"Unavailable tool" error message lists every available tool, burning thousands of tokens on repeated occurrences

Description

Related upstream trigger: #33618 (Qwen via OpenRouter sporadically emits phantom/invalid tool calls). This issue is about the cost of the error response, which is independent of that bug.

When a model emits a malformed/invalid tool call (e.g. a phantom unknown tool call, which happens occasionally with some models during parallel tool batching), the harness responds with an error message that enumerates the complete list of available tools:

The arguments provided to the tool are invalid: Model tried to call unavailable
tool 'unknown'. Available tools: <full list of 100+ tool names…>

In a session with many MCP servers configured (~130 tools in my setup), each of these error messages injects several thousand tokens into the context. The underlying model behavior can repeat dozens of times in a single session (observed with openrouter/qwen/qwen3.8-max), so this adds up to a very large amount of wasted tokens and cost — far more than the invalid call itself.

A secondary effect: when the invalid call is part of a parallel batch, sibling tool calls in the same batch sometimes get aborted ("Tool execution aborted"), forcing the agent to re-issue them — more tokens and slower progress.

Expected behavior

The error should be minimal and actionable, e.g.:

Tool 'unknown' is not available. See the system prompt for the list of available tools.

The model already has the tool list in its system prompt; repeating it in every error message provides no additional value and is very expensive.

Suggested fix

• Truncate/omit the Available tools: … enumeration from the error message (or cap it to a short prefix / first N names).
• Optionally, deduplicate: if the same invalid-tool error already occurred in the session, return an even shorter message on repeats.

Environment

• opencode CLI (latest at time of writing)
• Model: openrouter/qwen/qwen3.8-max (observed repeatedly), likely model-agnostic
• Setup: ~130 tools configured via MCP servers (supabase, atlascloud, cloudflare, agentmemory, etc.)