Gemini managed agents: the old harness dies October 5

If you built anything on Google's Gemini managed agents this summer, you have eleven days. Google's Gemini API release notes record that on September 17 it released antigravity-preview-09-2026, which "replaces and deprecates" antigravity-preview-05-2026. The deprecations page puts the shutdown of the old harness at October 5, 2026. The new harness is not a drop-in: the built-in tools changed shape.

What actually happened

A managed agent on the Gemini API is not just a model. It is a harness — the loop, the tool definitions, the sandbox, the compaction behavior. Google's documentation for the Antigravity agent describes it as a general-purpose managed agent with reasoning, code execution, file management and web browsing inside a Linux sandbox, with automatic context compaction at roughly 135k tokens and a max_total_tokens budget control where cached tokens do not count toward the limit.

The September release notes describe the change as architectural: parameter naming conventions and the file-editing approach in the built-in tools both moved. That matters more than a model swap, because your prompts, your evals and any code that parses tool calls were written against the old names. Google also shipped a Files API for moving data in and out of the agent sandbox and a Credentials API that lets you register a secret once and attach it to an MCP server or a sandbox environment variable so the model never sees the value — we covered the Credentials API separately.

One thing to be careful with: several roundups are circulating precise efficiency figures for the new harness — fewer output tokens on file edits, higher cache hit rates, better multi-turn completion. Google's own release notes and docs do not state those numbers, so we are not repeating them. The dated, verifiable facts are the release, the replacement and the October 5 shutdown.

Why an agent harness deprecation matters for your business

Your agent has a dependency you probably did not write down. Model IDs get pinned because everyone learned that lesson in 2024. Harness versions are newer and quieter, and they carry the tool contract. A harness EOL is a breaking change with a date on it, not a performance note.

Silent redirects are worse than errors. After a shutdown, requests to a retired preview typically get routed to the successor. An error you can alert on. A redirect means your agent keeps running and starts behaving differently — different tool parameter names, different file-edit semantics — with no failure to catch. That is the outage nobody pages for.

This is what preview means. Five months from release to shutdown is the deal you accepted. Build on preview endpoints where the upside is real, but treat every one as a thing that will move under you, and keep the integration behind one adapter so the migration is one file, not forty.

Do this week, in order: grep your codebase and config for antigravity-preview-05-2026, swap it to 09-2026 in a branch, run your eval set against the new harness before October 5, and diff the tool-call shapes your code depends on. If you do not have an eval set for the agent, that is the actual finding, and it is a bigger problem than this deprecation.

Sources: Gemini API release notes, Gemini API deprecations, Antigravity agent docs.