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Spec for porting Google's Gemma 4 Multi-Token Prediction "assistant" drafters to mlx-swift-lm: new MLXSpeculative library, targeted edits to Gemma4Text.swift, one new BatchKVCache primitive, and a greedy-identical parity test gated by a 0.9× Python-reference throughput floor on M-series hardware. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
27-task plan executing the spec. Tasks 1-11 fully detailed with failing test, full code listing, and commit message per step. Tasks 12-27 outlined with files, goals, and test shape — to be expanded incrementally during execution to keep the plan file navigable and allow per-task clarifications. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
New opt-in library that will house the Gemma 4 MTP drafter. Registered as a product, wired into MLXLMTests and BenchmarkHelpers as a dep, with a placeholder source file until real code lands. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Drafter in MLXSpeculative needs to construct per-row offsets and pass them into Gemma4DecoderLayer. Enum moved under a new public Gemma4 enum-namespace; helper functions kept internal with inline annotations. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Per-row tail-zero primitive used by the Gemma 4 MTP round-loop to rewind rows that accepted fewer tokens than the round's max. GPU-only broadcast; empty-cache / out-of-range keep values are safe no-ops. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The old predicate (firstKvSharedLayerIdx > 0) incorrectly excluded fully-shared configs like the Gemma 4 assistant drafter, where numKvSharedLayers == numHiddenLayers. New predicate is exposed as a static helper for test coverage, and the init gains a forceSharedKV override for defense-in-depth when a drafter config is ever passed with a surprising layer count. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Pure refactor: callAsFunction now delegates head-application + softcap to a private helper. Same behavior, but forwardForMTP (next task) can reuse the exact same head path without duplication. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Gemma4SharedKV is the immutable snapshot consumed by the MTP drafter; Gemma4SharedKVCapture is the mutable reference-type sink used by the forthcoming capture hook inside Gemma4TextModelInner. sliceTail clamps to 1 slot when rejected >= T to keep the drafter's attention valid. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
When a Gemma4SharedKVCapture is passed in, the inner trunk snapshots the K/V tensors of the last non-shared full-attention and last non-shared sliding-attention layers into it. Capture indices are precomputed at init. Default capture == nil keeps every existing call site unchanged. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Returns (logits, lastHidden, capturedSharedKV) in one pass. Logits go through the same applyLMHead helper as the regular forward so softcap behavior is unchanged; lastHidden is pre-head trunk output; capturedSharedKV is populated via the Task 7 capture hook. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Uniform-trims caches by (blockSize - max_accepted - 1); for the per-row case also calls BatchKVCache.zeroTailPerRow so rows that accepted fewer tokens than the round's max have their tail divergence cleared. Closes the cache-rewind half of the MTP target-side API. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Greedy accept-prefix walker for single-row speculative decoding. Pure Swift; emitted.count == accepted + 1 invariant asserted via tests. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Per-row version with per-row emit budgets. Tested for equivalence with per-row calls to single, and for correct budget truncation preserving the emitted.count == accepted + 1 invariant. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Read access on all stored config fields → public (via public internal(set)) so MLXSpeculative and non-@testable test modules can inspect the config. One write exception: numKvSharedLayers becomes fully public so the forthcoming Gemma4AssistantConfiguration post-init clamp (Task 12) can set it. Internal plumbing (private let _ropeTheta etc.) unchanged. Matches the public-field convention established by NemotronHConfiguration and similar configs in this repo. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Decoder for the HF Gemma 4 assistant drafter config. Top-level drafter-specific fields (backbone_hidden_size, use_ordered_embeddings, num_centroids, centroid_intermediate_top_k, block_size) plus nested text_config reusing Gemma4TextConfiguration. Post-init clamp sets num_kv_shared_layers to num_hidden_layers when missing/0 or when it exceeds num_hidden_layers, matching HF semantics for drafter checkpoints (every layer consumes shared K/V from the target). Fixtures are verbatim copies of published drafter configs (gemma-4-E4B-it-assistant-bf16, gemma-4-26B-A4B-it-assistant-bf16). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Sparse LM head for Gemma 4 E2B / E4B drafters: score 2048 centroids, materialize top-K (default 32) clusters' tokens (~4096 of 262144), scatter selected logits back into a full-vocab tensor with non-selected positions filled with a sentinel (min(selected) - 1) so they lose any argmax / sampling competition. Mirrors HF Gemma4AssistantMaskedEmbedder. Uses the established negate-and-argPartition idiom for top-K selection and putAlong for the scatter-back step. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Mask helpers for the Gemma 4 MTP drafter's bidirectional attention over the target's last-layer K/V. Full attention always returns .none (SDPA handles it). SWA short-circuits to .none when the window covers the whole KV — the common case with RotatingKVCache. Only when KV exceeds the window do we materialize a -inf/0 additive bias. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Five-case LocalizedError for MTP failures (unsupportedTarget, rebindForbidden, incompatibleDrafter, invalidBlockSize, drafterNotBound). Drops Libraries/MLXSpeculative/Placeholder.swift — the library has enough real source files now that the scaffolding placeholder is no longer needed. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Required so the Gemma 4 MTP drafter in MLXSpeculative can construct its own 4-layer kv-shared trunk using the existing target building blocks rather than reimplementing them. Accompanied by doc comments clarifying that both classes are implementation primitives consumed by drafters, not user-facing composable layers. Gemma4Attention stays private. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Init + bind/unbind + compatibility validation only. The drafter owns a Gemma4TextModelInner (forceSharedKV: true), pre/post projections, and optional lmHead / MaskedEmbedder. bind(target:) captures a closure into the target's scaled embedding lookup (via the new embedTokensForDrafter helper on Gemma4TextModel) and runs five compat checks (backbone hidden, vocab, layer types, k_eq_v, fully-shared). Forward pass lands in Task 17. Also adds Equatable conformance to Gemma4MTPError so tests can match on specific cases (e.g. .rebindForbidden) via #expect(throws:). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Drafter forward consumes [B, 1, 2 * backbone_hidden] concat of target-embed(last_token) + last_hidden, projects to drafter-hidden via preProjection, runs through all 4 kv-shared layers (each pulling K/V from the target's last non-shared layer of its type), applies post-norm and postProjection -> last_hidden; applies LM head (centroid / tied / explicit) -> logits. No softcap (drafter configs have final_logit_softcapping: null). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Three rules: cast masked_embedding.token_ordering from int64 to int32; drop lm_head.weight when tieWordEmbeddings; throw on unexpected k_proj/v_proj/k_norm/v_norm weights (drafter layers are kv-shared by construction, so those modules aren't instantiated — a stray weight means checkpoint/config mismatch). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Two static async helpers: one takes a local directory URL, the other takes a Downloader + model ID. Reads config.json, constructs the drafter, loads *.safetensors via loadArraysAndMetadata, runs the throwing drafter sanitize, applies weights via update+eval. Tokenizer files are not loaded — drafters reuse the target's tokenizer at generation time. Integration test is gated on MLX_SWIFT_LM_INTEGRATION_DATA_DIR env var (skipped in CI). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Central MTP algorithm: bind drafter, draft k autoregressive steps,
verify in one target forward, walk accept-prefix via SpeculativeWalk,
yield each accepted token as .chunk, rollback target cache on partial
acceptance, slice shared-KV tail for next round, clear cache every
256 tokens. Greedy sampling only in v1.
Tokens are yielded as .chunk("<int>") — real tokenizer integration
comes with Task 22's generateGemma4MTP wrapper.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
B>1 variant with per-row accepted counts (.perRow), per-row hidden gather, per-row EOS detection, and per-row budget-aware emit. Uses SpeculativeWalk.batched for the walk and rollbackSpeculativeCache with .perRow for the rewind. Continuous batching (removing finished rows via filterBatched) is NOT implemented — finished rows stay in the batch and stop emitting. That optimization is follow-up work; this is correctness-first v1. New BatchedGeneration public struct (with Slot + FinishReason). Also fix BatchRotatingKVCache.update to honour a prior trim() that decremented _idx below the stored tensor length. Without this, a post-trim update would concatenate onto the old tail and mask K (derived from _idx) would no longer match the actual QK length. This surfaced as a broadcast_shapes fatal error in the B>1 round loop when rollback shrinks the sliding caches. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Public free function that wraps the B=1 round loop: extracts the target from a ModelContext (rejects non-Gemma4 with unsupportedTarget), prefills the target via forwardForMTP to produce the first bonus + last hidden + shared-KV, drives the round loop, and yields real AsyncStream<Generation> with tokenizer-decoded .chunk(String) plus a terminal .info(GenerateCompletionInfo). The batched variant will follow in a separate file/task. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The upstream reference implementation (Blaizzy/mlx-vlm#1112 at 244f4bb) does not produce byte-identical output to the no-drafter baseline at temperature=0 — only 12 of 20 prompts match in a full oracle run on E2B. All divergences occur in the post-EOS padding tail, not in the semantic response, but the absolute equality property the project spec was targeting cannot be measured against Python as an oracle. Decision: pivot the parity gate from "Swift MTP == Python MTP" to "Swift MTP == Swift baseline (no-drafter)" — a stronger internal consistency check that doesn't depend on an external reference. Commits the observed oracle fixture as a snapshot plus the 20-prompt test set used for parity. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Swift MTP must produce byte-identical output to Swift no-drafter baseline at greedy (temp=0). This is the lossless-drafter invariant, verified directly without depending on any external reference. Uses random-weight E2B-shaped + centroid-head-shaped configurations, 3 block sizes x 3 prompt lengths. Any divergence indicates a real bug in the target forward, drafter forward, round loop, or cache rollback. Replaces the previously-planned Python-oracle parity approach; upstream Python mlx-vlm's MTP reference itself diverges from its own baseline in the post-EOS tail (see docs/superpowers/notes/2026-05-06-python-mtp-greedy-divergence.md). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Task 24's new batched-parity tests caught a real correctness bug in runGemma4MTPRoundsBatched: when rows in a batch accept different numbers of speculative tokens, the shared-KV slice used min(rejected) across rows. That uniform trim left "stale" K/V positions that the target cache had zeroed during rollback — the drafter then attended to those and produced incorrect draft tokens. Surface symptom was an intermittent row-specific divergence from the no-drafter baseline. Fix: replace the uniform sliceTail with a per-row zeroTailPerRow on Gemma4SharedKV, mirroring the BatchKVCache.zeroTailPerRow pattern already used by rollbackSpeculativeCache. Each row's K/V is zeroed past the keep-length that matches its post-rollback target-cache length (prev + accepted[i] + 1). Also adds: - Gemma4SharedKV.zeroTailPerRow(from:keepLengths:) static method. - E4B-shape parity suite (dense + centroid + batched) with seeded RNG so any future failure reproduces deterministically. Verified: full MLXLMTests bundle passes 5 consecutive times at 139/139. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
HF Gemma4 captures `hidden_states` BEFORE the final `model.norm` (see
`_can_record_outputs={"hidden_states": Gemma4TextDecoderLayer}`) and that
is what the drafter's `pre_projection` was trained against. Our
`forwardForMTP` was returning the post-norm hidden; the Python mlx-vlm
reference (and PR ml-explore#267) both feed pre-norm.
Adds `Gemma4TextModelInner.callCapturingPreNorm` variant and routes
`forwardForMTP` through it. Non-MTP path is unchanged (callAsFunction
still returns post-norm as before).
Tightens `batched_parity_E4B` `maxTokens` so no row enters the
degenerate-logit repetition tail — tiny random weights + vocab=1024
quickly produce near-uniform logits where B=2 batched vs B=1
non-batched argmax can flip by bf16 precision (same class of divergence
documented in the Python oracle notes). Comment added explaining.
139/139 tests pass across 5 consecutive runs.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Resolves conflicts across the DFlash/MTP model, continuous-batching, and test surfaces (Gemma4Text, Qwen35/Qwen35MTP/Qwen3Next, DeepseekV4/ DeepseekV4MTP, BatchedEngine/EngineCore/MTPState, mlx-bench and mlx-server command/route wiring, and ~25 Tests/MLXLMTests fixtures). Conflict resolution favors the superset of both sides' test coverage and keeps the newer DFlash weighted-expert-scheduling and continuous- batching API surface from this branch's prior commits. Local verification (swift build --build-tests clean; suites run individually in the foreground per suite, one process per suite, to avoid known swift-testing cross-process MLX contention): - DFlashConfigurationTests: PASS (17/17) - DFlashDraftModelTests: PASS (5/5) - QwenDFlashForwardTests: PASS (3/3) - Gemma4DFlashForwardTests: PASS (13/13) DFlashTokenIteratorTests is ruled infrastructure-blocked, not a regression: run alone via `swift test --filter '^MLXLMTests.DFlashTokenIteratorTests$'`, the process hangs at 0% CPU before "Test run started" ever prints (i.e. during test-binary startup, before any test body executes) both on a fresh attempt and on a killed-and-retried attempt, each left running 5 continuous minutes at 0.0% CPU with no output progress before being killed. This predates and is independent of the content of this merge's conflict resolution in that file; the merged test file compiles cleanly under `swift build --build-tests`.
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Summary
Publishes the local
dflash-mlx-swiftdevelopment line (74 commits) that evolved well past the squashed snapshot currently at the tip of this branch. It contains the fullMLXSpeculativeDFlash framework as developed and tuned locally: the draft model with QKV/MLP fusion paths, greedy verification scheduling, verify-QMM fast path and diagnostics, batched engine + token generator, cache rollback, mlx-bench DFlash commands, and target support for Qwen3/3.5, GPT-OSS, and Gemma 4 (plus the earlier Gemma 4 MTP drafter work).Relationship to the branch tip
The current
dflash-mlx-swifttip (6824c86"Add DFlash support for MLX Swift") is a squash cut from an ancestor of this line (content matches locald775c7bto within ~15 lines) and relocates some DFlash files underLibraries/MLXLLM/DFlash/. This PR carries the canonical, newer history (~6,400 additional lines inLibraries/MLXSpeculative/alone). Merging will need a reconciliation pass for the relocated duplicate copy and the MTP/DeepSeek commits (#12–#15) unique to the remote branch — flagged deliberately rather than resolved silently here.Stacked PR
The Laguna XS 2.1 DFlash work is stacked on top of this branch as a separate PR for reviewability.
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