Merge overlapping intervals: every AI model's reply, tested

Write a JavaScript function mergeIntervals(intervals) that takes an array of [start, end] pairs (numbers, start ≤ end, in any order) and returns a new array in which overlapping or touching intervals are merged, sorted by start.
[1, 3] and [3, 5] touch, so they merge into [1, 5]. Don't change the input array.
Reply with the whole function in one ```javascript code block: plain JavaScript for Node.js 22, no imports, no TypeScript.

mergeIntervals([[1, 3], [2, 6], [8, 10], [15, 18]])  // → [[1, 6], [8, 10], [15, 18]]
mergeIntervals([[1, 4], [4, 5]])  // → [[1, 5]]
mergeIntervals([])  // → []
mergeIntervals([[5, 7], [1, 2]])  // → [[1, 2], [5, 7]]
mergeIntervals([[1, 10], [2, 3], [4, 5]])  // → [[1, 10]]
mergeIntervals([[1, 2], [2, 2], [2, 3]])  // → [[1, 3]]
(() => { const input = [[3, 4], [1, 2]]; mergeIntervals(input); return input; })()  // → [[3, 4], [1, 2]]

Exactly what this prompt's replies are checked against, with every other prompt of our test runs.

1. GPT-6 Luna

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     const sorted = intervals
       .map(([start, end]) => [start, end])
       .sort((a, b) => a[0] - b[0]);
   
     const merged = [];
   
     for (const [start, end] of sorted) {
       const last = merged[merged.length - 1];
   
       if (!last || start > last[1]) {
         merged.push([start, end]);
       } else {
         last[1] = Math.max(last[1], end);
       }
     }
   
     return merged;
   }
   ```

   449 tokens in, 130 out · 1.5 s · $0.0001 · 1 message on Pro · answered by openai/gpt-6-luna via OpenAI · September 27, 2026

2. Claude Haiku 5.5

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     const sorted = intervals
       .map(([start, end]) => [start, end])
       .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
   
     const merged = [];
     for (const [start, end] of sorted) {
       const last = merged[merged.length - 1];
       if (last && start <= last[1]) {
         last[1] = Math.max(last[1], end);
       } else {
         merged.push([start, end]);
       }
     }
     return merged;
   }
   ```

   622 tokens in, 185 out · 1.3 s · $0.0002 · 1 message on Pro · answered by anthropic/claude-haiku-5.5 via Anthropic · October 7, 2026

3. GLM 5.3 Flash

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     // Sort a copy by start so the input array is left unchanged
     const sorted = [...intervals].sort((a, b) => a[0] - b[0]);
   
     const merged = [];
     for (const [start, end] of sorted) {
       const last = merged[merged.length - 1];
       // Merge when overlapping ([1,4] & [2,5]) or touching ([1,3] & [3,5])
       if (last && start <= last[1]) {
         last[1] = Math.max(last[1], end);
       } else {
         merged.push([start, end]);
       }
     }
     return merged;
   }
   ```
   
   Example: `mergeIntervals([[5, 7], [1, 3], [3, 5], [8, 10]])` → `[[1, 7], [8, 10]]`.

   463 tokens in, 287 out (82 of them reasoning) · 9.6 s · $0.0002 · 1 message on Pro · answered by z-ai/glm-5.3-flash via Wafer · September 27, 2026

4. DeepSeek V4.1 Flash

   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

5. GLM 5.3

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     const sorted = [...intervals].sort((a, b) => a[0] - b[0]);
     const result = [];
     for (const [start, end] of sorted) {
       const last = result[result.length - 1];
       if (last && start <= last[1]) {
         last[1] = Math.max(last[1], end);
       } else {
         result.push([start, end]);
       }
     }
     return result;
   }
   ```

   486 tokens in, 114 out (1 of them reasoning) · 0.8 s · $0.0008 · 1 message on Pro · answered by z-ai/glm-5.3 via Wafer · September 27, 2026

6. GPT-6.1 Sol

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     const sorted = intervals.map(([start, end]) => [start, end]);
     sorted.sort((a, b) => a[0] - b[0]);
   
     const merged = [];
   
     for (const [start, end] of sorted) {
       const last = merged[merged.length - 1];
   
       if (last && start <= last[1]) {
         last[1] = Math.max(last[1], end);
       } else {
         merged.push([start, end]);
       }
     }
   
     return merged;
   }
   ```

   451 tokens in, 127 out · 2.0 s · $0.0011 · 1 message on Pro · answered by openai/gpt-6.1-sol via OpenAI · September 29, 2026

7. Gemini 3.8 Flash

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     if (!intervals || intervals.length <= 1) {
       return intervals ? intervals.map(interval => [...interval]) : [];
     }
   
     // Clone and sort by start time, breaking ties by end time
     const sorted = intervals
       .map(interval => [...interval])
       .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
   
     const merged = [sorted[0]];
   
     for (let i = 1; i < sorted.length; i++) {
       const current = sorted[i];
       const lastMerged = merged[merged.length - 1];
   
       // Overlapping or touching intervals
       if (current[0] <= lastMerged[1]) {
         lastMerged[1] = Math.max(lastMerged[1], current[1]);
       } else {
         merged.push(current);
       }
     }
   
     return merged;
   }
   ```

   477 tokens in, 229 out · 3.8 s · $0.0012 · 1 message on Pro · answered by google/gemini-3.8-flash via Google · September 27, 2026

8. Claude Haiku 4.5

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     if (intervals.length === 0) return [];
   
     // Sort intervals by start time, then by end time
     const sorted = intervals.map(interval => [...interval]).sort((a, b) => {
       return a[0] - b[0] || a[1] - b[1];
     });
   
     const merged = [sorted[0]];
   
     for (let i = 1; i < sorted.length; i++) {
       const current = sorted[i];
       const last = merged[merged.length - 1];
   
       // If current interval overlaps or touches the last merged interval
       if (current[0] <= last[1]) {
         // Merge by extending the end of the last interval
         last[1] = Math.max(last[1], current[1]);
       } else {
         // No overlap, add as new interval
         merged.push(current);
       }
     }
   
     return merged;
   }
   ```

   481 tokens in, 239 out · 2.0 s · $0.0017 · 1 message on Pro · answered by anthropic/claude-haiku-4.5 via Amazon Bedrock · September 27, 2026

9. Gemini 4 Argon

   Passed: All 7 tests passed.

   ```javascript
   function mergeIntervals(intervals) {
     if (!intervals || intervals.length === 0) {
       return [];
     }
   
     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 current = sorted[i];
       const last = merged[merged.length - 1];
   
       if (current[0] <= last[1]) {
         last[1] = Math.max(last[1], current[1]);
       } else {
         merged.push(current);
       }
     }
   
     return merged;
   }
   ```

   698 tokens in, 198 out · 3.4 s · $0.0017 · 1 message on Pro · answered by google/gemini-4-argon via Google AI Studio · October 15, 2026

10. Mistral Large 4

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (intervals.length === 0) return [];
    
      const sorted = intervals.slice().sort((a, b) => a[0] - b[0]);
      const merged = [];
    
      let start = sorted[0][0];
      let end = sorted[0][1];
    
      for (let i = 1; i < sorted.length; i++) {
        const s = sorted[i][0];
        const e = sorted[i][1];
    
        if (s <= end) {
          if (e > end) end = e;
        } else {
          merged.push([start, end]);
          start = s;
          end = e;
        }
      }
    
      merged.push([start, end]);
      return merged;
    }
    ```

    456 tokens in, 727 out (552 of them reasoning) · 8.1 s · $0.0018 · 1 message on Pro · answered by mistralai/mistral-large-4-0 via Mistral · October 9, 2026

11. DeepSeek V4 Pro

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!Array.isArray(intervals) || intervals.length === 0) return [];
    
      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]) {
          if (end > last[1]) last[1] = end;
        } else {
          merged.push([start, end]);
        }
      }
    
      return merged;
    }
    ```

    461 tokens in, 492 out (313 of them reasoning) · 6.8 s · $0.0021 · 1 message on Pro · answered by deepseek/deepseek-v4-pro-0813 via Wafer · October 8, 2026

12. GPT-6 Sol

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      const sorted = intervals
        .map(([start, end]) => [start, end])
        .sort((a, b) => a[0] - b[0]);
    
      const merged = [];
    
      for (const [start, end] of sorted) {
        const last = merged[merged.length - 1];
    
        if (last && start <= last[1]) {
          last[1] = Math.max(last[1], end);
        } else {
          merged.push([start, end]);
        }
      }
    
      return merged;
    }
    ```

    449 tokens in, 130 out · 1.7 s · $0.0022 · 1 message on Pro · answered by openai/gpt-6-sol via OpenAI · September 27, 2026

13. Claude Sonnet 5.5

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      const sorted = intervals
        .map(([start, end]) => [start, end])
        .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
    
      const result = [];
      for (const [start, end] of sorted) {
        const last = result[result.length - 1];
        if (last && start <= last[1]) {
          if (end > last[1]) last[1] = end;
        } else {
          result.push([start, end]);
        }
      }
      return result;
    }
    ```

    623 tokens in, 180 out · 1.4 s · $0.0033 · 1 message on Pro · answered by anthropic/claude-sonnet-5.5 via Anthropic · September 28, 2026

14. Claude Sonnet 5

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!intervals || intervals.length === 0) return [];
    
      const sorted = intervals
        .map(([a, b]) => [a, b])
        .sort((x, y) => x[0] - y[0]);
    
      const result = [sorted[0]];
    
      for (let i = 1; i < sorted.length; i++) {
        const [start, end] = sorted[i];
        const last = result[result.length - 1];
    
        if (start <= last[1]) {
          last[1] = Math.max(last[1], end);
        } else {
          result.push([start, end]);
        }
      }
    
      return result;
    }
    ```

    619 tokens in, 216 out · 2.7 s · $0.0034 · 1 message on Pro · answered by anthropic/claude-sonnet-5 via Claude Platform on AWS · September 27, 2026

15. Kimi K3

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      const sorted = intervals
        .map(([start, end]) => [start, end])
        .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
    
      const merged = [];
      for (const [start, end] of sorted) {
        const last = merged[merged.length - 1];
        if (last && start <= last[1]) {
          last[1] = Math.max(last[1], end);
        } else {
          merged.push([start, end]);
        }
      }
      return merged;
    }
    ```

    536 tokens in, 152 out (4 of them reasoning) · 2.0 s · $0.0036 · 1 message on Pro · answered by moonshotai/kimi-k3 via Together · September 27, 2026

16. Grok 4.7

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!intervals || intervals.length === 0) return [];
    
      const sorted = intervals
        .map((iv) => [iv[0], iv[1]])
        .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
    
      const merged = [[sorted[0][0], sorted[0][1]]];
    
      for (let i = 1; i < sorted.length; i++) {
        const last = merged[merged.length - 1];
        const curr = sorted[i];
        if (curr[0] <= last[1]) {
          if (curr[1] > last[1]) last[1] = curr[1];
        } else {
          merged.push([curr[0], curr[1]]);
        }
      }
    
      return merged;
    }
    ```

    1,684 tokens in, 547 out (349 of them reasoning) · 6.1 s · $0.0045 · 1 message on Pro · answered by x-ai/grok-4.7 via xAI · October 2, 2026

17. Claude Opus 5.5

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!Array.isArray(intervals) || intervals.length === 0) return [];
    
      const sorted = intervals
        .map(([start, end]) => [start, end])
        .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
    
      const result = [sorted[0]];
    
      for (let i = 1; i < sorted.length; i++) {
        const last = result[result.length - 1];
        const [start, end] = sorted[i];
    
        if (start <= last[1]) {
          if (end > last[1]) last[1] = end;
        } else {
          result.push([start, end]);
        }
      }
    
      return result;
    }
    ```

    621 tokens in, 234 out · 3.7 s · $0.0077 · 1 message on Pro · answered by anthropic/claude-opus-5.5 via Claude Platform on AWS · September 27, 2026

18. Gemini 3.1 Pro

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!intervals || intervals.length === 0) {
        return [];
      }
    
      const sorted = [...intervals].sort((a, b) => a[0] - b[0]);
      const result = [];
      
      let current = [sorted[0][0], sorted[0][1]];
    
      for (let i = 1; i < sorted.length; i++) {
        const next = sorted[i];
        
        if (next[0] <= current[1]) {
          current[1] = Math.max(current[1], next[1]);
        } else {
          result.push(current);
          current = [next[0], next[1]];
        }
      }
      
      result.push(current);
      return result;
    }
    ```

    477 tokens in, 780 out (584 of them reasoning) · 8.6 s · $0.0103 · 1 message on Pro · answered by google/gemini-3.1-pro-preview via Google · September 27, 2026

19. GPT-6 Astra

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      const sorted = intervals.map(([start, end]) => [start, end]);
      sorted.sort((a, b) => a[0] - b[0]);
    
      const merged = [];
    
      for (const [start, end] of sorted) {
        const last = merged[merged.length - 1];
    
        if (last && start <= last[1]) {
          last[1] = Math.max(last[1], end);
        } else {
          merged.push([start, end]);
        }
      }
    
      return merged;
    }
    ```

    449 tokens in, 127 out · 2.6 s · $0.0108 · 1 message on Pro · answered by openai/gpt-6-astra via OpenAI · September 27, 2026

20. Claude Fable 5.1

    Passed: All 7 tests passed.

    ```javascript
    function mergeIntervals(intervals) {
      if (!Array.isArray(intervals) || intervals.length === 0) return [];
    
      // Copy so the input (and its inner pairs) is never mutated
      const sorted = intervals
        .map(([start, end]) => [start, end])
        .sort((a, b) => a[0] - b[0] || a[1] - b[1]);
    
      const result = [sorted[0]];
    
      for (let i = 1; i < sorted.length; i++) {
        const [start, end] = sorted[i];
        const last = result[result.length - 1];
    
        if (start <= last[1]) {
          // Overlapping or touching: extend the current interval
          if (end > last[1]) last[1] = end;
        } else {
          result.push([start, end]);
        }
      }
    
      return result;
    }
    ```

    621 tokens in, 275 out · 5.3 s · $0.0214 · 1 message on Pro · answered by anthropic/claude-fable-5.1 via Anthropic · September 27, 2026

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