Clarify js_repl binding reuse guidance (#13803)

## Summary

Clarify the `js_repl` prompt guidance around persistent bindings and
redeclaration recovery.

This updates the generated `js_repl` instructions in
`core/src/project_doc.rs` to prefer this order when a name is already
bound:

1. Reuse the existing binding
2. Reassign a previously declared `let`
3. Pick a new descriptive name
4. Use `{ ... }` only for short-lived scratch scope
5. Reset the kernel only when a clean state is actually needed

The prompt now also explicitly warns against wrapping an entire cell in
block scope when the goal is to reuse names across later cells.

## Why

The previous wording still left too much room for low-value workarounds
like whole-cell block wrapping. In downstream browser rollouts, that
pattern was adding tokens and preventing useful state reuse across
`js_repl` cells.

This change makes the preferred behavior more explicit without changing
runtime semantics.

## Scope

- Prompt/documentation change only
- No runtime behavior changes
- Updates the matching string-backed `project_doc` tests

codex-rs/core/src/project_doc.rs

@@ -62,7 +62,7 @@ fn render_js_repl_instructions(config: &Config) -> Option<String> {
     section.push_str("- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n");
     section.push_str("- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n");
     section.push_str("- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n");
-    section.push_str("- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, reuse the binding, pick a new name, wrap in `{ ... }` for block scope, or reset the kernel with `js_repl_reset`.\n");
+    section.push_str("- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, first reuse the existing binding, reassign a previously declared `let`, or pick a new descriptive name. Use `{ ... }` only for a short temporary block when you specifically need local scratch names; do not wrap an entire cell in block scope if you want those names reusable later. Reset the kernel with `js_repl_reset` only when you need a clean state.\n");
     section.push_str("- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n");
 
     if config.features.enabled(Feature::JsReplToolsOnly) {
@@ -490,7 +490,7 @@ mod tests {
         let res = get_user_instructions(&cfg, None, None)
             .await
             .expect("js_repl instructions expected");
-        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, reuse the binding, pick a new name, wrap in `{ ... }` for block scope, or reset the kernel with `js_repl_reset`.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
+        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, first reuse the existing binding, reassign a previously declared `let`, or pick a new descriptive name. Use `{ ... }` only for a short temporary block when you specifically need local scratch names; do not wrap an entire cell in block scope if you want those names reusable later. Reset the kernel with `js_repl_reset` only when you need a clean state.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
         assert_eq!(res, expected);
     }
 
@@ -509,7 +509,7 @@ mod tests {
         let res = get_user_instructions(&cfg, None, None)
             .await
             .expect("js_repl instructions expected");
-        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, reuse the binding, pick a new name, wrap in `{ ... }` for block scope, or reset the kernel with `js_repl_reset`.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Do not call tools directly; use `js_repl` + `codex.tool(...)` for all tool calls, including shell commands.\n- MCP tools (if any) can also be called by name via `codex.tool(...)`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
+        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, first reuse the existing binding, reassign a previously declared `let`, or pick a new descriptive name. Use `{ ... }` only for a short temporary block when you specifically need local scratch names; do not wrap an entire cell in block scope if you want those names reusable later. Reset the kernel with `js_repl_reset` only when you need a clean state.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Do not call tools directly; use `js_repl` + `codex.tool(...)` for all tool calls, including shell commands.\n- MCP tools (if any) can also be called by name via `codex.tool(...)`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
         assert_eq!(res, expected);
     }
 
@@ -528,7 +528,7 @@ mod tests {
         let res = get_user_instructions(&cfg, None, None)
             .await
             .expect("js_repl instructions expected");
-        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, reuse the binding, pick a new name, wrap in `{ ... }` for block scope, or reset the kernel with `js_repl_reset`.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
+        let expected = "## JavaScript REPL (Node)\n- Use `js_repl` for Node-backed JavaScript with top-level await in a persistent kernel.\n- `js_repl` is a freeform/custom tool. Direct `js_repl` calls must send raw JavaScript tool input (optionally with first-line `// codex-js-repl: timeout_ms=15000`). Do not wrap code in JSON (for example `{\"code\":\"...\"}`), quotes, or markdown code fences.\n- Helpers: `codex.tmpDir`, `codex.tool(name, args?)`, and `codex.emitImage(imageLike)`.\n- `codex.tool` executes a normal tool call and resolves to the raw tool output object. Use it for shell and non-shell tools alike. Nested tool outputs stay inside JavaScript unless you emit them explicitly.\n- `codex.emitImage(...)` adds one image to the outer `js_repl` function output each time you call it, so you can call it multiple times to emit multiple images. It accepts a data URL, a single `input_image` item, an object like `{ bytes, mimeType }`, or a raw tool response object with exactly one image and no text. It rejects mixed text-and-image content.\n- Example of sharing an in-memory Playwright screenshot: `await codex.emitImage({ bytes: await page.screenshot({ type: \"jpeg\", quality: 85 }), mimeType: \"image/jpeg\" })`.\n- Example of sharing a local image tool result: `await codex.emitImage(codex.tool(\"view_image\", { path: \"/absolute/path\" }))`.\n- When encoding an image to send with `codex.emitImage(...)` or `view_image`, prefer JPEG at about 85 quality when lossy compression is acceptable; use PNG when transparency or lossless detail matters. Smaller uploads are faster and less likely to hit size limits.\n- Top-level bindings persist across cells. If a cell throws, prior bindings remain available and bindings that finished initializing before the throw often remain usable in later cells. For code you plan to reuse across cells, prefer declaring or assigning it in direct top-level statements before operations that might throw. If you hit `SyntaxError: Identifier 'x' has already been declared`, first reuse the existing binding, reassign a previously declared `let`, or pick a new descriptive name. Use `{ ... }` only for a short temporary block when you specifically need local scratch names; do not wrap an entire cell in block scope if you want those names reusable later. Reset the kernel with `js_repl_reset` only when you need a clean state.\n- Top-level static import declarations (for example `import x from \"./file.js\"`) are currently unsupported in `js_repl`; use dynamic imports with `await import(\"pkg\")`, `await import(\"./file.js\")`, or `await import(\"/abs/path/file.mjs\")` instead. Imported local files must be ESM `.js`/`.mjs` files and run in the same REPL VM context. Bare package imports always resolve from REPL-global search roots (`CODEX_JS_REPL_NODE_MODULE_DIRS`, then cwd), not relative to the imported file location. Local files may statically import only other local relative/absolute/`file://` `.js`/`.mjs` files; package and builtin imports from local files must stay dynamic. `import.meta.resolve()` returns importable strings such as `file://...`, bare package names, and `node:...` specifiers. Local file modules reload between execs, while top-level bindings persist until `js_repl_reset`.\n- Avoid direct access to `process.stdout` / `process.stderr` / `process.stdin`; it can corrupt the JSON line protocol. Use `console.log`, `codex.tool(...)`, and `codex.emitImage(...)`.";
         assert_eq!(res, expected);
     }