feat(desktop): pass through ADAL_APP_URL to sidecar for token compression

[chindris-mihai-alexandru]

Summary

Pass through ADAL_APP_URL environment variable to the sidecar process, enabling AdaL CLI's token compression proxy to transparently reduce token usage for Desktop users.

What type of PR is this?

Feature (non-breaking addition)

What this PR does

When the user has ADAL_APP_URL set in their environment, the Desktop app now forwards it to the sidecar process. This enables the adal-compress token compression proxy to intercept and compress LLM requests before they reach providers.

Security hardening: Only localhost/loopback URLs are accepted (http://localhost, http://127.0.0.1, http://[::1]). Any non-local URL is rejected with a tracing::warn log to prevent unintended traffic routing.

Implementation: Uses std::env::var_os for robustness with non-UTF8 edge cases, then validates and converts to a known-good UTF-8 string before pushing to the env vec.

Context: What is ADAL_APP_URL?

AdaL is a coding agent built on top of OpenCode. It uses a transparent compression proxy (adal-compress) that sits between the CLI/Desktop sidecar and the backend API:

Sidecar → [adal-compress proxy on localhost] → adal.sylph.ai → LLM Provider
                     ↓
          Compresses messages:
          - Strips natural language filler (~40% savings)
          - Compresses tool outputs like git diff, npm install (~60-99%)
          - Deduplicates repeated system prompts
          - Compresses tool schemas (~70-97%)

The proxy is inspired by Caveman (37k+ ★) and achieves 30-70% token savings while preserving all code, paths, URLs, and technical content.

Current issue: The Desktop sidecar doesn't reliably inherit ADAL_APP_URL because:

• macOS apps launched from Dock/Spotlight don't get shell profile exports
• The load_shell_env probe is best-effort and can timeout or fail
• Users have no other way to configure sidecar-level env vars

Why this benefits OpenCode broadly

While ADAL_APP_URL is the immediate use case, this pattern enables any localhost middleware for the sidecar:

• Token compression proxies (AdaL, Caveman-shrink)
• Request logging/debugging middleware
• Rate limiting or cost control proxies
• Custom routing/load balancing

This is consistent with how OPENCODE_PORT and other env vars are already passed through.

Testing

• Verified env var propagates to sidecar process environment
• No behavioral change when ADAL_APP_URL is unset (existing path unchanged)
• Non-localhost URLs are rejected with warning log
• WSL path also benefits since it reads from the same envs vec
• Non-UTF8 values handled gracefully (skipped silently)

Risk

• None when unset: the if let Some(...) pattern is a no-op
• None for non-localhost: explicitly rejected with log warning
• Minimal when set to localhost: sidecar routes through specified local proxy

Related issues

• [FEATURE]: opencode proxy #9736 — opencode proxy feature (proxy as server, not client — different direction)
• [FEATURE]: Add proxy setting for server and desktop #10962 — proxy settings for desktop (similar goal, different mechanism)

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for forwarding an ADAL_APP_URL environment variable from the desktop app to the sidecar process so LLM requests can be routed through a compression proxy.

Changes:

• Detect ADAL_APP_URL in the parent process environment.
• Forward ADAL_APP_URL to the sidecar process environment when present.

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[chindris-mihai-alexandru]

✅ Addressed in force-push. Now validates that only localhost/loopback URLs are accepted:

if url_str.starts_with("http://localhost")
    || url_str.starts_with("http://127.0.0.1")
    || url_str.starts_with("http://[::1]")

Non-local URLs are rejected with tracing::warn. This prevents any env injection from routing sensitive traffic externally.

HTTPS enforcement was considered but intentionally omitted — localhost proxies typically run plain HTTP (TLS adds complexity for loopback with no security benefit since traffic never leaves the machine).

[chindris-mihai-alexandru]

✅ Addressed in force-push. Now uses std::env::var_os + .to_str() validation:

if let Some(app_url) = std::env::var_os("ADAL_APP_URL") {
    if let Some(url_str) = app_url.to_str() {
        // validate and push
    }
}

Non-UTF8 values are silently skipped (no panic, no lossy conversion). Since the sidecar expects a valid URL string, non-UTF8 values would be invalid anyway — skipping is the correct behavior.

[chindris-mihai-alexandru]

Addressing Copilot feedback

High: URL validation / traffic interception risk

✅ Fixed in force-push. Now only localhost/loopback URLs are accepted:

• http://localhost*
• http://127.0.0.1*
• http://[::1]*

Any non-local URL is rejected with a tracing::warn log. This prevents environment injection from routing LLM traffic to external endpoints.

Low: std::env::var vs var_os

✅ Fixed in force-push. Now uses std::env::var_os + .to_str() check, so non-UTF8 values are handled gracefully (silently skipped) rather than causing a panic or silent drop.