fix(sequentialthinking): bound the stderr thought box - #5435

Closes #5433

Description

The stderr thought box from #5205 padded every row to the thought's longest line, so a thought with one long line and many short ones logged their product. Copilot found it reviewing the v2.3.0 milestone merge PR #5430: a 60 KB thought wrote 400 million characters and pushed RSS to 878 MB. A somewhat larger one would exhaust memory, from a single valid call with logging on by default.

The box is now bounded:

• Width: at most MAX_BOX_TEXT_WIDTH (100). Longer lines wrap onto further rows, and so does a header longer than that (a long branchId, which would otherwise set every row's width). A cut never splits a surrogate pair.
• Height: at most MAX_BOX_ROWS (1000) rows of thought text. Past that the box ends with one row, … N more lines not shown. Padding every short line to the width would otherwise still log about 100 times the thought's size.

#5205's behaviour is kept: one row per line, any line ending ends a row, no \r inside a row, every row the same width (#5108), and colour sized from the visible header (#5000). Only stderr changes. The stored thought and the tool result are unchanged.

Server Details

• Server: sequentialthinking
• Changes to: the thought log on stderr (lib.ts, formatThought); README; patch changeset

Motivation and Context

#5433. It is a regression from #5205, in this release (v2.3.0, #5425), so it is fixed before the release.

How Has This Been Tested?

Measured on the module (SequentialThinkingServer.processThought, stderr counted). The thought is one line of n characters, then n - 1 one-character lines:

Inspector CLI 2.13.0, tools/call sequentialthinking with the n = 5,000 thought, both eras:

candidate legacy  exit=0 {"thoughtNumber":1,"thoughtHistoryLength":1} stderr 25,105,119 chars, 5,006 lines
candidate modern  exit=0 {"thoughtNumber":1,"thoughtHistoryLength":1} stderr 25,105,119 chars, 5,006 lines
branch    legacy  exit=0 {"thoughtNumber":1,"thoughtHistoryLength":1} stderr 112,232 chars, 1,007 lines, "… 4049 more lines not shown"
branch    modern  exit=0 {"thoughtNumber":1,"thoughtHistoryLength":1} stderr 112,232 chars, 1,007 lines, "… 4049 more lines not shown"

LLM client: Claude Code 2.1.318, headless, --strict-mcp-config naming this branch's build (no era switch). It was asked for one sequentialthinking call with a 300-character line and three short lines, and returned {"thoughtNumber":1,"totalThoughts":1,"nextThoughtNeeded":false,"branches":[],"thoughtHistoryLength":1}.

Tests: thought-logging.test.ts adds:

• wrapping at the capped width;
• Copilot's exact case staying bounded;
• the row cap's summary row, including a line the cap cuts partway;
• a thought that exactly fills the cap, with no summary row;
• every row the same width;
• a long branchId header wrapping and staying linear;
• an emoji at the cut not split.

The #5205 "very many lines" test now expects the capped box. The suite is 138 passed; lib.ts coverage is 100 / 98.11 / 100 / 100. npm run local:gate: EXIT=0.

Breaking Changes

None. Only the stderr diagnostic's layout changes for thoughts with lines over 100 characters or more than 1000 rows.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

Checklist

• I have read the MCP Protocol Documentation (not applicable to the protocol surface: stderr only)
• My changes follow MCP security best practices
• I have updated the server's README accordingly
• I have added a changeset (npm run changeset) if this changes what a TypeScript server publishes
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling (not applicable: no new failure path)
• I have documented all environment variables and configuration options (not applicable: none added)

🤖 Generated with Claude Code