How to fix Agent executor error and Message truncation failure in Antigravity?

You open Antigravity (v1.20.5), send the first prompt, and instantly hit: “agent executor error: could not convert a single message before hitting truncation.” The agent won’t run any tools, and Gemini fails even at the start of a new session. This points to a context window overflow at turn 1, usually caused by a bloated system prompt, enabled tools returning long outputs by default, or persistent memory dumping too much text into the first message. Some users report the same error after the latest update, while older versions work fine.

How to fix Agent executor error and Message truncation failure in Antigravity?

The error “agent executor error: could not convert a single message before hitting truncation” means the agent tried to build the first request but couldn’t even fit a single message (system + tool manifests + memory + your user prompt) into the model’s context limit. Gemini is particularly strict here; Claude may tolerate more overhead, which is why it appears “okay” for similar tasks.

If your runs also stop entirely with a different error, see this targeted Antigravity guide: agent terminated errors in Antigravity.

Solution Overview

Aspect	Detail
Root Cause	Context window overflow at the very first turn, amplified by v1.20.5 changes (tool metadata, memory, or system instructions)
Primary Fix	Roll back Antigravity to v1.19.6 and disable auto-updates; then trim tool outputs and system/memory configs
Complexity	Easy
Estimated Time	10–20 minutes

How to fix Agent executor error and Message truncation failure in Antigravity?

Step-by-Step Solution

1. Roll back to Antigravity v1.19.6

• The forum consensus fix is to revert from v1.20.5 to v1.19.6. Uninstall v1.20.5, then install v1.19.6 from the official releases for your OS.
• After installing, open Antigravity and confirm the version (Help/About or Settings/About).

2. Turn off auto-updates

• In Antigravity: Settings > Updates > set Auto-update to Off.
• This prevents v1.20.5 from re-installing while you work.

3. Trim the first-turn context to avoid truncation

• Disable high-noise tools for initial runs. Keep only the tool you need for your first prompt; add others gradually.
• Shorten the system prompt. Keep it focused and avoid long operational manifest text.
• Minimize persistent memory. If the app maintains MEMORY.md or similar, clear it or reduce it to a brief summary.
• Limit default tool outputs. For tools that list directories, read files, or search logs, cap lines/bytes and filter aggressively.

Example principles you can apply when configuring tools:

• Return only the last N lines of a file instead of the whole file.
• For directory listings, return names only, not detailed metadata.
• For searches, return the top 5 matches, not full file dumps.

4. Start a fresh session after changes

• Create a new workspace/session so the agent doesn’t pull old memory or cached tool outputs into turn 1.

5. Pick a model with headroom (temporary)

• If Gemini still fails at turn 1, try Claude for this project while you keep Gemini for smaller prompts.
• For Gemini token budgeting guidance, review Google’s token guidance: Gemini tokens and context.

6. Update again only when a patch is confirmed

• When a fixed release beyond v1.20.5 is announced, upgrade and re-enable auto-updates.

Read More: how to approach 500 errors in AI backends

Alternative Fixes & Workarounds

• Split the task: Kick off with a minimal prompt (“Identify files to inspect”) and only then enable tools for targeted files.
• Pre-summarize large artifacts: Paste a short summary instead of full logs or markdown files.
• Keep Gemini on a smaller “Flash” variant for simple steps; switch back to Pro for critical reasoning with trimmed inputs.
• If your toolchain emits verbose “thought” or “reasoning” traces into messages, disable or shorten them.
• Clean caches/indexes if your workspace includes generated indexes that the agent tries to inject.

If config problems compound this issue in other tooling, see this quick reference on config errors: handling a failed read config issue.

Troubleshooting Tips

• Confirm it’s a first-turn overflow: if the error fires on the first user message, the system + tools + memory are already too large.
• Inspect which component is biggest: long system instructions, tool manifests, or memory. Remove anything not required.
• Test with an empty workspace: no MEMORY.md, no prior runs, minimal prompt.
• Try another model: if Claude runs with the same setup, the issue is token headroom in Gemini.
• Watch for tools that expand aggressively (full file dumps, recursive directory trees, or verbose search results). Cap them.

Best Practices

• Keep system instructions lean. Aim for short, specific rules, not policy essays.
• Add tools incrementally. Enable only what the current step needs.
• Cap tool outputs by lines/bytes and prefer summaries over raw dumps.
• Tame memory. Store short, rolling summaries instead of full transcripts or documents.
• Pin known-good versions for active projects, and upgrade in a test workspace first.

Final Thought

The error is almost always a first-turn context overflow. Rolling back to v1.19.6, disabling auto-updates, and tightening your system/tools/memory immediately restore stability. Keep prompts short, limit tool outputs, and upgrade only when a patched release is available. If a separate “agent terminated” message appears, use this Antigravity-specific help: Antigravity termination troubleshooting.