[BUG] Claude Managed Agent internal proxy throwing 503 when interacting with Git

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The sandbox git push proxy (local_proxy@127.0.0.1:35159) returns HTTP 503 on every push attempt — regardless of branch, pack size, or git config. The proxy is reachable (fetches work fine through it) and auth is valid (the same session's GitHub MCP tools authenticate against the same repo without issue). The 503 persisted across 10+ retries over 8 minutes with exponential backoff, making it impossible to push code via bash git as the agent's system prompt instructs.

Session: sesn_01Ri5Wt7p8gorMWFhefx3W3V
Environment: env_01KnQ5593XVM7QZphk5LiQMB
Agent: agent_01E4iVPYT4Eq5rArhc2ecbJ4
Proxy origin: http://local_proxy@127.0.0.1:35159/git/{user}/{repo}

Date: June 9 2026

What Should Happen?

git push origin HEAD:main should succeed when git fetch origin main works and the credentials are valid. The agent should be able to commit and push to main via bash git as instructed by its system prompt, without needing to fall back to the GitHub MCP push_files API.

Error Messages/Logs

error: RPC failed; HTTP 503 curl 22 The requested URL returned error: 503
send-pack: unexpected disconnect while reading sideband packet
fatal: the remote end hung up unexpectedly
Everything up-to-date

Steps to Reproduce

1. Start a Managed Agents session with a GitHub repository resource attached
2. Make changes and commit locally
3. Attempt to push:

git push origin HEAD:main
→ Returns HTTP 503

1. Confirm fetch works (proving proxy is reachable and auth is valid):

git fetch origin main
→ Succeeds

1. Rule out branch protection — push a single-line file to a new branch:

git checkout -b tmp-probe origin/main
echo "probe" > probe.txt && git add probe.txt && git commit -m "probe"
git push origin tmp-probe:tmp-probe
→ Same 503

1. Rule out pack size — the test branch pack was 66KB, full pack ~400KB. Both fail.

1. Rule out git config:

git config http.postBuffer 1048576000
git config http.version HTTP/1.1
git push origin HEAD:main
→ Still 503

1. Retry with backoff — 10+ attempts over 8 minutes, all 503.

Workaround: files were pushed via the GitHub MCP push_files tool (Contents API),
which bypasses the proxy. This worked but required 13 sequential commits instead
of one atomic push.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

managed-agents-2026-04-01

Platform

Anthropic API

Operating System

Other

Terminal/Shell

Other

Additional Information

_No response_