feat: add allow_tmpdir permission for scoped tmpdir access

[elithrar]

This PR adds an allow_tmpdir permission that allows file operations in the system's temporary directory without requiring external_directory approval prompts.

The permission.allow_tmpdir: boolean (defaults to false) and is in both global/merged config and per-agent config/schemas.

specifically:

• external_directory defaults to "ask", prompting for permission on any file operation outside the current working directory
• In CI/CD and CLI environments, this prompt causes indefinite hangs (see Opencode Hangs when used as CLI tool #5888)
• The existing workaround (external_directory: "allow") permits access to ALL external directories, which is overly broad
• Requiring users to create a custom agent just to allow tmpdir access is high friction

security:

• Symlink resolution prevents traversal attacks (critical on macOS where /tmp → /private/tmp)
• Default false maintains current security model (opt-in)
• Only affects tmpdir; other external directories still follow external_directory settings

tests:

• Filesystem.tmpdir(), isInTmpdir(), containsResolved() utilities tested for symlink resolution and non-existent paths
• Bash tool tests verify allow_tmpdir: true permits tmpdir workdir while external_directory: deny is set
• Bash tool tests verify allow_tmpdir: true does not bypass external_directory for non-tmpdir paths

usage example:

{
  "$schema": "https://opencode.ai/config.json",
  "permission": {
    "allow_tmpdir": true,
    "external_directory": "ask"
  }
}

Or per-agent:

{
  "agent": {
    "build": {
      "permission": {
        "allow_tmpdir": true
      }
    }
  }
}

related:

• [FEATURE]: allow /tmp or $TMPDIR folder access option #4743 - Feature request for tmpdir access
• Opencode Hangs when used as CLI tool #5888 - CLI mode hangs due to external_directory prompts
• Split external_directory permission into read vs write #5395 - Split external_directory into read vs write

[rekram1-node]

@elithrar do you think is is preferable to something like:

"external_directory": {
  "/tmp*": "allow",
  "*": "deny"
  }

@thdxr has been working on a revamp of our permissions and part of it was making every permission (that makes sense) "glob-able" like this

Other examples for other permissions:

"read": {
  "*.md": "allow",
  "*.env": "deny"
}

[elithrar]

TBH I actually like both + tried to leave room for custom permission objects.

thoughts:

• tmpdir is widely used and isn’t always /tmp/ depending on your OS (yes, users can use {env:var} but extra cognitive burden
• tmpdir is “safe” - trust opencode to use it as a sandbox (and not follow symlinks, etc)
• glob-style patterns can be additive - use tmpdir as the sandbox but read-only other dirs or file patterns - with more risk if you get it slightly off / too permissive
• I otherwise like the { glob, ask | allow | deny } API proposed and feels easy to tackle.

As a user: I want opencode to be able to work within the cwd and use tmpdir as a sandbox.

I do not want it to accidentally (through config) end up having wider access to the rest of my machine. Especially in an enterprise env, but also as a regular privacy conscious user.

[rekram1-node]

hmm okay good point about the tmpdir differences across different os, I will send this to Dax since he has been thinking about these things lately

[thdxr]

let me merge my permissions branch (it's close) and i'll look at this