[Bug]: query params with nested objects are encoded wrong

[sugiruu]

Describe the bug

When a @query parameter uses @query(#{explode: true}) on a model type containing complex objects, the generated JS/C# client produces [object Object] in the URL instead of bracket-expanded key-value pairs.

Reproduction

TypeSpec

Playground link

import "@typespec/http";
using TypeSpec.Http;

model Condition {
  field: string;
  value: string;
}

model SearchFilter {
  items?: Condition[];
}

model ItemCollection { data: unknown[]; }

@service
@route("/api/v1")
namespace Example {
  @route("/items")
  interface Items {
    @get list(
      @query(#{explode: true}) filter?: SearchFilter,
    ): ItemCollection;
  }
}

Steps to reproduce

npx tsp compile main.tsp

Inspect the URI template expansion in the generated client:

import { parse } from "uri-template";

const filter = {
  items: [
    { field: "status", value: "active" },
    { field: "type",   value: "admin"  }
  ]
};

console.log(parse("/api/v1/items{?filter*}").expand({ filter }));
// Output: /api/v1/items?items=%5Bobject%20Object%5D&items=%5Bobject%20Object%5D

Expected behavior

The same output I would receive by using qs.stringify:

import qs from "qs";

const queryString = qs.stringify(
  { filter: { items: [{ field: "status", value: "active" }] } },
  { encode: false }
);
// filter[items][0][field]=status&filter[items][0][value]=active

GET /api/v1/items?filter[items][0][field]=status&filter[items][0][value]=active&filter[items][1][field]=type&filter[items][1][value]=admin

Actual behavior

GET /api/v1/items?items=%5Bobject%20Object%5D&items=%5Bobject%20Object%5D

http-client-csharp

The C# emitter has the same issue. The generated RestClient calls:

uri.AppendQuery("filter", TypeFormatters.ConvertToString(filter), true);

outputs

GET /api/v1/items?filter=Example.SearchFilter

Checklist

• Follow our Code of Conduct
• 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.

[github-actions]

🎯 Issue Triage

When @query(#{explode: true}) is used on a model with nested complex objects (e.g., arrays of objects), both the JS and C# client emitters fail to properly serialize the query parameter — the JS client produces [object Object] via RFC 6570 URI template expansion, and the C# client serializes the whole object as its type name.

Category: Bug
Area: emitter:client:all

🔗 [Playground Reproduction]((typespec.io/redacted)

📋 Reproduction code

import "`@typespec/http`";
using TypeSpec.Http;

model Condition {
  field: string;
  value: string;
}

model SearchFilter {
  items?: Condition[];
}

model ItemCollection { data: unknown[]; }

`@service`
`@route`("/api/v1")
namespace Example {
  `@route`("/items")
  interface Items {
    `@get` list(
      `@query`(#{explode: true}) filter?: SearchFilter,
    ): ItemCollection;
  }
}

🔍 Analysis

Affected components:

• http-client-js: Uses uri-template for RFC 6570 expansion ({?filter*}). RFC 6570's explode operator only handles flat objects — arrays of objects get coerced to strings, yielding %5Bobject%20Object%5D.
• http-client-csharp: Uses TypeFormatters.ConvertToString(filter) which calls the object's .ToString() method, outputting the class name instead of a serialized query string.

Root cause: Neither emitter implements "deep object" serialization (OpenAPI style: deepObject, explode: true) for complex nested types. RFC 6570 URI templates do not natively support deep object serialization — this requires custom query-string building logic (similar to qs.stringify).

Impact: Any API using @query(#{explode: true}) with a model containing nested objects or arrays of objects will generate broken client code that produces malformed URLs at runtime.

📝 Suggested next steps

• Decide on the target serialization format for nested query objects (e.g., filter[items][0][field]=status bracket notation vs. alternatives)
• Update http-client-js to detect when a query param value is a nested object/array and apply deep serialization instead of relying on the RFC 6570 expander
• Update http-client-csharp to generate proper serialization code for complex query param types instead of calling TypeFormatters.ConvertToString
• Add test coverage for nested object models with explode: true in both emitters

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