diff --git a/e2e/responses.test.ts b/e2e/responses.test.ts
index bfd45cb..1351163 100644
--- a/e2e/responses.test.ts
+++ b/e2e/responses.test.ts
@@ -374,4 +374,82 @@ describe("openai-responses adapter through runInference", () => {
       { type: "text", text: "still here" },
     ]);
   });
+
+  test("parallel function_call items assemble arguments from done envelopes when deltas are absent", async () => {
+    harness = setupHarness({ adapters: registry });
+    harness.scenario.onTool("read_file", () => ({ ok: true }));
+    const stream = harness.scenario.createStream();
+    harness.scenario.whenRequestMatches(() => true, stream);
+    stream.enqueueAll(
+      [
+        sse({
+          type: "response.output_item.added",
+          output_index: 0,
+          item: {
+            type: "function_call",
+            id: "fc_1",
+            call_id: "call_1",
+            name: "read_file",
+          },
+        }),
+        sse({
+          type: "response.output_item.added",
+          output_index: 1,
+          item: {
+            type: "function_call",
+            id: "fc_2",
+            call_id: "call_2",
+            name: "read_file",
+          },
+        }),
+        sse({
+          type: "response.function_call_arguments.done",
+          item_id: "fc_1",
+          arguments: '{"path":"first.ts"}',
+        }),
+        sse({
+          type: "response.output_item.done",
+          output_index: 0,
+          item: {
+            type: "function_call",
+            id: "fc_1",
+            call_id: "call_1",
+            name: "read_file",
+            arguments: '{"path":"first.ts"}',
+          },
+        }),
+        sse({
+          type: "response.output_item.done",
+          output_index: 1,
+          item: {
+            type: "function_call",
+            id: "fc_2",
+            call_id: "call_2",
+            name: "read_file",
+            arguments: '{"path":"second.ts"}',
+          },
+        }),
+        sse({
+          type: "response.completed",
+          response: { usage: { input_tokens: 10, output_tokens: 10 } },
+        }),
+      ],
+      { startAt: 1 },
+    );
+    const events = await collect(harness, [userTurn("read both")]);
+    expect(doneEvent(events).data.turn.content).toEqual([
+      {
+        type: "tool_call",
+        id: "call_1",
+        name: "read_file",
+        arguments: { path: "first.ts" },
+      },
+      {
+        type: "tool_call",
+        id: "call_2",
+        name: "read_file",
+        arguments: { path: "second.ts" },
+      },
+    ]);
+  });
 });
diff --git a/src/protocol/iterator.ts b/src/protocol/iterator.ts
index ec157ad..471de6e 100644
--- a/src/protocol/iterator.ts
+++ b/src/protocol/iterator.ts
@@ -36,6 +36,10 @@ const FunctionCallArgsDeltaEvent = type({
   item_id: "string > 0",
   delta: "string",
 });
+const FunctionCallArgsDoneEvent = type({
+  item_id: "string > 0",
+  arguments: "string",
+});
 
 const OutputItemAddedEvent = type({
   item: {
@@ -43,6 +47,7 @@ const OutputItemAddedEvent = type({
     "id?": "string > 0",
     "call_id?": "string > 0",
     "name?": "string > 0",
+    "arguments?": "string",
   },
 });
 
@@ -51,6 +56,7 @@ const OutputItemDoneEvent = type({
     type: "string",
     "id?": "string > 0",
     "encrypted_content?": "string",
+    "arguments?": "string",
   },
 });
 
@@ -132,11 +138,22 @@ function toInferenceUsage(
 // tool calls interleave, and lets `response.output_item.done` attach an
 // encrypted-reasoning signature to the exact thinking block it belongs to.
 // `kind` is recorded so a signature is only emitted against a real thinking
-// block.
-type BlockKind = "text" | "thinking" | "tool_call" | "refusal";
+// block. A tool call's arguments may arrive only on `output_item.added`,
+// `function_call_arguments.done`, or `output_item.done` (parallel calls on
+// some backends skip deltas entirely); `hasEmittedArgs` lets those envelopes
+// fill in arguments once without repeating what deltas already streamed.
+type BlockEntry =
+  | { index: number; kind: "text" | "thinking" | "refusal" }
+  | {
+      index: number;
+      kind: "tool_call";
+      callId: string;
+      hasEmittedArgs: boolean;
+    };
+type ToolCallBlockEntry = Extract<BlockEntry, { kind: "tool_call" }>;
 type ResponsesBlockIndexer = {
   nextIndex: number;
-  items: Map<string, { index: number; kind: BlockKind }>;
+  items: Map<string, BlockEntry>;
 };
 
 export function createResponsesBlockIndexer(): ResponsesBlockIndexer {
@@ -146,7 +163,7 @@ export function createResponsesBlockIndexer(): ResponsesBlockIndexer {
 function getOrAssignBlockIndex(
   state: ResponsesBlockIndexer,
   itemId: string,
-  kind: BlockKind,
+  kind: "text" | "thinking" | "refusal",
 ): number {
   const existing = state.items.get(itemId);
   if (existing !== undefined) return existing.index;
@@ -156,6 +173,41 @@ function getOrAssignBlockIndex(
   return index;
 }
 
+function registerToolCallBlock(
+  state: ResponsesBlockIndexer,
+  itemId: string,
+  callId: string,
+): ToolCallBlockEntry {
+  const existing = state.items.get(itemId);
+  if (existing?.kind === "tool_call") return existing;
+  const entry: ToolCallBlockEntry = {
+    index: state.nextIndex,
+    kind: "tool_call",
+    callId,
+    hasEmittedArgs: false,
+  };
+  state.nextIndex += 1;
+  state.items.set(itemId, entry);
+  return entry;
+}
+
+function toolCallDeltaEvent(
+  entry: ToolCallBlockEntry,
+  argumentFragment: string,
+): InferenceEvent {
+  entry.hasEmittedArgs = true;
+  return {
+    type: "inference.tool_call.delta",
+    seq: 0,
+    data: {
+      callId: entry.callId,
+      argumentFragment,
+      partial: EMPTY_PARTIAL,
+      index: entry.index,
+    },
+  };
+}
+
 function protocolMismatch(
   provider: string,
   message: string,
@@ -305,6 +357,7 @@ export function parseResponse(
             parsed,
           );
         }
+        const entry = registerToolCallBlock(indexer, item.id, item.call_id);
         events.push({
           type: "inference.tool_call.start",
           seq,
@@ -312,9 +365,12 @@ export function parseResponse(
             callId: item.call_id,
             name: decodeToolName(item.name),
             partial: EMPTY_PARTIAL,
-            index: getOrAssignBlockIndex(indexer, item.id, "tool_call"),
+            index: entry.index,
           },
         });
+        if (item.arguments !== undefined && item.arguments.length > 0) {
+          events.push(toolCallDeltaEvent(entry, item.arguments));
+        }
       } else if (item.type === "reasoning") {
         // A reasoning item with no id cannot be pre-registered, and the
         // signature it will later carry on output_item.done could never be
@@ -392,6 +448,32 @@ export function parseResponse(
             },
           });
         }
+      } else if (item.type === "function_call") {
+        // Without an announced block the arguments have no tool_call.start
+        // to attach to; dropping them would finalize the turn without the
+        // call.
+        if (item.id === undefined) {
+          throw protocolMismatch(
+            provider,
+            "output_item.done function_call item is missing id",
+            parsed,
+          );
+        }
+        const existing = indexer.items.get(item.id);
+        if (existing?.kind !== "tool_call") {
+          throw protocolMismatch(
+            provider,
+            `output_item.done function_call for item_id ${item.id} was never announced by output_item.added`,
+            parsed,
+          );
+        }
+        if (
+          !existing.hasEmittedArgs &&
+          item.arguments !== undefined &&
+          item.arguments.length > 0
+        ) {
+          events.push(toolCallDeltaEvent(existing, item.arguments));
+        }
       }
       return events;
     }
@@ -410,7 +492,7 @@ export function parseResponse(
       // tool_call.start to route it against, not a fresh block to
       // synthesize silently.
       const existing = indexer.items.get(validated.item_id);
-      if (existing === undefined) {
+      if (existing?.kind !== "tool_call") {
         throw protocolMismatch(
           provider,
           `function_call_arguments.delta for item_id ${validated.item_id} was never announced by output_item.added`,
@@ -418,20 +500,29 @@ export function parseResponse(
         );
       }
       if (validated.delta.length > 0) {
-        const blockIndex = existing.index;
-        events.push({
-          type: "inference.tool_call.delta",
-          seq,
-          // The harness routes argument fragments by a per-stream
-          // placeholder keyed to the block index registered on the start
-          // event.
-          data: {
-            callId: String(blockIndex),
-            argumentFragment: validated.delta,
-            partial: EMPTY_PARTIAL,
-            index: blockIndex,
-          },
-        });
+        events.push(toolCallDeltaEvent(existing, validated.delta));
+      }
+      return events;
+    }
+    case "response.function_call_arguments.done": {
+      const validated = FunctionCallArgsDoneEvent(parsed);
+      if (validated instanceof type.errors) {
+        throw protocolMismatch(
+          provider,
+          `function_call_arguments.done failed schema validation: ${validated.summary}`,
+          parsed,
+        );
+      }
+      const existing = indexer.items.get(validated.item_id);
+      if (existing?.kind !== "tool_call") {
+        throw protocolMismatch(
+          provider,
+          `function_call_arguments.done for item_id ${validated.item_id} was never announced by output_item.added`,
+          parsed,
+        );
+      }
+      if (!existing.hasEmittedArgs && validated.arguments.length > 0) {
+        events.push(toolCallDeltaEvent(existing, validated.arguments));
       }
       return events;
     }
@@ -738,7 +829,7 @@ export function parseJSONResponse(
           type: "inference.tool_call.delta",
           seq,
           data: {
-            callId: String(index),
+            callId: item.call_id,
             argumentFragment: item.arguments,
             partial: EMPTY_PARTIAL,
             index,
diff --git a/src/responses.test.ts b/src/responses.test.ts
index 52f90ca..eba41da 100644
--- a/src/responses.test.ts
+++ b/src/responses.test.ts
@@ -400,6 +400,174 @@ describe("Responses parser — event schema validation", () => {
       ProtocolMismatchError,
     );
   });
+
+  test("function_call_arguments.done or output_item.done for an unannounced function_call throws", () => {
+    const adapter = createOpenAIResponsesAdapter(source, {});
+    adapter.buildRequest(turns, "model", {});
+    expect(() =>
+      adapter.parseResponse(
+        JSON.stringify({
+          type: "response.function_call_arguments.done",
+          item_id: "never_announced",
+          arguments: '{"cmd":"ls"}',
+        }),
+      ),
+    ).toThrow(ProtocolMismatchError);
+    expect(() =>
+      adapter.parseResponse(
+        JSON.stringify({
+          type: "response.output_item.done",
+          item: {
+            type: "function_call",
+            id: "never_announced",
+            arguments: '{"cmd":"ls"}',
+          },
+        }),
+      ),
+    ).toThrow(ProtocolMismatchError);
+  });
+});
+
+describe("Responses parser — parallel function_call arguments", () => {
+  type ToolCallDelta = Extract<
+    InferenceEvent,
+    { type: "inference.tool_call.delta" }
+  >;
+
+  // Each path must carry the real callId, not the block-index placeholder.
+  test("routes parallel function_call arguments across added, delta, arguments.done, and output_item.done without duplication", () => {
+    const adapter = createOpenAIResponsesAdapter(source, {});
+    adapter.buildRequest(turns, "model", {});
+
+    const frames = [
+      {
+        type: "response.output_item.added",
+        item: {
+          type: "function_call",
+          id: "fc_added",
+          call_id: "call_added",
+          name: "read_file",
+          arguments: '{"path":"a.ts"}',
+        },
+      },
+      {
+        type: "response.output_item.added",
+        item: {
+          type: "function_call",
+          id: "fc_delta",
+          call_id: "call_delta",
+          name: "read_file",
+          arguments: "",
+        },
+      },
+      {
+        type: "response.output_item.added",
+        item: {
+          type: "function_call",
+          id: "fc_args_done",
+          call_id: "call_args_done",
+          name: "read_file",
+        },
+      },
+      {
+        type: "response.output_item.added",
+        item: {
+          type: "function_call",
+          id: "fc_item_done",
+          call_id: "call_item_done",
+          name: "read_file",
+        },
+      },
+      {
+        type: "response.function_call_arguments.delta",
+        item_id: "fc_delta",
+        delta: '{"path":"b.ts"}',
+      },
+      {
+        type: "response.function_call_arguments.done",
+        item_id: "fc_added",
+        arguments: '{"path":"a.ts"}',
+      },
+      {
+        type: "response.function_call_arguments.done",
+        item_id: "fc_delta",
+        arguments: '{"path":"b.ts"}',
+      },
+      {
+        type: "response.function_call_arguments.done",
+        item_id: "fc_args_done",
+        arguments: '{"path":"c.ts"}',
+      },
+      {
+        type: "response.output_item.done",
+        item: {
+          type: "function_call",
+          id: "fc_added",
+          call_id: "call_added",
+          name: "read_file",
+          arguments: '{"path":"a.ts"}',
+        },
+      },
+      {
+        type: "response.output_item.done",
+        item: {
+          type: "function_call",
+          id: "fc_delta",
+          call_id: "call_delta",
+          name: "read_file",
+          arguments: '{"path":"b.ts"}',
+        },
+      },
+      {
+        type: "response.output_item.done",
+        item: {
+          type: "function_call",
+          id: "fc_args_done",
+          call_id: "call_args_done",
+          name: "read_file",
+          arguments: '{"path":"c.ts"}',
+        },
+      },
+      {
+        type: "response.output_item.done",
+        item: {
+          type: "function_call",
+          id: "fc_item_done",
+          call_id: "call_item_done",
+          name: "read_file",
+          arguments: '{"path":"d.ts"}',
+        },
+      },
+    ];
+
+    const emitted = frames.flatMap((frame) =>
+      adapter.parseResponse(JSON.stringify(frame)),
+    );
+    const deltas = emitted.filter(
+      (e): e is ToolCallDelta => e.type === "inference.tool_call.delta",
+    );
+
+    expect(
+      deltas.map((d) => ({
+        callId: d.data.callId,
+        argumentFragment: d.data.argumentFragment,
+        index: d.data.index,
+      })),
+    ).toEqual([
+      { callId: "call_added", argumentFragment: '{"path":"a.ts"}', index: 0 },
+      { callId: "call_delta", argumentFragment: '{"path":"b.ts"}', index: 1 },
+      {
+        callId: "call_args_done",
+        argumentFragment: '{"path":"c.ts"}',
+        index: 2,
+      },
+      {
+        callId: "call_item_done",
+        argumentFragment: '{"path":"d.ts"}',
+        index: 3,
+      },
+    ]);
+  });
 });
 
 describe("Responses parser — non-streaming failure states", () => {