diff --git a/packages/ai/src/providers/mistral.bounded-stream.test.ts b/packages/ai/src/providers/mistral.bounded-stream.test.ts
index 62eb6e2607fa..b0936068a807 100644
--- a/packages/ai/src/providers/mistral.bounded-stream.test.ts
+++ b/packages/ai/src/providers/mistral.bounded-stream.test.ts
@@ -169,7 +169,7 @@ describe("Mistral bounded-stream-read direct (synthetic ReadableStream)", () =>
 });
 
 type MistralTerminalFixture = {
-  finishReason: "stop" | "length" | "error" | "tool_calls" | null;
+  finishReason: string | null;
   done: boolean;
   abort?: boolean;
   toolArguments?: string[];
@@ -269,6 +269,8 @@ describe("Mistral terminal ownership through the installed SDK and real HTTP/SSE
     { name: "EOF without a provider terminal", finishReason: null, done: false },
     { name: "DONE without a provider terminal", finishReason: null, done: true },
     { name: "a provider error terminal", finishReason: "error", done: true },
+    { name: "a filtered provider terminal", finishReason: "content_filter", done: true },
+    { name: "an unknown provider terminal", finishReason: "provider_guardrail", done: true },
     { name: "malformed arguments on a tool terminal", finishReason: "tool_calls", done: true },
   ] as const)("rejects $name without executable calls", async (fixture) => {
     const { result, events } = await streamMistralTerminalFixture({
@@ -279,24 +281,31 @@ describe("Mistral terminal ownership through the installed SDK and real HTTP/SSE
     expect(result.stopReason).toBe("error");
     if (fixture.finishReason === null) {
       expect(result.errorMessage).toBe("Mistral stream ended without a terminal finish reason");
+    } else if (fixture.finishReason === "tool_calls") {
+      expect(result.errorMessage).toContain("invalid JSON arguments");
     } else {
-      expect(result.errorMessage).toBeTruthy();
+      expect(result.errorMessage).toBe(`Provider finish_reason: ${fixture.finishReason}`);
     }
+    expect(events).toContain("error");
     expect(events).not.toContain("toolcall_end");
     expect(result.content).not.toContainEqual(expect.objectContaining({ type: "toolCall" }));
     expect(result.content).toContainEqual({ type: "text", text: "Safe partial answer" });
   });
 
-  it.each(["length", "stop"] as const)(
-    "preserves a %s terminal and visible text without finalizing partial tools",
-    async (finishReason) => {
+  it.each([
+    { finishReason: "length", stopReason: "length" },
+    { finishReason: "model_length", stopReason: "length" },
+    { finishReason: "stop", stopReason: "stop" },
+  ] as const)(
+    "preserves a $finishReason terminal and visible text without finalizing partial tools",
+    async ({ finishReason, stopReason }) => {
       const { result, events } = await streamMistralTerminalFixture({
         finishReason,
         done: true,
         toolArguments: ['{"action":"delete_all"'],
         text: "Safe partial answer",
       });
-      expect(result.stopReason).toBe(finishReason);
+      expect(result.stopReason).toBe(stopReason);
       expect(result.content).toEqual([{ type: "text", text: "Safe partial answer" }]);
       expect(events).not.toContain("toolcall_end");
       expect(events).toContain("done");
diff --git a/packages/ai/src/providers/mistral.ts b/packages/ai/src/providers/mistral.ts
index 381a93929894..7dbbe1004b1c 100644
--- a/packages/ai/src/providers/mistral.ts
+++ b/packages/ai/src/providers/mistral.ts
@@ -23,7 +23,6 @@ import type {
   Message,
   Model,
   SimpleStreamOptions,
-  StopReason,
   StreamFunction,
   StreamOptions,
   TextContent,
@@ -43,6 +42,7 @@ import { projectProviderError } from "../utils/provider-error.js";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.js";
 import { createSseByteGuard } from "../utils/streaming-byte-guard.js";
 import { stripSystemPromptCacheBoundary } from "../utils/system-prompt-cache-boundary.js";
+import { mapOpenAIStopReason } from "./openai-stop-reason.js";
 import { buildBaseOptions, clampMaxTokensToModel } from "./simple-options.js";
 import {
   describeToolResultMediaPlaceholder,
@@ -206,7 +206,7 @@ export const streamMistral: StreamFunction<"mistral-conversations", MistralOptio
       }
 
       if (output.stopReason === "aborted" || output.stopReason === "error") {
-        throw new Error("An unknown error occurred");
+        throw new Error(output.errorMessage ?? "An unknown error occurred");
       }
 
       stream.push({ type: "done", reason: output.stopReason, message: output });
@@ -619,7 +619,13 @@ async function consumeChatStream(
 
     if (choice.finishReason) {
       terminalFinishReason = choice.finishReason;
-      output.stopReason = mapChatStopReason(choice.finishReason);
+      const { stopReason, errorMessage } = mapOpenAIStopReason(
+        choice.finishReason === "model_length" ? "length" : choice.finishReason,
+      );
+      output.stopReason = stopReason;
+      if (errorMessage) {
+        output.errorMessage = errorMessage;
+      }
     }
 
     const delta = choice.delta;
@@ -1018,22 +1024,4 @@ function mapToolChoice(
   };
 }
 
-function mapChatStopReason(reason: string | null): StopReason {
-  if (reason === null) {
-    return "stop";
-  }
-  switch (reason) {
-    case "stop":
-      return "stop";
-    case "length":
-    case "model_length":
-      return "length";
-    case "tool_calls":
-      return "toolUse";
-    case "error":
-      return "error";
-    default:
-      return "stop";
-  }
-}
 /* oxlint-disable max-lines -- TODO: split this grandfathered oversized file. */