refactor(draft): codify proposal row layout

server/CMakeLists.txt

@@ -1069,6 +1069,12 @@ if(DFLASH27B_TESTS)
         list(APPEND _raw_unit_test_targets test_recurrent_snapshot)
     endif()
 
+    # Pure proposal-shape contract test: no ggml, GPU, or model files.
+    add_executable(test_draft_contract test/test_draft_contract.cpp)
+    target_include_directories(test_draft_contract PRIVATE
+        ${CMAKE_CURRENT_SOURCE_DIR}/src)
+    add_test(NAME draft_contract COMMAND test_draft_contract)
+
     if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/test_server_unit.cpp")
         set(_server_unit_sources
             test/test_unit_main.cpp
@@ -1230,6 +1236,9 @@ if(DFLASH27B_TESTS)
     # 'make check' — build every registered unit-test target, including the
     # lightweight feature gate added on main, then run the complete CTest set.
     set(_check_deps ${_raw_unit_test_targets})
+    if(TARGET test_draft_contract)
+        list(APPEND _check_deps test_draft_contract)
+    endif()
     if(TARGET test_platform_compat)
         list(APPEND _check_deps test_platform_compat)
     endif()

server/src/draft/draft_contract.h

@@ -0,0 +1,31 @@
+#pragma once
+
+namespace dflash::common {
+
+// Draft artifacts expose one of two row layouts. Existing DFlash and DS4
+// DSpark artifacts include the accepted seed in row zero; proposal-only
+// artifacts such as Bonsai rely on the runtime to prepend the accepted anchor
+// before target verification.
+enum class DraftProposalLayout {
+    SeedThenProposals,
+    ProposalsOnly,
+};
+
+struct DraftProposalShape {
+    int draft_rows = 0;
+    DraftProposalLayout layout = DraftProposalLayout::SeedThenProposals;
+
+    constexpr int first_proposal_row() const noexcept {
+        return layout == DraftProposalLayout::ProposalsOnly ? 0 : 1;
+    }
+
+    constexpr int proposal_count() const noexcept {
+        const int count = draft_rows - first_proposal_row();
+        return count > 0 ? count : 0;
+    }
+
+    // Verification always includes the already accepted anchor exactly once.
+    constexpr int verify_width() const noexcept { return 1 + proposal_count(); }
+};
+
+}  // namespace dflash::common

server/src/internal.h

@@ -25,6 +25,7 @@
 #include "gguf.h"
 
 #include "dflash27b.h"
+#include "draft/draft_contract.h"
 
 namespace dflash::common {
 
@@ -314,6 +315,11 @@ struct DraftWeights {
     int n_target_layers = DFLASH27B_DRAFT_N_TARGET_LAYERS;  // captured target layers (5)
     std::vector<int> capture_layer_ids;                     // explicit captured target-layer ids (GGUF dflash.target_layer_ids); empty = derive from count
     int mask_token_id   = DFLASH27B_DRAFT_MASK_TOKEN_ID;    // noise mask token
+    DraftProposalLayout proposal_layout = DraftProposalLayout::SeedThenProposals;
+
+    DraftProposalShape proposal_shape() const {
+        return DraftProposalShape{block_size, proposal_layout};
+    }
 
     // Optional Domino causal correction head. When present, greedy chain
     // speculative decode corrects each draft token with a lightweight GRU

server/test/test_draft_contract.cpp

@@ -0,0 +1,28 @@
+#include "draft/draft_contract.h"
+
+using namespace dflash::common;
+
+constexpr DraftProposalShape legacy{16, DraftProposalLayout::SeedThenProposals};
+static_assert(legacy.first_proposal_row() == 1);
+static_assert(legacy.proposal_count() == 15);
+static_assert(legacy.verify_width() == 16);
+
+constexpr DraftProposalShape native{4, DraftProposalLayout::ProposalsOnly};
+static_assert(native.first_proposal_row() == 0);
+static_assert(native.proposal_count() == 4);
+static_assert(native.verify_width() == 5);
+
+constexpr DraftProposalShape anchor_only{1, DraftProposalLayout::SeedThenProposals};
+static_assert(anchor_only.proposal_count() == 0);
+static_assert(anchor_only.verify_width() == 1);
+
+constexpr DraftProposalShape empty_native{0, DraftProposalLayout::ProposalsOnly};
+static_assert(empty_native.proposal_count() == 0);
+static_assert(empty_native.verify_width() == 1);
+
+constexpr DraftProposalShape default_shape{};
+static_assert(default_shape.layout == DraftProposalLayout::SeedThenProposals);
+static_assert(default_shape.proposal_count() == 0);
+static_assert(default_shape.verify_width() == 1);
+
+int main() { return 0; }