DeepSeek V4.1 Flash · Coding

DeepSeek V4.1 Flash for coding: our test runs

DeepSeek V4.1 Flash ran our 5 coding prompts in llmwise, through the same pipeline your messages take. Here's every reply as it came, whether it passed, what it cost and what it counts as on Pro, next to the other models.

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Based on 5 of our test runs on October 8, 2026, through OpenRouter with the app's own prompt and settings. Updated October 15, 2026.

Short answer

In our test runs on October 8, 2026, DeepSeek V4.1 Flash passed 5 of 5 of our coding prompts (both hard ones): tied 1st of 20 models. Each reply counted as 1 message on Pro, where it has 60 a day, and cost $0.0018 to run on average.

Prompt by prompt

Each of the 5 coding prompts, and how DeepSeek V4.1 Flash's reply did.

DeepSeek V4.1 Flash on each coding prompt
Prompt	Result	Cost	Time
Turn a title into a URL slugEveryday	Passed: All 7 tests passed.	$0.0004	1.3 s
Parse a duration like “1h 30m”Everyday	Passed: All 10 tests passed.	$0.0011	2.5 s
Merge overlapping intervalsEveryday	Passed: All 7 tests passed.	$0.0003	0.6 s
Evaluate an arithmetic expression, no evalHard	Passed: All 15 tests passed.	$0.0030	5.8 s
Parse CSV with quoted fieldsHard	Passed: All 8 tests passed.	$0.0042	9.1 s

Every coding reply from DeepSeek V4.1 Flash

Turn a title into a URL slug

An everyday prompt: “Turn a title into a URL slug”, in full.

Passed: All 7 tests passed.

```javascript
function slugify(title) {
  return title
    .normalize('NFD')            // split accented letters into base + combining mark
    .replace(/[\u0300-\u036f]/g, '') // drop the combining marks (é → e, ñ → n, ü → u)
    .toLowerCase()
    .replace(/[^a-z0-9]+/g, '-')  // every run of anything else becomes one hyphen
    .replace(/^-+|-+$/g, '');     // trim leading/trailing hyphens
}
```

Note: `normalize('NFD')` handles the accents, but a few letters don't decompose that way (`ø`, `ß`, `đ`, `æ`). If you need those, add an explicit map before the cleanup:

```javascript
const SPECIAL = { ø: 'o', ß: 'ss', đ: 'd', æ: 'ae', œ: 'oe', þ: 'th', ł: 'l' };
```

with `title.replace(/[øßđæœþł]/gi, c => SPECIAL[c.toLowerCase()])` inserted before `.toLowerCase()`.

522 tokens in, 276 out (23 of them reasoning) · 1.3 s · $0.0004 · 1 message on Pro · answered by deepseek/deepseek-v4.1-flash via Together · October 7, 2026

Parse a duration like “1h 30m”

An everyday prompt: “Parse a duration like “1h 30m””, in full.

Passed: All 10 tests passed.

```javascript
function parseDuration(text) {
  if (typeof text !== 'string') return null;
  const match = /^\s*(?:(\d+)h)?\s*(?:(\d+)m)?\s*(?:(\d+)s)?\s*$/.exec(text);
  if (!match) return null;
  const [, h, m, s] = match;
  if (h === undefined && m === undefined && s === undefined) return null;
  return (h ? Number(h) * 3600 : 0) + (m ? Number(m) * 60 : 0) + (s ? Number(s) : 0);
}
```

505 tokens in, 890 out (746 of them reasoning) · 2.5 s · $0.0011 · 1 message on Pro · answered by deepseek/deepseek-v4.1-flash via Together · October 7, 2026

Merge overlapping intervals

An everyday prompt: “Merge overlapping intervals”, in full.

Passed: All 7 tests passed.

```javascript
function mergeIntervals(intervals) {
  if (!Array.isArray(intervals) || intervals.length === 0) return [];

  // Copy each pair so the input array (and its pairs) are never mutated.
  const sorted = intervals
    .map(([start, end]) => [start, end])
    .sort((a, b) => a[0] - b[0] || a[1] - b[1]);

  const merged = [sorted[0]];

  for (let i = 1; i < sorted.length; i++) {
    const [start, end] = sorted[i];
    const last = merged[merged.length - 1];

    if (start <= last[1]) {
      // Overlapping or touching: extend the current interval.
      if (end > last[1]) last[1] = end;
    } else {
      merged.push([start, end]);
    }
  }

  return merged;
}
```

489 tokens in, 216 out (6 of them reasoning) · 0.6 s · $0.0003 · 1 message on Pro · answered by deepseek/deepseek-v4.1-flash via Together · October 8, 2026

Evaluate an arithmetic expression, no eval

A hard prompt: “Evaluate an arithmetic expression, no eval”, in full.

Passed: All 15 tests passed.

```javascript
function evaluate(expression) {
  if (typeof expression !== 'string') {
    throw new Error('Expression must be a string');
  }

  // ---- Tokenizer ----
  const tokens = [];
  let i = 0;
  const isDigit = (c) => c >= '0' && c <= '9';

  while (i < expression.length) {
    const ch = expression[i];

    if (ch === ' ' || ch === '\t') {
      i++;
      continue;
    }

    // number: digits with an optional single decimal point (at least one digit)
    if (isDigit(ch) || ch === '.') {
      let j = i;
      let digits = 0;
      while (j < expression.length && isDigit(expression[j])) {
        j++;
        digits++;
      }
      if (j < expression.length && expression[j] === '.') {
        j++;
        while (j < expression.length && isDigit(expression[j])) {
          j++;
          digits++;
        }
      }
      if (digits === 0) {
        throw new Error('Invalid number in expression');
      }
      tokens.push({ type: 'num', value: parseFloat(expression.slice(i, j)) });
      i = j;
      continue;
    }

    if ('+-*/^()'.indexOf(ch) !== -1) {
      tokens.push({ type: ch });
      i++;
      continue;
    }

    throw new Error('Unexpected character: ' + ch);
  }

  if (tokens.length === 0) {
    throw new Error('Empty expression');
  }

  // ---- Parser (recursive descent) ----
  let pos = 0;
  const peek = () => tokens[pos];
  const isOp = (tok, ch) => tok !== undefined && tok.type === ch;

  // expr := term (('+' | '-') term)*          left-associative
  function parseExpression() {
    let value = parseTerm();
    while (isOp(peek(), '+') || isOp(peek(), '-')) {
      const op = tokens[pos++].type;
      const rhs = parseTerm();
      value = op === '+' ? value + rhs : value - rhs;
    }
    return value;
  }

  // term := unary (('*' | '/') unary)*        left-associative
  function parseTerm() {
    let value = parseUnary();
    while (isOp(peek(), '*') || isOp(peek(), '/')) {
      const op = tokens[pos++].type;
      const rhs = parseUnary();
      value = op === '*' ? value * rhs : value / rhs;
    }
    return value;
  }

  // unary := ('-' | '+') unary | power
  // unary minus is looser than '^', so -2^2 === -(2^2) === -4
  function parseUnary() {
    if (isOp(peek(), '-')) {
      pos++;
      return -parseUnary();
    }
    if (isOp(peek(), '+')) {
      pos++;
      return parseUnary();
    }
    return parsePower();
  }

  // power := primary ('^' unary)?             right-associative
  // the exponent is a unary, which permits 2^-1 and 2^3^2 = 2^(3^2)
  function parsePower() {
    const base = parsePrimary();
    if (isOp(peek(), '^')) {
      pos++;
      const exponent = parseUnary();
      return Math.pow(base, exponent);
    }
    return base;
  }

  // primary := number | '(' expr ')'
  function parsePrimary() {
    const tok = peek();
    if (tok === undefined) {
      throw new Error('Unexpected end of expression');
    }
    if (tok.type === 'num') {
      pos++;
      return tok.value;
    }
    if (tok.type === '(') {
      pos++;
      const value = parseExpression();
      if (!isOp(peek(), ')')) {
        throw new Error('Missing closing parenthesis');
      }
      pos++;
      return value;
    }
    throw new Error('Unexpected token: ' + tok.type);
  }

  const result = parseExpression();
  if (pos !== tokens.length) {
    throw new Error('Unexpected token: ' + tokens[pos].type);
  }
  return result;
}
```

591 tokens in, 2,442 out (1,491 of them reasoning) · 5.8 s · $0.0030 · 1 message on Pro · answered by deepseek/deepseek-v4.1-flash via Together · October 8, 2026

Parse CSV with quoted fields

A hard prompt: “Parse CSV with quoted fields”, in full.

Passed: All 8 tests passed.

```javascript
function parseCsv(text) {
  const rows = [];
  let row = [];
  let field = '';
  let inQuotes = false;
  let pending = false; // true if there is an in-progress field/row
  let i = 0;
  const n = text.length;

  while (i < n) {
    const ch = text[i];

    if (inQuotes) {
      if (ch === '"') {
        if (text[i + 1] === '"') {
          field += '"';
          i += 2;
        } else {
          inQuotes = false;
          i += 1;
        }
      } else {
        field += ch;
        i += 1;
      }
      continue;
    }

    if (ch === '"' && field === '') {
      // A quote only opens a quoted field when it is the first char of the field.
      inQuotes = true;
      pending = true;
      i += 1;
      continue;
    }

    if (ch === ',') {
      row.push(field);
      field = '';
      pending = true;
      i += 1;
      continue;
    }

    if (ch === '\n' || ch === '\r') {
      if (ch === '\r' && text[i + 1] === '\n') i += 1; // treat CRLF as one break
      row.push(field);
      rows.push(row);
      row = [];
      field = '';
      pending = false;
      i += 1;
      continue;
    }

    field += ch;
    pending = true;
    i += 1;
  }

  // A line break at the very end leaves "pending" false, so no empty trailing row.
  if (pending) {
    row.push(field);
    rows.push(row);
  }

  return rows;
}
```

522 tokens in, 3,473 out (3,074 of them reasoning) · 9.1 s · $0.0042 · 1 message on Pro · answered by deepseek/deepseek-v4.1-flash via Together · October 8, 2026

How it compares on coding

DeepSeek V4.1 Flash beside the job's picks among every model. 18 models share the hard-problems pick, DeepSeek V4.1 Flash among them.

DeepSeek V4.1 Flash beside other models on coding
Model	Passed	On Pro	Cost per reply
GLM 5.3Our pick: hard problems (shared), best value	5 of 5	250 a month on Pro	$0.0015
GLM 5.3 FlashOur pick: hard problems (shared), everyday	5 of 5	60 a day on Pro	$0.0003
DeepSeek V4.1 FlashOur pick: hard problems (shared)	5 of 5	60 a day on Pro	$0.0018

On Pro: Pro's count on each model. Cost: what OpenRouter charged us per reply, on average.

Every limit is published. Paid plans also have a monthly fair-use limit on AI cost: Plus $3.75, Pro $7.50, Max $20, Ultra $42, Studio $85. Using every message on your plan at typical sizes stays under it; very large messages and heavy research use it faster. Every limit, explained.

DeepSeek V4.1 Flash next to each model on coding

DeepSeek V4.1 Flash against each model on the same 5 coding prompts. One run each, through OpenRouter: a wait depends on the provider and the load that day, so a lead under 10% counts as close.

Against Claude Fable 5.1: DeepSeek V4.1 Flash answered 2.2× sooner (2.5 s to 5.5 s), cost 23.2× less, and passed 5 of 5 to its 5.
Against Claude Opus 5.5: DeepSeek V4.1 Flash answered 2.9× sooner (2.5 s to 7.2 s), cost 9.3× less, and passed 5 of 5 to its 5.
Against Claude Sonnet 5.5: DeepSeek V4.1 Flash answered 1.3× later (2.5 s to 2.0 s), cost 3.4× less, and passed 5 of 5 to its 5.
Against Claude Sonnet 5: DeepSeek V4.1 Flash answered within 10% of its time (2.5 s to 2.7 s), cost 5.5× less, and passed 5 of 5 to its 5.
Against Claude Haiku 5.5: DeepSeek V4.1 Flash answered 1.6× sooner (2.5 s to 3.9 s), cost 4.1× more, and passed 5 of 5 to its 5.
Against Claude Haiku 4.5: DeepSeek V4.1 Flash answered within 10% of its time (2.5 s to 2.3 s), cost 1.5× less, and passed 5 of 5 to its 3. Only DeepSeek V4.1 Flash passed Evaluate an arithmetic expression, no eval; Parse CSV with quoted fields.
Against GPT-6 Astra: DeepSeek V4.1 Flash answered 3.3× sooner (2.5 s to 8.2 s), cost 12.2× less, and passed 5 of 5 to its 5.
Against GPT-6.1 Sol: DeepSeek V4.1 Flash answered 1.9× sooner (2.5 s to 4.8 s), cost 1.3× less, and passed 5 of 5 to its 5.
Against GPT-6 Sol: DeepSeek V4.1 Flash answered 1.9× sooner (2.5 s to 4.8 s), cost 2.9× less, and passed 5 of 5 to its 5.
Against GPT-6 Luna: DeepSeek V4.1 Flash answered 1.8× sooner (2.5 s to 4.6 s), cost 6.4× more, and passed 5 of 5 to its 5.
Against Gemini 4 Argon: DeepSeek V4.1 Flash answered 1.4× sooner (2.5 s to 3.5 s), cost 1.3× less, and passed 5 of 5 to its 5.
Against Gemini 3.1 Pro (preview): DeepSeek V4.1 Flash answered 4.1× sooner (2.5 s to 10.3 s), cost 10.6× less, and passed 5 of 5 to its 5.
Against Gemini 3.8 Flash: DeepSeek V4.1 Flash answered 1.7× sooner (2.5 s to 4.2 s), cost within 10% of it, and passed 5 of 5 to its 5.
Against DeepSeek V4 Pro: DeepSeek V4.1 Flash answered 48.0× sooner (2.5 s to 119.6 s), cost 13.1× less, and passed 5 of 5 to its 5.
Against Grok 4.7: DeepSeek V4.1 Flash answered 12.7× sooner (2.5 s to 31.6 s), cost 13.4× less, and passed 5 of 5 to its 5.
Against Kimi K3: DeepSeek V4.1 Flash answered 1.7× sooner (2.5 s to 4.1 s), cost 3.4× less, and passed 5 of 5 to its 5.
Against GLM 5.3: DeepSeek V4.1 Flash answered 2.4× later (2.5 s to 1.1 s), cost 1.2× more, and passed 5 of 5 to its 5.
Against GLM 5.3 Flash: DeepSeek V4.1 Flash answered 3.4× sooner (2.5 s to 8.4 s), cost 7.3× more, and passed 5 of 5 to its 5.
Against Mistral Large 4: DeepSeek V4.1 Flash answered 14.8× sooner (2.5 s to 36.8 s), cost 4.0× less, and passed 5 of 5 to its 4. Only DeepSeek V4.1 Flash passed Evaluate an arithmetic expression, no eval.

How these runs were done

Tests. Automatic. The function runs against the prompt's tests in a separate Node.js process with a time limit and no file, network or child-process access; it passes when every test passes.

How the runs were done, and every coding prompt.

DeepSeek V4.1 Flash, and coding, elsewhere

Questions

Is DeepSeek V4.1 Flash good for coding?

In our test runs it passed 5 of 5 coding prompts, tied 1st of the 20 models in llmwise. Every reply is on this page, so you can judge them yourself.

How many of my messages does a coding reply from DeepSeek V4.1 Flash use?

1 message each on Pro, where it has 60 a day on Pro. The price of a message is fixed and shown before you send it, however long the reply.

How were these runs done?

The same way for every model: each prompt sent through llmwise's own pipeline, each reply checked the same way. The methods page has every prompt and how each is scored.

Claude, GPT, Gemini, DeepSeek, Grok, Kimi, GLM, and Mistral, in one chat.

See what a message costs before you send it. Free is 5 messages to try; sign in with an email link, no password or card.

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