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90. Runtime-neutral sandbox tool hooks contract ​

Date: 2026-08-18

Status ​

Accepted

Context ​

fullsend run drives agents through a pluggable runtime.Runtime (runtimes.md), but the sandbox tool hooks — Tirith command scanning, SSRF and canary checks, secret redaction, unicode normalization, context suppression, tool allowlist — were only reachable through a Claude-specific ClaudeHooksBootstrap type assertion, and their wiring lived solely in the generated Claude Code settings.json. A second runtime (OpenCode, #1260) would therefore ship with no sandbox tool hooks, and every further runtime (Cursor CLI #6319, pi, …) would face the same choice: re-implement the hooks or drop them. Related runner code also branched on rt.Name() == "claude" and hardcoded Claude artifact paths.

The credential-isolation architecture (ADR 0017, ADR 0025) makes the OpenShell sandbox, its L7 egress policy, and credential placeholders the primary security boundary; the hooks are defense-in-depth. The recommendation on #1260 was that a runtime may lack them only if that absence is explicit. Longer-term hook layers (ACP proxy chains #587, tool proxies #5243) remain open.

Decision ​

The sandbox tool hooks are a runtime-neutral contract, not a Claude Code feature:

  • The hook scripts (internal/security/hooks/*.py) are the portable artifact. security.HookFiles returns them and security.HookPlan returns their wiring — ordered HookGroup{Phase, Tools, Scripts} entries in PreToolUse/PostToolUse phases with Claude tool names as the canonical vocabulary. Claude's GenerateClaudeSettings is rendered from HookPlan so the two cannot diverge. The stdin/stdout/exit-code wire protocol is documented in runtimes.md.
  • The bootstrap extension is runtime.SandboxHooksBootstrap (carrying security.SandboxHookConfig). Every runtime's Bootstrap SHOULD honour it by installing the scripts (installHookScripts, any directory) and wiring HookPlan through its own interception mechanism (Claude Code: settings.json; OpenCode: tool.execute.before/after plugin; pi: tool_call/tool_result extension). A runtime that cannot MUST record the absence in the security feature matrix in docs/runtimes.md.

Consequences ​

  • New runtimes get Tirith/SSRF/canary/redaction parity by writing a thin adapter over the existing scripts instead of re-implementing scanners.
  • The Claude runtime is unchanged in behaviour: same scripts, same settings.json, same claude-debug.log and CLAUDE.md bridge.
  • Tool-name translation (Claude Bash vs lowercase bash, #608) becomes an adapter responsibility; the plan keeps Claude names as the vocabulary.
  • The contract is versioned (v1 = the scripts' current stdin/stdout fields). Verifying it against Claude Code surfaced two pre-existing gaps the runtimes.md matrix now records instead of a blanket ✓: the runner's hook wiring was not loaded from where it was written (#6358 — since fixed via --settings), and the PostToolUse payload differs (tool_response, hookSpecificOutput.updatedToolOutput, parallel execution — #6357).
  • Alongside this decision the runner's remaining Claude-specific branches were replaced by optional capability interfaces (DebugLogNamer, ContextBridger) — an implementation detail recorded in architecture.md, not a separate architectural decision.
  • The security matrix and config-key matrix in docs/runtimes.md gain a column per runtime and become part of a runtime PR's definition of done.
  • ACP proxies (#587) or tool proxies (#5243) can later supersede per-runtime adapters without changing the scripts or the plan.

Done (#6357): PostToolUse contract v2 — scripts read tool_response (fallback tool_result), replace via hookSpecificOutput.updatedToolOutput, and enforce unicode → canary → suppress → redact in posttool_chain.py. See runtimes.md.

Done (#608): The canonical Claude tool-name vocabulary is recorded once in security.CanonicalClaudeTools (with security.LegacyClaudeTools for names agents and adapters still use), mirrored into tool_allowlist_pretool.py and kept identical by a Go test; HookPlan tools and the pi adapter's maps are tested against it. The allowlist hook stays exact-match and fail-closed — no case-insensitive allowing — but a blocked name that is a case variant of an allowlisted entry is reported as a normalization gap (tool_name_unnormalized when the adapter did not translate, allowlist_entry_unnormalized when the allowlist is the non-canonical side, tool_name_case_collision when neither spelling is a Claude tool; all ALLOWLIST_HOOK_ERROR, severity high) rather than as a forbidden tool (tool_blocked, critical). MCP names are matched verbatim. The pi adapter's maps are held to canonical-or-legacy names (ls → LS), a deliberate relaxation of "canonical only". See runtimes.md.