EARLY ACCESS · v0.9

Clean-room your LLM tool use.

Smoke-test agents against deterministic replays of your real upstream traffic. No live API calls. No surprises.

Recorded upstream behavior, not synthesized. Sturdier architecture, not smarter models. Gostly proxies your real API traffic, learns from it, and replays it deterministically across environments or in your CI/CD — for tests, for local development, for agent runtimes calling production at machine speed.

Sign up for a free trial See how it works

─ architectureinside your perimeter

the readiness gap: 83% / 29%
the silent rot: 70%
the verification tax: $14.2K

─ THE FAILURE MODE

Mocks are an unmaintained interface contract you wrote by hand. Then forgot about.

The contract between your service and someone else's API is the most load-bearing thing in your test suite — and the part with the least engineering rigor. Four failure modes show up on every team that grows past two services.

FIXTURES

Hand-written stubs go stale before the PR merges.

Every team writes the same brittle JSON. “Mocks are static, but reality evolves. The mock becomes a lie.”

DRIFT

Tests pass locally. Production fails because the API changed.

Three-week-old stubs against an upstream that added a new field. CI is green; reality isn’t. 75% of APIs don’t conform to their own spec (APIContext).

COVERAGE

5xx, 429, timeouts, partial responses get skipped.

Edge-case fixtures are written when a flaky test forces them — at 2 AM, by whoever’s on call. 70% of production API failures pass CI checks (InstaTunnel).

AGENTS

Multi-step flows are where stateful mocking falls apart.

An agent retrying the same upstream four ways needs the response trajectory to evolve. A static stub set says yes to everything, then the orchestration silently diverges.

─ DRIFT DETECTION

When the upstream changes, the diff is a row. Not a Slack thread.

A schema-diff loop compares the current capture window against the prior accepted baseline per (method, route). Every change becomes a drift event that’s actionable and audit-logged.

§ 08 BUILT FOR THREE ROOMS

Three audiences. One architecture. The same answer in every room.

The platform team wants tests that don't depend on someone else's status page. The agent operator wants a deterministic substitute for the runtime that's suddenly calling production APIs. The regulated buyer wants the architecture to be the answer, not a Trust Pack PDF. Gostly is the same product to all three.

▸ platform team

Tests that don’t depend on someone else’s status page.

Industry surveys put 70 % of production API failures inside changes that passed CI against stale fixtures. Gostly replays the recording the operator approved — bit-for-bit. The 2 a.m. flaky run because a third-party rate-limited a build slave stops happening.

API MOCKING USE CASES →

▸ agent operator

Verifiable replay for agent loops at machine speed.

Cisco’s 2026 State of AI Security found 83 % of organizations plan agentic AI deployments, but only 29 % feel secure enough to ship. The gap is verifiability. Gostly gives the operator a deterministic substitute — record the conversation once, replay it in evaluations forever. The LLM is not in the substitute’s hot path.

AGENT RUNTIME PATTERNS →

▸ regulated buyer

Structural redaction the procurement reviewer can read.

Twenty-two RLS tables. Sixteen-header floor. SAML, OIDC, RBAC, audit log — shipped, not roadmap. Four-hour offline grace through platform outages. The architecture, not a Trust Pack PDF, is the answer to “what happens if your platform is down?”

SECURITY MODEL →

─ READY WHEN YOU ARE

Recorded once. Replayed verbatim. Verifiable by construction.

Drop the proxy in. Run a real workload through it once. Have a byte-stable mock library by the end of the afternoon — plus drift detection, an SSRF-guarded webhook replay, and the audit trail to defend all of it.

Get started free Review the security model