OpenClaw Bug: Vertex Gemini 3.1 Pro Preview Returns Blank Replies in 2026.4.5

A fresh community report says OpenClaw 2026.4.5 can return a blank assistant message when using google-vertex/gemini-3.1-pro-preview, even though the run appears to complete normally.

The symptom is operationally nasty because it does not look like a loud provider failure:

• the request reaches Vertex AI
• stopReason is recorded as stop
• no obvious fallback is triggered
• but the saved assistant text is empty and output tokens stay at 0

If you run OpenClaw in production-like workflows, this is the kind of bug that wastes time because teams often misdiagnose it as GCP auth, quota, or regional Vertex setup, while the same model can still work in Vertex Playground.

• Source issue: openclaw/openclaw#62027

TL;DR

If google-vertex/gemini-3.1-pro-preview in OpenClaw 2026.4.5 keeps finishing with a blank assistant reply:

1. Confirm the model is specifically google-vertex/gemini-3.1-pro-preview.
2. Check whether the run ends with stopReason=stop, empty assistant text, and output tokens = 0.
3. Cross-check with google-vertex/gemini-3-flash-preview or Vertex Playground in the same environment.
4. Treat this as a likely OpenClaw runtime/provider handling bug before spending hours on IAM or quota debugging.
5. Use a lower-risk workaround until upstream clarifies the fix.

What the bug looks like in practice

According to the issue report, the affected path looks like this:

• provider: google-vertex
• model: gemini-3.1-pro-preview
• OpenClaw version: 2026.4.5
• result: request reaches Vertex AI, but the assistant reply is an empty string
• recorded outcome: stopReason: stop, output tokens: 0

That combination matters.

A hard provider failure is usually easier to debug because you get a visible error. Here, the run can look successful-but-empty, which is worse for operators because it can silently poison automations, summaries, or chat flows downstream.

How to verify you are hitting this exact issue

Use this order so you do not chase the wrong root cause.

1) Confirm it is model-specific, not a total Vertex outage

The issue report says two important cross-checks are true in the same general environment:

• Vertex Playground can use Gemini 3.1 Pro
• google-vertex/gemini-3-flash-preview can work in the same OpenClaw install

If your environment shows the same split, the probability goes up that this is not a broad Vertex authentication or networking problem.

2) Inspect the OpenClaw session result

Look for a run where all of the following are true:

• provider is google-vertex
• model is gemini-3.1-pro-preview
• assistant text is ""
• stopReason is stop
• output tokens are 0

That pattern matches the community report much better than a generic “model unavailable” or “permission denied” failure.

3) Re-test with a nearby working model

A practical sanity check is to swap only the model and keep the rest of the chain unchanged:

• same OpenClaw install
• same Vertex auth
• same region
• same prompt
• test google-vertex/gemini-3-flash-preview

If Flash returns normal text while 3.1 Pro Preview stays blank, you have a much stronger case for treating this as a model-specific runtime handling problem inside OpenClaw.

Why this matters beyond one bad reply

A blank reply bug is more damaging than it first appears.

Common downstream effects include:

• chat sessions that appear to “hang” or “say nothing” without a visible error
• automations that interpret an empty model response as a valid answer
• operators wasting time rotating keys, checking quotas, or changing regions
• false confidence because the run technically ends with stop

In other words: this is not just a quality issue. It can become an observability and incident response problem.

Lowest-risk workarounds for now

Choose the least invasive option that keeps your workflow moving.

Workaround A: Switch the affected workflow to a nearby working Vertex model

If your use case does not strictly require Gemini 3.1 Pro Preview today, the simplest unblock is to route that workflow to a model that is already known to produce normal text in your install, such as:

• google-vertex/gemini-3-flash-preview

This is the safest workaround because it avoids patching your gateway or changing authentication.

Workaround B: Reproduce once in Vertex Playground, then stop blaming IAM

When operations teams hit blank outputs, they often burn time on the wrong layer.

If the same prompt works in Vertex Playground while OpenClaw stores an empty reply, that is useful evidence to document internally. It tells you to stop over-investing in:

• credential rotation
• quota speculation
• regional guesswork
• unrelated firewall debugging

Workaround C: Avoid using this model in unattended production flows until upstream clarifies behavior

If you rely on scheduled jobs, inbox triage, or long-running chat automations, a “successful blank response” is risky because it may not trigger the same alerting path as a hard failure.

Until the upstream issue is clarified, it is reasonable to avoid this exact model in:

• cron-driven jobs
• customer-facing chat surfaces
• summary pipelines
• any workflow where empty output can be mistaken for a valid completion

What upstream likely needs to clarify

From the current report alone, the failure looks less like “Vertex never returned anything” and more like “OpenClaw finished the run without surfacing usable text”.

That means maintainers will likely need to inspect one or more of these layers:

• how the google-vertex provider parses Gemini 3.1 Pro Preview responses
• whether response text is arriving in a different structure than older models
• whether token accounting / candidate extraction differs for this preview model
• whether a no-text candidate is being treated as a successful final answer

What to watch before calling it fixed

When an upstream fix lands, do not just look for the absence of crashes. Re-test for the actual broken symptom:

• assistant text is non-empty
• output tokens are greater than 0
• the response appears in normal OpenClaw chat history
• repeated prompts behave consistently, not intermittently blank

FAQ for search and empty-output triage

If stopReason=stop, why can’t operators treat the run as a success?

Because the real question is not whether the pipeline ended, but whether it produced usable assistant text.

If the final record shows all three of these together:

• stopReason=stop
• empty assistant text
• output tokens = 0

then the safer interpretation is successful termination without a usable answer, not a healthy completion.

When should teams compare models first instead of continuing to debug IAM, quota, or region setup?

If Vertex Playground works in the same environment, and google-vertex/gemini-3-flash-preview also works in the same OpenClaw install, while only gemini-3.1-pro-preview returns blanks, then model comparison deserves priority over infrastructure debugging.

That split is already strong evidence that the failure surface is not the broad Vertex foundation layer.

If someone searches for gemini 3.1 pro preview empty reply, what are the first three branches this page should help them split?

Help them separate these first:

1. whether all Vertex models fail or only 3.1 Pro Preview fails
2. whether the provider actually errors or the run ends with stop and no text
3. whether Vertex Playground still returns text in the same environment

Those three checks are the fastest path from “generic Vertex outage suspicion” to “OpenClaw handling issue for a specific preview model”.

Exact searches this Vertex Gemini empty-reply page should answer next

Use this section when a reader arrives from a narrow Gemini 3.1 Pro Preview or Vertex AI query and needs to separate a blank assistant turn from normal model latency, quota, or prompt problems.

• Google Vertex Gemini 3.1 Pro Preview empty assistant reply: capture the raw provider response before retrying so the team can see whether candidates, parts, or finish reasons are missing.
• Vertex Gemini returns empty response in OpenClaw 2026.4: compare the same prompt against another Gemini model to separate model-preview behavior from OpenClaw adapter handling.
• fix Gemini 3.1 Pro Preview blank response OpenClaw: pin a known-good model or route affected tasks away from the preview until the adapter preserves enough diagnostics.

Turn empty-reply traffic into evidence-based triage

If you arrived because Google Vertex Gemini 3.1 Pro Preview returns an empty assistant reply, do not just retry the same prompt. Split the issue into three evidence layers: whether the model API returned candidates, whether the OpenClaw runtime dropped content, and whether the channel adapter treated an empty message as a successful delivery.

The minimal triage path is one raw response, one runtime trace, one final channel payload, plus the model name, region, request time, and safety or finish reason. Classify it as an upstream model issue only when all three layers show empty content. Otherwise, fix parsing, fallback routing, or empty-reply guardrails first.

Empty-reply triage before switching models

If you arrived here by searching “Gemini 3.1 Pro empty assistant reply” or “Vertex AI returns blank response in OpenClaw”, treat the first pass as a response-shape investigation, not an immediate model replacement. Use this order:

1. capture the raw Vertex response body, status code, and finish reason before OpenClaw normalizes it;
2. compare whether the same prompt returns text through the Vertex console, direct API call, and OpenClaw runtime;
3. check whether safety ratings, candidate filtering, tool-call output, or streamed chunks are being dropped by the adapter;
4. retry once with a smaller prompt and no tools to separate context pressure from provider behavior;
5. only move traffic to a fallback model after you know whether the blank answer is provider-side, adapter-side, or prompt-shape-specific.

This helps operators preserve high-intent Gemini traffic: the page gives them a safe diagnostic path before they rotate credentials, downgrade models, or hide the underlying adapter bug.

Related reading

• OpenClaw Bug: Google usageMetadata is not mapped into usage.input/output on Gemini Code Assist and CLI flows
• OpenClaw Bug: ACPX 0.4.x may fail to create sessions silently even though the MCP stream comes up
• OpenClaw Bug: ACP Session transcripts may exist on disk while session history APIs still return empty

Search-entry triage: what to check when Gemini returns an empty assistant reply

If you landed here from searches like “Gemini empty assistant reply”, “Vertex Gemini 3.1 Pro empty response”, or “OpenClaw Google provider no content”, split the issue into four layers:

1. Check whether the model returned empty content or the provider mapping produced empty content: inspect the raw Vertex response for candidates, finish reason, safety fields, and tool calls.
2. Remember that usageMetadata is not readable text: token accounting proves the request was processed, not that an assistant message was assembled correctly.
3. Check safety, truncation, and multimodal parts: empty text can come from a safety block, a candidate parts shape change, or a response that only includes non-text parts.
4. Preserve a raw response sample: provider, model version, request id, and candidate parts are more useful than only saying “the reply was empty”.

Do not debug this class of bug only at the UI fallback layer. The shortest path is to compare the raw Vertex response, the OpenClaw provider mapping, and the final assistant message.

Related reading

• Troubleshooting OpenClaw Agents: What to Check When Tasks Stall or Tools Misbehave
• OpenClaw Update Guide (2026-02-02): What to Check Before and After You Upgrade
• What to Check When Google Provider usageMetadata Is Not Mapped to usage input/output
• How to Build Self-Healing Infrastructure with OpenClaw

Production alert rules for blank Gemini replies

If this page is part of an incident runbook, turn the symptom into an alert instead of relying on humans to notice an empty chat bubble. A useful first rule is: model equals google-vertex/gemini-3.1-pro-preview, finish reason equals stop, assistant text length is zero, and output tokens are zero.

That rule is intentionally narrow. It avoids paging people for ordinary provider errors, while still catching the dangerous case where OpenClaw records the run as completed but downstream automation receives no usable answer.

For higher-value workflows, add two more labels to the event before you route it:

• whether another Vertex model returned text in the same window
• whether the raw Vertex response contained text parts, non-text parts, or no candidate text at all

Those labels make later debugging faster because they separate a provider outage, a model-specific response-shape change, and an OpenClaw message assembly bug.

On-call decision tree for blank Gemini replies

If this incident appears during an on-call window, do not treat the blank assistant message as an ordinary slow-model timeout. Split it this way first:

1. The HTTP request succeeded, but assistant content is empty: preserve the raw provider response before retrying, then compare it with the assistant message that OpenClaw saved.
2. Only Vertex Gemini 3.1 Pro Preview is affected: treat it as a model/provider compatibility boundary before escalating it as a full chat-system outage.
3. Switching to another Gemini, OpenAI, or Anthropic model restores output: route production traffic away from the affected preview model first, then keep the failed sample for provider-mapping debugging.

What to re-test before declaring the issue closed

A single non-empty reply is not enough evidence for closure. Before removing the workaround, verify these four checks:

1. similar prompts repeatedly produce non-empty assistant text, not intermittent blanks;
2. the raw provider response, OpenClaw normalized result, and final chat history all agree;
3. the workaround or compatibility patch does not break usage accounting, tool calls, or multi-turn context;
4. the incident record keeps the blank-response sample, affected model version, temporary route, and rollback condition.

Source

• Community bug report: openclaw/openclaw#62027