ROADMAP-04 — P0 — Calculator emits inf/nan into the prompt; exponent guard uses bare literals #96

Series: ROADMAP — filed from the 2026-10-27 whole-project assessment · Backlog task: M14-T04 · Priority: P0 · Effort: extra small

Problem

Two related nits in the otherwise well-bounded AST calculator:

1. Float results are never checked with math.isfinite, so overflow paths like 1e308 * 10**3 produce inf/nan strings that flow into the prompt (and into history).
2. The exponent-blast-radius guard at line 59 hardcodes bare literals 1000 / 100 instead of referencing MAX_EXPONENT, so a future change to the constant desynchronizes it.

Evidence

• agentkit/tools/builtin/calculator.py:59 — abs(left) > 1000 and right > 100 bare constants
• agentkit/tools/builtin/calculator.py:14, 66–74 — existing MAX_RESULT_BITS / integer blast-radius guards to model the fix on

Suggested fix / acceptance criteria

• Non-finite float results (and intermediate values) produce a structured ToolResult(ok=False, error=...), never inf/nan text
• Exponent guard constants reference MAX_EXPONENT (or a derived bound); behaviour unchanged at current values
• Unit tests for 1e308 * 10**3, 0 ** 0-edge and deep-division paths

Suggested branch / commit

fix/calculator-finite-guard · fix(tools): reject non-finite calculator results and tie exponent guard to MAX_EXPONENT