PromptGate aims for predictable error semantics across every surface. This page is the reference.

REST surfaces (Gateway, Wrapper, Proxy)

All REST error responses share a shape:

{
  "ok": false,
  "error": "<human-readable message>",
  "detail": "<optional, more context>",
  "scope": "<optional, e.g. 'minute' for rate limits>",
  "retry_after": 38
}

The ok: false boolean is the canonical “this is an error” signal. Everything else is contextual.

HTTP status codes

Status	Cause	Client behaviour
400 Bad Request	Wrong project type for this surface (e.g. /v1/chat/completions against an ai_gateway project), malformed body.	Don’t retry. Fix the client.
401 Unauthorized	Missing / malformed / invalid token, token revoked.	Refresh the token. Don’t retry with the same one.
402 Payment Required	Pre-flight budget kill-switch — the request’s estimated cost would push month-to-date spend over a monthly cap (endpoint monthly_budget_usd or token monthly_budget_usd_cap). The upstream call was never made.	Wait for next month, raise the cap, or wait for spend to roll off.
403 Forbidden	Token belongs to a different project, or lacks the required scope.	Use a token with the right scope or reach out for one.
404 Not Found	Endpoint slug doesn’t exist or is inactive. Unknown alias in Wrapper.	Don’t retry; the resource is gone.
405 Method Not Allowed	API Gateway endpoint’s allowed_methods doesn’t include the request’s method.	Don’t retry.
422 Unprocessable Entity	Validation failure: input schema, content length, PII block, prompt injection, keyword blocklist, per-request token limit, SSRF block.	Don’t retry; fix the input.
429 Too Many Requests	Rate limit hit (per-minute or per-hour).	Wait Retry-After seconds, then retry.
500 Internal Server Error	Unexpected exception. Should never see this — file an issue.	Retry once after a delay; if persists, report.
502 Bad Gateway	Upstream provider failure after failover, output schema validation failure, OAuth refresh failure, upstream HTTP unreachable.	Retry with exponential backoff.
503 Service Unavailable	A required provider is disabled gateway-wide.	Don’t retry; admin needs to re-enable.

Error message taxonomy by surface

Gateway / Wrapper API

Message	Meaning
Missing Bearer token.	No Authorization header.
Invalid token format.	Doesn’t start with pg_live_ / pg_test_.
Invalid or revoked token.	Token doesn’t exist or is_active=false.
Missing required scope: 'chat'. Token has: models.	Scope mismatch.
Token does not belong to this project.	Cross-project use.
This project is not an AI Gateway. Use /api/{uuid}/v1/chat/completions for AI Wrapper projects.	Wrong project type.
Endpoint 'foo' not found or inactive.	Slug doesn’t exist or is_active=false.
Rate limit exceeded.	429 with Retry-After header + body fields scope and retry_after.
Request exceeds endpoint per-request token limit: ~12000 tokens estimated, cap is 8000.	Per-request token cap (422).
Endpoint monthly budget would be exceeded: ~$24.99 spent + ~$0.05 estimated for this request, budget $25.00. Resets at the start of next month.	Endpoint pre-flight budget gate (402).
Token monthly budget would be exceeded: ~$4.99 spent + ~$0.02 estimated for this request, cap $5.00. Resets at the start of next month.	Token-level pre-flight budget gate (402), enforced cross-surface.
Request blocked: E-Mail detected in input.	PII filter, block mode.
Request blocked: prompt injection pattern detected.	Prompt injection guardrail.
Request blocked: keyword 'foo' detected in input.	Keyword blocklist.
Request blocked: content length 80012 exceeds maximum 50000.	Content-length guardrail.
Validation failed: ...	Input or output JSON schema.
Provider call failed: ...	Upstream error after failover (502).
Provider 'openai' is disabled in this gateway.	Provider disabled (503).

Proxy API

Message	Meaning
Method 'POST' is not allowed for this endpoint.	Method not in allowed_methods.
Upstream URL blocked by SSRF guard.	SSRF check failed (with detail).
Upstream OAuth connection failed.	OAuth refresh failure (with detail).
Upstream unreachable.	Network / timeout to upstream.
Endpoint 'foo' not found or inactive.	Unknown slug.

The proxy also passes upstream status codes through — if your upstream returned 404, the client sees 404 (not wrapped). The wrapping kicks in only for PromptGate-side errors.

JSON-RPC surfaces (MCP, Control Plane)

JSON-RPC errors are in-protocol — the HTTP status is 200 even on protocol errors:

HTTP/1.1 200 OK

{
  "jsonrpc": "2.0", "id": 1,
  "error": { "code": -32602, "message": "Unknown tool: nonexistent" }
}

The HTTP layer only emits non-200 for transport-level errors:

HTTP status	Cause
400	Body isn’t valid JSON / parse error. Body has the JSON-RPC envelope.
401 / 403	Bearer token missing / wrong scope. Body is plain {ok: false, error: ...} — no JSON-RPC envelope.
204	Notification (envelope without id). No body.

JSON-RPC error codes

Code	Standard meaning	Used for
-32700	Parse error	Malformed JSON in request body
-32600	Invalid request	Missing method, etc.
-32601	Method not found	Unknown JSON-RPC method
-32602	Invalid params	Unknown tool, missing argument, bad type, bad enum value
-32603	Internal error	Provider failure, upstream MCP server unreachable, malformed upstream response
-32000 to -32099	Server-defined	Reserved; PromptGate currently propagates these from upstream MCP servers verbatim

Control Plane specific

The Control Plane uses -32602 (invalid params) for almost every business-rule failure:

• Missing required argument: project_uuid
• Project not found: <uuid>
• Invalid project_type: banana
• Unknown scope(s): root_all
• Token not found: 99999
• Endpoint not found: <uuid>
• is_active must be boolean

-32603 for upstream / internal failures.

Retry guidance

Status	Retryable?	How
401	No	Fix the token.
403	No	Fix the scope or project.
404	No	Fix the URL.
405	No	Use an allowed method.
422	No	Fix the input.
429	Yes, after Retry-After seconds	Linear backoff is fine.
500	Sometimes	Retry once; if persists, report.
502	Yes, with exponential backoff	Provider was flaky; retry 1-3 times.
503	No	Admin needs to re-enable the provider.
JSON-RPC -32603	Sometimes	If it’s an upstream provider issue, retry. If schema/internal, don’t.

Idempotency

PromptGate’s API is not idempotent in the strict sense — every call to POST /api/{uuid}/{slug} runs the full pipeline again. There’s no idempotency key.

For destructive operations via the Control Plane:

• pg_revoke_token is idempotent (revoking an already-revoked token returns OK).
• pg_set_endpoint_active is idempotent.
• pg_create_token is not — each call mints a fresh row.
• pg_rotate_token is not — each call rotates again, invalidating the previous plaintext.

Observing errors

Every error path writes to:

• gateway_logs — for runtime errors (provider failure, guardrail block, rate limit, budget).
• audit_logs — for state-changing errors (token revoked, OAuth refresh failed, etc.).

So you can filter both for post-hoc analysis. Webhooks fire on audit insertions, so you can route specific errors to alerting in real time.

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That’s the full API Reference. Continue to:

• Plugins — coming soon.
• Cookbook — task-oriented walkthroughs.

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