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phases/6-cli-runtime-resume/step3.md

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+# Step 3: device-oom-policy
+
+## Read First
+- /AGENTS.md
+- /PLAN.md
+- /PROGRESS.md
+- /docs/HARNESS.md
+- /docs/IMPLEMENTATION_PLAN.md
+- /docs/ARCHITECTURE.md
+- /docs/CONVERSION_POLICY.md
+- /docs/TOOLCHAIN.md
+- /phases/1-core-runtime-contracts/step1.md
+
+## Task
+Implement runtime device selection, CUDA fail-fast behavior, auto CPU fallback behavior, and OOM retry policy hooks.
+
+This step should be tested with mocks and small CUDA smoke checks only where safe.
+
+## Sprint Contract
+- Done means: runtime policy enforces explicit CUDA fail-fast, auto fallback warning, and configurable OOM retry reductions.
+- Hard thresholds: no silent CPU fallback for explicit CUDA; tests do not require exhausting VRAM; GTX 1070 Ti constraints remain documented.
+- Files owned: `src/pdftomd/runtime.py`, tests, `docs/TOOLCHAIN.md` if behavior changes, `PROGRESS.md`, phase index.
+- Dependencies: Runtime config options.
+
+## Acceptance Criteria
+```powershell
+python scripts\validate_workspace.py
+.\venv\python.exe -m pytest tests
+```
+
+## Verification
+1. Run the acceptance commands.
+2. Confirm CUDA smoke test instructions still work separately.
+3. Update `PROGRESS.md` and this phase index.
+
+## Do Not
+- Do not intentionally trigger real GPU OOM in tests.
+- Do not change PyTorch pins without updating `docs/TOOLCHAIN.md`.
+- Do not hide runtime warnings.