diff --git a/tests/error-paths.test.ts b/tests/error-paths.test.ts
new file mode 100644
index 0000000..df88c9c
--- /dev/null
+++ b/tests/error-paths.test.ts
@@ -0,0 +1,244 @@
+import { env, runInDurableObject, SELF } from "cloudflare:test"
+import { describe, expect, it } from "vitest"
+import { createFrameCodec } from "../src/wire/frame-codec.ts"
+import type { ClientFrame, ServerFrame } from "../src/wire/frames.ts"
+
+// WHY: these tests pin three load-bearing invariants in the server's error paths:
+//   1. Atomicity — a failed mutation leaves no partial state; nothing is applied
+//      and no delta frames leak to subscribers.
+//   2. Fail-loud — the response to a failed operation is always a `rejected` frame
+//      with the cause, never a silent drop or a `committed`.
+//   3. Exactly-once in failure — a retried rejected txId replays the rejection from
+//      the dedup record without re-running the handler.
+//
+// A regression in any of these paths (rolled-back op leaving a row, a `d` frame
+// arriving after rejection, a retry re-executing) would ship silently without these
+// tests. They are also the verification baseline that plans 004 and 005 build on.
+
+const codec = createFrameCodec()
+
+async function openWs(path: string): Promise<WebSocket> {
+  const res = await SELF.fetch(`https://example.com${path}`, { headers: { Upgrade: "websocket" } })
+  expect(res.status).toBe(101)
+  const ws = res.webSocket
+  if (!ws) throw new Error("no webSocket on 101 response")
+  ws.accept()
+  return ws
+}
+
+function collectUntil(ws: WebSocket, done: (f: ServerFrame) => boolean, timeoutMs = 2000): Promise<Array<ServerFrame>> {
+  return new Promise((resolve, reject) => {
+    const out: Array<ServerFrame> = []
+    const timer = setTimeout(() => reject(new Error(`timeout; got [${out.map((f) => f.t).join(",")}]`)), timeoutMs)
+    const onMsg = (e: MessageEvent): void => {
+      out.push(codec.decode(e.data as ArrayBuffer) as ServerFrame)
+      if (done(out[out.length - 1]!)) {
+        clearTimeout(timer)
+        ws.removeEventListener("message", onMsg)
+        resolve(out)
+      }
+    }
+    ws.addEventListener("message", onMsg)
+  })
+}
+
+function send(ws: WebSocket, frame: ClientFrame): void {
+  ws.send(codec.encode(frame))
+}
+
+/** Subscribe and wait for the initial snap-end so later frames are post-sub. */
+async function subscribe(ws: WebSocket, subId: string): Promise<void> {
+  send(ws, { t: "sub", subId, collection: "messages" })
+  await collectUntil(ws, (f) => f.t === "snap-end")
+}
+
+describe("mutation error paths (atomicity, fail-loud, exactly-once)", () => {
+  it("execute throws → rejected, rolled back, no deltas", async () => {
+    const ws = await openWs("/sync/err-exec-throw")
+    await subscribe(ws, "s1")
+
+    // Insert key "a" successfully.
+    send(ws, { t: "mut", txId: "t1", collection: "messages", ops: [{ type: "insert", key: "a", cols: { id: "a", body: "v1" } }] })
+    await collectUntil(ws, (f) => f.t === "committed")
+
+    // Insert the SAME key "a" — SQLite PRIMARY KEY throws inside transactionSync.
+    send(ws, { t: "mut", txId: "t2", collection: "messages", ops: [{ type: "insert", key: "a", cols: { id: "a", body: "v2" } }] })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+
+    const last = frames[frames.length - 1]!
+    expect(last.t).toBe("rejected")
+    expect(frames.some((f) => f.t === "d")).toBe(false) // no delta for the failed write
+
+    // Via server-side inspection: original row is intact.
+    const room = "err-exec-throw"
+    const rows = await runInDurableObject(env.SYNC_DO.get(env.SYNC_DO.idFromName(room)), (_i, s) => {
+      return Array.from(s.storage.sql.exec("SELECT body FROM messages WHERE id='a'"))
+    })
+    expect(rows).toHaveLength(1)
+    expect((rows[0] as { body: string }).body).toBe("v1")
+    ws.close()
+  })
+
+  it("execute throw rolls back the WHOLE multi-op batch", async () => {
+    const subWs = await openWs("/sync/err-multi-rollback")
+    await subscribe(subWs, "s1")
+
+    const ws = await openWs("/sync/err-multi-rollback")
+    // Two ops: op 2 duplicates the PK of op 1 — the whole batch must roll back.
+    send(ws, {
+      t: "mut",
+      txId: "t1",
+      collection: "messages",
+      ops: [
+        { type: "insert", key: "x", cols: { id: "x", body: "first" } },
+        { type: "insert", key: "x", cols: { id: "x", body: "dup" } },
+      ],
+    })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    expect(frames[frames.length - 1]!.t).toBe("rejected")
+
+    // op 1 must also be rolled back (count = 0).
+    const room = "err-multi-rollback"
+    const rows = await runInDurableObject(env.SYNC_DO.get(env.SYNC_DO.idFromName(room)), (_i, s) => {
+      return Array.from(s.storage.sql.exec("SELECT COUNT(*) as cnt FROM messages WHERE id='x'"))
+    })
+    expect((rows[0] as { cnt: number }).cnt).toBe(0)
+
+    // No delta frames arrived on the subscribed socket either.
+    // We collect with a small timeout (no terminal frame to wait for).
+    let deltaArrived = false
+    subWs.addEventListener("message", (e) => {
+      const f = codec.decode(e.data as ArrayBuffer) as ServerFrame
+      if (f.t === "d") deltaArrived = true
+    })
+    await new Promise((r) => setTimeout(r, 100))
+    expect(deltaArrived).toBe(false)
+    ws.close()
+    subWs.close()
+  })
+
+  it("authorize failure on op 2 rejects whole batch before any write", async () => {
+    const ws = await openWs("/sync/err-authz-op2")
+    await subscribe(ws, "s1")
+
+    send(ws, {
+      t: "mut",
+      txId: "t1",
+      collection: "messages",
+      ops: [
+        { type: "insert", key: "ok1", cols: { id: "ok1", body: "fine" } },
+        { type: "insert", key: "bad", cols: { id: "bad", body: "FORBIDDEN" } },
+      ],
+    })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const last = frames[frames.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(last.t).toBe("rejected")
+    expect(last.error.message).toMatch(/forbidden/)
+    expect(frames.some((f) => f.t === "d")).toBe(false)
+
+    // "ok1" must not exist server-side (authorize runs before any execute).
+    const room = "err-authz-op2"
+    const rows = await runInDurableObject(env.SYNC_DO.get(env.SYNC_DO.idFromName(room)), (_i, s) => {
+      return Array.from(s.storage.sql.exec("SELECT COUNT(*) as cnt FROM messages WHERE id='ok1'"))
+    })
+    expect((rows[0] as { cnt: number }).cnt).toBe(0)
+    ws.close()
+  })
+
+  it("unknown mutation collection → rejected", async () => {
+    const ws = await openWs("/sync/err-unknown-mut")
+    send(ws, { t: "mut", txId: "u1", collection: "nonexistent", ops: [{ type: "insert", key: "a", cols: { id: "a", body: "x" } }] })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const last = frames[frames.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(last.t).toBe("rejected")
+    expect(last.error.message).toMatch(/no mutation handler/)
+    ws.close()
+  })
+
+  it("a retried REJECTED txId replays the rejection without re-running", async () => {
+    const ws = await openWs("/sync/err-retry-rejected")
+    const frame: ClientFrame = { t: "mut", txId: "u1", collection: "nonexistent", ops: [{ type: "insert", key: "a", cols: { id: "a", body: "x" } }] }
+
+    send(ws, frame)
+    const first = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const r1 = first[first.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(r1.t).toBe("rejected")
+
+    // Retry the exact same frame — must replay the rejection from the dedup record.
+    send(ws, frame)
+    const second = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const r2 = second[second.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(r2.t).toBe("rejected")
+    expect(r2.error.message).toBe(r1.error.message)
+    ws.close()
+  })
+})
+
+describe("command error paths", () => {
+  it("command execute throws → rejected with message", async () => {
+    const ws = await openWs("/sync/err-cmd-boom")
+    send(ws, { t: "call", txId: "c1", name: "boom", args: {} })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const last = frames[frames.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(last.t).toBe("rejected")
+    expect(last.error.message).toMatch(/command boom/)
+    ws.close()
+  })
+
+  it("unknown command → rejected with UNKNOWN_COMMAND code", async () => {
+    const ws = await openWs("/sync/err-cmd-unknown")
+    send(ws, { t: "call", txId: "c2", name: "nope", args: {} })
+    const frames = await collectUntil(ws, (f) => f.t === "rejected" || f.t === "committed")
+    const last = frames[frames.length - 1]! as Extract<ServerFrame, { t: "rejected" }>
+    expect(last.t).toBe("rejected")
+    expect(last.error.code).toBe("UNKNOWN_COMMAND")
+    ws.close()
+  })
+})
+
+describe("retention floor reconnect reset", () => {
+  it("a since below the retention floor gets reset + fresh snapshot", async () => {
+    const room = "err-floor-reset"
+    const ws = await openWs(`/sync/${room}`)
+
+    // Insert three rows. We'll prune the first two change log rows so floor=3,
+    // which makes since="1" strictly below floor-1=2, triggering a reset.
+    send(ws, { t: "mut", txId: "t1", collection: "messages", ops: [{ type: "insert", key: "a", cols: { id: "a", body: "alpha" } }] })
+    await collectUntil(ws, (f) => f.t === "committed")
+
+    send(ws, { t: "mut", txId: "t2", collection: "messages", ops: [{ type: "insert", key: "b", cols: { id: "b", body: "beta" } }] })
+    const c2frames = await collectUntil(ws, (f) => f.t === "committed")
+    const secondSeq = Number((c2frames.find((f) => f.t === "committed") as Extract<ServerFrame, { t: "committed" }>).seq)
+
+    send(ws, { t: "mut", txId: "t3", collection: "messages", ops: [{ type: "insert", key: "c", cols: { id: "c", body: "gamma" } }] })
+    await collectUntil(ws, (f) => f.t === "committed")
+    ws.close()
+
+    // Simulate retention pruning: delete the first two change rows, leaving
+    // floor=3. The client's pretend cursor is since="1" < floor-1=2 → reset.
+    await runInDurableObject(env.SYNC_DO.get(env.SYNC_DO.idFromName(room)), (_i, s) => {
+      s.storage.sql.exec("DELETE FROM _sync_changes WHERE seq <= ?", secondSeq)
+    })
+
+    // Open a new socket and subscribe with since="1" — strictly below floor-1.
+    const ws2 = await openWs(`/sync/${room}`)
+    send(ws2, { t: "sub", subId: "s1", collection: "messages", since: "1" })
+    // Expect: reset for s1, then snap frames for both rows, then snap-end.
+    const frames = await collectUntil(ws2, (f) => f.t === "snap-end")
+
+    const resetIdx = frames.findIndex((f) => f.t === "reset")
+    expect(resetIdx).toBeGreaterThanOrEqual(0)
+    const resetFrame = frames[resetIdx] as Extract<ServerFrame, { t: "reset" }>
+    expect(resetFrame.sub).toBe("s1")
+
+    const snapFrames = frames.filter((f) => f.t === "snap")
+    expect(snapFrames.length).toBe(3) // all three rows in the fresh snapshot
+
+    const snapEnd = frames[frames.length - 1]! as Extract<ServerFrame, { t: "snap-end" }>
+    expect(snapEnd.t).toBe("snap-end")
+    // reset must come before snap frames.
+    const firstSnapIdx = frames.findIndex((f) => f.t === "snap")
+    expect(resetIdx).toBeLessThan(firstSnapIdx)
+    ws2.close()
+  })
+})
diff --git a/tests/test-worker.ts b/tests/test-worker.ts
index 74d874c..f939d66 100644
--- a/tests/test-worker.ts
+++ b/tests/test-worker.ts
@@ -88,6 +88,12 @@ export class SyncTestDO extends SyncDurableObject<unknown, Claims> {
           .defineCommand({
             name: "echo",
             execute: ({ args }) => ({ echoed: args }),
+          })
+          .defineCommand({
+            name: "boom",
+            execute: () => {
+              throw new Error("command boom")
+            },
           }),
       )
     })