Lesson 478 of 2116
Codex With Custom Tools And MCP
Codex's real power shows when you connect it to your own tools — internal APIs, datastores, ticketing systems — usually via Model Context Protocol.
Lesson map
What this lesson covers
Learning path
The main moves in order
- 1Out of the box, Codex sees a sandbox
- 2MCP
- 3custom tool
- 4tool authorization
Concept cluster
Terms to connect while reading
Section 1
Out of the box, Codex sees a sandbox
Default Codex has shell, file system, and HTTP. It does not know your Linear, your Datadog, your internal docs API, or your billing system. Connecting those requires custom tools — usually exposed via Model Context Protocol (MCP), which both Codex CLI and Codex Cloud speak natively in 2026.
The MCP shape
- 1A tool advertises a schema — name, description, input parameters
- 2The agent reads the schema and decides when to invoke
- 3Each call returns structured output the agent can reason over
- 4Auth happens at the MCP server, not in the prompt
- 5Tool calls are logged so you can audit what the agent did
Compare the options
| Use case | Tool to expose | Why |
|---|---|---|
| Triage incoming bugs | Linear search and create | Codex can file bugs against the right team |
| Investigate on-call alerts | Datadog query and Sentry fetch | Codex can correlate logs and stack traces |
| Onboarding doc updates | Notion search and update | Codex can keep docs synced with code |
| Vendor email triage | Inbox search and label | Codex can categorize before a human reads |
Applied exercise
- 1List the three tools you wish your AI had access to
- 2For each, write a one-line description and the input parameters
- 3Pick the most-likely-safe one and stand up an MCP server for it
- 4Watch what Codex does with it — note what was useful, what was dangerous
Key terms in this lesson
The big idea: Codex without your tools is a generic engineer. Codex with your tools is a teammate.
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