diff --git a/crates/storage-boundary-test/tests/write_boundary.rs b/crates/storage-boundary-test/tests/write_boundary.rs
new file mode 100644
index 00000000..1dccf9e9
--- /dev/null
+++ b/crates/storage-boundary-test/tests/write_boundary.rs
@@ -0,0 +1,256 @@
+//! Cross-boundary integration test for the WRITABLE half of the storage
+//! contract.
+//!
+//! Loads the built synthetic writer bridge (from either the sibling repo or
+//! the local `artifacts/extensions/` staging dir) and drives its
+//! `storage-write-dispatch@2.0.0` exports through the WIT component boundary
+//! via wasmtime, proving that transactional / DDL / DML calls round-trip
+//! ACROSS the interface: args are encoded into the component's linear memory
+//! by the canonical ABI and the resultset (rowids + counts) is decoded back
+//! out, not merely exercised by the component's native Rust logic. This is
+//! the mirror-image counterpart to `boundary.rs` (which exercises the
+//! read-side `storage-dispatch` interface against the sqlite component).
+//!
+//! Strategy: identical to the read-side test. The bridge is a plain writer
+//! component with NO imports (its `writer` world only exports
+//! storage-write-dispatch), so we just instantiate it and call the exports
+//! directly. `handle = 1` and `catalog = 1` are the fixed values the bridge
+//! serves (it does not model a callback registry / catalog table).
+
+use wasmtime::component::{Component, Linker, ResourceTable};
+use wasmtime::{Config, Engine, Store};
+
+mod bindings {
+    wasmtime::component::bindgen!({
+        path: "wit-write",
+        world: "write-boundary-test",
+    });
+}
+
+use bindings::duckdb::extension as ext;
+use bindings::WriteBoundaryTest;
+use ext::types::Duckvalue;
+
+/// Store state. The writer bridge's `writer` world imports nothing from the
+/// `duckdb:extension` package, but the wasip2 stdlib pulls in `wasi:io` /
+/// `wasi:cli` / `wasi:filesystem` implicitly (Mutex + OnceLock allocate,
+/// panic-time stderr goes through wasi-cli); satisfy those with a real (but
+/// idle) `WasiCtx` so instantiation succeeds. The dispatch calls never touch
+/// them.
+struct State {
+    table: ResourceTable,
+    wasi: wasmtime_wasi::WasiCtx,
+}
+
+impl Default for State {
+    fn default() -> Self {
+        State {
+            table: ResourceTable::new(),
+            wasi: wasmtime_wasi::WasiCtxBuilder::new().build(),
+        }
+    }
+}
+
+impl wasmtime_wasi::WasiView for State {
+    fn ctx(&mut self) -> wasmtime_wasi::WasiCtxView<'_> {
+        wasmtime_wasi::WasiCtxView {
+            ctx: &mut self.wasi,
+            table: &mut self.table,
+        }
+    }
+}
+
+const HANDLE: u32 = 1;
+const CATALOG: u32 = 1;
+
+fn locate_writer_bridge_wasm() -> std::path::PathBuf {
+    // 1. explicit env var wins (CI).
+    if let Ok(p) = std::env::var("DUCKLINK_WRITER_BRIDGE_WASM") {
+        return std::path::PathBuf::from(p);
+    }
+    // 2. staged copy in the local artifacts dir (developer convenience:
+    //    `cp .../synthetic_mutating_ducklink_bridge_dynlink.wasm
+    //     ducklink/artifacts/extensions/synthetic_mutating_ducklink.wasm`).
+    let manifest = env!("CARGO_MANIFEST_DIR");
+    let staged = std::path::Path::new(manifest)
+        .join("../../artifacts/extensions/synthetic_mutating_ducklink.wasm");
+    if staged.exists() {
+        return staged;
+    }
+    // 3. sibling checkout (default dev layout: ~/git/synthetic-mutating-vtab-
+    //    ducklink-bridge/ next to ~/git/ducklink/).
+    let home = std::env::var("HOME").expect("HOME env var");
+    let sibling = std::path::PathBuf::from(home).join(
+        "git/synthetic-mutating-vtab-ducklink-bridge/target/wasm32-wasip2/release/\
+         synthetic_mutating_ducklink_bridge_dynlink.wasm",
+    );
+    if sibling.exists() {
+        return sibling;
+    }
+    panic!(
+        "writer bridge wasm not found. tried:\n\
+        \x20 * $DUCKLINK_WRITER_BRIDGE_WASM (unset)\n\
+        \x20 * {}\n\
+        \x20 * {}\n\
+        build the bridge with:\n\
+        \x20 (cd ~/git/synthetic-mutating-vtab-ducklink-bridge && \
+        cargo build --target wasm32-wasip2 --release)\n\
+        then either set DUCKLINK_WRITER_BRIDGE_WASM=<path> or copy the wasm to \
+        the staged location.",
+        staged.display(),
+        sibling.display(),
+    );
+}
+
+fn columns_kv() -> Vec<ext::types::Columndef> {
+    vec![
+        ext::types::Columndef {
+            name: "key".to_string(),
+            logical: ext::types::Logicaltype::Text,
+        },
+        ext::types::Columndef {
+            name: "value".to_string(),
+            logical: ext::types::Logicaltype::Text,
+        },
+    ]
+}
+
+fn row(k: &str, v: &str) -> Vec<Duckvalue> {
+    vec![Duckvalue::Text(k.to_string()), Duckvalue::Text(v.to_string())]
+}
+
+fn setup() -> (Store<State>, WriteBoundaryTest) {
+    let mut config = Config::new();
+    config.wasm_component_model(true);
+    let engine = Engine::new(&config).expect("engine");
+
+    // The `writer` world imports nothing from duckdb:extension, but the
+    // wasip2 stdlib the bridge is built against imports wasi:io / wasi:cli /
+    // wasi:filesystem. Satisfy them with the standard wasi linker.
+    let mut linker: Linker<State> = Linker::new(&engine);
+    wasmtime_wasi::p2::add_to_linker_sync(&mut linker).expect("wasi add_to_linker");
+    let mut store = Store::new(&engine, State::default());
+
+    let wasm_path = locate_writer_bridge_wasm();
+    let bytes = std::fs::read(&wasm_path)
+        .unwrap_or_else(|e| panic!("read {}: {e}", wasm_path.display()));
+    assert_eq!(
+        &bytes[0..8],
+        &[0x00, 0x61, 0x73, 0x6d, 0x0d, 0x00, 0x01, 0x00],
+        "not a wasm component (bad magic) at {}",
+        wasm_path.display()
+    );
+    let component = Component::new(&engine, &bytes).expect("Component::new");
+    let instance =
+        WriteBoundaryTest::instantiate(&mut store, &component, &linker).expect("instantiate");
+    (store, instance)
+}
+
+/// Full happy path: begin -> create -> insert -> update -> delete -> commit.
+/// Every dispatch call round-trips through the canonical ABI; the assertions
+/// check the returned counts / rowids the component emits.
+#[test]
+fn write_dispatch_crosses_wit_boundary() {
+    let (mut store, instance) = setup();
+    let wd = instance.duckdb_extension_storage_write_dispatch();
+
+    // begin_transaction across the boundary.
+    let txn = wd
+        .call_begin_transaction(&mut store, HANDLE, CATALOG)
+        .expect("begin_transaction host call")
+        .expect("begin_transaction component result");
+    assert!(txn >= 1, "txn handle must be non-zero, got {txn}");
+
+    // create_table across the boundary.
+    let cols = columns_kv();
+    wd.call_create_table(&mut store, HANDLE, txn, "kv_store", &cols)
+        .expect("create_table host call")
+        .expect("create_table component result");
+
+    // insert_rows across the boundary. Two rows, count=2.
+    let rows = vec![row("alpha", "1"), row("beta", "2")];
+    let inserted = wd
+        .call_insert_rows(&mut store, HANDLE, txn, "kv_store", &rows)
+        .expect("insert_rows host call")
+        .expect("insert_rows component result");
+    assert_eq!(inserted, 2, "insert returned {inserted}, want 2");
+
+    // update_rows across the boundary. The bridge assigns rowids 1..=N in
+    // monotonic order; update rowid 1 in place, count=1.
+    let update_row = vec![row("alpha", "one")];
+    let updated = wd
+        .call_update_rows(
+            &mut store,
+            HANDLE,
+            txn,
+            "kv_store",
+            &[1_i64],
+            &update_row,
+        )
+        .expect("update_rows host call")
+        .expect("update_rows component result");
+    assert_eq!(updated, 1, "update returned {updated}, want 1");
+
+    // delete_rows across the boundary. Delete rowid 2, count=1.
+    let deleted = wd
+        .call_delete_rows(&mut store, HANDLE, txn, "kv_store", &[2_i64])
+        .expect("delete_rows host call")
+        .expect("delete_rows component result");
+    assert_eq!(deleted, 1, "delete returned {deleted}, want 1");
+
+    // commit_transaction across the boundary.
+    wd.call_commit_transaction(&mut store, HANDLE, txn)
+        .expect("commit_transaction host call")
+        .expect("commit_transaction component result");
+}
+
+/// Rollback path: begin -> insert -> rollback. After rollback, a new txn
+/// starts from the pre-rollback committed state (the bridge's committed
+/// state was already populated by `write_dispatch_crosses_wit_boundary`,
+/// so this test starts with a fresh setup and checks that a rollback
+/// leaves no writes visible in the next txn).
+#[test]
+fn write_dispatch_rollback_crosses_wit_boundary() {
+    let (mut store, instance) = setup();
+    let wd = instance.duckdb_extension_storage_write_dispatch();
+
+    // begin_transaction + create_table + insert 3 rows.
+    let txn = wd
+        .call_begin_transaction(&mut store, HANDLE, CATALOG)
+        .expect("begin_transaction host call")
+        .expect("begin_transaction component result");
+    let cols = columns_kv();
+    wd.call_create_table(&mut store, HANDLE, txn, "kv_store", &cols)
+        .expect("create_table host call")
+        .expect("create_table component result");
+    let rows = vec![row("k1", "v1"), row("k2", "v2"), row("k3", "v3")];
+    let inserted = wd
+        .call_insert_rows(&mut store, HANDLE, txn, "kv_store", &rows)
+        .expect("insert_rows host call")
+        .expect("insert_rows component result");
+    assert_eq!(inserted, 3, "insert returned {inserted}, want 3");
+
+    // rollback_transaction across the boundary.
+    wd.call_rollback_transaction(&mut store, HANDLE, txn)
+        .expect("rollback_transaction host call")
+        .expect("rollback_transaction component result");
+
+    // Open a new txn and delete a rowid the rolled-back txn had inserted.
+    // The rollback should have discarded the shadow, so the delete of a
+    // never-committed rowid returns 0 (nothing to delete).
+    let txn2 = wd
+        .call_begin_transaction(&mut store, HANDLE, CATALOG)
+        .expect("begin_transaction (rerun) host call")
+        .expect("begin_transaction (rerun) component result");
+    let deleted = wd
+        .call_delete_rows(&mut store, HANDLE, txn2, "kv_store", &[1_i64, 2_i64, 3_i64])
+        .expect("delete_rows (post-rollback) host call")
+        .expect("delete_rows (post-rollback) component result");
+    assert_eq!(
+        deleted, 0,
+        "post-rollback delete returned {deleted}, want 0 (shadow was discarded)"
+    );
+    wd.call_rollback_transaction(&mut store, HANDLE, txn2)
+        .expect("cleanup rollback host call")
+        .expect("cleanup rollback component result");
+}
diff --git a/crates/storage-boundary-test/wit-write/boundary.wit b/crates/storage-boundary-test/wit-write/boundary.wit
new file mode 100644
index 00000000..863d8add
--- /dev/null
+++ b/crates/storage-boundary-test/wit-write/boundary.wit
@@ -0,0 +1,12 @@
+package boundary-test:write@0.1.0;
+
+use duckdb:extension/types@2.0.0;
+use duckdb:extension/storage-write-dispatch@2.0.0;
+
+// Host-side world for the write-boundary integration test. Mirrors the
+// synthetic ducklink writer bridge's `synthetic:writer/writer` world: exports
+// only storage-write-dispatch@2.0.0. No imports (the bridge is a plain writer
+// component that does not call any host-side interfaces).
+world write-boundary-test {
+  export duckdb:extension/storage-write-dispatch@2.0.0;
+}
diff --git a/crates/storage-boundary-test/wit-write/deps.toml b/crates/storage-boundary-test/wit-write/deps.toml
new file mode 100644
index 00000000..0ab11cfc
--- /dev/null
+++ b/crates/storage-boundary-test/wit-write/deps.toml
@@ -0,0 +1,6 @@
+[package]
+name = "boundary-test:write"
+version = "0.1.0"
+
+[dependencies]
+"duckdb:extension" = { path = "deps/duckdb-extension" }
diff --git a/crates/storage-boundary-test/wit-write/deps/duckdb-extension/deps.toml b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/deps.toml
new file mode 100644
index 00000000..f0d09d11
--- /dev/null
+++ b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/deps.toml
@@ -0,0 +1,3 @@
+[package]
+name = "duckdb:extension"
+version = "2.0.0"
diff --git a/crates/storage-boundary-test/wit-write/deps/duckdb-extension/storage-write-dispatch.wit b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/storage-write-dispatch.wit
new file mode 100644
index 00000000..5fa6a1fd
--- /dev/null
+++ b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/storage-write-dispatch.wit
@@ -0,0 +1,48 @@
+package duckdb:extension@2.0.0;
+
+use types;
+
+// Host -> component callbacks for the WRITABLE half of a storage backend
+// (additive, 2.1.0). The read-only `storage-dispatch` (attach/list/scan) is left
+// INTACT; this adds the mutating path -- transactions, DDL, and DML -- for
+// storage backends that can write through. Exported by writable storage
+// components via the separate `duckdb-extension-storage-write` world.
+//
+// Every call carries `handle` = the callback-handle the component passed to
+// storage.register-storage; `catalog` is the catalog handle from
+// storage-dispatch.storage-attach.
+interface storage-write-dispatch {
+  use types.{duckerror, duckvalue, columndef};
+
+  // Begin a transaction on `catalog`; returns a transaction handle.
+  begin-transaction: func(handle: u32, catalog: u32) -> result<u32, duckerror>;
+
+  commit-transaction: func(handle: u32, txn: u32) -> result<_, duckerror>;
+  rollback-transaction: func(handle: u32, txn: u32) -> result<_, duckerror>;
+
+  // CREATE TABLE within `txn`.
+  create-table: func(handle: u32,
+                     txn: u32,
+                     table: string,
+                     columns: list<columndef>) -> result<_, duckerror>;
+
+  // Append rows; returns the number inserted.
+  insert-rows: func(handle: u32,
+                    txn: u32,
+                    table: string,
+                    rows: list<list<duckvalue>>) -> result<u64, duckerror>;
+
+  // Delete rows by row-id; returns the number deleted.
+  delete-rows: func(handle: u32,
+                    txn: u32,
+                    table: string,
+                    rowids: list<s64>) -> result<u64, duckerror>;
+
+  // Update rows by row-id (parallel `rowids` / `rows`); returns the number
+  // updated.
+  update-rows: func(handle: u32,
+                    txn: u32,
+                    table: string,
+                    rowids: list<s64>,
+                    rows: list<list<duckvalue>>) -> result<u64, duckerror>;
+}
diff --git a/crates/storage-boundary-test/wit-write/deps/duckdb-extension/types.wit b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/types.wit
new file mode 100644
index 00000000..2da11dfc
--- /dev/null
+++ b/crates/storage-boundary-test/wit-write/deps/duckdb-extension/types.wit
@@ -0,0 +1,173 @@
+package duckdb:extension@2.0.0;
+
+interface types {
+  // ESCAPE-HATCH logical type. The `complex` arm carries a DuckDB type-expression
+  // string (e.g. "INTEGER[]", "STRUCT(a INTEGER, b VARCHAR)"); the core resolves it
+  // to a real logical type. This makes `logicaltype` a `variant` (was an `enum`); the
+  // fieldless arms keep their discriminant order, so the canonical-ABI is appended-to,
+  // not reordered. Any FUTURE type rides this arm with no further contract bump.
+  variant logicaltype {
+    boolean,
+    int64,
+    uint64,
+    float64,
+    text,
+    blob,
+    int32,
+    timestamp,
+    int8,
+    int16,
+    uint8,
+    uint16,
+    uint32,
+    float32,
+    date,
+    time,
+    timestamptz,
+    decimal,
+    interval,
+    uuid,
+    complex(string),
+  }
+
+  // DuckDB DECIMAL is a HUGEINT-backed scaled integer:
+  //   value = ((upper as i128) << 64 | lower as i128), interpreted with
+  //   `width` total digits and `scale` fractional digits.
+  record decimalvalue {
+    lower: u64,
+    upper: u64,
+    width: u8,
+    scale: u8,
+  }
+
+  // DuckDB INTERVAL: months + days + microseconds components.
+  record intervalvalue {
+    months: s32,
+    days: s32,
+    micros: s64,
+  }
+
+  // 128-bit UUID. hi/lo are the logical big-endian halves of the UUID value
+  // (NOT the sign-flipped physical hugeint storage DuckDB uses internally).
+  record uuidvalue {
+    hi: u64,
+    lo: u64,
+  }
+
+  // ESCAPE-HATCH composite value. `type-expr` is a DuckDB type string (e.g.
+  // "INTEGER[]" / "STRUCT(a INTEGER, b VARCHAR)") and `json` is the value rendered
+  // as JSON (e.g. "[10,20,30]" / {"a":1,"b":"hi"}). This is a FLAT record -- it does
+  // NOT reference `duckvalue`, so there is no recursive WIT cycle. The core
+  // reconstructs the real LIST/STRUCT vector from the JSON via the duckdb C vector
+  // API (which has no recursion limit).
+  record complexvalue {
+    type-expr: string,
+    json: string,
+  }
+
+  record funcarg {
+    name: option<string>,
+    logical: logicaltype,
+  }
+
+  flags funcflags {
+    deterministic,
+    commutative,
+    stateless,
+    sideeffecting,
+    deprecated,
+  }
+
+  record funcopts {
+    description: option<string>,
+    tags: list<string>,
+    attributes: funcflags,
+  }
+
+  record columndef {
+    name: string,
+    logical: logicaltype,
+  }
+
+  record extopts {
+    description: option<string>,
+    tags: list<string>,
+  }
+
+  variant duckvalue {
+    null,
+    boolean(bool),
+    int64(s64),
+    uint64(u64),
+    float64(f64),
+    text(string),
+    blob(list<u8>),
+    int32(s32),
+    timestamp(s64),
+    int8(s8),
+    int16(s16),
+    uint8(u8),
+    uint16(u16),
+    uint32(u32),
+    float32(f32),
+    date(s32),
+    time(s64),
+    timestamptz(s64),
+    decimal(decimalvalue),
+    interval(intervalvalue),
+    uuid(uuidvalue),
+    complex(complexvalue),
+  }
+
+  type resultset = list<list<duckvalue>>;
+  type rowbatch = list<list<duckvalue>>;
+
+  record invokeinfo {
+    rowindex: option<u64>,
+    iswindow: bool,
+  }
+
+  variant duckerror {
+    invalidargument(string),
+    unsupported(string),
+    invalidstate(string),
+    io(string),
+    internal(string),
+  }
+
+  variant configerror {
+    invalidkey(string),
+    typemismatch(string),
+    unavailable(string),
+    internalconfig(string),
+  }
+
+  enum loglevel {
+    trace,
+    debug,
+    info,
+    warn,
+    error,
+  }
+
+  record logfield {
+    key: string,
+    value: string,
+  }
+
+  record loadresult {
+    name: string,
+    version: option<string>,
+    requires: list<capabilitykind>,
+  }
+
+  enum capabilitykind {
+    scalar,
+    table,
+    aggregate,
+    pragma,
+    macro,
+    catalog,
+    file-format,
+  }
+}