Known differences

ServiceNow

A ServiceNow instance that answers the Table, CMDB Instance, Aggregate, Service Catalog, Knowledge and Attachment APIs pysnc and pysnow call, plus an MCP server.

Vendor reference: https://developer.servicenow.com/dev.do#!/reference/api/latest/rest/ ↗. Machine-readable: compat/servicenow.json.

Surfaces

Kind	Path	Notes
REST	`/api/now/table`	Table API, also under `/api/now/v1/table` and `/api/now/v2/table`
REST	`/api/now/cmdb`	CMDB Instance API, read-only; Aggregate API under `/api/now/stats`
REST	`/api/sn_sc/servicecatalog`	Service Catalog API
REST	`/api/sn_km_api/knowledge`	Knowledge API
REST	`/api/now/attachment`	Attachment API
OAuth	`/oauth_token.do`	`password`, `refresh_token` and `client_credentials` grants
MCP	`/mcp`	table tools and the `servicenow-mcp` tool set

API versions: Table API (unversioned, v1, v2), CMDB Instance API (unversioned, v1)

Supported

Encoded queries in sysparm_query (^, ^OR, ^NQ, LIKE, STARTSWITH, IN, ISEMPTY, BETWEEN, ORDERBY, dot-walks and javascript:gs.* date helpers)
sysparm_fields, sysparm_limit, sysparm_offset, sysparm_display_value and sysparm_exclude_reference_link, with X-Total-Count and a Link header on lists
ServiceNow's {"result": ...} responses and {"error": {"message", "detail"}, "status": "failure"} errors
Basic and bearer auth, and OAuth token exchange at /oauth_token.do
pysnc and pysnow against a live server
Aggregate API counts, averages, sums, min and max with sysparm_group_by and sysparm_having
Users, groups, CMDB, incidents, problems, changes, SLAs, Service Catalog requests, approvals and knowledge articles

Not supported

Tables outside the documented table list
CMDB Instance API writes; that API is read-only

Known differences and test guidance

Scenario	Difference from ServiceNow	In your tests
Unknown query conditions	Conditions on unknown fields are ignored, as ServiceNow does, so the query returns rows instead of an error.	Do not expect a 400 for a misspelled field in `sysparm_query`.
Writes on a self-run instance	Client writes, through REST and MCP, are refused unless the host enables them.	Enable writes on the instance you run before testing create, update or delete.
Attachment size	An uploaded file is kept up to 1 MiB.	Do not test large attachments against the replica.
Record ids	`sys_id`s are deterministic 32-hex values, stable across runs.	Read ids from the API; do not hard-code ids from a real instance.
Featured and most-viewed articles	Knowledge articles are ranked by ticket links and `sys_view_count`.	Do not assert on a specific ranking order.

Fault injection

Key	What the client sees
`throttle`	429 `{"error": "rate_limited", "message": "Too Many Requests"}` with `Retry-After`
`fail_next`	The next N calls fail as `throttle` does, then clear
`quota_limit`	429 `{"error": "quota_exceeded"}` once the per-window quota is spent
`error_rate`	500 for the given fraction of calls
`latency_ms`	Fixed added latency on every data-plane request

Applies to every system

The hosted data plane is read-only: a write is refused with 403 writes_disabled. A SOQL, GraphQL or search POST is a read and is answered.
Faults are injected through POST /_admin/faults on a simulator you run yourself; POST /_admin/faults/reset clears them.
Rate limits do not happen on their own unless a page says so. Use fault injection to exercise a client's backoff.
Distributions come from aggregated metadata sketches of data Eon backs up; no customer records; all Era data is simulated.