[BUG] Syntax highlighting loses multi-line string state — docstring continuation lines render as code

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When Claude Code renders a code diff (Edit/Write preview) or code block in the CLI containing a multi-line triple-quoted Python string (e.g. a docstring), only the first line is colored as a string. Every subsequent line of the same string is highlighted as if it were code — words like and/in/for/not/is, numbers, and punctuation inside the prose all pick up keyword/identifier/operator colors.

The highlighter appears to be stateless across line boundaries: it re-tokenizes each line independently and doesn't remember that a previous line opened a """ string that is still unterminated, so the "inside a string" state is lost after the first newline.

What Should Happen?

All lines of a multi-line string literal — from the opening """ through the closing """ — should be colored uniformly as a string, matching how the first line renders (and how editors / bat / pygments render it).

Steps to Reproduce

1. Have Claude display or diff a Python file with a function that has a multi-line docstring, e.g.:

def average(values):
    """Return the mean of a sequence of numbers.

    The values are summed and then divided by the count. If the
    sequence is empty this returns None rather than raising, so
    callers can treat 0 items and 1 item the same way.
    """
    return sum(values) / len(values) if values else None

1. Observe the rendered output in the terminal.
2. The opening line ("""Return the mean of a sequence of numbers.) is correctly string-colored; docstring lines 2–4 are colored as code — words like and, If, is, None, and the numbers 0/1 take on keyword/number colors instead of string color.

Expected vs Actual

• Expected: the whole docstring block is one uniform string color.
• Actual: only the opening line is string-colored; continuation lines fall back to code highlighting.

Environment

• Claude Code CLI (latest)
• OS: Linux 6.17
• Not terminal-specific — reproduces in both:
• GNOME Terminal 3.52.0 (VTE 0.76.0), VTE_VERSION=7600
• PyCharm Community 2026.1 integrated terminal (JediTerm), build PY-261.24374.152
• TERM=xterm-256color, COLORTERM unset

Additional Notes

Because it reproduces in both a native VTE terminal and JetBrains' JediTerm, this is Claude Code's highlighter losing lexer state across newlines, not an emulator quirk. Likely affects any language whose literals span lines: Python """/''', JS/TS template literals, C/CSS block comments /* … */, HTML comments. A fix would need the tokenizer to thread lexer state from one line into the next rather than highlighting each line in isolation.

Claude Model

Opus

Is this a regression?

I don't know