[Bug]: default value of `explode` in the query parameter became `false` contrary to OpenAPI v3 speficiation

[ogugu9]

Describe the bug

From #4167, the default value of explode in the query parameter became false.
However, according to the OpenAPI v3 specification, the default value of explode in the query parameter is true.

https://swagger.io/docs/specification/v3_0/serialization/#query-parameters
https://spec.openapis.org/oas/v3.0.2#fixed-fields-9

Here's what I think:

• Expected behavior: explode is NOT specified in OpenAPI. It is interpreted as true by OpenAPI specification.
• Actual behavior: explode is explicitly specified as false.

I tried to create #7148 , but it is still incomplete.

Reproduction

It was not reproduced until v.0.58.x.
It is reproduced from v.0.59.x to latest version.

v0.58.x: https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBIdHRwOwoKbW9kZWwgU3RvcmUgewogIG5hbWU6IHN0cmluZzsKICBhZGRyZXNzOiBBxgk7Cn3INccSxTdzdHJlZXTMOWNpdHnKEH0KCkByb3V0ZSgiL3PEbXMiKQppbnRlcmZhY2XmAIDGSi8vIGBmaWx0ZXJgIGRvZXMgbm90IGV4cGxpY2l0bHkg5ADLaWZ5IGDEFG9kZWAgaW4gT3BlbkFQSS7GQFNvyBEgxWNwcmV0cyBpdCBhcyB0cnVlIGJ5IGRlZmF1bHTEMmxpc3QoQHF1ZXJ5IMZ66AC4KTrmAJpbXeQAwg%3D%3D&e=%40typespec%2Fopenapi3&options=%7B%7D&version=0.58.x
v0.59.x: https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBIdHRwOwoKbW9kZWwgU3RvcmUgewogIG5hbWU6IHN0cmluZzsKICBhZGRyZXNzOiBBxgk7Cn3INccSxTdzdHJlZXTMOWNpdHnKEH0KCkByb3V0ZSgiL3PEbXMiKQppbnRlcmZhY2XmAIDGSi8vIGBmaWx0ZXJgIGV4cGxpY2l0bHkg5ADCaWZ5IGDEFG9kZTogZmFsc2VgIGluIE9wZW5BUEkuxj5JdCBpcyBjb250cmFyeSB0byBkZWZhdWx0IGJlaGF2aW9yIG9myDQgdjPHW2ljYXRpb27ERWxpc3QoQHF1ZXJ5IOYAi%2BgAySk65gCrW13kANM%3D&e=%40typespec%2Fopenapi3&options=%7B%7D&version=0.59.x
v1.0.x: https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBIdHRwOwoKbW9kZWwgU3RvcmUgewogIG5hbWU6IHN0cmluZzsKICBhZGRyZXNzOiBBxgk7Cn3INccSxTdzdHJlZXTMOWNpdHnKEH0KCkByb3V0ZSgiL3PEbXMiKQppbnRlcmZhY2XmAIDGSi8vIGBmaWx0ZXJgIGV4cGxpY2l0bHkg5ADCaWZ5IGDEFG9kZTogZmFsc2VgIGluIE9wZW5BUEkuxj5JdCBpcyBjb250cmFyeSB0byBkZWZhdWx0IGJlaGF2aW9yIG9myDQgdjPHW2ljYXRpb27ERWxpc3QoQHF1ZXJ5IOYAi%2BgAySk65gCrW13kANM%3D&e=%40typespec%2Fopenapi3&options=%7B%7D&version=1.0.x

Checklist

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• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[timotheeguerin]

hi @ogugu9 as discussed on the PR it is by design that in TypeSpec explode is false by default. It align with the uri template spec where explode needs to be opt-in with the * operator.
So it is accurate that we set explode: false in OpenAPI3.0 if you don't specify it as we do not share the same default.

[ogugu9]

@timotheeguerin
Fmm...
In my understanding, the default behavior is deferent between query parameter and path parameter according to OpenAPI v3 specification.

I'll quote the actual text below:

Path Parameter

The default serialization method is style: simple and explode: false

ref. https://swagger.io/docs/specification/v3_0/serialization/#path-parameters:~:text=The%20default%20serialization%20method%20is%20style%3A%20simple%20and%20explode%3A%20false

Query Parameter

The default serialization method is style: form and explode: true

ref. https://swagger.io/docs/specification/v3_0/serialization/#query-parameters:~:text=The%20default%20serialization%20method%20is%20style%3A%20form%20and%20explode%3A%20true

Is there a possibility that there is confusion...?

[ogugu9]

Actually, our product behavior was changed by deference of explode in openapi.yaml that I showed in playground.
I'll show code-sandbox or something that shows reproduction later.

[timotheeguerin]

Yeah path and query have different default in OpenAPI 3. But its always false in TypeSpec. We are not trying to be an exact match to the openapi spec but to what makes most sense in our language.

Code	Result in openapi 3.0	Array serialize example
@path pathParam: string[]	explode not set as false is already the default	/foo/1,2,3/bar
@query queryParam: string[]	Set explode: false because otherwise it the default would assume true which is different from the meaning we have defined in TypeSpec	/foo?queryParam=1,2,3

Playground

If there is still some confusion here can we you agree with

1. The meaning in TypeSpec(No Openapi involved) is that the explode property is always false by default. (Parameter are serialized following the https://datatracker.ietf.org/doc/html/rfc6570 according to their type without the explode modifier)
2. Given a meaning in Typespec the openapi should match the exact same serialization

[microsoft-github-policy-service]

Hi @@ogugu9. Since there hasn't been recent engagement, we're going to close this out. Please feel free to reopen if you have any further questions or concerns.