---

title: Runner must run mutants in child processes, not threads, loading the project once per worker. status: open text_ref: plan:running-mutants/in-process-or-subprocess tags:

• strict-modeling
• confidence:0.82
• confidence-band:medium
• provenance:plan-running-mutants-in-process-or-subprocess
• lane:strict

semantic_text: Runner must run mutants in child processes, not threads, loading the project once per worker. logic_claims:

• CLAIM-31AEF4005052471E

semantic_clauses:

• Runner must run mutants in child processes, not threads, loading the project once per worker.

semantic_inventory_version: kibi.semantic-inventory.v1 semantic_source_field: semantic_text semantic_source_hash: 17dfa32c38b40ed8a035e360a68060eb3132d591f931395118441d6c8a6da8b2 semantic_inventory:

• claim_key: CLAIM-31AEF4005052471E claim_text: Runner must run mutants in child processes, not threads, loading the project once per worker role: normative status: modeled span: start: 0 end: 92 rationale: Tests under mutation can write temp directories, spawn processes, and exhaust the stack or heap; process isolation contains that. origin: kind: import ref: plan:running-mutants/in-process-or-subprocess recorded_at: '2026-10-29T16:05:42.779Z' firmness: firm required_evidence: asserted id: REQ-AUTO-43894D6940BD6965 type: req --- Runner must run mutants in child processes, not threads, loading the project once per worker.

Context

Tests under mutation can write temp directories, spawn processes, and exhaust the stack or heap; process isolation contains that.

Source

The first draft uses child processes and pays the load cost once per worker, not once per mutant.

Source: Mutalog design plan: first draft - running-mutants/in-process-or-subprocess