Dead loop: infinite empty assistant messages (tokens=0) after tool call with empty text #28507

Description

After an LLM returns a tool call with empty/no text content, the agent loop enters an infinite cycle creating empty assistant messages (tokens: {input:0, output:0}) without ever invoking the LLM. The only recovery path is manual compaction.

Reproduction

1. Start an interactive session with a custom provider (in our case, glm-5 via llm-router)
2. LLM returns a tool call (e.g., Read a PNG file) with empty text in the response
3. Tool executes successfully, result is sent back to the LLM
4. On the next loop iteration, the LLM returns an empty/errored response (tokens: {input:0, output:0})
5. Loop continues indefinitely creating empty assistant messages

Session evidence: ses_1bba6f1c4ffe55rFTzStkU2fyM — 162 total messages, two dead zones:

• msg[143-149]: 7 empty assistant messages, all tokens=0
• msg[155-161]: 7 empty assistant messages, all tokens=0
• Recovery at msg[150] and msg[162] via compaction

Root Cause Analysis

The exit condition (prompt.ts L1268-1276)

The while(true) loop at prompt.ts:1248 breaks only when ALL four conditions are true:

if (
  lastAssistant?.finish &&                              // (1) finish is truthy
  !["tool-calls"].includes(lastAssistant.finish) &&     // (2) not tool-calls
  !hasToolCalls &&                                      // (3) no pending tools
  lastUser.id < lastAssistant.id                        // (4) ID ordering
) {
  break
}

When the LLM returns an empty response after a tool result, finish may be "unknown" or unset (condition 1 fails), and the loop continues.

Why safety nets don't catch it

1. DOOM_LOOP_THRESHOLD (processor.ts:424) — Only detects 3 consecutive identical tool calls (same tool name + same input). Does NOT catch empty responses or different tools.

2. isOverflow() (overflow.ts:20-31) — When tokens = {input:0, output:0}, count = 0, so isOverflow() returns false. The compaction safety net never triggers.

3. No max-empty-steps guard — There is no mechanism to detect "N consecutive steps with zero tokens" and force-exit the loop.

Recovery

Only compaction recovers the session — it regenerates context and creates an assistant message with finish: "stop", which passes the exit check.

Suggested Fix

Add a guard in the runLoop to detect consecutive empty responses:

// In the while(true) loop, after processor returns:
if (handle.message.tokens.input === 0 && handle.message.tokens.output === 0) {
  emptyStepCount++
  if (emptyStepCount >= 3) {
    yield* slog.warn("exiting loop: consecutive empty responses")
    break
  }
} else {
  emptyStepCount = 0
}

Alternatively, isOverflow() could treat consecutive zero-token responses as a degenerate case that should trigger compaction or loop exit.

Environment

• opencode: v1.15.5 (installed via Homebrew)
• Provider: custom (glm-5 via llm-router, OpenAI-compatible API)
• OS: macOS (darwin)

Related Issues

• Web: client clock skew can cause repeated assistant responses / prompt loop #28339 — Clock skew causes ID ordering failure in the same exit condition
• Streaming response validation fails when provider returns empty delta.role instead of "assistant" #28427 — Provider returns empty delta.role, stream error prevents finish from being set
• Loop Compacted opencode --version 1.15.5 #28426 — Compaction loop issue