diff --git a/CHANGELOG.md b/CHANGELOG.md
index 167d884..b3b97d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ## [0.3.1] — Unreleased
 
-Stricter UTF-8 tail handling, a guard against Python-style option names in `filterRelevant`, and a typed `SmartApplyError` for smartapply refusals. No change to `generateDiff` or to what `smartapply` writes.
+Stricter UTF-8 tail handling, a guard against Python-style option names in `filterRelevant`, a typed `SmartApplyError` for smartapply refusals, and a new default model to replace a retired one. No change to `generateDiff` or to what `smartapply` writes.
 
 ### Behavior changes
 
@@ -10,6 +10,8 @@ Stricter UTF-8 tail handling, a guard against Python-style option names in `filt
 - **A truncated UTF-8 tail keeps its valid prefix at any file length.** In 0.3.0 an incomplete trailing sequence was tolerated only when the buffer was exactly 8192 bytes; any other length took the binary path. Now a file that ends in a legal partial character is text with the partial character dropped (`0x61 0xC3` and `0x61 0xE0 0xA0` are `"a"`). Tails that can never become a legal character stay binary (`0x61 0xE0 0x80`, `0x61 0xED 0xA0`, `0x61 0xF0 0x80`, `0x61 0xF4 0x90`; leads C0, C1, F5-FF). Some short files that 0.3.0 skipped as binary are now sent to the decision model.
 - **`smartapply` refusals throw `SmartApplyError` instead of a plain `Error`.** When smartapply refuses a model rewrite (it changed lines the diff did not add or remove, and the edit could not be replayed from the `+`/`-` lines), the thrown object is now a `SmartApplyError`, exported from the package root and matching patchling-py. What changed: the error's class and constructor (`err.constructor === SmartApplyError`, `instanceof SmartApplyError` is true; it is still `instanceof Error`), `err.name` (`'SmartApplyError'`, was `'Error'`), and therefore how it prints (`String(err)` and the first line of `err.stack` read `SmartApplyError: smartapply: refusing to write …` instead of `Error: …`). Code that checks `err.name === 'Error'`, `err.constructor === Error`, or snapshots the printed error will see the difference. What did not change: the message text (`smartapply: refusing to write "<path>": the model changed lines the diff did not add or remove`, so matching on `err.message` keeps working), when a refusal happens, and provider/LLM errors, which still propagate as the original error object and are never wrapped.
 
+- **Default model is now `deepseek/deepseek-v4.1-flash-1017`.** The 0.3.0 default, `xiaomi/mimo-v2.5-pro-ultraspeed`, has been retired by NanoGPT and returns HTTP 410, so `generateDiff` and `smartapply` failed whenever `GPTDIFF_MODEL` was unset and no `model` was passed. `DEFAULT_MODEL` (and its `.d.ts` literal type) is now `deepseek/deepseek-v4.1-flash-1017`, the October 17, 2026 snapshot of DeepSeek V4.1 Flash as listed on NanoGPT ($0.10 / $0.60 per 1M input / output tokens, 1M context). NanoGPT lists no non-Flash V4.1 model. patchling-py uses the same default. Calls that pass `model` or set `GPTDIFF_MODEL` are unchanged. The `filterRelevant` default (`xiaomi/mimo-v2.6-flash`) is unchanged. The verified catalog snapshot (`test/fixtures/demo-models.json`) adds the new id and was re-checked on 2026-10-21.
+
 ### Docs and tests
 
 - README documents the snake_case guard and the UTF-8 tail rule; `filter.d.ts` JSDoc updated to match.
diff --git a/README.md b/README.md
index 26d17df..287fb8b 100644
--- a/README.md
+++ b/README.md
@@ -59,7 +59,7 @@ The browser demos explicitly use `xiaomi/mimo-v2.6-flash` for diff generation
 and repair. The landing-page picker also pins `xiaomi/mimo-v2.6-pro` and
 `z-ai/glm-5.3-flash`. These IDs, capabilities, and prices were checked against
 [NanoGPT's public catalog](https://nano-gpt.com/api/v1/models?detailed=true)
-on 2026-10-13; the offline snapshot is in `test/fixtures/demo-models.json`.
+on 2026-10-13 (re-checked 2026-10-21, when the default `deepseek/deepseek-v4.1-flash-1017` was added); the offline snapshot is in `test/fixtures/demo-models.json`.
 The decision demo requests MiMo's supported `none` reasoning effort.
 Library environment defaults below are independent of the demo choices.
 
@@ -71,7 +71,7 @@ Configuration comes from environment variables (Node) or `setEnv(...)` overrides
 | --- | --- | --- |
 | `GPTDIFF_LLM_API_KEY` | NanoGPT API key (`sk-nano-…`) | — (required for live calls) |
 | `GPTDIFF_LLM_BASE_URL` | OpenAI-compatible base URL | `https://nano-gpt.com/api/v1/` |
-| `GPTDIFF_MODEL` | Model id | `xiaomi/mimo-v2.5-pro-ultraspeed` |
+| `GPTDIFF_MODEL` | Model id | `deepseek/deepseek-v4.1-flash-1017` |
 | `GPTDIFF_FILTER_MODEL` | Cheap model for `filterRelevant` | `xiaomi/mimo-v2.6-flash` |
 
 ```js
diff --git a/landing/lib/env.js b/landing/lib/env.js
index 2728268..b556f77 100644
--- a/landing/lib/env.js
+++ b/landing/lib/env.js
@@ -36,5 +36,5 @@ export function getEnv(name, fallback = undefined) {
   return fallback;
 }
 
-export const DEFAULT_MODEL = 'xiaomi/mimo-v2.5-pro-ultraspeed';
+export const DEFAULT_MODEL = 'deepseek/deepseek-v4.1-flash-1017';
 export const DEFAULT_BASE_URL = 'https://nano-gpt.com/api/v1/';
diff --git a/src/env.d.ts b/src/env.d.ts
index a0b3528..e8605fb 100644
--- a/src/env.d.ts
+++ b/src/env.d.ts
@@ -12,5 +12,5 @@ export function setEnv(name: string, value: string | undefined): void;
  * @returns {string | undefined}
  */
 export function getEnv(name: string, fallback?: string): string | undefined;
-export const DEFAULT_MODEL: "xiaomi/mimo-v2.5-pro-ultraspeed";
+export const DEFAULT_MODEL: "deepseek/deepseek-v4.1-flash-1017";
 export const DEFAULT_BASE_URL: "https://nano-gpt.com/api/v1/";
diff --git a/src/env.js b/src/env.js
index 2728268..b556f77 100644
--- a/src/env.js
+++ b/src/env.js
@@ -36,5 +36,5 @@ export function getEnv(name, fallback = undefined) {
   return fallback;
 }
 
-export const DEFAULT_MODEL = 'xiaomi/mimo-v2.5-pro-ultraspeed';
+export const DEFAULT_MODEL = 'deepseek/deepseek-v4.1-flash-1017';
 export const DEFAULT_BASE_URL = 'https://nano-gpt.com/api/v1/';
diff --git a/test/default_model.test.js b/test/default_model.test.js
new file mode 100644
index 0000000..a5e23ca
--- /dev/null
+++ b/test/default_model.test.js
@@ -0,0 +1,30 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { DEFAULT_MODEL } from '../src/index.js';
+import { DEFAULT_MODEL as LANDING_DEFAULT_MODEL } from '../landing/lib/env.js';
+
+const RETIRED = 'xiaomi/mimo-v2.5-pro-ultraspeed'; // NanoGPT returns HTTP 410
+const catalog = JSON.parse(readFileSync(new URL('./fixtures/demo-models.json', import.meta.url), 'utf8'));
+
+test('DEFAULT_MODEL is deepseek/deepseek-v4.1-flash-1017', () => {
+  assert.equal(DEFAULT_MODEL, 'deepseek/deepseek-v4.1-flash-1017');
+  assert.equal(LANDING_DEFAULT_MODEL, DEFAULT_MODEL);
+});
+
+test('retired xiaomi/mimo-v2.5-pro-ultraspeed is not the default', () => {
+  assert.notEqual(DEFAULT_MODEL, RETIRED);
+  assert.notEqual(LANDING_DEFAULT_MODEL, RETIRED);
+});
+
+test('DEFAULT_MODEL is in the verified NanoGPT catalog fixture and cheap', () => {
+  const m = catalog.text[DEFAULT_MODEL];
+  assert.ok(m, `${DEFAULT_MODEL} missing from test/fixtures/demo-models.json`);
+  assert.ok(m.pricing.prompt <= 0.5 && m.pricing.completion <= 1, 'default should stay cheap');
+});
+
+test('src/env.d.ts declares the new default literal', () => {
+  const dts = readFileSync(new URL('../src/env.d.ts', import.meta.url), 'utf8');
+  assert.match(dts, /DEFAULT_MODEL: "deepseek\/deepseek-v4\.1-flash-1017";/);
+  assert.ok(!dts.includes(RETIRED));
+});
diff --git a/test/fixtures/demo-models.json b/test/fixtures/demo-models.json
index 46cb215..1d3d756 100644
--- a/test/fixtures/demo-models.json
+++ b/test/fixtures/demo-models.json
@@ -1,10 +1,10 @@
 {
-  "verifiedOn": "2026-10-13",
+  "verifiedOn": "2026-10-21",
   "sources": {
     "text": "https://nano-gpt.com/api/v1/models?detailed=true"
   },
   "catalogSha256": {
-    "nanogpt-models.json": "0ff41c8e54be2645c386bd00a712dca92e8b9103468ea28a0fd0a7e12c2aa89e"
+    "nanogpt-models.json": "e4a5241a0498b8d0db13d8bb667aaafa1d3d2b162a49d917de2412f66b634f7a"
   },
   "text": {
     "z-ai/glm-5.3-flash": {
@@ -125,6 +125,45 @@
         "currency": "USD",
         "unit": "per_million_tokens"
       }
+    },
+    "deepseek/deepseek-v4.1-flash-1017": {
+      "created": 1792195200,
+      "context_length": 1048576,
+      "max_output_tokens": 393216,
+      "architecture": {
+        "modality": "text+image->text",
+        "input_modalities": [
+          "text",
+          "image"
+        ],
+        "output_modalities": [
+          "text"
+        ]
+      },
+      "capabilities": {
+        "vision": true,
+        "video_input": false,
+        "audio_input": false,
+        "reasoning": true,
+        "tool_calling": true,
+        "parallel_tool_calls": false,
+        "structured_output": false,
+        "pdf_upload": false,
+        "batch_api": true
+      },
+      "reasoning_efforts": [
+        "none",
+        "low",
+        "high",
+        "max"
+      ],
+      "pricing": {
+        "prompt": 0.09999999999999999,
+        "completion": 0.6,
+        "cacheReadInputPer1kTokens": 3e-06,
+        "currency": "USD",
+        "unit": "per_million_tokens"
+      }
     }
   }
 }
diff --git a/test/live.test.js b/test/live.test.js
index 4182720..75e48f5 100644
--- a/test/live.test.js
+++ b/test/live.test.js
@@ -5,7 +5,7 @@
  *   GPTDIFF_LIVE=1
  *   GPTDIFF_LLM_API_KEY=<your NanoGPT key>   (e.g. sk-nano-... from OAuth sign-in)
  *
- * Model defaults to xiaomi/mimo-v2.5-pro-ultraspeed (override with GPTDIFF_MODEL).
+ * Model defaults to deepseek/deepseek-v4.1-flash-1017 (override with GPTDIFF_MODEL).
  * Base URL defaults to https://nano-gpt.com/api/v1/ (override GPTDIFF_LLM_BASE_URL).
  *
  * Run with:  npm run test:live