> ## Documentation Index > Fetch the complete documentation index at: https://docs.mcp-b.ai/llms.txt > Use this file to discover all available pages before exploring further. # Codex site tools compatibility > Dated comparison of Codex site tools in ChatGPT's built-in browser with the WebMCP Community Group draft. OpenAI calls its WebMCP implementation **site tools**. The [official site tools documentation](https://learn.chatgpt.com/docs/webmcp) owns availability, setup, security, and supported features. This page records an independent compatibility snapshot from August 27, 2026. It is not an OpenAI release document or a formal conformance claim. ## Authoritative sources | Source | Owns | | ----------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------- | | [OpenAI site tools documentation](https://learn.chatgpt.com/docs/webmcp) | Product availability, setup, security, API example, limits | | [OpenAI Help Center](https://help.openai.com/en/articles/20001423-using-site-tools-in-the-chatgpt-desktop-app) | User-facing availability, scope, and controls | | [ChatGPT built-in browser](https://learn.chatgpt.com/docs/browser) | Browser profiles and supported Codex surfaces | | [WebMCP Community Group draft](https://webmachinelearning.github.io/webmcp/) | Proposed WebMCP interfaces, algorithms, and security model | | [Runme and WebMCP with Codex](https://developers.openai.com/blog/automating-repetitive-work-at-openai-with-codex) | First-party example of a WebMCP workflow | OpenAI documents site tools for ChatGPT Work and Codex in the ChatGPT desktop app's built-in browser. This snapshot does not cover Chrome, Codex CLI, or the Codex IDE extension. Check OpenAI's documentation for current model, account, workspace, and rollout requirements. ## Test scope | Field | Value | | -------------- | ------------------------------------------------------------------------------------------- | | Date | August 27, 2026 | | Agent | Codex with GPT-5.6 Sol | | Browser | ChatGPT desktop app's built-in browser; build identifier unavailable | | Host | macOS | | Test page | `http://127.0.0.1:41739` top-level page with `window.isSecureContext === true` | | Runtime | `document.modelContext` provided by ChatGPT's built-in browser, without MCP-B or a polyfill | | Draft baseline | WebMCP Draft Community Group Report dated August 26, 2026 | One manual probe produced the observations below. Its [fixtures and captured results](https://github.com/WebMCP-org/npm-packages/tree/main/docs/research/codex-site-tools-2026-08-27) are preserved in this repository. The test covered imperative registration, discovery, execution, results, registration lifecycle, declarative markup, a same-origin iframe, invalid registrations, origin options, and the `tools` Permissions Policy. It did not run the Web Platform Tests or test every app build, account configuration, origin, or active-call cancellation path. Agent integration results came from Codex's agent-facing tool bridge. Page API results came from JavaScript calls inside the test page. The probe inspected but did not call the undocumented `codexGetTools` or `codexExecuteTool` properties. ## Agent integration | Capability | Observed on August 27, 2026 | Source | | -------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- | ----------------------- | | Top-level imperative tools | Discovered and invoked tools registered with `document.modelContext.registerTool()` | OpenAI and manual probe | | Tool listing | Included name, title, description, input schema, `readOnlyHint`, `untrustedContentHint`, origin, and page URL | Manual probe | | Dynamic registration | Refreshed the agent's tool list after registration and abort-based removal | Manual probe | | Page lifecycle | Removed tools after navigation | OpenAI and manual probe | | Inputs | Passed a JavaScript object to the tool callback | Manual probe | | Results | Preserved JSON-compatible objects, arrays, primitives, and `null`; converted `undefined` to `null` | Manual probe | | Non-JSON results | Rejected `BigInt` and circular values | Manual probe | | Declarative tools | Did not discover tools declared through form attributes | OpenAI and manual probe | | Iframe tools | OpenAI documents them as unsupported; the tested same-origin child had no `document.modelContext`, so it could not register a tool | OpenAI and manual probe | | Disabled `tools` policy | Discovered a registration from a response with `Permissions-Policy: tools=()` | Manual probe | The draft's execution algorithm does not validate calls against `inputSchema`. In Codex agent-bridge probes, missing required values, wrong types, extra properties, and out-of-range values reached the handler. Validate and authorize every call in application code. On the agent path, `undefined` became `null`. The draft's public execution path treats `undefined`, `BigInt`, and circular results as serialization failures. It does not specify the browser agent's result transport. ## Page API comparison The draft defines `getTools()` and `executeTool()` for in-page JavaScript agents. A browser agent receives an implementation-defined observation instead, and the draft does not prescribe the format used to expose tools to it. This table compares direct page calls only. | Surface | August 26 draft | Observed in Codex | | -------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------- | | `document.modelContext` | Same-object `ModelContext` in secure contexts | Repeated reads returned the same object | | `navigator.modelContext` | Not defined | `undefined` | | Event interface | `ModelContext` extends `EventTarget` and exposes `ontoolchange` | `addEventListener()` and `ontoolchange` were absent | | `registerTool()` | Resolves with `undefined` after registration; supports registration `signal` | Resolved with `undefined`; abort-based removal worked | | `getTools()` | Returns entries with a deep-copied object `inputSchema`, owning `window`, and origin | Returned a serialized JSON schema and origin, without `window` | | `executeTool()` input and result | Accepts an optional object input, defaults to `{}`, and resolves to a JSON string | Accepted an object, rejected a JSON string, and resolved to a JSON string; omitted input was untested | | Tool callback | Receives `(input, { signal })` | Received the input, but callback options did not contain an `AbortSignal` | | `tools` Permissions Policy | Keeps the interface exposed, but methods reject `NotAllowedError` when denied; default allowlist is `'self'` | The interface remained present and `registerTool()` resolved under `Permissions-Policy: tools=()` | | `exposedTo` | Lists cross-origin callers that may discover and execute a registration; rejects invalid or non-trustworthy URLs with `SecurityError` | Accepted `http://insecure.example`; cross-document behavior was untested because the child lacked `modelContext` | | `fromOrigins` | Requests tools from matching cross-origin descendant documents; rejects invalid or non-trustworthy URLs with `SecurityError` | Accepted `http://insecure.example`; cross-document behavior was untested | | Registration errors | Duplicate, empty, or invalid names and empty descriptions reject with `InvalidStateError`; schema serialization errors propagate | Named cases rejected with a plain `Object` whose `name` and `message` were `null` and JSON form was `{}`; a circular schema rejected with `Error` | The object also exposed `codexGetTools` and `codexExecuteTool` during the test. They are undocumented product internals, not WebMCP APIs. Do not call them or use their presence for feature detection. ## Compatibility notes | Constraint | Compatibility requirement | | -------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- | | Feature detection | Check that `document.modelContext?.registerTool` is a function. | | Registration location | Register site tools in the top-level page. | | Callback options | Treat callback options as optional and use `options?.signal` when present. The draft requires it. | | Input enforcement | Validate and authorize inputs in application code. | | Results | Return JSON-compatible values. | | Cleanup | Use the registration `AbortSignal` to remove tools when their application scope ends. | | Origin and policy controls | The tested build accepted non-trustworthy origin options and allowed registration under `tools=()`. Cross-document enforcement was not tested. | | Product internals | Use only documented WebMCP properties, not `codexGetTools` or `codexExecuteTool`. | These observations describe the tested build. Recheck the [official site tools documentation](https://learn.chatgpt.com/docs/webmcp) and this page's date before depending on them. ## Codex with Chrome DevTools MCP [Chrome DevTools MCP](https://github.com/ChromeDevTools/chrome-devtools-mcp) is a separate integration that lets Codex inspect a Chrome instance through MCP. Its experimental WebMCP category exposes tool discovery and execution; see the [WebMCP tool reference](https://github.com/ChromeDevTools/chrome-devtools-mcp/blob/main/docs/tool-reference.md#webmcp). This integration is separate from site tools in ChatGPT's built-in browser. Codex site tools use the current page directly and do not require the MCP-B local relay. For other implementations, see [WebMCP resources and status](/explanation/design/spec-status-and-limitations) and the [package index](/packages/index).