diff --git a/Project.toml b/Project.toml
index 6b41141..c753c79 100644
--- a/Project.toml
+++ b/Project.toml
@@ -4,7 +4,7 @@ keywords = ["Swagger", "OpenAPI", "REST"]
 license = "MIT"
 desc = "OpenAPI server and client helper for Julia"
 authors = ["JuliaHub Inc."]
-version = "0.2.8"
+version = "0.2.9"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"
diff --git a/src/client.jl b/src/client.jl
index 906192b..20b88c2 100644
--- a/src/client.jl
+++ b/src/client.jl
@@ -136,6 +136,23 @@ function set_param(params::Dict{String,String}, name::String, value; collection_
     end
 end
 
+# Escape one query parameter value for the URL. The OAS `form` style keeps the
+# list separator comma literal (`?color=blue,black,brown` in the specification's
+# Style Examples, per RFC6570 form-style expansion) while percent-encoding the
+# items themselves. `set_param` joins collections before the item boundaries
+# reach this point, so escaping each comma-separated segment individually
+# preserves the separators. The spaceDelimited and pipeDelimited separators are
+# percent-encoded to `%20`/`%7C`, which is what the specification requires for
+# those styles. A comma inside a scalar value stays literal, which is valid in
+# query strings (RFC3986 sub-delim) and percent-decodes identically.
+escape_query_param_value(value::AbstractString) =
+    join((escapeuri(part) for part in split(value, ','; keepempty=true)), ",")
+
+query_string(query::Dict{String,String}) = join(
+    (string(escapeuri(name), "=", escape_query_param_value(value)) for (name, value) in query),
+    "&",
+)
+
 prep_args(ctx::Ctx) = prep_args(Val(ctx.client.httplib), ctx)
 
 response(::Type{Nothing}, resp::HTTPLibResponse, body) = nothing::Nothing
@@ -178,7 +195,7 @@ function do_request(ctx::Ctx, stream::Bool=false; stream_to::Union{Channel,Nothi
     end
     # append query params if needed
     if !isempty(ctx.query)
-        resource_path = string(URIs.URI(URIs.URI(resource_path); query=escapeuri(ctx.query)))
+        resource_path = string(URIs.URI(URIs.URI(resource_path); query=query_string(ctx.query)))
     end
 
     body, kwargs = prep_args(ctx)
diff --git a/test/param_deserialize.jl b/test/param_deserialize.jl
index 2a9b7a6..d30ba3e 100644
--- a/test/param_deserialize.jl
+++ b/test/param_deserialize.jl
@@ -128,3 +128,52 @@ end
     end
 
 end
+
+@testset "Query parameter serialization" begin
+    # The OAS `form` style (RFC6570) keeps the collection separator comma
+    # literal — `?color=blue,black,brown` in the specification's Style
+    # Examples — while percent-encoding the items. Previously the whole query
+    # value was escaped, turning separators into `%2C`, which spec-compliant
+    # servers read as a single item containing commas.
+    import OpenAPI.Clients: set_param, query_string
+
+    @testset "form-style csv keeps literal separators" begin
+        params = Dict{String,String}()
+        set_param(params, "status", ["pending", "sold"]; style="form", is_explode=false)
+        @test params["status"] == "pending,sold"
+        @test query_string(params) == "status=pending,sold"
+    end
+
+    @testset "items are percent-encoded individually" begin
+        params = Dict{String,String}()
+        set_param(params, "tags", ["a b", "c/d", "x&y"]; style="form", is_explode=false)
+        @test query_string(params) == "tags=a%20b,c%2Fd,x%26y"
+    end
+
+    @testset "space and pipe separators are percent-encoded" begin
+        # per the specification's Style Examples, spaceDelimited and
+        # pipeDelimited separators must be percent-encoded (%20 / %7C)
+        params = Dict{String,String}()
+        set_param(params, "s", ["blue", "black"]; collection_format="ssv")
+        set_param(params, "p", ["blue", "black"]; collection_format="pipes")
+        query = query_string(params)
+        @test occursin("s=blue%20black", query)
+        @test occursin("p=blue%7Cblack", query)
+    end
+
+    @testset "scalar values" begin
+        params = Dict{String,String}()
+        set_param(params, "q", "a b&c")
+        @test query_string(params) == "q=a%20b%26c"
+        # a comma in a scalar stays literal: valid per RFC3986 and
+        # percent-decodes identically on the server
+        params = Dict{String,String}()
+        set_param(params, "note", "a,b")
+        @test query_string(params) == "note=a,b"
+    end
+
+    @testset "keys are escaped" begin
+        params = Dict{String,String}("a key" => "v")
+        @test query_string(params) == "a%20key=v"
+    end
+end