[leehack]

Summary

• Keep LlamaArgumentException.name when the LiteRT-LM worker reports an argument error (LiteRtLmErrorResponse.argumentName, rebuilt in _exceptionForErrorResponse). invalidValue is not carried: it is an arbitrary object and may not cross the isolate port.
• Reword the raw-prompt refusal per case: "the prompt is empty", or "the prompt holds only the model's start token, which the LiteRT-LM runtime adds itself" (only possible when the bundle has a start token). The old text said the runtime "cannot prefill" it, which overstated the limit.
• From #1861 (Send the raw prompt unchanged from native LiteRT-LM generate (#1853) #1864 delta audit), 0.19.3 plan #1874, lane 6.

Production-readiness scope

• User-facing scope: Callers of native LiteRT-LM generate() get error.name == 'prompt' through the worker, and a message that names the case.
• Supported platforms/paths: Native LiteRT-LM (worker isolate).
• Unsupported or intentionally unavailable paths: Web LiteRT-LM does not use this path.
• Out of scope / follow-ups: #1861.

Completeness checklist

• Declared scope is fully implemented, or explicitly reduced above.
• Public API docs, changelog entries are updated where relevant (both changelogs under Unreleased).
• Regression coverage covers the original issue.
• Security/privacy review completed: only an argument name crosses the port.

PR type guidance

• Bugfix PR: root cause: _exceptionForErrorResponse rebuilt LlamaArgumentException from the message only. Tests: worker_messages_test.dart (name captured) and litert_lm_backend_test.dart (name rebuilt). Platforms: native LiteRT-LM.

Test Plan

• dart run tool/prepare_workspace.dart
• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm -j 1 test/unit/backends/litert_lm (1536 pass), isolated TMPDIR
• dart test -p chrome N/A: native-only files
• coverage-lib N/A: changed lines are all executed by the new tests

Matrix Evidence

Matrix row	Scope covered	Platform / model / backend	Result	Evidence / notes
N/A	worker error kinds	macOS VM	pass	mutants below

Mutants killed: drop name: in _exceptionForErrorResponse; drop argumentName: in LiteRtLmErrorResponse._from; default the name to 'prompt' at either end (unnamed errors stay unnamed); invert the empty/start-token-only branch.

Review Notes

• Independent review status: pending (high-risk by path: backendRuntime)
• CI status / head SHA: 2ebe814

High-risk regression review

• Classification: high-risk (backendRuntime, by path)
• Implementation task: this lane (l193-sweep)
• Independent blocking QA task: pending
• Exact head / current base: 2ebe814 / f78cafa
• Production-branch deletion, bypass, or miswire proof: both new tests fail when either line is removed
• Affected-family real-model/artifact evidence: N/A: error plumbing, no model behavior
• Explicit unavailable-family or other N/A evidence: N/A
• Known PR-caused P1 regressions: 0
• Unresolved review threads: 0

Refs #1861, Refs #1874

[leehack]

Landed on main via batch integration PR #1895 (rebase merge, main ba4e0e1b; tree identical to the audited integration head 5ffd1224).