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+---
+rule: mcp-server-must-be-verified-at-session-start
+title: MCP server availability must be verified at session start
+category: mcp
+severity: error
+version: 1
+---
+
+# MCP server availability must be verified at session start
+
+## Rule
+
+When an MCP server is configured in `.mcp.json`, the agent must at session start verify
+that the server's tools appear in the active deferred-tools list. If they are missing,
+the agent must name the missing server and stop MCP-dependent work until the problem
+is resolved or a workaround is chosen and declared.
+
+## Why
+
+MCP servers are started by the Claude Code harness when a session initializes. If a
+server fails to start — due to a startup error, a configuration problem, or a timing
+issue — its tools do not appear in the deferred-tools list. The harness does not report
+this failure explicitly. An agent that proceeds as if the tools are available will
+spend the session diagnosing what appears to be a tool-call error but is actually a
+server-startup failure.
+
+Early detection saves the entire session from misdirected debugging.
+
+## How to verify
+
+At session start, before using any MCP-dependent tool:
+
+1. Note which servers are configured in `.mcp.json`.
+2. Check whether each server's tools appear in the deferred-tools list
+   (visible in the `system-reminder` block at session start).
+3. If a server's tools are absent: report it immediately.
+
+> "WARNING: MCP server '[name]' is configured in .mcp.json but its tools are not
+> registered in this session. MCP-dependent work for this server is paused.
+> Likely causes: startup error, missing config, or harness timeout. Diagnose before
+> continuing."
+
+4. Offer a diagnostic path: verify the server command runs without error,
+   check configuration files, check for BOM or encoding issues in the server script.
+
+## What NOT to do
+
+- Do not proceed with MCP-dependent tasks assuming the tools will appear later.
+- Do not silently skip MCP steps without reporting why.
+- Do not attempt to call MCP tools whose server is not confirmed active.
+- Do not diagnose the absence as a tool-call error — diagnose it as a startup failure.
+
+## Applies to
+
+All CURABIS projects that configure MCP servers in `.mcp.json`.
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