Litert Conversion Workflow

LiteRT-LM conversion workflow

A conversion is done when three things hold, in this order:

1. the bundle loads and generates through the LiteRT-LM engine (not just the raw interpreter),
2. output quality is gated against the source model — a floor gate plus a task-level parity check, not a smoke test,
3. it holds up on the deployment path it claims: the target backend, the target device, and multi-turn conversation.

Each step can pass while the next one fails. A bundle that converts can die at engine creation; an engine that generates can be quantization garbage; a model that answers 8/8 single-turn can crash on message two. The gates exist because every one of these has happened.

Scope: models producing a .litertlm bundle consumed by the LiteRT-LM engine (pip install litert-lm) — text LLMs and vision-language models (a VLM bundle is an LLM bundle plus two vision graphs; references/vlm-conversion.md covers the delta). Standalone/classic .tflite models go through the gpu-clean-conversion → accuracy-safe-quantization → on-device-verification lane; this skill is the LM sibling and reuses their discipline where it applies.

Step 0: classify before you convert

Look at config.json (model_type, layer_types, MoE fields, size) before running anything. Architecture decides everything downstream, and some structures die at a known point no recipe can route around. references/architecture-walls.md is the lookup table: structure → where it dies (export / load / engine / backend) → the error signature you'll see. Check it first; it turns a day of debugging into a table lookup.

Then pick the lane in references/recipe-selector.md: plain dense decoders ride a standard export; hybrids (SSM / linear-attention / short-conv) need state-aware export plus executor metadata; reasoning models need template care; MoE and MLA are currently walls.

Loop

1. Export with a minimal, extraction-safe template. The single most common ship-killer is not math — it is the chat template. Export with use_jinja_template=False and a minimal ChatML-style template swapped in, so the bundle carries plain prefix/suffix markers and no Jinja at all. Vendor templates routinely call Python methods (.get(), .startswith(), .strip()) that the runtime's minijinja renderer does not implement — such a bundle imports fine and dies on the first message. Details, the multi-turn prefix contract, and the tokenizer traps that pair with this: references/template-tokenizer-traps.md.

2. Check the bundle before measuring anything.

python -m litert_lm_builder.litertlm_peek_main --litertlm_file model.litertlm

prompt_templates-only = safe. A jinja_prompt_template carrying Python-method calls = the first-message crasher. Also confirm the stop tokens and (for hybrids) the executor-metadata section are present.

3. Quantize on the LLM lane. int8 dynamic is the safe default; int4 must be blockwise (block-32 quality, block-128 for ~4B / iPhone section limits) — channelwise int4 collapses decoders while still passing smoke tests. Conv/scan layers of hybrids stay float. The decision facts and the recipe mechanics: references/recipe-selector.md §Quantization, plus the accuracy-safe-quantization skill for the general ladder.

4. Gate quality — floor, parity, then structure. Run the gate stack in references/verification-gates.md:

• 8-question floor gate on the engine (CPU, then the target backend). Catches collapse, never proves parity.
• Task parity (e.g. GSM8K n≥100) against the source model, same prompt and extraction both sides. Reasoning models need max-tokens ≥ 2048 or int4 falsely looks degraded.
• First-token length sweep — chat-templated prompt lengths are chunked by the engine's prefill planner, and state-carrying models corrupt at specific lengths while answering perfectly at others.
• Multi-turn — single-turn evals structurally cannot catch template-contract violations that kill message two.

5. Gate on the target device. Desktop GPU pass ≠ mobile GPU pass (different delegates, different compilers). Record device, backend, runtime version, and speeds next to the quality numbers — the on-device-verification skill's rules apply unchanged.

6. Publish behind the gate. The upload step must mechanically refuse unless the quality report passed — a collapsed quant that reaches a public repo costs more than every hour the gates cost. Card conventions and the publish checklist: references/verification-gates.md §Publish.

Symptom router

Watch for

• Pin and record the toolchain. litert-torch / litert-converter / litert-lm-builder / ai-edge-quantizer / transformers versions decide what exports and what runs; several walls in the references are version-bounded facts. A conversion note without versions is not reproducible.
• The runtime moves under you. A wall table entry is a dated fact, not a law — re-test walls on each runtime/converter release, in both directions: walls fall (blocked architectures start working) and regressions appear (bundles that ran stop running). Both have happened in the same release.
• Never diagnose quality without a control. Run the same eval on the source model with the same prompt and extraction; a broken harness undermeasures everyone and reads as a conversion bug.
• The engine adds behavior the graph does not have — start-token prepending, prefill chunking, cross-conversation caching. When engine output differs from a raw-graph replay, reproduce the engine's exact token stream before blaming the graph.
• One variable at a time at ship time. Repair published artifacts from the published files, not from local experiments; verify by checksum that what you gated is what you shipped.

Output layout

Ship each conversion as a recipe, the same split the model-recipe skills use — export, verification, and repair each separately re-runnable:

<model>/
  convert_<model>.py       export driver: flags, template swap, quant recipe
  templates/<model>.jinja  the minimal template the bundle embeds
  verify/                  gate scripts + their JSON results
  README.md                versions, recipe, gate numbers (device + backend
                           + runtime named), known limitations — honest ones

State known limitations on the card in actionable form ("prompts whose templated length lands on 33–37 tokens can end the reply early; adding or removing a word avoids it" — a real example). An honest limitation note survives contact with users; a hidden one becomes an issue report.