From 3815558af6840d529ca4ef6e8a24cd93fa7b8d14 Mon Sep 17 00:00:00 2001
From: Aly Raffauf <aly@aly.codes>
Date: Fri, 2 Oct 2026 11:43:48 -0400
Subject: [PATCH] perf(stream): validate delta maps without rebuilding them

OTP 28 medians used 2.4% fewer reductions for text and 4.9% fewer with reasoning; plain-text timing was unchanged.
Keep string-key validation; all 212 Gleam tests passed.
---
 src/albedo/openai_api/albedo_openai_json.erl | 17 ++++++++++++++++-
 src/albedo/openai_api/chat.gleam             |  7 ++++++-
 test/openai_api/albedo_openai_json_test.erl  |  7 +++++++
 test/openai_api/stream_test.gleam            |  8 ++++++++
 4 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/albedo/openai_api/albedo_openai_json.erl b/src/albedo/openai_api/albedo_openai_json.erl
index 065a7be..b3955e1 100644
--- a/src/albedo/openai_api/albedo_openai_json.erl
+++ b/src/albedo/openai_api/albedo_openai_json.erl
@@ -1,5 +1,5 @@
 -module(albedo_openai_json).
--export([encode/1, null/0, flatten/1, data_url/2, base64_string/1, semantically_empty/1]).
+-export([encode/1, null/0, flatten/1, data_url/2, base64_string/1, semantically_empty/1, object_fields/1]).
 
 encode(Value) -> json:encode(Value).
 
@@ -15,6 +15,21 @@ semantically_empty({}) -> true;
 semantically_empty(Value) when is_map(Value) -> map_size(Value) =:= 0;
 semantically_empty(_) -> false.
 
+%% Validate the typed key contract while retaining the original map and values.
+object_fields(Value) when is_map(Value) ->
+    case has_string_keys(maps:iterator(Value)) of
+        true -> {ok, Value};
+        false -> {error, #{}}
+    end;
+object_fields(_) -> {error, #{}}.
+
+has_string_keys(Iterator) ->
+    case maps:next(Iterator) of
+        none -> true;
+        {Key, _, Next} when is_binary(Key) -> has_string_keys(Next);
+        _ -> false
+    end.
+
 %% Binaries at least this large cross process boundaries by reference.
 -define(SHARED, 512).
 
diff --git a/src/albedo/openai_api/chat.gleam b/src/albedo/openai_api/chat.gleam
index bc2720f..09adbce 100644
--- a/src/albedo/openai_api/chat.gleam
+++ b/src/albedo/openai_api/chat.gleam
@@ -120,7 +120,7 @@ fn choice_decoder() -> decode.Decoder(Choice) {
 }
 
 fn delta_decoder() -> decode.Decoder(Delta) {
-  use fields <- decode.then(decode.dict(decode.string, decode.dynamic))
+  use fields <- decode.then(decode.new_primitive_decoder("Dict", object_fields))
   case unsupported_field(fields) {
     Some(field) ->
       decode.failure(
@@ -154,6 +154,11 @@ fn delta_decoder() -> decode.Decoder(Delta) {
   }
 }
 
+@external(erlang, "albedo_openai_json", "object_fields")
+fn object_fields(
+  value: dynamic.Dynamic,
+) -> Result(Dict(String, dynamic.Dynamic), Dict(String, dynamic.Dynamic))
+
 fn non_empty_string() -> decode.Decoder(Option(String)) {
   decode.optional(decode.string)
   |> decode.map(fn(value) {
diff --git a/test/openai_api/albedo_openai_json_test.erl b/test/openai_api/albedo_openai_json_test.erl
index f6dce5c..8ec993c 100644
--- a/test/openai_api/albedo_openai_json_test.erl
+++ b/test/openai_api/albedo_openai_json_test.erl
@@ -7,3 +7,10 @@ semantically_empty_test_() ->
      || Value <- [null, nil, undefined, <<>>, [], {}, #{}]] ++
     [?_assertEqual(false, albedo_openai_json:semantically_empty(Value))
      || Value <- [false, true, 0, <<"x">>, [null], {null}, #{<<"id">> => null}]].
+
+object_fields_test_() ->
+    [?_assertEqual({ok, Value}, albedo_openai_json:object_fields(Value))
+     || Value <- [#{}, #{<<"content">> => <<"hello">>, <<"opaque">> => [1, #{}]}]] ++
+    [?_assertEqual({error, #{}}, albedo_openai_json:object_fields(Value))
+     || Value <- [null, [], 42, <<"x">>, #{content => <<"hello">>},
+                  #{<<"content">> => <<"hello">>, 1 => null}]].
diff --git a/test/openai_api/stream_test.gleam b/test/openai_api/stream_test.gleam
index def48a2..6d4e7e3 100644
--- a/test/openai_api/stream_test.gleam
+++ b/test/openai_api/stream_test.gleam
@@ -215,6 +215,14 @@ pub fn chat_ignores_only_empty_unknown_fields_test() -> Nil {
   })
 }
 
+pub fn chat_rejects_non_object_deltas_test() -> Nil {
+  list.each(["null", "[]", "42", "\"x\"", "false"], fn(value) {
+    let data = "{\"choices\":[{\"index\":0,\"delta\":" <> value <> "}]}"
+    let assert Error(types.InvalidEvent(_)) =
+      send(stream.new(types.ChatCompletions), "", data)
+  })
+}
+
 pub fn chat_requires_finish_and_suppresses_partial_tools_on_limits_test() -> Nil {
   assert send(stream.new(types.ChatCompletions), "", "[DONE]")
     == Error(types.InvalidEvent("[DONE] before chat finish reason"))
-- 
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