. Validate number ownership, inbound and outbound directions, SIP authentication, international routes, transfers, and failure behavior.
5. Inspect the compliance package. Reconcile Synthflow's differing public lists; for Vapi, confirm the BAA, provider selection, retention, and the HIPAA/ZDR constraint.
6. Run identical test calls. Compare audible behavior, tools, transfers, grading evidence, repeatability, and the cost of simulations or voice tests.

FAQ

How much does Synthflow cost compared with Vapi?

Synthflow starts at $30,000 annually; Vapi starts with $0.05 per minute for hosting. Those figures are not like-for-like. Synthflow does not publish its allowance or overage, and Vapi does not include provider or telephony costs in the hosting rate. Synthflow pricing, accessed 2026-09-08; Vapi pricing, accessed 2026-08-18.

Is Synthflow still self-serve?

Not for new subscriptions on the reviewed public pages. The pricing page shows only Enterprise with Contact Sales, and the docs say older Pro, Growth, and Agency plans are no longer available to new customers. Synthflow pricing and concurrency docs, accessed 2026-09-08.

Can Vapi replace Synthflow for a small deployment?

Vapi offers a self-serve buying path, but feature equivalence is not established from the reviewed sources. Budget at least the $0.05 hosting line plus the selected providers and telephony, then test the required call flows. Vapi pricing, accessed 2026-08-18.

What would 10,000 minutes cost on each?

Neither complete total is public. Synthflow's floor equals $2,500 per month, but its allowance and overage are unknown. Vapi hosting computes to $500, before speech, model, voice, and telephony. ThunderPhone's published engine arithmetic for context is $200/$500/$900 before setup-dependent telephony and optional charges.

Do Synthflow and Vapi support SIP trunking?

Yes, both publish SIP paths. Synthflow lists SIP trunking under Enterprise telephony choices. Vapi documents named and generic any-provider integrations with gateway and authentication details. Synthflow pricing, accessed 2026-09-08; Vapi SIP docs, accessed 2026-08-18.

Which has the clearer public compliance posture?

Both require qualification. Synthflow's public lists differ between its docs and homepage. Vapi consistently states SOC 2, HIPAA, and PCI on the reviewed pages, but prices HIPAA separately, makes it mutually exclusive with Zero Data Retention, and does not state GDPR there. Synthflow Enterprise docs and homepage, accessed 2026-09-08; Vapi homepage, pricing, and HIPAA docs, accessed 2026-08-18.

Sources and freshness

For the line-by-line pricing breakdowns, see Synthflow pricing explained and Vapi pricing explained. Related product context is available in Synthflow alternatives, Vapi alternatives, and ThunderPhone vs. Vapi.

Synthflow pricing, billing, usage, Enterprise, concurrency, and compliance claims were checked September 8, 2026. Vapi claims retain their August 18 access date. ThunderPhone facts were checked against its published pricing on September 10, 2026. This comparison is documentation-based: no Synthflow quote, vendor account, provider bill, invoice, or live call was tested. Confirm the signed Synthflow allowance and the complete Vapi provider stack before relying on either total.