Definitions
Tool Output Truncation: When an Agent Reasons Over a Result It Never Fully Saw
Tool output truncation is the failure mode where a tool call succeeds but the result handed back to the agent has already been cut down to a size cap, with no signal in the payload that anything is missing. Grounded in published framework defaults (Codex's 10 KiB/256-line cap, Claude Code's 25,000-token default) and a 2026 audit documenting 91 silent failures in a real tool-calling pipeline, this entry covers the mechanism, why it fails silently rather than loudly, and the structural fix: completeness flags, not longer logs.