Message transforms

transforms and plugins are two request fields openrouter.ai defines for automatic prompt compression and content-processing add-ons (PDF parsing, web search, and similar). We accept both fields — so that existing client code does not have to strip them out or branch on which gateway it's talking to — but we do not implement what they do.

{
  "model": "google/gemma-4-26b-a4b",
  "transforms": ["middle-out"],
  "messages": [ /* ... */ ]
}

Why we don't have this

Automatic middle-of-conversation truncation trades correctness for convenience: it silently drops or shortens messages you sent, on your behalf, so the request doesn't fail. That's a reasonable default for a router in front of dozens of providers with wildly different context windows. It's a worse default for a service with two catalog models where you can just check the number:

curl -s "https://openrouter.myip.co.kr/api/v1/models" | jq '.data[] | {id, context_length}'

{ "id": "google/gemma-4-26b-a4b", "context_length": 32768 }
{ "id": "lgai/exaone-4.0-32b", "context_length": 32768 }

If you need to fit a long conversation into 32,768 tokens, you decide what to drop — we don't do it invisibly.

What actually happens with an oversized prompt

The behaviour differs depending on whether the candidate that would receive it is local or external, because only local candidates get a pre-flight context check (see Local GPU models):

None of these are billed — a request that fails before producing tokens costs nothing.

What you can do instead

• Trim in your own application code. You have the full message history; you know which parts matter. A summarizing pass, a sliding window, or dropping old tool results are all things you control precisely, instead of a generic middle-out heuristic guessing on your behalf.
• Check context length before you send. context_length from GET /models (above) tells you the ceiling for each catalog model.
• Use models[] as a size-aware fallback. If you list several candidates, the chain already skips local candidates that can't fit the prompt — see Model fallbacks.

Compatibility note

transforms and plugins are in our list of gateway-consumed keys (see Request parameters), which means they are read and stripped before the request is forwarded upstream — they never leak into the payload sent to the serving engine. Sending them is harmless; it just isn't doing what it would do against openrouter.ai.