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Provider passthrough

Problem — A provider ships a knob lm15's canonical Config does not have, or returns metadata the canonical Response does not model. You need both without forking your code off lm15's types — and without the escape hatch quietly becoming the API.

Keys loaded as in recipe 01.

Recipe

Config.extensions is a dict of raw provider fields. The adapter merges it into the request payload after the canonical fields — provider spelling, your responsibility. You can see exactly what goes on the wire without spending a token: every LM exposes build_request.

import json

from lm15 import Config, LMRouter, Message, Request, tool

router = LMRouter()

config = Config(max_tokens=300, extensions={"max_tool_calls": 1, "service_tier": "default"})
request = Request(model="gpt-4.1-mini", messages=(Message.user("Hi"),), config=config)

wire = router.lm("gpt-4.1-mini").build_request(request, stream=False)
print(wire.url)
print(json.dumps(json.loads(wire.body), indent=2))

https://api.openai.com/v1/responses
{
  "model": "gpt-4.1-mini",
  "input": […],
  "stream": false,
  "max_output_tokens": 300,
  "max_tool_calls": 1,
  "service_tier": "default"
}

max_tokens became max_output_tokens — canonical, translated. The two extension keys went through verbatim. lm15 does not validate them; a typo'd key is the provider's 400, not lm15's.

max_tool_calls is one of the four blessed extension knobs (below). It caps total tool invocations across a Responses API run. Live:

def get_weather(city: str) -> str:
    """Current weather for a city."""
    return "22 C, clear"

def get_time(city: str) -> str:
    """Current local time in a city."""
    return "14:05"

weather, clock = tool(get_weather), tool(get_time)

request = Request(
    model="gpt-4.1-mini",
    messages=(Message.user("What is the weather AND the local time in Oslo?"),),
    tools=(weather, clock),
    config=Config(extensions={"max_tool_calls": 1}),
)
response = router.complete(request)
print([(c.name, c.input) for c in response.tool_calls])
print(response.finish_reason)

[('get_weather', {'city': 'Oslo'}), ('get_time', {'city': 'Oslo'})]
tool_call

Honest note: this run still emitted two parallel function calls — the cap governs invocations the provider executes server-side, not how many function calls the model emits in one turn (that is canonical ToolChoice(parallel=False)). How do you know the knob was accepted at all? The response echoes it — which is what Response.provider_data is for. It holds the raw provider response body, verbatim:

print(response)
pd = response.provider_data
print(len(pd), "keys")
for key in ("id", "model", "max_tool_calls", "service_tier", "status"):
    print(key, "=", pd.get(key))

Response(
    message=Message(role='assistant', parts=(ToolCallPart(…), ToolCallPart(…)), …),
    …
    provider_data=<dict: 35 keys>,
)
35 keys
id = resp_04c181fea396…
model = gpt-4.1-mini-2025-04-14
max_tool_calls = 1
service_tier = default
status = completed

The repr elides it (<dict: 35 keys>) because raw payloads are noise until you ask for them; the data itself is complete.

The blessed knob on the Anthropic side is the thinking object's display field — canonical Reasoning maps to {type, budget_tokens} only, because no other provider has a display concept. The whole wire object goes in extensions:

request = Request(
    model="claude-sonnet-4-5",
    messages=(Message.user("Why is the sky blue? One sentence."),),
    config=Config(
        max_tokens=4096,
        extensions={"thinking": {"type": "enabled", "budget_tokens": 2048, "display": "summarized"}},
    ),
)
wire = router.lm("claude-sonnet-4-5").build_request(request, stream=False)
print(json.dumps(json.loads(wire.body)["thinking"], indent=2))

{
  "type": "enabled",
  "budget_tokens": 2048,
  "display": "summarized"
}

How it works

extensions and provider_data are deliberately asymmetric — different directions, different ownership, different names:

  • extensions (request): you wrote it; you are asking the adapter to forward it. Merged into the payload after canonical fields, so an extension key can override what lm15 built — useful (the thinking example) and sharp.
  • provider_data (response): the provider produced it; the adapter preserves it verbatim. Response fields lm15 could not map are also flagged here under _lm15_unmapped (path + type), so nothing is silently dropped.

A single name like extra would suggest the two round-trip. They do not: echoing provider_data back as extensions is almost always a bug. See design rationale.

The discipline is INV-049 in the spec: exactly four extensions passthrough keys are blessed — permanent, not burn-down debt — because each is provider syntax for a capability no canonical key can express without inventing semantics the other providers lack: Anthropic thinking display (two spec cases), OpenAI max_tool_calls, and OpenAI include for code-interpreter outputs (provider-executed tool traces live in provider_data by mapping rule MAP-1, never in canonical parts — see mapping rules). Everything else you put in extensions is your contract with the provider, not lm15's. Promoting a blessed key to canonical later is an additive spec change.

Variations

  • Async mirror. AsyncLMRouter is identical: await router.complete(request); extensions and provider_data are plain data, no sync/async split.
  • Reserved keys. Each adapter pops the keys it interprets itself before forwarding: all three reserve prompt_caching; OpenAI also reserves cache and compat/openai_compat/openai_responses_compat; Gemini reserves output (modality switch) and transport (live sessions). Anything else passes through untouched.
  • Shapes differ per provider. The same recipe, three raw payloads:

for model in ("claude-sonnet-4-5", "gemini-3-flash-preview"):
    r = router.complete(Request(model=model, messages=(Message.user("Say ok."),)))
    print(model, sorted(r.provider_data.keys()))

claude-sonnet-4-5 ['content', 'id', 'model', 'role', 'stop_details', 'stop_reason', 'stop_sequence', 'type', 'usage']
gemini-3-flash-preview ['candidates', 'modelVersion', 'responseId', 'usageMetadata']

Code that reads provider_data is provider-specific by construction. Branch on router.resolve(model).provider, not on key sniffing. - Gemini nesting. Gemini extensions merge at the payload top level; a generationConfig sub-field must be passed as the full nested object, e.g. extensions={"generationConfig": {"responseLogprobs": True}} — and it replaces the generationConfig lm15 built, canonical knobs included. Prefer canonical fields; reach for extensions only for what they cannot say. - Other endpoints. BatchRequest, ImageGenerationRequest, SpeechGenerationRequest, and LiveConfig carry the same extensions field, and their responses the same provider_data (see recipes 12 and 13).

See also