fix(sse): emit valid concatenable kiro tool_calls.arguments deltas (#4855)

Integrated into release/v3.8.37 — cherry-picked defining commit onto release tip; CHANGELOG re-merged; tests green.

open-sse/executors/kiro.ts

@@ -27,6 +27,8 @@ type KiroStreamState = {
   hasToolCalls: boolean;
   toolCallIndex: number;
   seenToolIds: Map<string, number>;
+  toolArgsEmitted: Map<string, string>;
+  toolArgsBuffered: Map<string, { toolIndex: number; canonical: string }>;
   totalContentLength?: number;
   contextUsagePercentage?: number;
   hasContextUsage?: boolean;
@@ -125,6 +127,56 @@ function crc32(buf: Uint8Array) {
   return (crc ^ 0xffffffff) >>> 0;
 }
 
+/**
+ * Flush buffered tool arguments at finish boundaries.
+ *
+ * Kiro/CodeWhisperer streams toolUseEvent.input as PARTIAL OBJECTS that grow over time
+ * (e.g. {command:"cat /home"} then {command:"cat /home/wxsys"}). Re-stringifying each one
+ * and emitting it as an OpenAI argument delta produces overlapping prefixes that
+ * concatenate into unparseable garbage downstream ("Unterminated string").
+ *
+ * Fix: defer object-form payloads into state.toolArgsBuffered keyed by toolCallId, keep
+ * only the latest canonical, and emit ONCE here as the complete arguments string (the
+ * final object is the source of truth — intermediate states are noise). String-form
+ * payloads are already concatenable deltas and are emitted incrementally.
+ */
+export function flushBufferedToolArgs(
+  state: Pick<KiroStreamState, "toolArgsBuffered" | "toolArgsEmitted">,
+  controller: { enqueue: (chunk: Uint8Array) => void },
+  ctx: { responseId: string; created: number; model: string }
+): void {
+  if (!state.toolArgsBuffered || state.toolArgsBuffered.size === 0) return;
+  const { responseId, created, model } = ctx;
+  for (const [toolCallId, info] of state.toolArgsBuffered) {
+    const alreadyEmitted = state.toolArgsEmitted.get(toolCallId) || "";
+    if (info.canonical && info.canonical !== alreadyEmitted) {
+      const argsChunk: JsonRecord = {
+        id: responseId,
+        object: "chat.completion.chunk",
+        created,
+        model,
+        choices: [
+          {
+            index: 0,
+            delta: {
+              tool_calls: [
+                {
+                  index: info.toolIndex,
+                  function: { arguments: info.canonical },
+                },
+              ],
+            },
+            finish_reason: null,
+          },
+        ],
+      };
+      controller.enqueue(TEXT_ENCODER.encode(`data: ${JSON.stringify(argsChunk)}\n\n`));
+      state.toolArgsEmitted.set(toolCallId, info.canonical);
+    }
+  }
+  state.toolArgsBuffered.clear();
+}
+
 function buildKiroFinishChunk(
   state: KiroStreamState,
   responseId: string,
@@ -310,6 +362,8 @@ export class KiroExecutor extends BaseExecutor {
       hasToolCalls: false,
       toolCallIndex: 0,
       seenToolIds: new Map(),
+      toolArgsEmitted: new Map(),
+      toolArgsBuffered: new Map(),
     };
 
     const transformStream = new TransformStream(
@@ -469,46 +523,56 @@ export class KiroExecutor extends BaseExecutor {
                 }
 
                 if (toolInput !== undefined) {
-                  let argumentsStr;
-
                   if (typeof toolInput === "string") {
-                    argumentsStr = toolInput;
-                  } else if (typeof toolInput === "object") {
-                    argumentsStr = JSON.stringify(toolInput);
-                  } else {
-                    continue;
-                  }
+                    // String-form payloads are already concatenable incremental deltas —
+                    // emit immediately and track what we've sent.
+                    state.toolArgsEmitted.set(
+                      toolCallId,
+                      (state.toolArgsEmitted.get(toolCallId) || "") + toolInput
+                    );
 
-                  const argsChunk = {
-                    id: responseId,
-                    object: "chat.completion.chunk",
-                    created,
-                    model,
-                    choices: [
-                      {
-                        index: 0,
-                        delta: {
-                          tool_calls: [
-                            {
-                              index: toolIndex,
-                              function: {
-                                arguments: argumentsStr,
+                    const argsChunk = {
+                      id: responseId,
+                      object: "chat.completion.chunk",
+                      created,
+                      model,
+                      choices: [
+                        {
+                          index: 0,
+                          delta: {
+                            tool_calls: [
+                              {
+                                index: toolIndex,
+                                function: {
+                                  arguments: toolInput,
+                                },
                               },
-                            },
-                          ],
+                            ],
+                          },
+                          finish_reason: null,
                         },
-                        finish_reason: null,
-                      },
-                    ],
-                  };
-                  chunkIndex++;
-                  controller.enqueue(TEXT_ENCODER.encode(`data: ${JSON.stringify(argsChunk)}\n\n`));
+                      ],
+                    };
+                    chunkIndex++;
+                    controller.enqueue(
+                      TEXT_ENCODER.encode(`data: ${JSON.stringify(argsChunk)}\n\n`)
+                    );
+                  } else if (typeof toolInput === "object" && toolInput !== null) {
+                    // Object-form payloads are PARTIAL OBJECTS that grow over time. Buffer
+                    // the latest canonical and flush once at a finish boundary, otherwise the
+                    // overlapping JSON prefixes concatenate into unparseable garbage.
+                    state.toolArgsBuffered.set(toolCallId, {
+                      toolIndex,
+                      canonical: JSON.stringify(toolInput),
+                    });
+                  }
                 }
               }
             }
 
             // Handle messageStopEvent
             if (eventType === "messageStopEvent") {
+              flushBufferedToolArgs(state, controller, { responseId, created, model });
               state.stopSeen = true;
             }
 
@@ -576,6 +640,10 @@ export class KiroExecutor extends BaseExecutor {
         },
 
         flush(controller) {
+          // Flush any buffered tool arguments (partial-object payloads) before finishing —
+          // idempotent against toolArgsEmitted if messageStopEvent already flushed them.
+          flushBufferedToolArgs(state, controller, { responseId, created, model });
+
           // Emit finish chunk if not already sent
           if (!state.finishEmitted) {
             state.finishEmitted = true;

tests/unit/kiro-tool-args-streaming.test.ts

@@ -0,0 +1,167 @@
+/**
+ * Regression test for Kiro/CodeWhisperer streaming tool_calls.arguments deltas.
+ *
+ * Kiro streams toolUseEvent.input as PARTIAL OBJECTS that grow over time
+ * (e.g. {command:"cat /home"} then {command:"cat /home/wxsys"}). The old executor
+ * re-stringified each partial object and emitted it as an OpenAI argument delta, so the
+ * overlapping JSON prefixes concatenated downstream into unparseable garbage
+ * ("Unterminated string"). The fix buffers object-form payloads keyed by toolCallId, keeps
+ * only the latest canonical, and flushes ONCE at a finish boundary. String-form payloads
+ * remain concatenable incremental deltas.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { flushBufferedToolArgs } = await import("../../open-sse/executors/kiro.ts");
+
+type ArgsBuffered = Map<string, { toolIndex: number; canonical: string }>;
+type State = { toolArgsEmitted: Map<string, string>; toolArgsBuffered: ArgsBuffered };
+
+function makeMockController() {
+  const enqueued: string[] = [];
+  const decoder = new TextDecoder();
+  return {
+    enqueued,
+    enqueue(bytes: Uint8Array) {
+      enqueued.push(decoder.decode(bytes));
+    },
+  };
+}
+
+function parseSSEData(sseLines: string[]) {
+  return sseLines
+    .map((line) => {
+      const m = line.match(/^data: (.+)\n\n$/);
+      return m ? JSON.parse(m[1]) : null;
+    })
+    .filter(Boolean);
+}
+
+function reconstructArguments(chunks: any[]) {
+  const perIndex = new Map<number, string>();
+  for (const chunk of chunks) {
+    const tc = chunk.choices?.[0]?.delta?.tool_calls?.[0];
+    if (!tc) continue;
+    const idx = tc.index as number;
+    const args = (tc.function?.arguments as string) || "";
+    perIndex.set(idx, (perIndex.get(idx) || "") + args);
+  }
+  return perIndex;
+}
+
+const CTX = { responseId: "resp_test", created: 1, model: "kiro" };
+
+test("flushes a single buffered tool call with valid concatenable JSON", () => {
+  const state: State = {
+    toolArgsEmitted: new Map(),
+    toolArgsBuffered: new Map([
+      [
+        "tool_abc",
+        {
+          toolIndex: 0,
+          canonical: '{"command":"cat /home/wxsys/Project/naskin/.impeccable.md"}',
+        },
+      ],
+    ]),
+  };
+  const controller = makeMockController();
+  flushBufferedToolArgs(state, controller, CTX);
+
+  const chunks = parseSSEData(controller.enqueued);
+  assert.equal(chunks.length, 1);
+
+  const args = reconstructArguments(chunks);
+  const final = args.get(0)!;
+  assert.doesNotThrow(() => JSON.parse(final));
+  assert.deepEqual(JSON.parse(final), {
+    command: "cat /home/wxsys/Project/naskin/.impeccable.md",
+  });
+
+  assert.equal(state.toolArgsBuffered.size, 0);
+  assert.equal(state.toolArgsEmitted.get("tool_abc"), final);
+});
+
+test("4 partial-object events produce a single valid flush (reported bug)", () => {
+  const state: State = { toolArgsEmitted: new Map(), toolArgsBuffered: new Map() };
+
+  const partialPayloads = [
+    { command: "cat /home" },
+    { command: "cat /home/wxsys" },
+    { command: "cat /home/wxsys/Project" },
+    { command: "cat /home/wxsys/Project/naskin/.impeccable.md" },
+  ];
+  // Each event overwrites the buffered canonical for the same toolCallId.
+  for (const input of partialPayloads) {
+    state.toolArgsBuffered.set("tool_abc", { toolIndex: 0, canonical: JSON.stringify(input) });
+  }
+
+  const controller = makeMockController();
+  flushBufferedToolArgs(state, controller, CTX);
+
+  const chunks = parseSSEData(controller.enqueued);
+  assert.equal(chunks.length, 1);
+
+  const final = reconstructArguments(chunks).get(0)!;
+  assert.doesNotThrow(() => JSON.parse(final));
+  assert.equal(JSON.parse(final).command, "cat /home/wxsys/Project/naskin/.impeccable.md");
+});
+
+test("flushes multiple concurrent tool calls preserving toolIndex", () => {
+  const state: State = {
+    toolArgsEmitted: new Map(),
+    toolArgsBuffered: new Map([
+      ["T1", { toolIndex: 0, canonical: '{"filePath":"/a/b/c.txt"}' }],
+      ["T2", { toolIndex: 1, canonical: '{"command":"ls"}' }],
+    ]),
+  };
+  const controller = makeMockController();
+  flushBufferedToolArgs(state, controller, CTX);
+
+  const chunks = parseSSEData(controller.enqueued);
+  assert.equal(chunks.length, 2);
+
+  const args = reconstructArguments(chunks);
+  assert.deepEqual(JSON.parse(args.get(0)!), { filePath: "/a/b/c.txt" });
+  assert.deepEqual(JSON.parse(args.get(1)!), { command: "ls" });
+});
+
+test("does not re-emit when canonical equals already-emitted (idempotent flush)", () => {
+  const state: State = {
+    toolArgsEmitted: new Map([["tool_abc", '{"command":"ls"}']]),
+    toolArgsBuffered: new Map([["tool_abc", { toolIndex: 0, canonical: '{"command":"ls"}' }]]),
+  };
+  const controller = makeMockController();
+  flushBufferedToolArgs(state, controller, CTX);
+
+  assert.equal(controller.enqueued.length, 0);
+  assert.equal(state.toolArgsBuffered.size, 0);
+});
+
+test("no-op when buffer is empty", () => {
+  const state: State = { toolArgsEmitted: new Map(), toolArgsBuffered: new Map() };
+  const controller = makeMockController();
+  flushBufferedToolArgs(state, controller, CTX);
+
+  assert.equal(controller.enqueued.length, 0);
+});
+
+test("OLD BUG: re-stringifying each partial object produces unparseable concat", () => {
+  const partials = [
+    { command: "cat /home" },
+    { command: "cat /home/wxsys" },
+    { command: "cat /home/wxsys/Project/naskin/.impeccable.md" },
+  ];
+  const buggyConcat = partials.map((p) => JSON.stringify(p)).join("");
+  assert.throws(() => JSON.parse(buggyConcat));
+});
+
+test("FIX: emitting only the final canonical produces parseable JSON", () => {
+  const partials = [
+    { command: "cat /home" },
+    { command: "cat /home/wxsys" },
+    { command: "cat /home/wxsys/Project/naskin/.impeccable.md" },
+  ];
+  const fixedConcat = JSON.stringify(partials[partials.length - 1]);
+  assert.doesNotThrow(() => JSON.parse(fixedConcat));
+  assert.equal(JSON.parse(fixedConcat).command, "cat /home/wxsys/Project/naskin/.impeccable.md");
+});