diff --git a/src/agents/openai-transport-stream.ts b/src/agents/openai-transport-stream.ts
index f53cad359101..75d54d5e5684 100644
--- a/src/agents/openai-transport-stream.ts
+++ b/src/agents/openai-transport-stream.ts
@@ -26,6 +26,7 @@ import { calculateCost } from "../llm/model-utils.js";
 import { resolveAzureDeploymentNameFromMap } from "../llm/providers/azure-deployment-map.js";
 import { convertMessages } from "../llm/providers/openai-completions.js";
 import { clampOpenAIPromptCacheKey } from "../llm/providers/openai-prompt-cache.js";
+import { mapOpenAIStopReason } from "../llm/providers/openai-stop-reason.js";
 import type { Api, Context, Model } from "../llm/types.js";
 import { createAssistantMessageEventStream } from "../llm/utils/event-stream.js";
 import { parseStreamingJson } from "../llm/utils/json-parse.js";
@@ -3101,7 +3102,9 @@ async function processOpenAICompletionsStream(
       hasReasoningUsageActivity = hasOpenAICompletionsReasoningUsageActivity(choiceUsage);
     }
     if (choice.finish_reason) {
-      const finishReasonResult = mapStopReason(choice.finish_reason);
+      const finishReasonResult = mapOpenAIStopReason(choice.finish_reason, {
+        allowSingularToolCall: true,
+      });
       output.stopReason = finishReasonResult.stopReason;
       if (finishReasonResult.stopReason === "stop") {
         sawStopFinishReason = true;
@@ -4478,32 +4481,6 @@ function hasOpenAICompletionsReasoningUsageActivity(
   );
 }
 
-function mapStopReason(reason: string | null) {
-  if (reason === null) {
-    return { stopReason: "stop" };
-  }
-  switch (reason) {
-    case "stop":
-    case "end":
-      return { stopReason: "stop" };
-    case "length":
-      return { stopReason: "length" };
-    case "function_call":
-    case "tool_call":
-    case "tool_calls":
-      return { stopReason: "toolUse" };
-    case "content_filter":
-      return { stopReason: "error", errorMessage: "Provider finish_reason: content_filter" };
-    case "network_error":
-      return { stopReason: "error", errorMessage: "Provider finish_reason: network_error" };
-    default:
-      return {
-        stopReason: "error",
-        errorMessage: `Provider finish_reason: ${reason}`,
-      };
-  }
-}
-
 export const testing = {
   getCompat,
   assertCodeModeResponsesToolSurface,
diff --git a/src/llm/providers/openai-completions.ts b/src/llm/providers/openai-completions.ts
index 0adc9fdb7a96..489d7f96e078 100644
--- a/src/llm/providers/openai-completions.ts
+++ b/src/llm/providers/openai-completions.ts
@@ -33,7 +33,6 @@ import type {
   Model,
   OpenAICompletionsCompat,
   SimpleStreamOptions,
-  StopReason,
   StreamFunction,
   StreamOptions,
   TextContent,
@@ -50,6 +49,7 @@ import { resolveCacheRetention } from "./cache-retention.js";
 import { isCloudflareProvider, resolveCloudflareBaseUrl } from "./cloudflare.js";
 import { buildCopilotDynamicHeaders, hasCopilotVisionInput } from "./github-copilot-headers.js";
 import { clampOpenAIPromptCacheKey } from "./openai-prompt-cache.js";
+import { mapOpenAIStopReason } from "./openai-stop-reason.js";
 import { buildBaseOptions } from "./simple-options.js";
 import { transformMessages } from "./transform-messages.js";
 
@@ -372,7 +372,7 @@ export const streamOpenAICompletions: StreamFunction<
         }
 
         if (choice.finish_reason) {
-          const finishReasonResult = mapStopReason(choice.finish_reason);
+          const finishReasonResult = mapOpenAIStopReason(choice.finish_reason);
           output.stopReason = finishReasonResult.stopReason;
           if (finishReasonResult.errorMessage) {
             output.errorMessage = finishReasonResult.errorMessage;
@@ -1243,34 +1243,6 @@ function parseChunkUsage(
   return usage;
 }
 
-function mapStopReason(reason: string): {
-  stopReason: StopReason;
-  errorMessage?: string;
-} {
-  if (reason === null) {
-    return { stopReason: "stop" };
-  }
-  switch (reason) {
-    case "stop":
-    case "end":
-      return { stopReason: "stop" };
-    case "length":
-      return { stopReason: "length" };
-    case "function_call":
-    case "tool_calls":
-      return { stopReason: "toolUse" };
-    case "content_filter":
-      return { stopReason: "error", errorMessage: "Provider finish_reason: content_filter" };
-    case "network_error":
-      return { stopReason: "error", errorMessage: "Provider finish_reason: network_error" };
-    default:
-      return {
-        stopReason: "error",
-        errorMessage: `Provider finish_reason: ${reason}`,
-      };
-  }
-}
-
 /**
  * Detect compatibility settings from provider and baseUrl for known providers.
  * Provider takes precedence over URL-based detection since it's explicitly configured.
diff --git a/src/llm/providers/openai-stop-reason.test.ts b/src/llm/providers/openai-stop-reason.test.ts
new file mode 100644
index 000000000000..3c7272c216b9
--- /dev/null
+++ b/src/llm/providers/openai-stop-reason.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import { mapOpenAIStopReason } from "./openai-stop-reason.js";
+
+describe("mapOpenAIStopReason", () => {
+  it.each([
+    ["stop", { stopReason: "stop" }],
+    ["end", { stopReason: "stop" }],
+    ["length", { stopReason: "length" }],
+    ["function_call", { stopReason: "toolUse" }],
+    ["tool_calls", { stopReason: "toolUse" }],
+    [null, { stopReason: "stop" }],
+  ] as const)("maps %s", (reason, expected) => {
+    expect(mapOpenAIStopReason(reason)).toEqual(expected);
+  });
+
+  it("keeps singular tool_call opt-in", () => {
+    expect(mapOpenAIStopReason("tool_call")).toEqual({
+      stopReason: "error",
+      errorMessage: "Provider finish_reason: tool_call",
+    });
+    expect(mapOpenAIStopReason("tool_call", { allowSingularToolCall: true })).toEqual({
+      stopReason: "toolUse",
+    });
+  });
+
+  it("surfaces provider errors and unknown reasons", () => {
+    expect(mapOpenAIStopReason("content_filter")).toEqual({
+      stopReason: "error",
+      errorMessage: "Provider finish_reason: content_filter",
+    });
+    expect(mapOpenAIStopReason("unexpected")).toEqual({
+      stopReason: "error",
+      errorMessage: "Provider finish_reason: unexpected",
+    });
+  });
+});
diff --git a/src/llm/providers/openai-stop-reason.ts b/src/llm/providers/openai-stop-reason.ts
new file mode 100644
index 000000000000..81777a8573f8
--- /dev/null
+++ b/src/llm/providers/openai-stop-reason.ts
@@ -0,0 +1,40 @@
+import type { StopReason } from "../types.js";
+
+export type OpenAIStopReasonResult = {
+  stopReason: StopReason;
+  errorMessage?: string;
+};
+
+export function mapOpenAIStopReason(
+  reason: string | null,
+  options?: { allowSingularToolCall?: boolean },
+): OpenAIStopReasonResult {
+  if (reason === null) {
+    return { stopReason: "stop" };
+  }
+
+  switch (reason) {
+    case "stop":
+    case "end":
+      return { stopReason: "stop" };
+    case "length":
+      return { stopReason: "length" };
+    case "function_call":
+    case "tool_calls":
+      return { stopReason: "toolUse" };
+    case "tool_call":
+      if (options?.allowSingularToolCall) {
+        return { stopReason: "toolUse" };
+      }
+      break;
+    case "content_filter":
+      return { stopReason: "error", errorMessage: "Provider finish_reason: content_filter" };
+    case "network_error":
+      return { stopReason: "error", errorMessage: "Provider finish_reason: network_error" };
+  }
+
+  return {
+    stopReason: "error",
+    errorMessage: `Provider finish_reason: ${reason}`,
+  };
+}