Yggdrasil MCP

Reasoning orchestration MCP server — Tree of Thoughts with multi-agent evaluation.

A fork of Anthropic's @modelcontextprotocol/server-sequential-thinking with critical bug fixes and an ambitious roadmap for advanced reasoning capabilities.

Why Yggdrasil?

In Norse mythology, Yggdrasil is the World Tree connecting all realms. This MCP embodies that metaphor:

• Branches = Different reasoning paths through possibility space
• Roots = Deep foundational analysis (first principles)
• Connections = Links between thoughts, revisions, and evaluations

Key Features

Current (v0.8.2)

• deep_planning tool — Structured multi-phase planning sessions (init → clarify → explore → evaluate → finalize)
• String coercion fix — Fixes Claude Code bug #3084 where MCP parameters are incorrectly serialized as strings
• Strict TypeScript — Full lint skill compliance with strictTypeChecked
• Break down complex problems into manageable steps
• Revise and refine thoughts as understanding deepens
• Branch into alternative paths of reasoning
• Adjust the total number of thoughts dynamically
• Generate and verify solution hypotheses

Roadmap

See .claude/plans/yggdrasil-roadmap.md for the full 5-phase roadmap including:

• Thought history retrieval and persistence (JSONL)
• Mermaid diagram export
• Branch evaluation with multi-agent support
• Codex/GPT-5.2 cross-model verification
• n8n workflow integration

Installation

Claude Code

claude mcp add --scope user yggdrasil "npx -y yggdrasil-mcp"

Or add to ~/.claude.json:

{
  "mcpServers": {
    "yggdrasil": {
      "command": "npx",
      "args": ["-y", "yggdrasil-mcp"]
    }
  }
}

Claude Desktop

Add to your Claude Desktop config:

{
  "mcpServers": {
    "yggdrasil": {
      "command": "npx",
      "args": ["-y", "yggdrasil-mcp"]
    }
  }
}

Local Development

{
  "mcpServers": {
    "yggdrasil": {
      "command": "node",
      "args": ["/path/to/yggdrasil-mcp/dist/index.js"]
    }
  }
}

Tool: sequential_thinking

Facilitates a detailed, step-by-step thinking process for problem-solving and analysis.

Parameters

Required

| Parameter | Type | Description | | ------------------- | ------- | -------------------------------------- | | thought | string | The current thinking step | | nextThoughtNeeded | boolean | Whether another thought step is needed | | thoughtNumber | integer | Current thought number (≥1) | | totalThoughts | integer | Estimated total thoughts needed (≥1) |

Optional

| Parameter | Type | Description | | ------------------- | ------- | -------------------------------------- | | isRevision | boolean | Whether this revises previous thinking | | revisesThought | integer | Which thought is being reconsidered | | branchFromThought | integer | Branching point thought number | | branchId | string | Branch identifier | | needsMoreThoughts | boolean | If more thoughts are needed |

Output

{
  "thoughtNumber": 3,
  "totalThoughts": 5,
  "nextThoughtNeeded": true,
  "branches": ["branch-a"],
  "thoughtHistoryLength": 3
}

Tool: deep_planning

Structured planning tool that manages multi-phase planning sessions. Complements sequential_thinking by tracking state while the LLM reasons deeply between phases.

Workflow

init → clarify* → explore+ → evaluate+ → finalize → done

Parameters

| Parameter | Type | Phases | Description | | ----------------- | ------ | ---------------- | ---------------------------------------------------- | | phase | enum | All | init, clarify, explore, evaluate, finalize | | problem | string | init | Problem statement | | context | string | init | Additional background | | constraints | string | init | JSON array of constraint strings | | question | string | clarify | Clarifying question | | answer | string | clarify | Answer to the question | | branchId | string | explore/evaluate | Unique approach identifier | | name | string | explore | Short approach name | | description | string | explore | Detailed approach description | | pros/cons | string | explore | JSON arrays of strings | | feasibility | number | evaluate | Score 0-10 | | completeness | number | evaluate | Score 0-10 | | coherence | number | evaluate | Score 0-10 | | risk | number | evaluate | Score 0-10 (lower is better) | | rationale | string | evaluate | Reasoning for scores | | recommendation | string | evaluate | pursue, refine, or abandon | | selectedBranch | string | finalize | Branch ID of chosen approach | | steps | string | finalize | JSON array of step objects | | risks | string | finalize | JSON array of risk objects | | assumptions | string | finalize | JSON array of strings | | successCriteria | string | finalize | JSON array of strings | | format | string | finalize | markdown (default) or json |

Output

{
  "sessionId": "dp-abc123",
  "phase": "explore",
  "status": "ok",
  "approachCount": 2,
  "evaluationCount": 0,
  "validNextPhases": ["explore", "evaluate", "clarify"],
  "message": "Approach recorded..."
}

Use Cases

Yggdrasil is designed for:

• Complex problem decomposition — Break down multi-step problems
• Iterative planning — Design with room for revision
• Course correction — Analysis that adapts as understanding deepens
• Scope discovery — Problems where the full scope isn't clear initially
• Context maintenance — Tasks requiring state across multiple steps
• Information filtering — Situations where irrelevant details need filtering

Configuration

Environment Variables

| Variable | Default | Description | | ------------------------- | ------- | ------------------------------ | | DISABLE_THOUGHT_LOGGING | false | Suppress stderr thought output |

The String Coercion Fix

This fork addresses a critical bug in Claude Code (#3084) where MCP parameters are serialized as strings regardless of their schema type.

The Problem

// Claude Code sends:
{ nextThoughtNeeded: "true", thoughtNumber: "5" }

// Instead of:
{ nextThoughtNeeded: true, thoughtNumber: 5 }

Using z.coerce.boolean() is dangerous:

z.coerce.boolean().parse('false'); // Returns TRUE! (non-empty string = truthy)

Our Solution

// Safe coercion that correctly handles "false" → false
const coerceBoolean = (val: unknown): boolean => {
  if (typeof val === 'boolean') return val;
  if (typeof val === 'string') {
    if (val.toLowerCase() === 'true') return true;
    if (val.toLowerCase() === 'false') return false;
  }
  throw new Error(`Cannot coerce "${val}" to boolean`);
};

// Applied via z.preprocess (NOT z.coerce)
const booleanSchema = z.preprocess(coerceBoolean, z.boolean());

Development

# Install dependencies
pnpm install

# Build
pnpm build

# Run tests
pnpm test

# Lint
pnpm lint

# Format
pnpm format

# Full quality check
pnpm check

# Watch mode
pnpm watch

Requirements

• Node.js ≥18 (.node-version pins to 24)
• pnpm (corepack-managed via packageManager field)

Upstream

This is a fork of @modelcontextprotocol/server-sequential-thinking.

We periodically sync relevant changes from upstream while maintaining our string coercion fix and additional features.

Changelog

v0.8.2 (2026-02-06)

• Fix: Allow session restart — init now works from any state including after completion
• 108 tests total, 97%+ coverage

v0.8.1 (2026-02-06)

• Fix: Step field normalization in deep_planning — accepts action/detail/name/info aliases alongside canonical title/description
• 101 tests total, 97%+ coverage

v0.8.0 (2026-02-06)

• New tool: deep_planning — Structured multi-phase planning sessions with weighted evaluation scoring
• Add optionalScoreSchema coercion utility for 0-10 score fields
• 93 tests total (51 planning + 28 coercion + 14 lib), 97%+ coverage

v0.7.5 (2026-02-06)

• Rename MCP tool from sequentialthinking to sequential_thinking (snake_case ecosystem alignment)

v0.7.2 (2026-01-31)

• Documentation: Task integration plan, updated TODO.md
• Scaffolding: LICENSE, .node-version, exports, packageManager field

v0.7.1 (2026-01-31)

• CI/CD: pnpm 9→10, Node 24.x for publish, renamed workflow to ci-cd.yml

v0.7.0 (2026-01-31)

• Full lint skill compliance with 6 additional ESLint plugins
• 90% test coverage threshold enforced (currently 98%+)
• Extracted coercion.ts module with 23 comprehensive tests
• Strict TypeScript checking (strictTypeChecked + stylisticTypeChecked)
• Node 24.x added to CI test matrix, TypeScript ^5.7.0

v0.6.3 (2026-01-30)

• Initial fork release from @modelcontextprotocol/server-sequential-thinking v0.6.2
• Critical fix: Claude Code string coercion bug #3084
• Safe z.preprocess coercion for boolean and number types
• GitHub Actions CI/CD with OIDC npm publishing
• TypeScript ironclad stack: ESLint 9.x, Prettier, Husky, lint-staged, Vitest 4.x

License

MIT License — see LICENSE file for details.

Contributing

Contributions are welcome! Please ensure:

1. All tests pass (pnpm test)
2. Linting passes (pnpm lint)
3. Code is formatted (pnpm format)
4. Version is incremented appropriately