sequential-thinking

name	sequential-thinking
description	Structured step-by-step reasoning specialist for complex, multi-step problems. Use when asked to: break down a hard problem, design an algorithm, plan an architecture, debug a non-obvious issue, analyze tradeoffs, or verify reasoning before committing to a solution. Trigger keywords: sequential-thinking, st. Do NOT use for simple lookups, straightforward code generation, or single-step questions — answer directly instead.
compatibility	Requires MCP server called sequentialthinking configured in .codex/mcp-config.json.

Sequential Thinking Skill

Description

Apply structured, explicit chain-of-thought reasoning using the sequentialthinking MCP tool. Each thought is a discrete, numbered step that can be revised, branched, or extended before a final answer is committed. Use when linear reasoning risks missing edge cases, assumptions need to be surfaced, or a problem has multiple valid approaches worth comparing.

Capabilities

• Decompose complex problems into explicit, numbered reasoning steps
• Generate and verify hypotheses before committing to a solution
• Revise earlier steps mid-chain when new information changes the analysis
• Branch the thought chain to explore and compare alternative approaches
• Surface hidden assumptions and edge cases through systematic analysis
• Reason through architectural tradeoffs, debugging puzzles, and algorithm correctness

Key Concepts

• Thought: A single discrete reasoning step — one idea, deduction, or check per thought; keep each thought focused and atomic.
• Revision: A thought that explicitly revisits and corrects an earlier step; use when downstream reasoning reveals an earlier assumption was wrong.
• Branch: A parallel reasoning path forked from an earlier thought to explore an alternative approach before choosing the best one.
• Hypothesis: A provisional conclusion generated mid-chain; must be verified against subsequent thoughts before being accepted as the final answer.
• totalThoughts: An estimate of steps needed — can be adjusted upward as complexity becomes clearer; prefer over-estimating to under-estimating.

Workflow

1. Assess complexity — before invoking, confirm the problem genuinely requires multi-step reasoning. If it is a simple lookup or single-step task, answer directly without sequential thinking.
2. Set initial scope — estimate totalThoughts; start conservatively and extend if needed.
3. Think step by step — emit one atomic thought per step; reference earlier step numbers when building on or revising prior reasoning.
4. Revise when needed — if a later step reveals an error in an earlier one, emit an explicit revision thought rather than silently correcting.
5. Branch for alternatives — when multiple valid approaches exist, branch and reason each to a local conclusion before selecting the best.
6. Verify hypothesis — before the final step, emit an explicit verification thought that checks the candidate answer against all prior reasoning.
7. Report using this structure:
   • Reasoning summary: brief narrative of the key steps and any revisions/branches
   • Answer: the final, verified conclusion
   • Confidence: note any remaining uncertainties or assumptions

Usage Examples

Algorithm Design

sequential-thinking Design an efficient algorithm for finding duplicates in a large dataset

Complex Debugging

st Debug why the authentication flow fails intermittently under load

Architecture Planning

st Plan microservices decomposition for an e-commerce platform

Root Cause Analysis

sequential-thinking Analyze why memory usage grows over time in this Node.js service

Tradeoff Analysis

st Compare event sourcing vs. CRUD for an audit-heavy financial application

Configuration

MCP server is called sequentialthinking and is configured in .codex/mcp-config.json. No environment variables or external service credentials are required — this is a local reasoning tool with no network dependency.

No connectivity probe is needed before use.

Best Practices

• Only invoke for genuinely complex problems — do not add overhead to simple questions
• Keep each thought atomic — one idea or deduction per step, not a paragraph of mixed reasoning
• Always revise explicitly — emit a revision thought rather than silently rewriting earlier steps
• Extend totalThoughts freely — it is better to use more steps than to compress reasoning and miss edge cases
• Verify before concluding — always emit an explicit verification step before the final answer
• Combine with other skills — use doc-search or web-search to gather facts first, then apply sequential thinking to reason over them

Limitations

• Increases response length and latency compared to direct answers — not suitable for time-sensitive or simple queries
• Thought quality depends on the problem being well-scoped; vague questions produce vague reasoning chains
• Cannot guarantee correctness for problems requiring external ground truth (e.g., current data, runtime state)