MCP Server Design

MCP Server Design for Agent Consumption

Design MCP tools that agents can discover, select correctly, and use efficiently. This skill covers the agent-facing surface of MCP servers — not protocol mechanics, but the design choices that determine whether agents pick the right tool on the first try.

Core Principles

1. Descriptions are routing signals. Tool descriptions work like skill frontmatter — they're always loaded into agent context and drive tool selection. Every word costs tokens; make each one count.

2. Names set expectations. Agents often select tools by name before reading descriptions. A name like get_test_results signals "start here" — even if the tool is niche.

3. Defer detail to knowledge tools. Don't bloat descriptions with situational patterns. Create on-demand knowledge endpoints that agents query when they need depth.

4. Align with vendor standards. Follow the MCP spec and Anthropic's naming conventions — they're designed for multi-server environments where naming collisions are real.

5. Test your claims. Design patterns are hypotheses until validated across models. Measure tool selection accuracy, false-positive calls, and steps to completion.

Quick Reference

Tool Descriptions

• Lead with a verb (what the tool does)
• Keep descriptions compact — every description loads into every session
• Put situational detail in parameter descriptions or knowledge tools
• Use "Niche" or similar qualifiers to signal low-frequency tools
• Cross-reference related tools ("Use X for most cases; this tool handles Y")

→ See references/tool-description-patterns.md

Tool Naming

• Format: {service}_{action}_{resource} (Anthropic standard)
• snake_case, action-oriented, specific
• Names that sound generic ("get_results") attract false-positive calls
• Names that sound specialized ("parse_uploaded_trx") naturally deprioritize

→ See references/tool-naming-conventions.md

Knowledge Tools

• Two-tier architecture: compact tool descriptions (always loaded) + knowledge endpoints (on demand)
• Knowledge tools carry repo-specific patterns, failure signatures, recommended tool sequences
• Agents call the knowledge tool first, then use the right data tools

→ See references/knowledge-tool-design.md

Tool Annotations

From the MCP spec (2025-11-25):

Mark all read-only tools as readOnlyHint: true. Required for Anthropic directory submission.

Agent Integration

• Skills that consume your MCP tools should use domain language, not tool names
• "Search the build logs" survives tool renames; "call azdo_search_log" doesn't
• CLI examples act as semantic bridges — self-describing syntax maps naturally to tool parameters
• CLI-as-skill: when your tool has both CLI and MCP modes, agents can use CLI via bash to avoid MCP context tax entirely

→ See references/agent-integration-patterns.md

When to Create a Knowledge Tool

Create a knowledge endpoint when:

• Multiple tools need context that doesn't fit in descriptions (repo-specific patterns, recommended sequences)
• The same guidance applies across tool families but varies by domain (e.g., per-repo failure patterns)
• Agents consistently make wrong first choices without domain context

Don't create one when:

• Tool descriptions alone are sufficient for correct selection
• The knowledge is static and small enough for a parameter description
• Only one tool needs the context (put it in that tool's parameter descriptions)

Validation

These patterns are hypotheses supported by evidence, not proven rules. Before adopting a specific threshold (word count, etc.), test it:

1. Pick a realistic task that exercises tool selection
2. Run it across 3+ models from different families
3. Change one variable (description length, name, etc.)
4. Measure: correct tool selection, false positives, steps to completion

→ See references/validation-methodology.md

References

• references/tool-description-patterns.md — description budgets, structure, routing signals
• references/tool-naming-conventions.md — naming conventions, traps, family naming
• references/knowledge-tool-design.md — on-demand knowledge architecture
• references/agent-integration-patterns.md — domain language, CLI bridges, INVOKES
• references/industry-alignment.md — vendor guidance, research findings
• references/validation-methodology.md — how to test MCP design claims
• references/server-comparisons.md — methodology for evaluating MCP servers + a working catalog of real-world comparisons
• references/mcp-wire-format-trim.md — measuring tools/list byte cost and the SDK-default-annotation, outputSchema-trim, and candidate-triage patterns for reducing it
• references/mcp-structured-content.md — when to use (and not use) UseStructuredContent, wire-compat discipline, error-path migration
• references/mcp-tool-routing-copy.md — discoverability through copy: route callers via descriptions and failure messages instead of growing the tool surface

See Also

• skill-builder/references/anti-patterns.md — "Re-documenting MCP tools" (the skill-author perspective)
• skill-builder/references/skill-patterns.md — INVOKES pattern for connecting skills to MCP tool families