The fourteen things skyl silently ignores

Why publish this at all#

The obvious move is to fix them and say nothing. Most of them have been fixed: provider/anthropic once ignored ProviderOptions entirely, leaving cache_control, top_k and every beta feature unreachable with no workaround. It once rebuilt tool schemas lossily enough to produce dangling $refs on one provider while the same skyl.Tool arrived intact on another.

What remains is genuinely hard to fix — mostly because a provider's wire format has no field for the thing.

So the choice is between a matrix that lists only the green cells, and one with a column for this. skyl's own design principles reject the first:

Publishing the gaps is the point. It is better to say "we don't support this" than to ship something that looks supported and quietly does the wrong thing.

A gap you know about costs you minutes. A gap you do not costs you a day.

The list#

The three that will actually bite you#

ToolResult.IsError on Gemini. The flag never reaches the wire, so a failed tool is indistinguishable from a successful one that happened to return that text. A model that thinks a tool succeeded will confidently build an answer on nothing. The fix is to put the failure in the content, where every provider carries it faithfully:

skyl.ToolErrorMessage(call.ID,
	"ERROR: get_weather is unavailable (HTTP 503). Do not call it again in this "+
		"conversation. Tell the user live weather data is unavailable.")

skyl.ToolErrorMessage(call.ID,
	"ERROR: get_weather is unavailable (HTTP 503). Do not call it again in this "+
		"conversation. Tell the user live weather data is unavailable.")

That text does three things the boolean cannot: names the tool, says what to do next, and works everywhere.

&Thinking{Enabled: false} on OpenAI. It does nothing. If you are turning reasoning off to control cost, it will not work there — the adapter ignores Thinking unless both Enabled and Effort are set, and an explicit "off" has no wire representation. Send reasoning_effort through ProviderOptions instead.

The shallow ProviderOptions merge. On openai, openaicompat and gemini, setting a nested object replaces the whole object:

// This destroys maxOutputTokens, temperature, topP, stopSequences AND
// thinkingConfig, because it replaces the entire generationConfig.
ProviderOptions: map[string]any{
	"generationConfig": map[string]any{"seed": 7},
}

// This destroys maxOutputTokens, temperature, topP, stopSequences AND
// thinkingConfig, because it replaces the entire generationConfig.
ProviderOptions: map[string]any{
	"generationConfig": map[string]any{"seed": 7},
}

You have to restate every sibling key skyl would have set. Anthropic is the exception: it applies options by JSON path, so "thinking.budget_tokens": 4096 sets one nested field without disturbing its neighbours.

How to defend against them#

The general shape is to make the ambiguous value unconstructable at your own boundary. Image takes exactly one of Data or URL, and setting both silently drops one — so do not expose a struct literal:

func imageFromData(mediaType string, data []byte) skyl.Part {
	return skyl.Image{MediaType: mediaType, Data: data}
}
func imageFromURL(url string) skyl.Part { return skyl.Image{URL: url} }

func imageFromData(mediaType string, data []byte) skyl.Part {
	return skyl.Image{MediaType: mediaType, Data: data}
}
func imageFromURL(url string) skyl.Part { return skyl.Image{URL: url} }

Neither function can produce the ambiguous case, so it cannot arise in your codebase.

One that looks like a bug and is not#

Unparseable SSE frames are skipped rather than treated as fatal. Providers interleave keep-alives and vendor-specific records, and failing an entire stream over one unrecognised line would make skyl brittle against every provider's next feature.

That is a design decision, and the one place skyl chooses tolerance over strictness.

Keeping this honest#

The feature matrix was written by reading the adapters, and it is accurate as of the commit that added it. If you find a cell that no longer matches the code, that is a bug — please report it. Making drift somebody's problem is the only way it gets fixed.