diff --git a/src/llm/providers/mistral.bounded-stream.test.ts b/src/llm/providers/mistral.bounded-stream.test.ts
new file mode 100644
index 000000000000..019ae45b8ea5
--- /dev/null
+++ b/src/llm/providers/mistral.bounded-stream.test.ts
@@ -0,0 +1,184 @@
+// Mistral provider tests cover bounded-stream-read helper (`createBoundedMistralFetcher`).
+import http from "node:http";
+import type { AddressInfo } from "node:net";
+import { describe, expect, it } from "vitest";
+import { createBoundedMistralFetcher } from "./mistral.js";
+
+const MAX = 16 * 1024 * 1024;
+const TOTAL = 18 * 1024 * 1024;
+
+async function readAllChunks(body: ReadableStream<Uint8Array> | null): Promise<{ total: number }> {
+  if (!body) {
+    return { total: 0 };
+  }
+  const reader = body.getReader();
+  let total = 0;
+  while (true) {
+    const { done, value } = await reader.read();
+    if (done) {
+      break;
+    }
+    if (value) {
+      total += value.byteLength;
+    }
+  }
+  return { total };
+}
+
+describe("Mistral bounded-stream-read real wire proof (loopback http.createServer)", () => {
+  it("caps an oversized body streamed chunked over real wire", async () => {
+    const fetcher = createBoundedMistralFetcher(MAX);
+    const CHUNK = 1024 * 1024;
+    const server = http.createServer((req, res) => {
+      res.writeHead(200, { "content-type": "application/octet-stream" });
+      let sent = 0;
+      const tick = setInterval(() => {
+        if (sent < 18) {
+          res.write(Buffer.alloc(CHUNK));
+          sent++;
+        } else {
+          clearInterval(tick);
+          res.end();
+        }
+      }, 1);
+    });
+    await new Promise<void>((resolve, reject) => {
+      server.once("error", reject);
+      server.listen(0, "127.0.0.1", () => {
+        resolve();
+      });
+    });
+    const port = (server.address() as AddressInfo).port;
+
+    let captured: Error | undefined;
+    let totalGot = 0;
+    try {
+      const response = await fetcher(`http://127.0.0.1:${port}/`);
+      // Wire framing merges TCP packets, so the reported size at throw time
+      // is between MAX (cap) and TOTAL (cap + last merged packet). Both
+      // bounds prove (a) cap fired (got > MAX) and (b) we did not buffer
+      // beyond the server's full 18 MiB (got < TOTAL).
+      try {
+        const result = await readAllChunks(response.body);
+        totalGot = result.total;
+      } catch (err) {
+        captured = err as Error;
+      }
+      expect(captured).toBeInstanceOf(Error);
+      const match = (captured as Error).message.match(
+        /mistral: stream body exceeds \d+ bytes \(got (\d+)\)/,
+      );
+      expect(match).not.toBeNull();
+      const got = Number(match![1]);
+      expect(got).toBeGreaterThan(MAX);
+      expect(got).toBeLessThan(TOTAL);
+      // Print to vitest stdout for PR-body real behavior proof capture.
+      console.log(
+        `[mistral bounded-stream proof] oversized path: cap=${MAX} reported=${got} server_total=${TOTAL}`,
+      );
+    } finally {
+      await new Promise<void>((resolve) => {
+        server.close(() => {
+          resolve();
+        });
+      });
+      if (totalGot > 0) {
+        // Use the value to satisfy strict unused rules without affecting asserts.
+        expect(totalGot).toBeGreaterThan(0);
+      }
+    }
+  });
+
+  it("returns a Response with exact bytes for normal-size responses on real wire", async () => {
+    const fetcher = createBoundedMistralFetcher(MAX);
+    const bodyText = 'data: {"choices":[{"delta":{"content":"hello"}}]}\n\ndata: [DONE]\n\n';
+    const server = http.createServer((req, res) => {
+      res.writeHead(200, { "content-type": "text/event-stream" });
+      res.end(bodyText);
+    });
+    await new Promise<void>((resolve, reject) => {
+      server.once("error", reject);
+      server.listen(0, "127.0.0.1", () => {
+        resolve();
+      });
+    });
+    const port = (server.address() as AddressInfo).port;
+
+    try {
+      const response = await fetcher(`http://127.0.0.1:${port}/`);
+      expect(response.status).toBe(200);
+      const { total } = await readAllChunks(response.body);
+      expect(total).toBe(Buffer.byteLength(bodyText, "utf8"));
+      console.log(
+        `[mistral bounded-stream proof] normal path: cap=${MAX} returned=${total} body=${JSON.stringify(bodyText)}`,
+      );
+    } finally {
+      await new Promise<void>((resolve) => {
+        server.close(() => {
+          resolve();
+        });
+      });
+    }
+  });
+});
+
+// Drive the bounded fetcher directly against a synthetic ReadableStream that
+// exceeds the cap. Bypasses any HTTP layer; proves the cap fires against an
+// unbounded chunked source, mirroring what the Mistral SDK's internal SSE
+// parser (`EventStream`) would see when a streaming body exceeds 16 MiB.
+describe("Mistral bounded-stream-read direct (synthetic ReadableStream)", () => {
+  it("caps an oversized synthetic ReadableStream at 16 MiB", async () => {
+    const fetcher = createBoundedMistralFetcher(MAX);
+    const CHUNK = 1024 * 1024;
+    let sent = 0;
+    const synthetic = new ReadableStream<Uint8Array>({
+      pull(controller) {
+        if (sent < 18) {
+          controller.enqueue(new Uint8Array(CHUNK));
+          sent++;
+        } else {
+          controller.close();
+        }
+      },
+    });
+    // Build the same shape `fetcher` expects from a real fetch(): a
+    // `Response` whose `body` is a ReadableStream.
+    const syntheticResponse = new Response(synthetic, {
+      status: 200,
+      headers: { "content-type": "application/octet-stream" },
+    });
+
+    let captured: Error | undefined;
+    try {
+      // Replace the fetcher's internal `fetch` call by exercising the
+      // post-fetchResponse code path directly: build a `Wrapped`
+      // that re-enters `fetcher` as if a real fetch returned our
+      // synthetic Response, by patching the global fetch.
+      const originalFetch = globalThis.fetch;
+      globalThis.fetch = (() => Promise.resolve(syntheticResponse)) as typeof globalThis.fetch;
+      try {
+        const wrapped = await fetcher("http://unused.invalid/");
+        try {
+          await readAllChunks(wrapped.body);
+        } catch (err) {
+          captured = err as Error;
+        }
+      } finally {
+        globalThis.fetch = originalFetch;
+      }
+      expect(captured).toBeInstanceOf(Error);
+      const match = (captured as Error).message.match(
+        /mistral: stream body exceeds \d+ bytes \(got (\d+)\)/,
+      );
+      expect(match).not.toBeNull();
+      const got = Number(match![1]);
+      // Synthetic stream chunks are exactly 1 MiB aligned, so cap+1 reads
+      // give exactly cap + 1 MiB = 16 MiB + 1 MiB = 17 825 792 bytes.
+      expect(got).toBe(16777216 + CHUNK);
+    } finally {
+      // Best-effort cleanup if the test threw mid-flight.
+      // No intervals to clear for this test; the synthetic stream closes
+      // automatically when `sent >= 18`.
+    }
+  });
+});
diff --git a/src/llm/providers/mistral.test.ts b/src/llm/providers/mistral.test.ts
index 8af0ec919ae2..4e50e08483d3 100644
--- a/src/llm/providers/mistral.test.ts
+++ b/src/llm/providers/mistral.test.ts
@@ -7,16 +7,26 @@ const mistralMockState = vi.hoisted(() => ({
   payloads: [] as unknown[],
 }));
 
-vi.mock("@mistralai/mistralai", () => ({
-  Mistral: class MockMistral {
-    chat = {
-      stream: vi.fn(async (payload: unknown) => {
-        mistralMockState.payloads.push(payload);
-        throw new Error("stop before network");
-      }),
-    };
-  },
-}));
+vi.mock("@mistralai/mistralai", async () => {
+  // Preserve real exports for everything except `Mistral`, so the new
+  // imports of `HTTPClient` and `Fetcher` introduced by the bounded-stream
+  // helper (`createBoundedMistralHttpClient`) resolve correctly. Only
+  // `Mistral` itself is overridden so the test can capture payloads without
+  // any actual HTTP traffic.
+  const actual =
+    await vi.importActual<typeof import("@mistralai/mistralai")>("@mistralai/mistralai");
+  return {
+    ...actual,
+    Mistral: class MockMistral {
+      chat = {
+        stream: vi.fn(async (payload: unknown) => {
+          mistralMockState.payloads.push(payload);
+          throw new Error("stop before network");
+        }),
+      };
+    },
+  };
+});
 
 import { streamMistral, streamSimpleMistral } from "./mistral.js";
 
diff --git a/src/llm/providers/mistral.ts b/src/llm/providers/mistral.ts
index c87536188757..6ff13cb61527 100644
--- a/src/llm/providers/mistral.ts
+++ b/src/llm/providers/mistral.ts
@@ -1,5 +1,5 @@
 // Mistral provider adapts Mistral streams and tool calls to the runtime.
-import { Mistral } from "@mistralai/mistralai";
+import { HTTPClient, Mistral, type Fetcher } from "@mistralai/mistralai";
 import type {
   ChatCompletionStreamRequest,
   ChatCompletionStreamRequestMessage,
@@ -7,6 +7,7 @@ import type {
   ContentChunk,
   FunctionTool,
 } from "@mistralai/mistralai/models/components";
+import { createSseByteGuard } from "../../agents/streaming-byte-guard.js";
 import { stripSystemPromptCacheBoundary } from "../../agents/system-prompt-cache-boundary.js";
 import { getEnvApiKey } from "../env-api-keys.js";
 import { calculateCost, clampThinkingLevel } from "../model-utils.js";
@@ -34,6 +35,63 @@ import { transformMessages } from "./transform-messages.js";
 const MISTRAL_TOOL_CALL_ID_LENGTH = 9;
 const MAX_MISTRAL_ERROR_BODY_CHARS = 4000;
 
+// 16 MiB cap on Mistral streaming success bodies, matching the
+// `PROVIDER_TEXT_RESPONSE_MAX_BYTES` / `PROVIDER_JSON_RESPONSE_MAX_BYTES`
+// 16 MiB cap used elsewhere. A hostile or malfunctioning Mistral-compatible
+// endpoint cannot exhaust memory by streaming an unbounded SSE body;
+// `createSseByteGuard` cancels the upstream reader and throws once the
+// accumulated byte count exceeds this cap.
+const MISTRAL_STREAM_BODY_MAX_BYTES = 16 * 1024 * 1024;
+
+/**
+ * Builds a `Fetcher` that wraps the default `fetch` with a 16 MiB byte cap
+ * on streamed response bodies. The wrapped `Response.body` exposes a
+ * `ReadableStream` whose chunks flow through `createSseByteGuard`, so the
+ * SDK's internal SSE parser (`EventStream` in
+ * `@mistralai/mistralai/lib/event-streams.ts`) reads exactly as it would on
+ * an unbounded body — but bounded.
+ *
+ * Bodyless responses (no `body` or no `getReader`) are returned unchanged so
+ * the SDK's error-path `res.arrayBuffer()` call still works.
+ */
+export function createBoundedMistralFetcher(
+  maxBytes: number = MISTRAL_STREAM_BODY_MAX_BYTES,
+): Fetcher {
+  return async (input, init) => {
+    const response = init == null ? await fetch(input) : await fetch(input, init);
+    if (!response.body || typeof response.body.getReader !== "function") {
+      return response;
+    }
+    const reader = response.body.getReader();
+    const guard = createSseByteGuard(reader, {
+      maxBytes,
+      onOverflow: ({ size, maxBytes: cap }) =>
+        new Error(`mistral: stream body exceeds ${cap} bytes (got ${size})`),
+    });
+    // Re-shape the response body so the SDK's `responseBody.getReader()`
+    // call inside `EventStream` resolves to a stream whose `read()` is
+    // routed through `guard.read()`. Cancellation is also forwarded.
+    const guardedStream = new ReadableStream<Uint8Array>({
+      async pull(controller) {
+        const { done, value } = await guard.read();
+        if (done) {
+          controller.close();
+          return;
+        }
+        controller.enqueue(value);
+      },
+      async cancel(reason) {
+        await guard.cancel(reason);
+      },
+    });
+    return new Response(guardedStream, {
+      status: response.status,
+      statusText: response.statusText,
+      headers: response.headers,
+    });
+  };
+}
+
 /**
  * Provider-specific options for the Mistral API.
  */
@@ -73,6 +131,14 @@ export const streamMistral: StreamFunction<"mistral-conversations", MistralOptio
       const mistral = new Mistral({
         apiKey,
         serverURL: model.baseUrl,
+        // Bound the streamed Mistral response body at 16 MiB so a hostile or
+        // malfunctioning endpoint cannot exhaust memory. The fetcher is
+        // injected via the SDK's `HTTPClient` (see
+        // `@mistralai/mistralai/lib/sdks.ts` `ClientSDK` constructor: when
+        // `httpClient` is passed, `ClientSDK.#httpClient` is set from it and
+        // every `chat.stream` / `complete` call routes through
+        // `HTTPClient.request` → `this.fetcher(req)`).
+        httpClient: new HTTPClient({ fetcher: createBoundedMistralFetcher() }),
       });
 
       const normalizeMistralToolCallId = createMistralToolCallIdNormalizer();