Test OmniRoute Fallbacks for Semantic Consistency, Not Just Availability

diegosouzapw/OmniRoute is an MIT-licensed AI gateway that advertises one endpoint across many providers and models, with quota-aware fallback and token compression.

A fallback can keep an endpoint available while changing the behavior behind it. For agentic workloads, that is a semantic failover problem.

Assumptions

Consider one request routed through a primary model and a fallback model. Both expose an OpenAI-compatible interface, but they may differ in:

• system-role precedence;
• tool-call JSON shape;
• context limits;
• streaming event order;
• refusal behavior;
• tokenization and truncation.

HTTP success does not prove workflow equivalence.

Versioned route envelope

route_id: coding-agent-v3
primary: provider-a/model-x
fallbacks:
  - provider-b/model-y
max_context_tokens: 32000
required_capabilities:
  - system_message
  - tool_calls
  - streaming
fixture_revision: 9b77c41

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Pin provider/model IDs and a fixture revision. A generic alias makes later failures hard to reconstruct.

Three fixtures

1. Tool-call shape

Ask for one deterministic tool call and validate exact required fields. Reject prose disguised as JSON.

2. Context boundary

Send a prompt just below the declared route limit, with a canary near the end. Verify both primary and fallback models preserve it or return an explicit context error. Silent truncation fails.

3. Streaming terminal state

Force provider failure after stream start. The gateway must not merge partial primary output with a fresh fallback response under one apparent completion. Emit an explicit retry or route-change event.

Event sequence

request accepted
-> primary selected
-> primary quota failure
-> fallback selected
-> response started
-> response completed

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Every event needs request ID, route revision, provider/model, attempt number, and terminal status. Consumers should deduplicate by request plus attempt, not by text content.

Acceptance matrix

Property Primary Fallback Required Valid tool schema yes yes yes System instruction honored yes yes yes Canary retained yes yes yes Stream has one terminal event yes yes yes Route identity observable yes yes yes

Availability is allowed to degrade. Semantics are not allowed to change silently.

Failure policy

If a fallback lacks a required capability, fail closed with a typed route_capability_unavailable error. Sending a lower-quality but syntactically successful answer may be worse than a visible outage when downstream tools can mutate state.

Limitations

I have not benchmarked OmniRoute, verified its provider count, or tested its compression claims. This protocol is derived from the repository’s stated gateway and fallback role plus standard distributed-systems invariants. Confirm current configuration syntax in the project documentation.

A multi-provider gateway is reliable when failover remains observable, bounded, and semantically compatible—not merely when it returns status 200.