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PreToolUse / PostToolUse Hooks — Deterministic Gates

Domain 3 · 20% ~12 min Hinglish narration

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Scenario anchor

Aap Aaranya IT BFSI ke ek large-scale Claude Code deployment ke architect hain — 500+ developers, production codebase, aur compliance team ka daily pressure. Ek developer ne accidentally Claude se ek destructive SQL DROP command generate karwaya aur directly execute ho gaya. Ab aapko ek deterministic gate chahiye jo har tool call ko — chahe Claude kitna bhi confident ho — intercept kare, validate kare, aur policy-violating calls ko block kare before execution. PreToolUse hooks exactly yahi karte hain: aapka API gateway interceptor, lekin Claude ke tool invocation pipeline ke andar.

Key Takeaways

PreToolUse hooks fire synchronously before any tool executes, giving you a deterministic veto point — exit code 0 passes the call through, non-zero blocks it and surfaces a structured error back into Claude's context.
Hook logic runs outside the model's probabilistic reasoning — it is imperative, auditable Python/shell code, making it the correct layer for compliance rules, secret-scanning, destructive-command detection, and rate-limit enforcement at enterprise scale.
PostToolUse hooks receive the tool's output and are suited for logging, output sanitisation, and downstream telemetry — never treat them as a substitute for PreToolUse-based prevention.
Memory anchor: PreToolUse hook = API gateway request interceptor — same interception point, same exit-code contract, same separation of policy from business logic.

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