diff --git a/apps/web/src/lib/agent-harness/model-stream.test.ts b/apps/web/src/lib/agent-harness/model-stream.test.ts
new file mode 100644
index 0000000000..ff7e990c65
--- /dev/null
+++ b/apps/web/src/lib/agent-harness/model-stream.test.ts
@@ -0,0 +1,230 @@
+import { expect, it } from '@jest/globals';
+import { z } from 'zod';
+import { TRPCError } from '@trpc/server';
+import { getHTTPStatusCodeFromError } from '@trpc/server/http';
+import { createOpenAICompatible } from '@ai-sdk/openai-compatible';
+import { streamText } from 'ai';
+import { boundedBody, streamHarnessModel } from './model-stream';
+
+const outputHeaders = {
+  'content-type': 'Text/Event-Stream; charset=utf-8',
+  'request-id': 'request-1',
+  authorization: 'upstream-secret',
+};
+const sse = (body: BodyInit | null) => new Response(body, { headers: outputHeaders });
+function invoke(
+  upstream: Response,
+  request = new AbortController(),
+  deadline = new AbortController()
+) {
+  const abort = new AbortController();
+  const signal = AbortSignal.any([request.signal, deadline.signal, abort.signal]);
+  const response = streamHarnessModel(
+    upstream,
+    new Headers({ 'cache-control': 'no-store', 'content-type': 'application/json' }),
+    {
+      signal,
+      abort,
+      failure: code => ({
+        error: { code, message: 'Safe error', retryable: code === 429 || code >= 500 },
+      }),
+      errorStatus: (error, invalid) => {
+        if (request.signal.aborted) return 499;
+        if (error instanceof TRPCError) return getHTTPStatusCodeFromError(error);
+        return error instanceof SyntaxError || error instanceof z.ZodError ? invalid : 503;
+      },
+    }
+  );
+  return { response, abort };
+}
+
+it.each(['', 'x-provider: private-field\nretry: private-retry\n\n'])(
+  'streams multiline SDK text, reasoning, usage, and identifiers (prefix %j)',
+  async prefix => {
+    const chunk = {
+      id: 'generation-1',
+      model: 'paid/model',
+      created: 1,
+      choices: [
+        {
+          index: 0,
+          delta: { content: 'Hello', reasoning_content: 'Thinking' },
+          finish_reason: 'stop',
+        },
+      ],
+      usage: { prompt_tokens: 3, completion_tokens: 2, total_tokens: 5, cost: 0.002 },
+    };
+    let upstream!: ReadableStreamDefaultController<Uint8Array>;
+    const cancelled = Promise.withResolvers<void>();
+    const { response, abort } = invoke(
+      sse(
+        new ReadableStream({
+          start(controller) {
+            upstream = controller;
+            const data = JSON.stringify(chunk, null, 2).replaceAll('\n', '\ndata: ');
+            controller.enqueue(new TextEncoder().encode(`${prefix}data: ${data}\n\n`));
+          },
+          cancel: () => cancelled.resolve(),
+        })
+      )
+    );
+    const reply = await response;
+    const raw = reply.clone().text();
+    const provider = createOpenAICompatible({
+      name: 'harness',
+      baseURL: 'https://unused.example',
+      fetch: async () => reply,
+    });
+    const result = streamText({
+      model: provider('paid/model'),
+      messages: [{ role: 'user', content: 'Hello' }],
+      maxOutputTokens: 128,
+      maxRetries: 0,
+    });
+    const reader = result.textStream.getReader();
+    expect((await reader.read()).value).toBe('Hello');
+    upstream.enqueue(new TextEncoder().encode('data: [DONE]\n\n'));
+    expect((await reader.read()).done).toBe(true);
+    expect((await result.response).id).toBe('generation-1');
+    expect(await result.reasoningText).toBe('Thinking');
+    expect(await result.usage).toMatchObject({ inputTokens: 3, outputTokens: 2 });
+    expect(await result.finishReason).toBe('stop');
+    expect(await raw).toBe(`data: ${JSON.stringify(chunk)}\n\ndata: [DONE]\n\n`);
+    expect(reply.headers.get('authorization')).toBeNull();
+    expect(reply.headers.get('request-id')).toBe('request-1');
+    expect(reply.headers.get('cache-control')).toBe('no-store');
+    expect(reply.headers.get('content-encoding')).toBe('identity');
+    await cancelled.promise;
+    expect(abort.signal.aborted).toBe(true);
+  }
+);
+it('preserves split UTF-8, tool extensions, and separate usage events', async () => {
+  const tool = {
+    id: 'call-1',
+    function: { name: 'lookup', arguments: '{}' },
+    extra_content: { google: { thought_signature: 'signature-1' } },
+  };
+  const delta = {
+    choices: [{ delta: { content: '界', reasoning: 'Thinking', tool_calls: [tool] } }],
+  };
+  const usage = {
+    choices: [],
+    usage: {
+      prompt_tokens: 3,
+      completion_tokens: 2,
+      cost: 0.002,
+      prompt_tokens_details: { cached_tokens: 1 },
+      completion_tokens_details: { reasoning_tokens: 1, accepted_prediction_tokens: 1 },
+    },
+  };
+  const data = `data: ${JSON.stringify(delta)}\n\ndata: ${JSON.stringify(usage)}\n\ndata: [DONE]\n\n`;
+  const bytes = new TextEncoder().encode(data);
+  const split = bytes.indexOf(0xe7) + 1;
+  const body = new ReadableStream<Uint8Array>({
+    start(controller) {
+      controller.enqueue(bytes.slice(0, split));
+      controller.enqueue(bytes.slice(split, split + 1));
+      controller.enqueue(bytes.slice(split + 1));
+      controller.close();
+    },
+  });
+  expect(await (await invoke(sse(body)).response).text()).toBe(data);
+});
+it.each([400, 401, 402, 403, 429, 503])('sanitizes terminal stream error %s', async code => {
+  const error = JSON.stringify({
+    error: { code, message: 'upstream-secret', metadata: { authorization: 'credential' } },
+  });
+  const { response, abort } = invoke(sse(`data: ${error}\n\ndata: [DONE]\n\n`));
+  const reply = await response;
+  const text = await reply.text();
+  expect(reply.status).toBe(200);
+  expect(text).toContain(`"code":${code}`);
+  expect(text).toContain(`"retryable":${code === 429 || code === 503}`);
+  expect(text).not.toMatch(/upstream-secret|credential|\[DONE\]/);
+  expect(abort.signal.aborted).toBe(true);
+});
+it.each([
+  ['data: private-malformed\n\n', 422],
+  [`data: ${'x'.repeat(1024 * 1024)}\n\n`, 413],
+  ['data: []\n\n', 422],
+  ['data: {"metadata":"private-malformed"}\n\n', 422],
+  ['event: error\ndata: {"choices":[]}\n\n', 422],
+  ['data: {"error":{"code":200,"message":"private-malformed"}}\n\n', 422],
+] as const)('rejects malformed or oversized streams (case %#)', async (data, code) => {
+  const text = await (await invoke(sse(data)).response).text();
+  expect(text).toContain(`"code":${code}`);
+  expect(text).not.toContain('private-malformed');
+});
+it.each([
+  { choices: ['private-malformed'] },
+  { choices: [{ delta: 'private-malformed' }] },
+  { choices: [{ delta: { content: ['private-malformed'] } }] },
+  { choices: [{ delta: { tool_calls: ['private-malformed'] } }] },
+  { usage: 'private-malformed' },
+  { usage: ['private-malformed'] },
+  { usage: { prompt_tokens: 'private-malformed' } },
+  { usage: { completion_tokens_details: { reasoning_tokens: 'private-malformed' } } },
+])('sanitizes malformed SDK payloads before forwarding (case %#)', async payload => {
+  const data = JSON.stringify({ choices: [], ...payload });
+  const { response, abort } = invoke(sse(`data: ${data}\n\ndata: [DONE]\n\n`));
+  const text = await (await response).text();
+  expect(text).not.toContain('private-malformed');
+  expect(text).toBe('data: {"error":{"code":422,"message":"Safe error","retryable":false}}\n\n');
+  expect(abort.signal.aborted).toBe(true);
+});
+it.each([0, 1])('counts cumulative bytes before decoding (extra bytes: %s)', async extra => {
+  const body = new ReadableStream<Uint8Array>({
+    start(controller) {
+      controller.enqueue(new Uint8Array(512 * 1024));
+      controller.enqueue(new Uint8Array(512 * 1024 + extra));
+      controller.close();
+    },
+  });
+  const result = new Response(boundedBody(body, new AbortController().signal)).arrayBuffer();
+  if (extra) await expect(result).rejects.toMatchObject({ code: 'PAYLOAD_TOO_LARGE' });
+  else expect((await result).byteLength).toBe(1024 * 1024);
+});
+it.each(['', 'data: [DONE]\n\n'])('preserves empty output %j', async data => {
+  const { response, abort } = invoke(sse(data));
+  expect(await (await response).text()).toBe(data);
+  expect(abort.signal.aborted).toBe(true);
+});
+it.each([
+  new Response('upstream-secret', { status: 429 }),
+  new Response('<html>private</html>', { headers: { 'content-type': 'text/html' } }),
+  sse(null),
+])('rejects HTTP failures, wrong media, and missing bodies (case %#)', async upstream => {
+  const reply = await invoke(upstream).response;
+  expect(reply.status).toBe(upstream.status === 429 ? 429 : 422);
+  expect(await reply.text()).not.toMatch(/upstream-secret|private/);
+});
+it('sanitizes a failed upstream read', async () => {
+  const body = new ReadableStream({
+    start(controller) {
+      controller.error(new Error('transport-secret'));
+    },
+  });
+  const { response, abort } = invoke(sse(body));
+  const text = await (await response).text();
+  expect(text).toContain('"code":503');
+  expect(text).not.toContain('transport-secret');
+  expect(abort.signal.aborted).toBe(true);
+});
+it.each(['request', 'reader', 'deadline'])('cancels and cleans up through the %s', async target => {
+  const cancelled = Promise.withResolvers<void>();
+  const body = new ReadableStream<Uint8Array>({ cancel: () => cancelled.resolve() });
+  const request = new AbortController();
+  const deadline = new AbortController();
+  const { response, abort } = invoke(sse(body), request, deadline);
+  const reply = await response;
+  if (target === 'reader') await reply.body!.cancel();
+  else {
+    const text = reply.text();
+    (target === 'request' ? request : deadline).abort(new Error('private'));
+    const output = await text;
+    expect(output).toContain(`"code":${target === 'request' ? 499 : 503}`);
+    expect(output).not.toContain('private');
+  }
+  await cancelled.promise;
+  expect(abort.signal.aborted).toBe(true);
+});
diff --git a/apps/web/src/lib/agent-harness/model-stream.ts b/apps/web/src/lib/agent-harness/model-stream.ts
new file mode 100644
index 0000000000..5e09575ac3
--- /dev/null
+++ b/apps/web/src/lib/agent-harness/model-stream.ts
@@ -0,0 +1,153 @@
+import 'server-only';
+import { z } from 'zod';
+import { TRPCError } from '@trpc/server';
+import { EventSourceParserStream } from 'eventsource-parser/stream';
+
+export function boundedBody(body: ReadableStream<Uint8Array>, signal: AbortSignal) {
+  let bytes = 0;
+  return body.pipeThrough(
+    new TransformStream<Uint8Array, Uint8Array>({
+      transform(chunk, controller) {
+        bytes += chunk.byteLength;
+        if (bytes > 1024 * 1024) throw new TRPCError({ code: 'PAYLOAD_TOO_LARGE' });
+        controller.enqueue(chunk);
+      },
+    }),
+    { signal }
+  );
+}
+const StreamEnvelope = z.object({ error: z.unknown().optional() });
+// Match @ai-sdk/openai-compatible's unexported chunk schema. Forward the original
+// JSON after validation so unknown provider extensions remain intact.
+const StreamEvent = z.object({
+  id: z.string().nullish(),
+  created: z.number().nullish(),
+  model: z.string().nullish(),
+  choices: z.array(
+    z.object({
+      delta: z
+        .object({
+          role: z.enum(['assistant', '']).nullish(),
+          content: z.string().nullish(),
+          reasoning_content: z.string().nullish(),
+          reasoning: z.string().nullish(),
+          tool_calls: z
+            .array(
+              z.object({
+                index: z.number().nullish(),
+                id: z.string().nullish(),
+                function: z.object({ name: z.string().nullish(), arguments: z.string().nullish() }),
+                extra_content: z
+                  .object({
+                    google: z.object({ thought_signature: z.string().nullish() }).nullish(),
+                  })
+                  .nullish(),
+              })
+            )
+            .nullish(),
+        })
+        .nullish(),
+      finish_reason: z.string().nullish(),
+    })
+  ),
+  usage: z
+    .object({
+      prompt_tokens: z.number().nullish(),
+      completion_tokens: z.number().nullish(),
+      total_tokens: z.number().nullish(),
+      prompt_tokens_details: z.object({ cached_tokens: z.number().nullish() }).nullish(),
+      completion_tokens_details: z
+        .object({
+          reasoning_tokens: z.number().nullish(),
+          accepted_prediction_tokens: z.number().nullish(),
+          rejected_prediction_tokens: z.number().nullish(),
+        })
+        .nullish(),
+    })
+    .nullish(),
+});
+const StreamError = z.object({ code: z.coerce.number().int().min(400).max(599) });
+export const mediaType = (headers: Headers) =>
+  headers.get('content-type')?.split(';')[0].trim().toLowerCase();
+
+type StreamContext = {
+  signal: AbortSignal;
+  abort: AbortController;
+  // Keep the caller's sanitized error contract without importing its authority dependencies.
+  failure: (status: number) => unknown;
+  errorStatus: (error: unknown, invalid: number) => number;
+};
+export async function streamHarnessModel(
+  response: Response,
+  headers: Headers,
+  { signal, abort, failure, errorStatus }: StreamContext
+) {
+  const requestId = response.headers.get('request-id');
+  if (requestId) headers.set('request-id', requestId);
+  if (!response.ok || mediaType(response.headers) !== 'text/event-stream' || !response.body) {
+    await response.body?.cancel().catch(() => undefined);
+    const status = response.ok ? 422 : response.status;
+    return Response.json(failure(status), { status, headers });
+  }
+  const reader = boundedBody(response.body, signal)
+    .pipeThrough(new TextDecoderStream('utf-8', { fatal: true }))
+    .pipeThrough(new EventSourceParserStream())
+    .getReader();
+  const encoder = new TextEncoder();
+  let cancelled = false;
+  const close = async () => {
+    abort.abort();
+    await reader.cancel().catch(() => undefined);
+    reader.releaseLock();
+  };
+  headers.set('content-type', 'text/event-stream');
+  headers.set('content-encoding', 'identity');
+  return new Response(
+    new ReadableStream<Uint8Array>({
+      async pull(controller) {
+        try {
+          signal.throwIfAborted();
+          const { done, value } = await reader.read();
+          if (cancelled) return;
+          if (done) {
+            controller.close();
+            await close();
+            return;
+          }
+          let data = value.data;
+          let terminal = data === '[DONE]';
+          if (!terminal) {
+            const json: unknown = JSON.parse(data);
+            const parsed = StreamEnvelope.parse(json);
+            if (parsed.error != null || value.event === 'error') {
+              const error = StreamError.safeParse(parsed.error);
+              data = JSON.stringify(failure(error.success ? error.data.code : 422));
+              terminal = true;
+            } else {
+              StreamEvent.parse(json);
+              data = JSON.stringify(json);
+            }
+          }
+          controller.enqueue(encoder.encode(`data: ${data}\n\n`));
+          if (terminal) {
+            controller.close();
+            await close();
+          }
+        } catch (error) {
+          if (!cancelled) {
+            controller.enqueue(
+              encoder.encode(`data: ${JSON.stringify(failure(errorStatus(error, 422)))}\n\n`)
+            );
+            controller.close();
+          }
+          await close();
+        }
+      },
+      async cancel() {
+        cancelled = true;
+        await close();
+      },
+    }),
+    { headers }
+  );
+}