Deterministic API mocks for CI and agents

Your CI is red again. Again, it’s not your code.

A vendor sandbox is down, a partner API is throttling your test run, or your agent sent a tool call you never recorded. Gostly records the services you depend on and replays them — deterministic, offline, and it answers requests you’ve never seen. Ship on your schedule, not your dependencies’.

On a structurally new request, most mocks miss — a 404, or a silent call to the real API. Gostly answers it — offline, from a twin it learned from your own traffic.

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gostly.internal / payments-api

the actual product · take the upstream offline, the tests stay green

the readiness gap: 83% / 29%
the contract gap: 17%
the verification tax: 45%

─ 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. Only 17% of teams contract-test that the API still matches its spec (Postman, 2025).

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. The 5xx that reaches prod is the one no fixture covered.

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.

─ ONE PROXY, EVERY CAPABILITY

Everything that ships in the box.

Verbatim replay is the spine. Around it: stateful flows, chaos, drift and repair, fidelity scoring, per-service adapters, TLS and WebSocket capture, PII redaction, webhook capture, and an audit trail — all inside your perimeter.

gostly.internal · tls handshake

encrypted capture ENCRYPTED

→ ClientHello

← ServerHello · certificate chain

✓ local CA trusted — once

🔒 TLS 1.3 established · traffic captured

· · · · · · · · · · · · · · · · · · · ·

→ upstream: client fingerprint matched

✓ HTTP/2 · looks like a real client

TLS capture & impersonation

All tiers

✓Trust the CA once, capture everything
✓Outbound fingerprint impersonation (Pro+)
✓HTTP/2 and HTTP/1.1

§ 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.

When a change passes CI against a stale fixture and still breaks in prod, the fixture was the problem. 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.

Runs entirely in your own infrastructure — your traffic never leaves your perimeter, and it is never mixed with another customer's data. Single sign-on (SAML and OIDC), role-based access control, and a full audit trail come standard on Team. Credentials are stripped before anything is written to disk. And because it keeps serving even when our platform is unreachable, the architecture — not a Trust Pack PDF — is the answer to “what happens if your vendor goes 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.

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