Apply when extracting code from a large monolith file into submodules. Covers barrel re-exports, _internals DI seam proxy patterns, CI invariant allowlist updates, and cross-file test verification. Prevents CI failures, broken imports, and test regressions from code extraction.
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name
safe-extraction
description
Apply when extracting code from a large monolith file into submodules. Covers barrel re-exports, _internals DI seam proxy patterns, CI invariant allowlist updates, and cross-file test verification. Prevents CI failures, broken imports, and test regressions from code extraction.
Re-linked to current knowledge entries (version 4). The original source ID c276dc6e... is no longer present in the active knowledge store. The skill body and behavior are unchanged; only source_knowledge_ids metadata was updated to point to current lessons about refactoring guards to separate functions and upgrade-safety test helpers, both directly relevant to the safe extraction protocol.
Safe Extraction Protocol
Follow every step in order. Do not skip steps.
When to use this skill
A source file exceeds team-agreed size thresholds (this repo uses <2000 lines per file per FR-005) and needs splitting
A subsystem (destructive-command, worktree-isolation, etc.) is being extracted to its own file
Code is being moved from one module to another without changing behavior
Benefit: Prevents the three most common extraction failure modes:
CI invariant check failures (new file paths not in allowlists)
Broken _internals DI seams (test mocks stop working)
Cross-file test regressions (other test files that consume the module)
Step 0 — Pre-extraction audit
Before moving ANY code, inventory every path-scoped artifact that references the source file:
Record EVERY test file that imports or spies on the source module. These must all pass after extraction.
Prefer the imports tool or repo_map action for comprehensive consumer discovery. Grep catches direct string matches but misses require() imports, dynamic import(), and re-exports through intermediate modules. Use grep as a secondary cross-check.
0d. Import graph
Use the imports tool or repo_map to find all consumers of exports from the source file.
Step 1 — Create the extracted module
Move the code block(s) to the new file(s)
Move all supporting types, constants, and helper functions used exclusively by the extracted code
Add necessary imports to the new file (from external dependencies)
Step 2 — Create barrel re-export (if preserving public API)
If consumers import from the original path, keep the original file as a barrel:
// src/hooks/guardrails.ts (barrel — preserves import path)// Use EXPLICIT named exports, not `export *`, to avoid naming conflicts// when multiple submodules export symbols with the same name.export {
_internals,
createGuardrailsHooks,
enforceSpecDriftGate,
} from'./guardrails/index';
export {
buildEffectiveRules,
checkFileAuthority,
getGlobMatcher,
} from'./guardrails/file-authority';
export {
createToolBeforeHandler,
normalizeToolInput,
} from'./guardrails/tool-before';
// etc.
Verify:bun run build succeeds. All existing imports still resolve.
Step 3 — Handle _internals DI seams
If the source module exports _internals for test injection:
3a. Direct functions stay in source _internals
Functions that remain in the source file stay as direct entries:
exportconst _internals = {
resolveEvidenceTaskId, // still in this file
loadPlanJsonOnly, // still in this file// ...
};
3b. Extracted functions need getter/setter proxies
Functions moved to the extracted module need proxy entries so test mocks propagate:
Type annotation: Always include an explicit type annotation on _internals objects to override as const readonly inference. Without it, as const makes properties readonly and test injection (_internals.fn = mockFn) fails at compile time:
CRITICAL: The extracted module's own production code must call mockable functions through _internals.fn(...), NOT through the direct function reference. If extractedFn internally calls otherExtractedFn, it must use _internals.otherExtractedFn() — otherwise test mocks set on _internals won't intercept the internal call. This is the same pattern the parent module follows.
Verify: Run ALL test files from Step 0c — not just the one explicitly in scope.
3c. Alternative: Factory parameter pattern (no _internals proxy needed)
When splitting a factory function into handler files (NOT extracting a subsystem with its own _internals), the getter/setter proxy is unnecessary. Instead:
Handler files export factory functions that receive dependencies as parameters
The orchestrator file calls these factories, passing closure-scoped config
The barrel re-exports only the top-level orchestrator API
// guardrails/tool-before.ts — handler fileexportfunctioncreateToolBeforeHandler(cfg: Config, deps: Deps) {
// Receives all dependencies as parameters — no _internals neededreturnfunctiontoolBefore(input: ToolInput) {
/* handler logic using cfg and deps */
};
}
// guardrails/index.ts — orchestratorexportfunctioncreateGuardrailsHooks(config: PluginConfig) {
const cfg = resolveConfig(config);
return {
toolBefore: createToolBeforeHandler(cfg, deps),
// ...
};
}
Use this pattern when:
Splitting a large factory function into handler files
The submodules don't have their own mockable functions
All dependencies can be passed as closure parameters
Use the _internals proxy pattern (3b) when:
Extracting a subsystem that tests mock independently
The source module exports _internals for test injection
Mockable functions are moving to the extracted module
Step 4 — Update CI invariant scripts
For EVERY path-scoped artifact found in Step 0a, add the new file paths:
# Example: check-invariants.sh
LEGACY_EXEMPTS=(
"src/hooks/guardrails.ts"# original (still exists as barrel)"src/hooks/guardrails/file-authority.ts"# NEW"src/hooks/guardrails/helpers.ts"# NEW"src/hooks/guardrails/index.ts"# NEW
)
Verify: Run the invariant check script locally if possible (bash on Windows may require WSL).
Step 4.5 — Re-capture SAST baseline
File-path moves alter SAST finding fingerprints, making pre-existing findings appear new. After extraction:
Run sast_scan with capture_baseline: true and the current phase number
Compare against the previous baseline to identify findings that moved (same rule, different file path)
Merge the updated baseline so pre-existing findings don't fail the gate
Verify: SAST scan on the new file paths shows only genuinely new findings, not relocated pre-existing ones.
Step 5 — Update documentation
Update any doc references that point to the old monolith for functions that moved:
grep -rn "<source-file-path>" docs/ *.md
Fix references to point to the new submodule location.
Step 6 — Verification checklist
bun run build succeeds
bun run typecheck succeeds (zero new type errors)
biome ci . passes
ALL test files from Step 0c pass (not just the one in scope)
CI invariant scripts updated for new paths
Documentation references updated
No runtime behavior changes (pure extraction)
Common mistakes
Mistake
Why it fails
Forgetting to update LEGACY_EXEMPTS
CI quality job fails on process.cwd() check for new file paths
Only testing the explicit test file
Cross-file regressions in OTHER consuming test files go undetected
Not creating getter/setter proxies for _internals
Test mocks on the parent module don't propagate to extracted functions
Moving helper functions without updating/re-exporting all consumers
Import errors in unrelated files that depended on the helper
Updating docs to reference new paths but missing some
Stale doc references confuse future readers
Relationship to other skills
safe-rename: Use when renaming symbols across the codebase. Use safe-extraction when moving code to new files.
mock-to-internals-migration: Use when converting test files from mock.module/vi.spyOn to _internals DI seam. May be needed as part of extraction if the source module's _internals changes.
subprocess-safety: Relevant if the extracted code calls spawn/spawnSync — ensure the _internals proxy preserves timeout/kill semantics.
