diff --git a/packages/ai/src/providers/openai-completions-tool-calls.ts b/packages/ai/src/providers/openai-completions-tool-calls.ts
new file mode 100644
index 000000000000..81672c4a7fc7
--- /dev/null
+++ b/packages/ai/src/providers/openai-completions-tool-calls.ts
@@ -0,0 +1,234 @@
+import { randomUUID } from "node:crypto";
+import { isRecord } from "@openclaw/normalization-core/record-coerce";
+import type { ChatCompletionChunk } from "openai/resources/chat/completions.js";
+
+type ChatCompletionToolCallDelta = ChatCompletionChunk.Choice.Delta.ToolCall;
+const MAX_BUFFERED_LEGACY_TOOL_CALL_ARGUMENT_BYTES = 256_000;
+const MAX_BUFFERED_LEGACY_FOLLOWING_DELTA_BYTES = 256_000;
+const MAX_BUFFERED_LEGACY_FOLLOWING_DELTAS = 1_024;
+
+type NormalizedOpenAICompletionsDelta = {
+  delta: ChatCompletionChunk.Choice.Delta;
+  toolCalls: ChatCompletionToolCallDelta[];
+};
+
+type OpenAICompletionsToolCallFinalizationOptions<TBlock extends object> = {
+  allowSilentToolCallPromotion?: boolean;
+  onConfirmedToolCall?: (block: TBlock, contentIndex: number) => void;
+};
+
+/** Normalize the SDK's legacy single-function lane into its modern tool-call shape. */
+export function createOpenAICompletionsToolCallDeltaNormalizer(): (
+  delta: ChatCompletionChunk.Choice.Delta,
+  finishReason: ChatCompletionChunk.Choice["finish_reason"],
+) => NormalizedOpenAICompletionsDelta[] {
+  let sawModernToolCall = false;
+  let pendingLegacyArgumentBytes = 0;
+  let pendingFollowingDeltaBytes = 0;
+  let pendingLegacyToolCall: ChatCompletionToolCallDelta | undefined;
+  const pendingFollowingDeltas: ChatCompletionChunk.Choice.Delta[] = [];
+
+  const takePendingFollowingDeltas = (): NormalizedOpenAICompletionsDelta[] => {
+    pendingFollowingDeltaBytes = 0;
+    return pendingFollowingDeltas.splice(0).map((delta) => ({ delta, toolCalls: [] }));
+  };
+
+  const bufferFollowingDelta = (delta: ChatCompletionChunk.Choice.Delta): void => {
+    const nextDeltaBytes = Buffer.byteLength(JSON.stringify(delta), "utf8");
+    if (
+      pendingFollowingDeltaBytes + nextDeltaBytes > MAX_BUFFERED_LEGACY_FOLLOWING_DELTA_BYTES ||
+      pendingFollowingDeltas.length >= MAX_BUFFERED_LEGACY_FOLLOWING_DELTAS
+    ) {
+      throw new Error("Exceeded legacy tool-call content buffer limit");
+    }
+    pendingFollowingDeltaBytes += nextDeltaBytes;
+    pendingFollowingDeltas.push(delta);
+  };
+
+  const withoutToolCalls = (
+    delta: ChatCompletionChunk.Choice.Delta,
+  ): ChatCompletionChunk.Choice.Delta => {
+    const ordinaryDelta = { ...delta };
+    delete ordinaryDelta.function_call;
+    delete ordinaryDelta.tool_calls;
+    return ordinaryDelta;
+  };
+
+  const hasObservableContent = (value: unknown): boolean => {
+    if (typeof value === "string") {
+      return value.length > 0;
+    }
+    if (Array.isArray(value)) {
+      return value.some(hasObservableContent);
+    }
+    if (isRecord(value)) {
+      return Object.entries(value).some(
+        ([field, nestedValue]) =>
+          field !== "type" &&
+          field !== "id" &&
+          field !== "index" &&
+          hasObservableContent(nestedValue),
+      );
+    }
+    return false;
+  };
+
+  const hasOrdinaryContent = (delta: ChatCompletionChunk.Choice.Delta): boolean =>
+    Object.entries(delta).some(([field, value]) => field !== "role" && hasObservableContent(value));
+
+  return (delta, finishReason) => {
+    const ordinaryDelta = withoutToolCalls(delta);
+    if (delta.tool_calls && delta.tool_calls.length > 0) {
+      const precedingDeltas = takePendingFollowingDeltas();
+      sawModernToolCall = true;
+      pendingLegacyArgumentBytes = 0;
+      pendingLegacyToolCall = undefined;
+      return [...precedingDeltas, { delta: ordinaryDelta, toolCalls: delta.tool_calls }];
+    }
+
+    const functionCall = delta.function_call;
+    if (sawModernToolCall) {
+      return [{ delta: ordinaryDelta, toolCalls: [] }];
+    }
+
+    const hadPendingLegacyCall = pendingLegacyToolCall !== undefined;
+    const leadingDeltas: NormalizedOpenAICompletionsDelta[] = [];
+    if (hasOrdinaryContent(ordinaryDelta)) {
+      if (hadPendingLegacyCall) {
+        // Keep every lane behind its provisional call; publishing text early
+        // permanently reverses the assistant's original tool/content order.
+        bufferFollowingDelta(ordinaryDelta);
+      } else {
+        leadingDeltas.push({ delta: ordinaryDelta, toolCalls: [] });
+      }
+    }
+
+    if (functionCall && (functionCall.name || functionCall.arguments)) {
+      // Legacy events are provisional until their terminal reason confirms no
+      // modern call superseded them. Coalesce fragments so hostile empty/name
+      // frames cannot create an unbounded queue or quadratic terminal parsing.
+      const nextFunctionName = pendingLegacyToolCall?.function?.name
+        ? undefined
+        : functionCall.name;
+      const nextArgumentBytes =
+        Buffer.byteLength(functionCall.arguments ?? "", "utf8") +
+        Buffer.byteLength(nextFunctionName ?? "", "utf8");
+      if (
+        pendingLegacyArgumentBytes + nextArgumentBytes >
+        MAX_BUFFERED_LEGACY_TOOL_CALL_ARGUMENT_BYTES
+      ) {
+        throw new Error("Exceeded tool-call argument buffer limit");
+      }
+      pendingLegacyArgumentBytes += nextArgumentBytes;
+      pendingLegacyToolCall ??= {
+        index: 0,
+        id: `call_${randomUUID().replaceAll("-", "").slice(0, 24)}`,
+        type: "function",
+        function: {},
+      };
+      const pendingFunction = (pendingLegacyToolCall.function ??= {});
+      if (nextFunctionName) {
+        pendingFunction.name = nextFunctionName;
+      }
+      if (functionCall.arguments) {
+        pendingFunction.arguments = (pendingFunction.arguments ?? "") + functionCall.arguments;
+      }
+    }
+
+    if (!pendingLegacyToolCall) {
+      return leadingDeltas.length > 0 ? leadingDeltas : [{ delta: ordinaryDelta, toolCalls: [] }];
+    }
+
+    if (finishReason === "function_call") {
+      const confirmedLegacyToolCall = pendingLegacyToolCall;
+      pendingLegacyToolCall = undefined;
+      pendingLegacyArgumentBytes = 0;
+      return [
+        ...leadingDeltas,
+        { delta: {}, toolCalls: [confirmedLegacyToolCall] },
+        ...takePendingFollowingDeltas(),
+      ];
+    }
+
+    if (finishReason) {
+      pendingLegacyArgumentBytes = 0;
+      pendingLegacyToolCall = undefined;
+      return [...leadingDeltas, ...takePendingFollowingDeltas()];
+    }
+
+    return leadingDeltas;
+  };
+}
+
+/** Publish only executable calls; streaming scratch state never belongs in replay. */
+export function finalizeOpenAICompletionsToolCalls<TBlock extends object>(
+  output: { content: TBlock[]; stopReason: string; errorMessage?: string },
+  options: OpenAICompletionsToolCallFinalizationOptions<TBlock> = {},
+): void {
+  const isToolCall = (block: TBlock) => (block as { type?: unknown }).type === "toolCall";
+  const hasToolCalls = output.content.some(isToolCall);
+
+  if (output.stopReason === "toolUse" && !hasToolCalls) {
+    output.stopReason = "stop";
+  }
+
+  if (
+    output.stopReason === "stop" &&
+    hasToolCalls &&
+    options.allowSilentToolCallPromotion !== false &&
+    !output.content.some((block) => {
+      const candidate = block as { type?: unknown; text?: unknown };
+      return (
+        candidate.type === "text" &&
+        typeof candidate.text === "string" &&
+        candidate.text.trim().length > 0
+      );
+    })
+  ) {
+    output.stopReason = "toolUse";
+  }
+
+  if (output.stopReason !== "toolUse") {
+    if (hasToolCalls) {
+      output.content = output.content.filter((block) => !isToolCall(block));
+    }
+    return;
+  }
+
+  for (const block of output.content) {
+    if (!isToolCall(block)) {
+      continue;
+    }
+    const toolCall = block as { name?: unknown; arguments?: unknown; partialArgs?: unknown };
+    let completeArguments: unknown;
+    try {
+      completeArguments =
+        typeof toolCall.partialArgs === "string" && toolCall.partialArgs.trim().length > 0
+          ? (JSON.parse(toolCall.partialArgs) as unknown)
+          : undefined;
+    } catch {
+      completeArguments = undefined;
+    }
+    if (
+      typeof toolCall.name !== "string" ||
+      toolCall.name.trim().length === 0 ||
+      !isRecord(completeArguments)
+    ) {
+      output.stopReason = "error";
+      output.errorMessage = "Provider returned an incomplete or malformed tool call";
+      output.content = output.content.filter((candidate) => !isToolCall(candidate));
+      return;
+    }
+    toolCall.arguments = completeArguments;
+  }
+
+  for (let contentIndex = 0; contentIndex < output.content.length; contentIndex += 1) {
+    const block = output.content[contentIndex];
+    if (!block || !isToolCall(block)) {
+      continue;
+    }
+    delete (block as { partialArgs?: string }).partialArgs;
+    delete (block as { streamIndex?: number }).streamIndex;
+    options.onConfirmedToolCall?.(block, contentIndex);
+  }
+}
diff --git a/packages/ai/src/providers/openai-completions.legacy-function-call.test.ts b/packages/ai/src/providers/openai-completions.legacy-function-call.test.ts
new file mode 100644
index 000000000000..53a79b012f09
--- /dev/null
+++ b/packages/ai/src/providers/openai-completions.legacy-function-call.test.ts
@@ -0,0 +1,700 @@
+import type { ChatCompletionChunk } from "openai/resources/chat/completions.js";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { configureAiTransportHost, getAiTransportHost } from "../host.js";
+import { createOpenAICompletionsTransportStreamFn } from "../transports/openai-completions-transport.js";
+import type { AssistantMessageEventStreamLike, Context, Model } from "../types.js";
+import { streamOpenAICompletions } from "./openai-completions.js";
+
+const model = {
+  id: "gpt-4",
+  name: "GPT-4",
+  api: "openai-completions",
+  provider: "openai",
+  baseUrl: "https://api.openai.com/v1",
+  reasoning: false,
+  input: ["text"],
+  cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+  contextWindow: 8_192,
+  maxTokens: 1_024,
+} satisfies Model<"openai-completions">;
+
+const context = {
+  messages: [{ role: "user", content: "Look up cats", timestamp: 1 }],
+  tools: [
+    {
+      name: "lookup",
+      description: "Look up a query",
+      parameters: {
+        type: "object",
+        properties: { query: { type: "string" } },
+        required: ["query"],
+      },
+    },
+  ],
+} satisfies Context;
+
+function chunk(
+  delta: ChatCompletionChunk.Choice.Delta,
+  finishReason: ChatCompletionChunk.Choice["finish_reason"] = null,
+): ChatCompletionChunk {
+  return {
+    id: "chatcmpl-legacy-fixture",
+    object: "chat.completion.chunk",
+    created: 0,
+    model: model.id,
+    choices: [{ index: 0, delta, finish_reason: finishReason }],
+  };
+}
+
+function installStream(chunks: ChatCompletionChunk[]): void {
+  const body = `${chunks.map((value) => `data: ${JSON.stringify(value)}\n\n`).join("")}data: [DONE]\n\n`;
+  const fetch = vi.fn<typeof globalThis.fetch>(async () => {
+    return new Response(body, {
+      status: 200,
+      headers: { "content-type": "text/event-stream" },
+    });
+  });
+  configureAiTransportHost({ buildModelFetch: () => fetch });
+}
+
+let previousHost: ReturnType<typeof getAiTransportHost>;
+
+beforeEach(() => {
+  previousHost = getAiTransportHost();
+});
+
+afterEach(() => {
+  configureAiTransportHost(previousHost);
+});
+
+const createManagedStream = createOpenAICompletionsTransportStreamFn();
+
+function createManagedFixtureStream(
+  fixtureModel: Model<"openai-completions">,
+  fixtureContext: Context,
+  fixtureOptions?: Parameters<typeof createManagedStream>[2],
+): AssistantMessageEventStreamLike {
+  const stream = createManagedStream(fixtureModel, fixtureContext, fixtureOptions);
+  if (stream instanceof Promise) {
+    throw new Error("OpenAI Chat transport must return its event stream synchronously");
+  }
+  return stream;
+}
+
+describe.each([
+  { name: "package", createStream: streamOpenAICompletions },
+  { name: "managed", createStream: createManagedFixtureStream },
+])("$name OpenAI Chat Completions stream", ({ createStream }) => {
+  it("preserves legacy function_call deltas and reassembles split arguments", async () => {
+    installStream([
+      chunk({ role: "assistant", function_call: { name: "lookup" } }),
+      chunk({ function_call: { arguments: '{"query":"ca' } }),
+      chunk({ function_call: { arguments: 'ts"}' } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content).toHaveLength(1);
+    expect(result.content[0]).toMatchObject({
+      type: "toolCall",
+      id: expect.stringMatching(/^call_[a-f0-9]{24}$/),
+      name: "lookup",
+      arguments: { query: "cats" },
+    });
+    expect(result.content[0]).not.toHaveProperty("partialArgs");
+    expect(result.content[0]).not.toHaveProperty("streamIndex");
+    expect(eventTypes).toContain("toolcall_start");
+    expect(eventTypes).toContain("toolcall_delta");
+  });
+
+  it("preserves a confirmed legacy call before later visible text", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"cats"}' } }),
+      chunk({ content: "Trailing commentary." }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content.map((block) => block.type)).toEqual(["toolCall", "text"]);
+    expect(result.content[1]).toMatchObject({ text: "Trailing commentary." });
+    expect(eventTypes.indexOf("toolcall_start")).toBeLessThan(eventTypes.indexOf("text_start"));
+  });
+
+  it("keeps content preceding a legacy call in the same provider delta", async () => {
+    installStream([
+      chunk({
+        content: "Commentary before the call.",
+        function_call: { name: "lookup", arguments: '{"query":"cats"}' },
+      }),
+      chunk({}, "function_call"),
+    ]);
+
+    const result = await createStream(model, context, { apiKey: "fixture-token" }).result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content.map((block) => block.type)).toEqual(["text", "toolCall"]);
+    expect(result.content[0]).toMatchObject({ text: "Commentary before the call." });
+  });
+
+  it("preserves a confirmed legacy call before later streamed reasoning", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"cats"}' } }),
+      chunk({ reasoning_content: "Reasoning after the call." } as ChatCompletionChunk.Choice.Delta),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream({ ...model, reasoning: true }, context, {
+      apiKey: "fixture-token",
+      reasoningEffort: "medium",
+    });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content.map((block) => block.type)).toEqual(["toolCall", "thinking"]);
+    expect(eventTypes.indexOf("toolcall_start")).toBeLessThan(eventTypes.indexOf("thinking_start"));
+  });
+
+  it("replays buffered visible text before a superseding modern call", async () => {
+    installStream([
+      chunk({ function_call: { name: "legacy", arguments: '{"query":"wrong"}' } }),
+      chunk({ content: "Visible before the modern call." }),
+      chunk({
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_modern",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"cats"}' },
+          },
+        ],
+      }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const observedStarts: string[] = [];
+    for await (const event of stream) {
+      if (event.type === "toolcall_start") {
+        const block = event.partial.content[event.contentIndex];
+        if (block?.type === "toolCall") {
+          observedStarts.push(block.id);
+        }
+      }
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content.map((block) => block.type)).toEqual(["text", "toolCall"]);
+    expect(result.content[0]).toMatchObject({ text: "Visible before the modern call." });
+    expect(observedStarts).toEqual(["call_modern"]);
+  });
+
+  it("releases buffered visible text when a legacy call is not confirmed", async () => {
+    installStream([
+      chunk({ function_call: { name: "legacy", arguments: '{"query":"discard"}' } }),
+      chunk({ content: "The provider supplied an answer instead." }),
+      chunk({}, "stop"),
+    ]);
+
+    const result = await createStream(model, context, { apiKey: "fixture-token" }).result();
+
+    expect(result.stopReason).toBe("stop");
+    expect(result.content).toEqual([
+      { type: "text", text: "The provider supplied an answer instead." },
+    ]);
+  });
+
+  it("keeps modern tool_calls authoritative when legacy data is also present", async () => {
+    installStream([
+      chunk({
+        function_call: { name: "legacy", arguments: '{"query":"wrong"}' },
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_modern",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"ca' },
+          },
+        ],
+      }),
+      chunk({ function_call: { arguments: "ignore this legacy continuation" } }),
+      chunk({ tool_calls: [{ index: 0, function: { arguments: 'ts"}' } }] }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const observedStarts: Array<{ id: string; name: string }> = [];
+    const observedArgumentDeltas: string[] = [];
+    for await (const event of stream) {
+      if (event.type === "toolcall_start") {
+        const block = event.partial.content[event.contentIndex];
+        if (block?.type === "toolCall") {
+          observedStarts.push({ id: block.id, name: block.name });
+        }
+      } else if (event.type === "toolcall_delta" && event.delta) {
+        observedArgumentDeltas.push(event.delta);
+      }
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content).toHaveLength(1);
+    expect(result.content[0]).toMatchObject({
+      type: "toolCall",
+      id: "call_modern",
+      name: "lookup",
+      arguments: { query: "cats" },
+    });
+    expect(observedStarts).toEqual([{ id: "call_modern", name: "lookup" }]);
+    expect(observedArgumentDeltas).toEqual(['{"query":"ca', 'ts"}']);
+  });
+
+  it("replaces an earlier legacy function_call when modern tool_calls arrive later", async () => {
+    installStream([
+      chunk({ function_call: { name: "legacy", arguments: '{"query":"wrong"}' } }),
+      chunk({
+        tool_calls: [
+          {
+            index: 1,
+            id: "call_modern",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"ca' },
+          },
+        ],
+      }),
+      chunk({ tool_calls: [{ index: 1, function: { arguments: 'ts"}' } }] }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const observedStarts: Array<{ id: string; name: string }> = [];
+    const observedArgumentDeltas: string[] = [];
+    for await (const event of stream) {
+      if (event.type === "toolcall_start") {
+        const block = event.partial.content[event.contentIndex];
+        if (block?.type === "toolCall") {
+          observedStarts.push({ id: block.id, name: block.name });
+        }
+      } else if (event.type === "toolcall_delta" && event.delta) {
+        observedArgumentDeltas.push(event.delta);
+      }
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content).toHaveLength(1);
+    expect(result.content[0]).toMatchObject({
+      type: "toolCall",
+      id: "call_modern",
+      name: "lookup",
+      arguments: { query: "cats" },
+    });
+    expect(observedStarts).toEqual([{ id: "call_modern", name: "lookup" }]);
+    expect(observedArgumentDeltas).toEqual(['{"query":"ca', 'ts"}']);
+  });
+
+  it("keeps ordinary text and stop finish reasons outside the tool-call lane", async () => {
+    installStream([chunk({ role: "assistant", content: "No tool needed." }), chunk({}, "stop")]);
+
+    const result = await createStream(model, context, { apiKey: "fixture-token" }).result();
+
+    expect(result.stopReason).toBe("stop");
+    expect(result.content).toEqual([{ type: "text", text: "No tool needed." }]);
+  });
+
+  it.each([
+    { finishReason: "length", visibleText: false, stopReason: "length" },
+    { finishReason: "content_filter", visibleText: false, stopReason: "error" },
+    { finishReason: "stop", visibleText: true, stopReason: "stop" },
+  ] as const)(
+    "does not publish completed modern calls discarded by a $finishReason terminal",
+    async ({ finishReason, visibleText, stopReason }) => {
+      installStream([
+        ...(visibleText ? [chunk({ content: "Visible final answer." })] : []),
+        chunk({
+          tool_calls: [
+            {
+              index: 0,
+              id: "call_unconfirmed",
+              type: "function",
+              function: { name: "lookup", arguments: '{"query":"discard"}' },
+            },
+          ],
+        }),
+        chunk({}, finishReason),
+      ]);
+
+      const stream = createStream(model, context, { apiKey: "fixture-token" });
+      const eventTypes: string[] = [];
+      for await (const event of stream) {
+        eventTypes.push(event.type);
+      }
+
+      const result = await stream.result();
+      expect(result.stopReason).toBe(stopReason);
+      expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+      expect(eventTypes).not.toContain("toolcall_end");
+    },
+  );
+
+  it.each([
+    { reason: "incomplete JSON", name: "lookup", arguments: '{"query":"cats"' },
+    { reason: "malformed JSON", name: "lookup", arguments: '{"query":}' },
+    { reason: "invalid string escape", name: "lookup", arguments: String.raw`{"query":"cats\q"}` },
+    { reason: "unescaped control character", name: "lookup", arguments: '{"query":"cats\n"}' },
+    { reason: "non-object JSON", name: "lookup", arguments: '["cats"]' },
+    { reason: "empty arguments", name: "lookup", arguments: "" },
+    { reason: "missing function name", name: "", arguments: '{"query":"cats"}' },
+  ] as const)(
+    "rejects an authoritative modern tool terminal with $reason",
+    async ({ name, arguments: rawArguments }) => {
+      installStream([
+        chunk({
+          tool_calls: [
+            {
+              index: 0,
+              id: "call_malformed",
+              type: "function",
+              function: { name, arguments: rawArguments },
+            },
+          ],
+        }),
+        chunk({}, "tool_calls"),
+      ]);
+
+      const stream = createStream(model, context, { apiKey: "fixture-token" });
+      const eventTypes: string[] = [];
+      for await (const event of stream) {
+        eventTypes.push(event.type);
+      }
+
+      const result = await stream.result();
+      expect(result.stopReason).toBe("error");
+      expect(result.errorMessage).toContain("incomplete or malformed tool call");
+      expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+      expect(eventTypes).not.toContain("toolcall_end");
+    },
+  );
+
+  it.each([
+    { reason: "incomplete JSON", arguments: '{"query":"cats"' },
+    { reason: "invalid string escape", arguments: String.raw`{"query":"cats\q"}` },
+    { reason: "unescaped control character", arguments: '{"query":"cats\n"}' },
+  ] as const)("rejects an authoritative legacy function terminal with $reason", async (value) => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: value.arguments } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+
+    const result = await stream.result();
+    expect(result.stopReason).toBe("error");
+    expect(result.errorMessage).toContain("incomplete or malformed tool call");
+    expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+    expect(eventTypes).not.toContain("toolcall_end");
+  });
+
+  it("rejects every parallel call when a sibling has incomplete arguments", async () => {
+    installStream([
+      chunk({
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_valid",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"cats"}' },
+          },
+          {
+            index: 1,
+            id: "call_invalid",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"dogs"' },
+          },
+        ],
+      }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+
+    const result = await stream.result();
+    expect(result.stopReason).toBe("error");
+    expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+    expect(eventTypes).not.toContain("toolcall_end");
+  });
+
+  it("removes provisional calls when the response is aborted mid-stream", async () => {
+    installStream([
+      chunk({
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_aborted",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"cats"}' },
+          },
+        ],
+      }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const abort = new AbortController();
+    const stream = createStream(model, context, { apiKey: "fixture-token", signal: abort.signal });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+      if (event.type === "toolcall_start") {
+        abort.abort();
+      }
+    }
+
+    const result = await stream.result();
+    expect(result.stopReason).toBe("aborted");
+    expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+    expect(eventTypes).not.toContain("toolcall_end");
+  });
+
+  it("retains an executable modern call after its confirmed tool terminal", async () => {
+    installStream([
+      chunk({
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_confirmed",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"cats"}' },
+          },
+        ],
+      }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const events: Array<{ type: string; toolCallId?: string }> = [];
+    for await (const event of stream) {
+      events.push({
+        type: event.type,
+        ...(event.type === "toolcall_end" ? { toolCallId: event.toolCall.id } : {}),
+      });
+    }
+
+    const result = await stream.result();
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content).toContainEqual({
+      type: "toolCall",
+      id: "call_confirmed",
+      name: "lookup",
+      arguments: { query: "cats" },
+    });
+    expect(events.filter((event) => event.type === "toolcall_end")).toEqual([
+      { type: "toolcall_end", toolCallId: "call_confirmed" },
+    ]);
+  });
+
+  it("preserves confirmed tool completion before following text blocks close", async () => {
+    installStream([
+      chunk({
+        tool_calls: [
+          {
+            index: 0,
+            id: "call_before_text",
+            type: "function",
+            function: { name: "lookup", arguments: '{"query":"cats"}' },
+          },
+        ],
+      }),
+      chunk({ content: "Trailing commentary." }),
+      chunk({}, "tool_calls"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+
+    expect((await stream.result()).stopReason).toBe("toolUse");
+    const toolEndIndex = eventTypes.indexOf("toolcall_end");
+    expect(toolEndIndex).toBeGreaterThanOrEqual(0);
+    expect(toolEndIndex).toBeLessThan(eventTypes.indexOf("done"));
+
+    const followingTextEndIndex = eventTypes.indexOf("text_end");
+    if (followingTextEndIndex >= 0) {
+      expect(toolEndIndex).toBeLessThan(followingTextEndIndex);
+    }
+  });
+
+  it.each([
+    { finishReason: "stop", stopReason: "stop" },
+    { finishReason: "length", stopReason: "length" },
+    { finishReason: "content_filter", stopReason: "error" },
+    { finishReason: "tool_calls", stopReason: "stop" },
+  ] as const)(
+    "discards provisional legacy fragments when the provider finishes with $finishReason",
+    async ({ finishReason, stopReason }) => {
+      installStream([
+        chunk({ function_call: { name: "lookup", arguments: '{"query":"discard"}' } }),
+        chunk({}, finishReason),
+      ]);
+
+      const stream = createStream(model, context, { apiKey: "fixture-token" });
+      const eventTypes: string[] = [];
+      for await (const event of stream) {
+        eventTypes.push(event.type);
+      }
+      const result = await stream.result();
+
+      expect(result.stopReason).toBe(stopReason);
+      expect(result.content.filter((block) => block.type === "toolCall")).toHaveLength(0);
+      expect(eventTypes).not.toContain("toolcall_start");
+      expect(eventTypes).not.toContain("toolcall_delta");
+    },
+  );
+
+  it("rejects oversized legacy arguments without publishing a provisional tool call", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: "x".repeat(256_001) } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("error");
+    expect(result.errorMessage).toContain("Exceeded tool-call argument buffer limit");
+    expect(eventTypes).not.toContain("toolcall_start");
+  });
+
+  it("rejects an oversized legacy function name before publishing a provisional call", async () => {
+    installStream([
+      chunk({ function_call: { name: "x".repeat(256_001), arguments: "{}" } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("error");
+    expect(result.errorMessage).toContain("Exceeded tool-call argument buffer limit");
+    expect(eventTypes).not.toContain("toolcall_start");
+  });
+
+  it("coalesces tiny provisional legacy fragments into one bounded executable delta", async () => {
+    const query = "x".repeat(1_100);
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"' } }),
+      ...Array.from(query, (character) =>
+        chunk({
+          content: "",
+          refusal: "",
+          reasoning_details: [],
+          function_call: { arguments: character },
+        } as ChatCompletionChunk.Choice.Delta),
+      ),
+      chunk({ function_call: { arguments: '"}' } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const argumentDeltas: string[] = [];
+    for await (const event of stream) {
+      if (event.type === "toolcall_delta" && event.delta) {
+        argumentDeltas.push(event.delta);
+      }
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("toolUse");
+    expect(result.content[0]).toMatchObject({
+      type: "toolCall",
+      name: "lookup",
+      arguments: { query },
+    });
+    expect(argumentDeltas).toEqual([JSON.stringify({ query })]);
+  });
+
+  it("bounds visible content buffered behind a provisional legacy call", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"cats"}' } }),
+      chunk({ content: "x".repeat(256_001) }),
+      chunk({}, "function_call"),
+    ]);
+
+    const stream = createStream(model, context, { apiKey: "fixture-token" });
+    const eventTypes: string[] = [];
+    for await (const event of stream) {
+      eventTypes.push(event.type);
+    }
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("error");
+    expect(result.errorMessage).toContain("Exceeded legacy tool-call content buffer limit");
+    expect(eventTypes).not.toContain("toolcall_start");
+    expect(eventTypes).not.toContain("text_start");
+  });
+
+  it("bounds the number of tiny deltas buffered behind a provisional legacy call", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"cats"}' } }),
+      ...Array.from({ length: 1_025 }, () => chunk({ content: "x" })),
+      chunk({}, "function_call"),
+    ]);
+
+    const result = await createStream(model, context, { apiKey: "fixture-token" }).result();
+
+    expect(result.stopReason).toBe("error");
+    expect(result.errorMessage).toContain("Exceeded legacy tool-call content buffer limit");
+    expect(result.content).toEqual([]);
+  });
+
+  it("generates a distinct legacy call id for each assistant response", async () => {
+    installStream([
+      chunk({ function_call: { name: "lookup", arguments: '{"query":"cats"}' } }),
+      chunk({}, "function_call"),
+    ]);
+
+    const first = await createStream(model, context, { apiKey: "fixture-token" }).result();
+    const second = await createStream(model, context, { apiKey: "fixture-token" }).result();
+
+    expect(first.content[0]).toMatchObject({ type: "toolCall" });
+    expect(second.content[0]).toMatchObject({ type: "toolCall" });
+    if (first.content[0]?.type === "toolCall" && second.content[0]?.type === "toolCall") {
+      expect(first.content[0].id).not.toBe(second.content[0].id);
+    }
+  });
+});
diff --git a/packages/ai/src/providers/openai-completions.ts b/packages/ai/src/providers/openai-completions.ts
index eb07a040301c..9157acd2732b 100644
--- a/packages/ai/src/providers/openai-completions.ts
+++ b/packages/ai/src/providers/openai-completions.ts
@@ -58,6 +58,10 @@ import { splitSystemPromptCacheBoundary } from "../utils/system-prompt-cache-bou
 import { resolveCacheRetention } from "./cache-retention.js";
 import { isCloudflareProvider, resolveCloudflareBaseUrl } from "./cloudflare.js";
 import { buildCopilotDynamicHeaders, hasCopilotVisionInput } from "./github-copilot-headers.js";
+import {
+  createOpenAICompletionsToolCallDeltaNormalizer,
+  finalizeOpenAICompletionsToolCalls,
+} from "./openai-completions-tool-calls.js";
 import { clampOpenAIPromptCacheKey } from "./openai-prompt-cache.js";
 import {
   resolveOpenAICompletionsResponseFormat,
@@ -203,6 +207,7 @@ export const streamOpenAICompletions: StreamFunction<
       const toolCallBlocksByIndex = new Map<number, StreamingToolCallBlock>();
       const toolCallBlocksById = new Map<string, StreamingToolCallBlock>();
       const toolCallBlocksByFirstId = new Map<string, StreamingToolCallBlock>();
+      const normalizeToolCallDeltas = createOpenAICompletionsToolCallDeltaNormalizer();
       const pendingReasoningDetailsByToolCallId = new Map<string, string>();
       const blocks = output.content as StreamingBlock[];
       // A block can be finished mid-stream (native reasoning sealed at the
@@ -249,10 +254,6 @@ export const streamOpenAICompletions: StreamFunction<
             partial: output,
           });
         } else if (block.type === "toolCall") {
-          // Finalize in-place and strip the scratch buffers so replay only
-          // carries parsed arguments.
-          delete block.partialArgs;
-          delete block.streamIndex;
           stream.push({
             type: "toolcall_end",
             contentIndex,
@@ -433,100 +434,107 @@ export const streamOpenAICompletions: StreamFunction<
         // Some OpenAI-compatible endpoints deliver a full `message` instead of
         // `delta` (including refusal-only turns with content: null). Normalize
         // the same way the managed agent transport does.
-        const choiceDelta =
+        const rawChoiceDelta =
           choice.delta ??
           (choice as { message?: ChatCompletionChunk["choices"][number]["delta"] }).message;
-        if (choiceDelta) {
-          // Some endpoints return reasoning in reasoning_content (llama.cpp),
-          // or reasoning (other openai compatible endpoints)
-          // Use the first non-empty reasoning field to avoid duplication
-          // (e.g., chutes.ai returns both reasoning_content and reasoning with same content)
-          const reasoningFields = ["reasoning_content", "reasoning", "reasoning_text"];
-          const deltaFields = choiceDelta as Record<string, unknown>;
-          const shouldEmitReasoning = Boolean(model.reasoning && options?.reasoningEffort);
-          let foundReasoningField: string | null = null;
-          for (const field of reasoningFields) {
-            const value = deltaFields[field];
-            if (typeof value === "string" && value.length > 0) {
-              foundReasoningField = field;
-              break;
-            }
-          }
-          if (foundReasoningField) {
-            reasoningTagTextPartitioner.markStrict();
-          }
-          if (shouldEmitReasoning && foundReasoningField) {
-            const delta = deltaFields[foundReasoningField];
-            if (typeof delta === "string" && delta.length > 0) {
-              const thinkingSignature =
-                model.provider === "opencode-go" && foundReasoningField === "reasoning"
-                  ? "reasoning_content"
-                  : foundReasoningField;
-              appendThinkingDelta(thinkingSignature, delta);
-            }
-          }
-          if (
-            choiceDelta.content !== null &&
-            choiceDelta.content !== undefined &&
-            choiceDelta.content.length > 0
-          ) {
-            appendPartitionedContent(choiceDelta.content, Boolean(foundReasoningField));
-          }
-
-          // Chat Completions can put safety/structured-output refusals in a
-          // top-level `refusal` field with content null. Surface that as
-          // visible text so the assistant turn is not empty.
-          const refusalText = typeof choiceDelta.refusal === "string" ? choiceDelta.refusal : "";
-          if (refusalText.length > 0) {
-            appendPartitionedContent(refusalText, Boolean(foundReasoningField));
-          }
-
-          if (choiceDelta.tool_calls && choiceDelta.tool_calls.length > 0) {
-            flushPartitionedContent();
-            // The tool-call lane is also a reasoning boundary; seal the thought
-            // before toolcall_start so thinking_end never trails the action.
-            sealNativeReasoningBeforeText();
-            rememberPendingCommentaryTags(
-              provisionalCommentaryTags,
-              tagPendingCommentaryText(output.content),
-            );
-            for (const toolCall of choiceDelta.tool_calls) {
-              const block = ensureToolCallBlock(toolCall);
-              if (!block.id && toolCall.id) {
-                block.id = toolCall.id;
-                toolCallBlocksById.set(toolCall.id, block);
-                rememberFirstToolCallById(toolCall.id, block);
+        if (rawChoiceDelta) {
+          for (const normalizedDelta of normalizeToolCallDeltas(
+            rawChoiceDelta,
+            choice.finish_reason,
+          )) {
+            const choiceDelta = normalizedDelta.delta;
+            // Some endpoints return reasoning in reasoning_content (llama.cpp),
+            // or reasoning (other openai compatible endpoints)
+            // Use the first non-empty reasoning field to avoid duplication
+            // (e.g., chutes.ai returns both reasoning_content and reasoning with same content)
+            const reasoningFields = ["reasoning_content", "reasoning", "reasoning_text"];
+            const deltaFields = choiceDelta as Record<string, unknown>;
+            const shouldEmitReasoning = Boolean(model.reasoning && options?.reasoningEffort);
+            let foundReasoningField: string | null = null;
+            for (const field of reasoningFields) {
+              const value = deltaFields[field];
+              if (typeof value === "string" && value.length > 0) {
+                foundReasoningField = field;
+                break;
               }
-              if (!block.name && toolCall.function?.name) {
-                block.name = toolCall.function.name;
-              }
-
-              let delta = "";
-              if (toolCall.function?.arguments) {
-                delta = toolCall.function.arguments;
-                block.partialArgs = (block.partialArgs ?? "") + toolCall.function.arguments;
-                block.arguments = parseStreamingJson(block.partialArgs);
-              }
-              stream.push({
-                type: "toolcall_delta",
-                contentIndex: getContentIndex(block),
-                delta,
-                partial: output,
-              });
             }
-          }
+            if (foundReasoningField) {
+              reasoningTagTextPartitioner.markStrict();
+            }
+            if (shouldEmitReasoning && foundReasoningField) {
+              const delta = deltaFields[foundReasoningField];
+              if (typeof delta === "string" && delta.length > 0) {
+                const thinkingSignature =
+                  model.provider === "opencode-go" && foundReasoningField === "reasoning"
+                    ? "reasoning_content"
+                    : foundReasoningField;
+                appendThinkingDelta(thinkingSignature, delta);
+              }
+            }
+            if (
+              choiceDelta.content !== null &&
+              choiceDelta.content !== undefined &&
+              choiceDelta.content.length > 0
+            ) {
+              appendPartitionedContent(choiceDelta.content, Boolean(foundReasoningField));
+            }
 
-          const reasoningDetails = (choiceDelta as { reasoning_details?: unknown })
-            .reasoning_details;
-          if (Array.isArray(reasoningDetails)) {
-            for (const detail of reasoningDetails) {
-              if (isEncryptedReasoningDetail(detail)) {
-                const serializedDetail = JSON.stringify(detail);
-                const matchingToolCall = toolCallBlocksByFirstId.get(detail.id);
-                if (matchingToolCall) {
-                  matchingToolCall.thoughtSignature = serializedDetail;
-                } else {
-                  pendingReasoningDetailsByToolCallId.set(detail.id, serializedDetail);
+            // Chat Completions can put safety/structured-output refusals in a
+            // top-level `refusal` field with content null. Surface that as
+            // visible text so the assistant turn is not empty.
+            const refusalText = typeof choiceDelta.refusal === "string" ? choiceDelta.refusal : "";
+            if (refusalText.length > 0) {
+              appendPartitionedContent(refusalText, Boolean(foundReasoningField));
+            }
+
+            const toolCallDeltas = normalizedDelta.toolCalls;
+            if (toolCallDeltas.length > 0) {
+              flushPartitionedContent();
+              // The tool-call lane is also a reasoning boundary; seal the thought
+              // before toolcall_start so thinking_end never trails the action.
+              sealNativeReasoningBeforeText();
+              rememberPendingCommentaryTags(
+                provisionalCommentaryTags,
+                tagPendingCommentaryText(output.content),
+              );
+              for (const toolCall of toolCallDeltas) {
+                const block = ensureToolCallBlock(toolCall);
+                if (!block.id && toolCall.id) {
+                  block.id = toolCall.id;
+                  toolCallBlocksById.set(toolCall.id, block);
+                  rememberFirstToolCallById(toolCall.id, block);
+                }
+                if (!block.name && toolCall.function?.name) {
+                  block.name = toolCall.function.name;
+                }
+
+                let delta = "";
+                if (toolCall.function?.arguments) {
+                  delta = toolCall.function.arguments;
+                  block.partialArgs = (block.partialArgs ?? "") + toolCall.function.arguments;
+                  block.arguments = parseStreamingJson(block.partialArgs);
+                }
+                stream.push({
+                  type: "toolcall_delta",
+                  contentIndex: getContentIndex(block),
+                  delta,
+                  partial: output,
+                });
+              }
+            }
+
+            const reasoningDetails = (choiceDelta as { reasoning_details?: unknown })
+              .reasoning_details;
+            if (Array.isArray(reasoningDetails)) {
+              for (const detail of reasoningDetails) {
+                if (isEncryptedReasoningDetail(detail)) {
+                  const serializedDetail = JSON.stringify(detail);
+                  const matchingToolCall = toolCallBlocksByFirstId.get(detail.id);
+                  if (matchingToolCall) {
+                    matchingToolCall.thoughtSignature = serializedDetail;
+                  } else {
+                    pendingReasoningDetailsByToolCallId.set(detail.id, serializedDetail);
+                  }
                 }
               }
             }
@@ -536,35 +544,46 @@ export const streamOpenAICompletions: StreamFunction<
 
       flushPartitionedContent();
 
-      for (const block of blocks) {
-        finishBlock(block);
-      }
+      let terminalError: Error | undefined;
       if (options?.signal?.aborted) {
-        throw transportAbortError(options.signal);
+        terminalError = transportAbortError(options.signal);
+      } else if (output.stopReason === "aborted") {
+        terminalError = new Error("Request was aborted");
+      } else if (output.stopReason === "error") {
+        terminalError = new Error(output.errorMessage || "Provider returned an error stop reason");
+      } else if (!hasFinishReason) {
+        terminalError = new Error("Stream ended without finish_reason");
       }
 
-      if (output.stopReason === "aborted") {
-        throw new Error("Request was aborted");
-      }
-      if (output.stopReason === "error") {
-        throw new Error(output.errorMessage || "Provider returned an error stop reason");
-      }
-      if (!hasFinishReason) {
-        throw new Error("Stream ended without finish_reason");
+      if (terminalError) {
+        for (const block of blocks) {
+          if (block.type !== "toolCall") {
+            finishBlock(block);
+          }
+        }
+        throw terminalError;
       }
 
-      const hasToolCalls = output.content.some((block) => block.type === "toolCall");
-      const hasVisibleText = output.content.some(
-        (block) => block.type === "text" && block.text.trim().length > 0,
-      );
-      if (output.stopReason === "toolUse" && !hasToolCalls) {
-        output.stopReason = "stop";
+      finalizeOpenAICompletionsToolCalls(output);
+      if (output.stopReason === "aborted" || output.stopReason === "error") {
+        for (const block of blocks) {
+          if (block.type !== "toolCall") {
+            finishBlock(block);
+          }
+        }
+        throw new Error(
+          output.errorMessage ||
+            (output.stopReason === "aborted"
+              ? "Request was aborted"
+              : "Provider returned an invalid tool call"),
+        );
       }
-      if (output.stopReason === "stop" && hasToolCalls && !hasVisibleText) {
-        output.stopReason = "toolUse";
-      }
-      if (hasToolCalls && output.stopReason !== "toolUse") {
-        output.content = output.content.filter((block) => block.type !== "toolCall");
+      // Tool completion is irreversible: confirm the terminal before closing
+      // blocks, then preserve their original text/thinking/tool event order.
+      for (const block of blocks) {
+        if (block.type !== "toolCall" || output.stopReason === "toolUse") {
+          finishBlock(block);
+        }
       }
       if (output.stopReason !== "toolUse") {
         clearPendingCommentaryText(provisionalCommentaryTags);
@@ -576,13 +595,14 @@ export const streamOpenAICompletions: StreamFunction<
       stream.push({ type: "done", reason: output.stopReason, message: output });
       stream.end();
     } catch (error) {
+      output.stopReason = options?.signal?.aborted ? "aborted" : "error";
+      finalizeOpenAICompletionsToolCalls(output, { allowSilentToolCallPromotion: false });
       for (const block of output.content) {
         delete (block as { index?: number }).index;
         // Streaming scratch buffers are only used during parsing; never persist them.
         delete (block as { partialArgs?: string }).partialArgs;
         delete (block as { streamIndex?: number }).streamIndex;
       }
-      output.stopReason = options?.signal?.aborted ? "aborted" : "error";
       output.errorMessage = formatProviderError(error);
       // Some providers via OpenRouter give additional information in this field.
       const rawMetadata = (error as { error?: { metadata?: { raw?: string } } })?.error?.metadata
diff --git a/packages/ai/src/transports/openai-completions-transport.ts b/packages/ai/src/transports/openai-completions-transport.ts
index f457fe4bc003..70edd3dcde8b 100644
--- a/packages/ai/src/transports/openai-completions-transport.ts
+++ b/packages/ai/src/transports/openai-completions-transport.ts
@@ -9,6 +9,10 @@ import { applyProviderReportedUsageCost, calculateCost } from "../model-utils.js
 import { convertMessages } from "../openai-completions-messages.js";
 import type { OpenAICompletionsOptions } from "../provider-options.js";
 import { resolveCacheRetention } from "../providers/cache-retention.js";
+import {
+  createOpenAICompletionsToolCallDeltaNormalizer,
+  finalizeOpenAICompletionsToolCalls,
+} from "../providers/openai-completions-tool-calls.js";
 import {
   isOpenAIGpt54MiniModel,
   isOpenAIGpt55Model,
@@ -353,7 +357,16 @@ export function createOpenAICompletionsTransportStreamFn(): StreamFn {
         });
         finalizeTransportStream({ stream, output, signal: options?.signal });
       } catch (error) {
-        failTransportStream({ stream, output, signal: options?.signal, error });
+        failTransportStream({
+          stream,
+          output,
+          signal: options?.signal,
+          error,
+          cleanup: () => {
+            output.stopReason = options?.signal?.aborted ? "aborted" : "error";
+            finalizeOpenAICompletionsToolCalls(output, { allowSilentToolCallPromotion: false });
+          },
+        });
       } finally {
         firstEventAbort?.dispose();
       }
@@ -408,6 +421,7 @@ async function processOpenAICompletionsStream(
   const provisionalCommentaryTags: PendingCommentaryTags = new Map();
   const toolCallBlockBytes = new WeakMap<ToolCallBlock, number>();
   const toolCallBlockIndices = new WeakMap<ToolCallBlock, number>();
+  const normalizeToolCallDeltas = createOpenAICompletionsToolCallDeltaNormalizer();
   let sawStopFinishReason = false;
   let sawNativeToolCallDelta = false;
   const blockIndex = () => output.content.length - 1;
@@ -669,132 +683,137 @@ async function processOpenAICompletionsStream(
         output.errorMessage = finishReasonResult.errorMessage;
       }
     }
-    const choiceDelta =
+    const rawChoiceDelta =
       choice.delta ??
       (choice as unknown as { message?: ChatCompletionChunk["choices"][number]["delta"] }).message;
-    if (!choiceDelta) {
+    if (!rawChoiceDelta) {
       emitReasoningUsageActivity(hasReasoningUsageActivity);
       await cooperativeScheduler.afterEvent();
       continue;
     }
-    const reasoningDeltas = getCompletionsReasoningDeltas(
-      choiceDelta as Record<string, unknown>,
-      compat.visibleReasoningDetailTypes,
-    );
-    const hasMirroredReasoning = reasoningDeltas.some((delta) => delta.kind === "thinking");
-    if (hasMirroredReasoning) {
-      reasoningTagTextPartitioner.markStrict();
-    }
-    if (choiceDelta.content) {
-      // Structured content can contain visible text and thinking blocks in the
-      // same delta, so route each extracted block through the normal stream path.
-      const contentDeltas = getCompletionsContentDeltas(choiceDelta.content);
-      for (const contentDelta of contentDeltas) {
-        if (contentDelta.kind === "text") {
-          const routedDeltas = hasMirroredReasoning
-            ? reasoningTagTextPartitioner.push(contentDelta.text)
-            : reasoningTagTextPartitioner.pushVisible(contentDelta.text);
-          for (const routedDelta of routedDeltas) {
-            appendPartitionedVisibleDelta(routedDelta);
-          }
-        } else {
-          reasoningTagTextPartitioner.markStrict();
-          appendRoutedContentDelta(contentDelta);
-        }
-      }
-    }
-    // Chat Completions can put safety/structured-output refusals in a top-level
-    // `refusal` field with content null. Surface that as visible text so the
-    // assistant turn is not empty (Responses path already routes refusal deltas).
-    const refusalText = typeof choiceDelta.refusal === "string" ? choiceDelta.refusal : "";
-    if (refusalText) {
-      const routedDeltas = hasMirroredReasoning
-        ? reasoningTagTextPartitioner.push(refusalText)
-        : reasoningTagTextPartitioner.pushVisible(refusalText);
-      for (const routedDelta of routedDeltas) {
-        appendPartitionedVisibleDelta(routedDelta);
-      }
-    }
-    for (const reasoningDelta of reasoningDeltas) {
-      if (reasoningDelta.kind === "thinking" && !emitReasoning) {
-        continue;
-      }
-      if (currentBlock?.type === "toolCall") {
-        queuePostToolCallDelta({ ...reasoningDelta });
-        continue;
-      }
-      if (reasoningDelta.kind === "text") {
-        appendTextDelta(reasoningDelta.text);
-      } else if (emitReasoning) {
-        appendThinkingDelta(reasoningDelta);
-      }
-    }
-    if (choiceDelta.tool_calls && choiceDelta.tool_calls.length > 0) {
-      sawNativeToolCallDelta = true;
-      flushReasoningTagTextPartitionerAtEnd();
-      rememberPendingCommentaryTags(
-        provisionalCommentaryTags,
-        tagPendingCommentaryText(output.content),
+    for (const normalizedDelta of normalizeToolCallDeltas(rawChoiceDelta, choice.finish_reason)) {
+      const choiceDelta = normalizedDelta.delta;
+      const reasoningDeltas = getCompletionsReasoningDeltas(
+        choiceDelta as Record<string, unknown>,
+        compat.visibleReasoningDetailTypes,
       );
-      for (const toolCall of choiceDelta.tool_calls) {
-        const streamIndex = typeof toolCall.index === "number" ? toolCall.index : undefined;
-        let block = streamIndex !== undefined ? toolCallBlocksByIndex.get(streamIndex) : undefined;
-        if (!block && toolCall.id) {
-          block = toolCallBlocksById.get(toolCall.id);
-        }
-        if (!block) {
-          const switchingToolCall = currentBlock?.type === "toolCall";
-          if (switchingToolCall) {
-            currentBlock = null;
-            flushPendingPostToolCallDeltas();
+      const hasMirroredReasoning = reasoningDeltas.some((delta) => delta.kind === "thinking");
+      if (hasMirroredReasoning) {
+        reasoningTagTextPartitioner.markStrict();
+      }
+      if (choiceDelta.content) {
+        // Structured content can contain visible text and thinking blocks in the
+        // same delta, so route each extracted block through the normal stream path.
+        const contentDeltas = getCompletionsContentDeltas(choiceDelta.content);
+        for (const contentDelta of contentDeltas) {
+          if (contentDelta.kind === "text") {
+            const routedDeltas = hasMirroredReasoning
+              ? reasoningTagTextPartitioner.push(contentDelta.text)
+              : reasoningTagTextPartitioner.pushVisible(contentDelta.text);
+            for (const routedDelta of routedDeltas) {
+              appendPartitionedVisibleDelta(routedDelta);
+            }
+          } else {
+            reasoningTagTextPartitioner.markStrict();
+            appendRoutedContentDelta(contentDelta);
           }
-          const initialSig = extractGoogleThoughtSignature(toolCall);
-          block = {
-            type: "toolCall",
-            id: toolCall.id || "",
-            name: toolCall.function?.name || "",
-            arguments: {},
-            partialArgs: "",
-            ...(initialSig ? { thoughtSignature: initialSig } : {}),
-          };
-          output.content.push(block);
-          toolCallBlockIndices.set(block, output.content.length - 1);
-          pushStreamEvent({
-            type: "toolcall_start",
-            contentIndex: toolCallBlockIndices.get(block) ?? -1,
-            partial: output,
-          });
         }
-        if (streamIndex !== undefined && !toolCallBlocksByIndex.has(streamIndex)) {
-          toolCallBlocksByIndex.set(streamIndex, block);
+      }
+      // Chat Completions can put safety/structured-output refusals in a top-level
+      // `refusal` field with content null. Surface that as visible text so the
+      // assistant turn is not empty (Responses path already routes refusal deltas).
+      const refusalText = typeof choiceDelta.refusal === "string" ? choiceDelta.refusal : "";
+      if (refusalText) {
+        const routedDeltas = hasMirroredReasoning
+          ? reasoningTagTextPartitioner.push(refusalText)
+          : reasoningTagTextPartitioner.pushVisible(refusalText);
+        for (const routedDelta of routedDeltas) {
+          appendPartitionedVisibleDelta(routedDelta);
         }
-        if (toolCall.id) {
-          block.id = toolCall.id;
-          toolCallBlocksById.set(toolCall.id, block);
+      }
+      for (const reasoningDelta of reasoningDeltas) {
+        if (reasoningDelta.kind === "thinking" && !emitReasoning) {
+          continue;
         }
-        currentBlock = block;
-        if (toolCall.function?.name) {
-          block.name = toolCall.function.name;
+        if (currentBlock?.type === "toolCall") {
+          queuePostToolCallDelta({ ...reasoningDelta });
+          continue;
         }
-        const deltaSig = extractGoogleThoughtSignature(toolCall);
-        if (deltaSig) {
-          block.thoughtSignature = deltaSig;
+        if (reasoningDelta.kind === "text") {
+          appendTextDelta(reasoningDelta.text);
+        } else if (emitReasoning) {
+          appendThinkingDelta(reasoningDelta);
         }
-        if (toolCall.function?.arguments) {
-          const nextArgumentBytes = measureUtf8Bytes(toolCall.function.arguments);
-          const currentBlockArgBytes = toolCallBlockBytes.get(block) ?? 0;
-          if (currentBlockArgBytes + nextArgumentBytes > MAX_TOOL_CALL_ARGUMENT_BUFFER_BYTES) {
-            throw new Error("Exceeded tool-call argument buffer limit");
+      }
+      const toolCallDeltas = normalizedDelta.toolCalls;
+      if (toolCallDeltas.length > 0) {
+        sawNativeToolCallDelta = true;
+        flushReasoningTagTextPartitionerAtEnd();
+        rememberPendingCommentaryTags(
+          provisionalCommentaryTags,
+          tagPendingCommentaryText(output.content),
+        );
+        for (const toolCall of toolCallDeltas) {
+          const streamIndex = typeof toolCall.index === "number" ? toolCall.index : undefined;
+          let block =
+            streamIndex !== undefined ? toolCallBlocksByIndex.get(streamIndex) : undefined;
+          if (!block && toolCall.id) {
+            block = toolCallBlocksById.get(toolCall.id);
+          }
+          if (!block) {
+            const switchingToolCall = currentBlock?.type === "toolCall";
+            if (switchingToolCall) {
+              currentBlock = null;
+              flushPendingPostToolCallDeltas();
+            }
+            const initialSig = extractGoogleThoughtSignature(toolCall);
+            block = {
+              type: "toolCall",
+              id: toolCall.id || "",
+              name: toolCall.function?.name || "",
+              arguments: {},
+              partialArgs: "",
+              ...(initialSig ? { thoughtSignature: initialSig } : {}),
+            };
+            output.content.push(block);
+            toolCallBlockIndices.set(block, output.content.length - 1);
+            pushStreamEvent({
+              type: "toolcall_start",
+              contentIndex: toolCallBlockIndices.get(block) ?? -1,
+              partial: output,
+            });
+          }
+          if (streamIndex !== undefined && !toolCallBlocksByIndex.has(streamIndex)) {
+            toolCallBlocksByIndex.set(streamIndex, block);
+          }
+          if (toolCall.id) {
+            block.id = toolCall.id;
+            toolCallBlocksById.set(toolCall.id, block);
+          }
+          currentBlock = block;
+          if (toolCall.function?.name) {
+            block.name = toolCall.function.name;
+          }
+          const deltaSig = extractGoogleThoughtSignature(toolCall);
+          if (deltaSig) {
+            block.thoughtSignature = deltaSig;
+          }
+          if (toolCall.function?.arguments) {
+            const nextArgumentBytes = measureUtf8Bytes(toolCall.function.arguments);
+            const currentBlockArgBytes = toolCallBlockBytes.get(block) ?? 0;
+            if (currentBlockArgBytes + nextArgumentBytes > MAX_TOOL_CALL_ARGUMENT_BUFFER_BYTES) {
+              throw new Error("Exceeded tool-call argument buffer limit");
+            }
+            toolCallBlockBytes.set(block, currentBlockArgBytes + nextArgumentBytes);
+            block.partialArgs += toolCall.function.arguments;
+            block.arguments = parseStreamingJson(block.partialArgs);
+            pushStreamEvent({
+              type: "toolcall_delta",
+              contentIndex: toolCallBlockIndices.get(block) ?? -1,
+              delta: toolCall.function.arguments,
+              partial: output,
+            });
           }
-          toolCallBlockBytes.set(block, currentBlockArgBytes + nextArgumentBytes);
-          block.partialArgs += toolCall.function.arguments;
-          block.arguments = parseStreamingJson(block.partialArgs);
-          pushStreamEvent({
-            type: "toolcall_delta",
-            contentIndex: toolCallBlockIndices.get(block) ?? -1,
-            delta: toolCall.function.arguments,
-            partial: output,
-          });
         }
       }
     }
@@ -807,14 +826,6 @@ async function processOpenAICompletionsStream(
   flushDeepSeekTextFilterAtEnd();
   currentBlock = null;
   flushPendingPostToolCallDeltas();
-  const hasToolCalls = output.content.some((block) => block.type === "toolCall");
-  const hasVisibleText = output.content.some(
-    (block) =>
-      block.type === "text" && typeof block.text === "string" && block.text.trim().length > 0,
-  );
-  if (output.stopReason === "toolUse" && !hasToolCalls) {
-    output.stopReason = "stop";
-  }
   // Promote complete silent tool-call-only responses when the stream finished
   // cleanly (reached post-loop). Two paths:
   //   sawStopFinishReason: explicit provider terminal (legacy DSML / #88791)
@@ -823,17 +834,18 @@ async function processOpenAICompletionsStream(
   //     DeepSeek V4). [DONE] tracking distinguishes clean termination from
   //     connection drops (EOF without [DONE] remains fail-closed).
   // Truncated streams throw before reaching this code.
-  if (
-    output.stopReason === "stop" &&
-    hasToolCalls &&
-    !hasVisibleText &&
-    (sawStopFinishReason || (sawNativeToolCallDelta && (options?.sawStreamDONE?.() ?? false)))
-  ) {
-    output.stopReason = "toolUse";
-  }
-  if (hasToolCalls && output.stopReason !== "toolUse") {
-    output.content = output.content.filter((block) => block.type !== "toolCall");
-  }
+  finalizeOpenAICompletionsToolCalls(output, {
+    allowSilentToolCallPromotion:
+      sawStopFinishReason || (sawNativeToolCallDelta && (options?.sawStreamDONE?.() ?? false)),
+    onConfirmedToolCall(block, contentIndex) {
+      pushStreamEvent({
+        type: "toolcall_end",
+        contentIndex,
+        toolCall: block,
+        partial: output,
+      });
+    },
+  });
   if (output.stopReason !== "toolUse") {
     clearPendingCommentaryText(provisionalCommentaryTags);
   }
diff --git a/src/agents/openai-transport-stream.deepseek-and-shaping.test.ts b/src/agents/openai-transport-stream.deepseek-and-shaping.test.ts
index 7a7c34a72d4b..40fc8273eb6d 100644
--- a/src/agents/openai-transport-stream.deepseek-and-shaping.test.ts
+++ b/src/agents/openai-transport-stream.deepseek-and-shaping.test.ts
@@ -101,7 +101,6 @@ describe("openai transport stream", () => {
         id: "call_native_1",
         name: "read",
         arguments: { path: "/tmp/native.md" },
-        partialArgs: '{"path":"/tmp/native.md"}',
       },
     ]);
     expect(JSON.stringify(events)).not.toContain("DSML");
@@ -144,7 +143,6 @@ describe("openai transport stream", () => {
         id: "call_native_1",
         name: "read",
         arguments: { path: "/tmp/native.md" },
-        partialArgs: '{"path":"/tmp/native.md"}',
       },
     ]);
   });
@@ -184,7 +182,6 @@ describe("openai transport stream", () => {
         id: "call_native_1",
         name: "read",
         arguments: { path: "/tmp/native.md" },
-        partialArgs: '{"path":"/tmp/native.md"}',
       },
       {
         type: "text",
@@ -236,7 +233,6 @@ describe("openai transport stream", () => {
         id: "call_native_1",
         name: "read",
         arguments: { path: "/tmp/native.md" },
-        partialArgs: '{"path":"/tmp/native.md"}',
       },
       {
         type: "text",
@@ -274,7 +270,6 @@ describe("openai transport stream", () => {
         id: expect.stringMatching(/^call_[0-9a-f]{24}$/),
         name: "session_status",
         arguments: { sessionKey: "current" },
-        partialArgs: '{"sessionKey":"current"}',
       },
     ]);
     expect(JSON.stringify(events)).not.toContain("DSML");
@@ -436,7 +431,6 @@ describe("openai transport stream", () => {
           id: expect.stringMatching(/^call_[0-9a-f]{24}$/),
           name: "session_status",
           arguments: { sessionKey: "current" },
-          partialArgs: '{"sessionKey":"current"}',
         },
       ]);
     }
@@ -465,7 +459,6 @@ describe("openai transport stream", () => {
         id: expect.stringMatching(/^call_[0-9a-f]{24}$/),
         name: "read",
         arguments: { path: "/tmp/native.md" },
-        partialArgs: '{"path":"/tmp/native.md"}',
       },
     ]);
   });