feat(fetch-use): default experimental.fetch_use to off

Eval runs showed fetch-use routing had no measurable impact on accuracy
versus native HttpClient, while adding latency on every webfetch call.
Flip the default so users must explicitly opt in via
experimental.fetch_use=true in opencode.json. The feature itself is
unchanged — only the default and accompanying docs/comments.

Author: bcode

packages/bcode-browser/skills/BROWSER.md

@@ -98,7 +98,7 @@ console.log("liveUrl for the user to watch:", liveUrl)
 
 Requires `BROWSER_USE_API_KEY` in the environment (the user should have set this before launching bcode). If absent, tell the user to get a key at https://browser-use.com and `export BROWSER_USE_API_KEY=...`.
 
-When `BROWSER_USE_API_KEY` is set, `webfetch` is automatically enhanced with `fetch-use` (Chrome TLS fingerprint + residential proxy + session cookies) — each request is free, but consumes a small amount of proxy bandwidth from the BU account. Disable in `opencode.json` with `experimental.fetch_use: false`.
+`webfetch` can be enhanced with `fetch-use` (Chrome TLS fingerprint + residential proxy + session cookies) when `BROWSER_USE_API_KEY` is set. Off by default — enable in `opencode.json` with `experimental.fetch_use: true`. Each request is free but consumes a small amount of proxy bandwidth from the BU account and adds latency vs. native HttpClient.
 
 ## Attaching to a target
 

packages/bcode-browser/test/fetch-use.test.ts

@@ -3,7 +3,7 @@
 // Unit: layer is constructible, `enabled` reflects BROWSER_USE_API_KEY presence.
 // Live: when the key is set, end-to-end POST to fetch.browser-use.com returns
 //       body bytes + content-type. Skipped without the key. Config-based
-//       opt-out (experimental.fetch_use=false) is enforced in webfetch.ts,
+//       opt-in (experimental.fetch_use=true) is enforced in webfetch.ts,
 //       not here.
 
 import { expect, test } from "bun:test"

packages/opencode/src/config/config.ts

@@ -265,7 +265,7 @@ export const Info = Schema.Struct({
       }),
       fetch_use: Schema.optional(Schema.Boolean).annotate({
         description:
-          "Route webfetch through the Browser Use fetch-use proxy when BROWSER_USE_API_KEY is set. Defaults to true; set false to opt out (still costs but uses native HttpClient instead).",
+          "Route webfetch through the Browser Use fetch-use proxy when BROWSER_USE_API_KEY is set. Defaults to false; set true to opt in (adds Chrome TLS fingerprint + residential proxy at the cost of extra latency).",
       }),
     }),
   ),

packages/opencode/src/tool/webfetch.ts

@@ -51,9 +51,10 @@ export const WebFetchTool = Tool.define(
 
           const timeout = Math.min((params.timeout ?? DEFAULT_TIMEOUT / 1000) * 1000, MAX_TIMEOUT)
 
-          // BrowserCode: route through fetch-use when BROWSER_USE_API_KEY is
-          // set and the user hasn't opted out via experimental.fetch_use=false.
-          const useFu = fetchUse.enabled && (yield* config.get()).experimental?.fetch_use !== false
+          // BrowserCode: route through fetch-use only when BROWSER_USE_API_KEY
+          // is set AND the user has opted in via experimental.fetch_use=true.
+          // Default is off: enabling adds latency without measurable accuracy gains.
+          const useFu = fetchUse.enabled && (yield* config.get()).experimental?.fetch_use === true
           const { arrayBuffer, contentType } = yield* (useFu
             ? fetchUse
                 .fetch(params.url, { timeoutMs: timeout })