Code Subtitle — Preview

What it does

Select code or a short comment, or place the cursor on a non-empty line, and run Code Subtitle: Show Subtitle to display a temporary, streamed explanation beside it.

Subtitles are aimed at experienced engineers reading OSS or reviewing code. They connect a visible mechanism to one useful responsibility, invariant, failure boundary, or tradeoff. For example, a request-ID guard can explain why a late response cannot overwrite the current view. The model is instructed to avoid syntax narration and unsupported design intent; comments alone receive a faithful short translation.

How to use it

Recorded in VS Code using a fixed-response demo provider. The animation shows the real extension UI; response text and timing are illustrative, not a live-model quality or speed measurement.

Use Shift+Alt+E on Windows/Linux or Ctrl+Alt+E on macOS, or pick Show Subtitle from the editor's right-click menu. Use Dismiss Subtitle or Esc to clear it. Selection changes, edits, editor switches, and the display deadline (10 to 30 seconds, scaled to the subtitle's length) also clear the subtitle. Clear Cache clears the in-memory results and cancels active generation.

When a request cannot complete for a reason you can fix in the editor (selection too large, unusable or overlong output, timeout, or a failed request), a short hint appears in the subtitle slot and disappears after a few seconds. Problems that need attention elsewhere, such as no available model or denied access, are shown as notifications.

The Windows/Linux default avoids the Alt+E menu-bar mnemonic that opens the Edit menu; it has not yet been verified on Windows or Linux hardware. To change the shortcut, open Preferences: Open Keyboard Shortcuts from the Command Palette. On macOS, you can also press Cmd+K, then Cmd+S. Search for Code Subtitle: Show Subtitle, select its pencil icon, press your preferred key combination, and press Enter. VS Code saves the override in your user keyboard settings. The same screen lets you change, remove, or reset the binding and inspect conflicts with other commands.

Model and language

The extension uses a language model exposed through VS Code. It has no API-key setting or separate backend. With auto, Copilot models are preferred; if Copilot is unavailable, the picker lists models from every installed provider. On first use, choose a model and complete that provider's VS Code consent or configuration flow if prompted. The vendor-qualified choice is remembered across restarts while it is still available; run Code Subtitle: Choose Model to change it. Sign-in, model access, configuration, and available quota may be required, depending on the provider.

Data and privacy

Only explicit commands send the selected text and up to five adjacent lines on each side. Optional semantic context also includes bounded language-service type information, documentation, and one-hop definition excerpts from the same workspace folder. It queries at most three selected identifiers, waits at most 600 ms, and includes up to 4,000 UTF-16 code units of extra evidence. Missing or slow providers fall back to the selection and adjacent lines. Untitled files and untrusted workspaces use that basic path as well.

Disable codeSubtitle.semanticContext in user settings to omit semantic evidence. Code Subtitle uses VS Code's standard provider APIs, including services supplied by TypeScript 7 (tsgo); the appropriate language extension must be active. No separate language server is launched.

Code Subtitle does not modify source files, persist code or responses, or send telemetry. Model-provider data handling is governed by that provider and your organization. See the VS Code Language Model API guide.

Output language follows VS Code's display language. Override it with codeSubtitle.outputLanguage. Set codeSubtitle.model to a bare Copilot model ID or a vendor-qualified vendor:id value for another provider; this explicit setting takes precedence over the remembered choice. Both are user-level settings.

Settings

{
  "codeSubtitle.outputLanguage": "auto",
  "codeSubtitle.model": "auto",
  "codeSubtitle.semanticContext": true,
  "codeSubtitle.timingLog": false
}

Set codeSubtitle.outputLanguage to a language tag such as ja or en. Set codeSubtitle.semanticContext to false when requests should contain only the selection and adjacent lines. Set codeSubtitle.timingLog to true to record content-free phase timings in the Code Subtitle Timing output channel; it records only request ids, phase names, and milliseconds, never code, prompts, subtitles, or file names.

Limitations

This preview requires desktop VS Code 1.135 or newer. Long lines and narrow editors can clip line-end decorations. Display readability, screen-reader behavior, live-model quality, and performance still require manual acceptance. Notebook, browser, and Remote environments are outside initial validation.

This is an early preview. The source is available under the MIT License.