Claude Fable 5.1 vs Claude Haiku 4.5

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

• Claude Fable 5.1

  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;
    while (i < expression.length) {
      const ch = expression[i];
      if (ch === " " || ch === "\t" || ch === "\n" || ch === "\r") { i++; continue; }
      if (ch >= "0" && ch <= "9") {
        let start = i;
        while (i < expression.length && expression[i] >= "0" && expression[i] <= "9") i++;
        if (expression[i] === ".") {
          i++;
          if (!(expression[i] >= "0" && expression[i] <= "9")) {
            throw new Error(`Invalid number at position ${start}`);
          }
          while (i < expression.length && expression[i] >= "0" && expression[i] <= "9") i++;
        }
        tokens.push({ type: "num", value: parseFloat(expression.slice(start, i)), pos: start });
        continue;
      }
      if ("+-*/^()".includes(ch)) {
        tokens.push({ type: ch, pos: i });
        i++;
        continue;
      }
      throw new Error(`Unexpected character '${ch}' at position ${i}`);
    }
    tokens.push({ type: "end", pos: expression.length });
  
    // ---- Parser ----
    let p = 0;
    const peek = () => tokens[p];
    const next = () => tokens[p++];
    const fail = (tok) =>
      new Error(
        tok.type === "end"
          ? "Unexpected end of expression"
          : `Unexpected token '${tok.type === "num" ? tok.value : tok.type}' at position ${tok.pos}`
      );
  
    // expr := term (('+'|'-') term)*
    function parseExpr() {
      let value = parseTerm();
      while (peek().type === "+" || peek().type === "-") {
        const op = next().type;
        const rhs = parseTerm();
        value = op === "+" ? value + rhs : value - rhs;
      }
      return value;
    }
  
    // term := unary (('*'|'/') unary)*
    function parseTerm() {
      let value = parseUnary();
      while (peek().type === "*" || peek().type === "/") {
        const op = next().type;
        const rhs = parseUnary();
        value = op === "*" ? value * rhs : value / rhs;
      }
      return value;
    }
  
    // unary := '-' unary | power
    function parseUnary() {
      if (peek().type === "-") {
        next();
        return -parseUnary();
      }
      return parsePower();
    }
  
    // power := atom ('^' unary)?   (right-associative; exponent may carry unary minus)
    function parsePower() {
      const base = parseAtom();
      if (peek().type === "^") {
        next();
        const exponent = parseUnary();
        return Math.pow(base, exponent);
      }
      return base;
    }
  
    // atom := number | '(' expr ')'
    function parseAtom() {
      const tok = next();
      if (tok.type === "num") return tok.value;
      if (tok.type === "(") {
        const value = parseExpr();
        const close = next();
        if (close.type !== ")") throw fail(close);
        return value;
      }
      throw fail(tok);
    }
  
    const result = parseExpr();
    if (peek().type !== "end") throw fail(peek());
    return result;
  }
  ```

  762 tokens in, 1,561 out (387 of them reasoning) · 15.7 s · $0.0874 · 1 message on Pro · answered by anthropic/claude-fable-5.1 via Anthropic · September 27, 2026

• Claude Haiku 4.5

  Failed: 14 of 15 tests passed. First failure: evaluate("-2 ^ 2"): Expected values to be strictly deep-equal:

  ```javascript
  function evaluate(expression) {
    const tokens = tokenize(expression);
    const parser = new Parser(tokens);
    const result = parser.parseExpression();
    
    if (parser.pos !== tokens.length) {
      throw new Error("Unexpected tokens after expression");
    }
    
    return result;
  }
  
  function tokenize(expression) {
    const tokens = [];
    let i = 0;
    
    while (i < expression.length) {
      const ch = expression[i];
      
      if (/\s/.test(ch)) {
        i++;
        continue;
      }
      
      if (/\d/.test(ch)) {
        let num = '';
        while (i < expression.length && /[\d.]/.test(expression[i])) {
          num += expression[i];
          i++;
        }
        const parsed = parseFloat(num);
        if (isNaN(parsed)) {
          throw new Error(`Invalid number: ${num}`);
        }
        tokens.push({ type: 'NUMBER', value: parsed });
        continue;
      }
      
      if ('+-*/^()'.includes(ch)) {
        tokens.push({ type: ch, value: ch });
        i++;
        continue;
      }
      
      throw new Error(`Unexpected character: ${ch}`);
    }
    
    return tokens;
  }
  
  class Parser {
    constructor(tokens) {
      this.tokens = tokens;
      this.pos = 0;
    }
    
    peek() {
      return this.tokens[this.pos];
    }
    
    consume() {
      return this.tokens[this.pos++];
    }
    
    parseExpression() {
      return this.parseAddSub();
    }
    
    parseAddSub() {
      let left = this.parseMulDiv();
      
      while (this.peek() && (this.peek().type === '+' || this.peek().type === '-')) {
        const op = this.consume().type;
        const right = this.parseMulDiv();
        left = op === '+' ? left + right : left - right;
      }
      
      return left;
    }
    
    parseMulDiv() {
      let left = this.parsePower();
      
      while (this.peek() && (this.peek().type === '*' || this.peek().type === '/')) {
        const op = this.consume().type;
        const right = this.parsePower();
        left = op === '*' ? left * right : left / right;
      }
      
      return left;
    }
    
    parsePower() {
      let left = this.parseUnary();
      
      if (this.peek() && this.peek().type === '^') {
        this.consume();
        const right = this.parsePower();
        left = Math.pow(left, right);
      }
      
      return left;
    }
    
    parseUnary() {
      if (this.peek() && this.peek().type === '-') {
        this.consume();
        return -this.parseUnary();
      }
      
      if (this.peek() && this.peek().type === '+') {
        this.consume();
        return this.parseUnary();
      }
      
      return this.parsePrimary();
    }
    
    parsePrimary() {
      const token = this.peek();
      
      if (!token) {
        throw new Error("Unexpected end of expression");
      }
      
      if (token.type === 'NUMBER') {
        this.consume();
        return token.value;
      }
      
      if (token.type === '(') {
        this.consume();
        const result = this.parseExpression();
        
        if (!this.peek() || this.peek().type !== ')') {
          throw new Error("Missing closing parenthesis");
        }
        
        this.consume();
        return result;
      }
      
      throw new Error(`Unexpected token: ${token.value}`);
    }
  }
  ```

  599 tokens in, 1,007 out · 7.6 s · $0.0056 · 1 message on Pro · answered by anthropic/claude-haiku-4.5 via Amazon Bedrock · September 27, 2026