diff --git a/.changeset/flat-tap-err-is-result.md b/.changeset/flat-tap-err-is-result.md
new file mode 100644
index 0000000..b891ca3
--- /dev/null
+++ b/.changeset/flat-tap-err-is-result.md
@@ -0,0 +1,17 @@
+---
+"unthrown": minor
+---
+
+Add two members, closing the only structural gaps surfaced by comparing the
+surface against boxed / neverthrow / byethrow:
+
+- **`flatTapErr`** (on `Result` and `AsyncResult`) — the error-channel mirror of
+  `flatTap`: runs a `Result`-returning effect on the error, keeps the original
+  error on the effect's success, and threads the effect's error otherwise
+  (`Result<T, E | E2>`). A throw becomes a `Defect`, like every other combinator.
+  Use it for a failable effect _during_ error handling (e.g. writing the error to
+  an audit log that may itself fail).
+- **`isResult(x)`** — a standalone type guard narrowing an `unknown` to
+  `Result<unknown, unknown>` (and `Result.isResult`). It checks the value carries
+  the `Result` prototype, so a plain `{ tag: "Ok" }` look-alike is not matched;
+  an `AsyncResult` is not a `Result`. For untyped interop boundaries.
diff --git a/CLAUDE.md b/CLAUDE.md
index b146a3f..a195881 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -47,7 +47,7 @@ was planned).
 
 - **Throw → defect.** Any value thrown by a callback inside a combinator
   (`map`, `flatMap`, `flatTap`, `bind`, `let`, `mapErr`, `orElse`, `recover`,
-  `tap*`, `recoverDefect`) is caught and converted to a `Defect`. Nothing
+  `tap*`, `flatTapErr`, `recoverDefect`) is caught and converted to a `Defect`. Nothing
   escapes a pipeline as a raw throw.
   This is what lets an HTTP adapter do a single `match({ ok, err, defect })`
   with **no surrounding `try/catch`**.
@@ -93,14 +93,18 @@ async work re-enters via `fromPromise` / `fromSafePromise` and composes with
   pure value). On `AsyncResult`, `bind`'s `f` may return a `Result` or an
   `AsyncResult`. A throw in either becomes a `Defect`; `Err`/`Defect`
   short-circuits/passes through. To go async, lift with `toAsync()`.
-- error: `mapErr`, `orElse`, `recover`, `tapErr`
+- error: `mapErr`, `orElse`, `recover`, `tapErr`, `flatTapErr` (the error-channel
+  mirror of `flatTap` — runs a `Result`-returning effect on the error, keeps the
+  original error, threads the effect's error)
 - defect: `recoverDefect`, `tapDefect`
 - eliminate: `match`, `unwrap`, `unwrapErr`, `unwrapOr`, `unwrapOrElse`,
   `getOrNull`, `getOrUndefined`
 - guards: methods `isOk`/`isErr`/`isDefect` **and** standalone
   `isOk`/`isErr`/`isDefect` both narrow (to `OkView`/`ErrView`/`DefectView`) — the
   methods are `this is …` type predicates, so `if (r.isErr()) r.error` compiles.
-  One narrowing concept, two call styles.
+  One narrowing concept, two call styles. Plus the standalone `isResult(x)` —
+  narrows an `unknown` to `Result<unknown, unknown>` (a prototype check, so a
+  plain `{ tag: "Ok" }` look-alike is not matched), for untyped boundaries.
 - constructors: `Ok`, `Err`, `Defect`
 - interop: `fromNullable`, `fromThrowable`, `fromPromise`, `fromSafePromise`
 - aggregate: `all` / `allAsync` take a **tuple/array** (a fixed tuple keeps
diff --git a/docs/guide/choosing-a-combinator.md b/docs/guide/choosing-a-combinator.md
index 7fe2c81..065507d 100644
--- a/docs/guide/choosing-a-combinator.md
+++ b/docs/guide/choosing-a-combinator.md
@@ -18,6 +18,7 @@ channel** and turns a thrown callback into a `Defect`.
 | try a fallback that returns a `Result`         | `orElse`           | Err     |
 | turn an error into a success value             | `recover`          | Err     |
 | run a side effect on the error                 | `tapErr`           | Err     |
+| run a **failable** side effect on the error    | `flatTapErr`       | Err     |
 | recover from a defect (rare)                   | `recoverDefect`    | Defect  |
 | observe a defect, e.g. log it                  | `tapDefect`        | Defect  |
 | handle all three channels at the edge          | `match`            | all     |
@@ -38,6 +39,12 @@ don't need).
 **replaces** the value with the callback's; `flatTap` **discards** the callback's
 value and keeps the original.
 
+**`tapErr` vs `flatTapErr`** — the error-channel mirror of `tap` vs `flatTap`.
+Both run only on `Err` and keep the original error on success. `tapErr` takes a
+`void` callback that can't fail; `flatTapErr` takes a `Result`-returning one and
+threads its error (a failable effect _during_ error handling — e.g. writing the
+error to an audit log that may itself fail).
+
 **`orElse` vs `recover`** — both run on `Err`. `recover` produces a plain success
 value (emptying the error channel to `never`); `orElse` produces another
 `Result` (which may still be an `Err`).
diff --git a/docs/guide/core-concepts.md b/docs/guide/core-concepts.md
index 96bae2f..e69d205 100644
--- a/docs/guide/core-concepts.md
+++ b/docs/guide/core-concepts.md
@@ -26,7 +26,7 @@ Every `Result` shares one method surface, grouped by the channel it touches:
 - **success** (runs on `Ok`): `map`, `flatMap`, `tap`, `flatTap`, `as`
 - **do-notation** (runs on `Ok`): `bind`, `let` — accumulate a named scope; see
   [Do Notation](./do-notation)
-- **error** (runs on `Err`): `mapErr`, `orElse`, `recover`, `tapErr`
+- **error** (runs on `Err`): `mapErr`, `orElse`, `recover`, `tapErr`, `flatTapErr`
 - **defect** (the only door to a `Defect`): `recoverDefect`, `tapDefect`
 - **eliminate**: `match`, `unwrap`, `unwrapErr`, `unwrapOr`, `unwrapOrElse`,
   `getOrNull`, `getOrUndefined`
@@ -98,6 +98,11 @@ if (r.isErr()) {
 }
 ```
 
+To narrow an **`unknown`** value (e.g. at an untyped boundary) to a `Result` in
+the first place, use the standalone `isResult(x)`. It checks the value carries
+the `Result` prototype, so a plain look-alike like `{ tag: "Ok" }` is **not**
+matched.
+
 ## Eliminating a Result
 
 Once you are ready to leave the `Result` world, pick the right exit:
diff --git a/packages/core/src/async-result.spec.ts b/packages/core/src/async-result.spec.ts
index db742b3..af6c91d 100644
--- a/packages/core/src/async-result.spec.ts
+++ b/packages/core/src/async-result.spec.ts
@@ -71,6 +71,7 @@ describe("AsyncResult: a throw in any combinator becomes a Defect", () => {
     expect((await asyncErr("e").orElse(t)).isDefect()).toBe(true);
     expect((await asyncErr("e").recover(t)).isDefect()).toBe(true);
     expect((await asyncErr("e").tapErr(t)).isDefect()).toBe(true);
+    expect((await asyncErr("e").flatTapErr(t)).isDefect()).toBe(true);
     expect((await asyncDefect().recoverDefect(t)).isDefect()).toBe(true);
     expect((await asyncDefect().tapDefect(t)).isDefect()).toBe(true);
   });
@@ -158,6 +159,29 @@ describe("AsyncResult error channel", () => {
     expect(seen).toEqual(["e"]);
     expect(r.unwrapErr()).toBe("e");
   });
+
+  it("flatTapErr keeps the original error when the effect succeeds", async () => {
+    const r = await asyncErr("e").flatTapErr(() => Ok("ignored"));
+    expect(r.unwrapErr()).toBe("e");
+  });
+
+  it("flatTapErr threads the effect's Err", async () => {
+    expect((await asyncErr("e").flatTapErr(() => Err("log_failed"))).unwrapErr()).toBe(
+      "log_failed",
+    );
+  });
+
+  it("flatTapErr composes an async effect via a qualified boundary, keeping the error", async () => {
+    const r = await asyncErr("e").flatTapErr(() => fromSafePromise(Promise.resolve("logged")));
+    expect(r.unwrapErr()).toBe("e");
+  });
+
+  it("flatTapErr does not run the effect on Ok or Defect", async () => {
+    const f = vi.fn(() => Ok(1));
+    expect((await asyncOk(1).flatTapErr(f)).unwrap()).toBe(1);
+    expect((await asyncDefect().flatTapErr(f)).isDefect()).toBe(true);
+    expect(f).not.toHaveBeenCalled();
+  });
 });
 
 describe("AsyncResult Defect channel", () => {
diff --git a/packages/core/src/constructors.spec.ts b/packages/core/src/constructors.spec.ts
index 89ee897..75ff30c 100644
--- a/packages/core/src/constructors.spec.ts
+++ b/packages/core/src/constructors.spec.ts
@@ -1,6 +1,16 @@
 import { describe, expect, it } from "vitest";
 
-import { Defect, Err, isDefect, isErr, isOk, Ok, type Result, UnwrapError } from "./index.js";
+import {
+  Defect,
+  Err,
+  isDefect,
+  isErr,
+  isOk,
+  isResult,
+  Ok,
+  type Result,
+  UnwrapError,
+} from "./index.js";
 
 const boom = new Error("boom");
 const defectOf = (cause: unknown): Result<number, never> =>
@@ -69,6 +79,27 @@ describe("standalone guards narrow and expose the relevant field", () => {
   });
 });
 
+describe("isResult narrows an unknown value to a Result", () => {
+  it("is true for every Result variant", () => {
+    expect(isResult(Ok(1))).toBe(true);
+    expect(isResult(Err("e"))).toBe(true);
+    expect(isResult(defectOf(boom))).toBe(true);
+  });
+
+  it("is false for look-alikes, primitives, and unrelated objects", () => {
+    expect(isResult({ tag: "Ok", value: 1 })).toBe(false); // structural look-alike
+    expect(isResult(null)).toBe(false);
+    expect(isResult(undefined)).toBe(false);
+    expect(isResult(42)).toBe(false);
+    expect(isResult("Ok")).toBe(false);
+    expect(isResult({})).toBe(false);
+  });
+
+  it("is false for an AsyncResult (not a Result)", () => {
+    expect(isResult(Ok(1).toAsync())).toBe(false);
+  });
+});
+
 describe("method guards narrow (parity with the standalone guards)", () => {
   it("r.isOk() narrows to OkView and exposes value", () => {
     const r: Result<number, string> = Ok(7);
diff --git a/packages/core/src/core.ts b/packages/core/src/core.ts
index 1446804..f1df819 100644
--- a/packages/core/src/core.ts
+++ b/packages/core/src/core.ts
@@ -166,6 +166,17 @@ class Res<T, E> {
     }
   }
 
+  flatTapErr<E2>(this: Result<T, E>, f: (error: E) => Result<unknown, E2>): Result<T, E | E2> {
+    if (this.tag !== "Err") return this;
+    try {
+      const r = f(this.error);
+      // Keep the original error on the effect's success; an Err/Defect threads through.
+      return r.tag === "Ok" ? this : passThrough(r);
+    } catch (cause) {
+      return defectRes(cause);
+    }
+  }
+
   recoverDefect<U, E2>(
     this: Result<T, E>,
     f: (cause: unknown) => Result<U, E2>,
@@ -307,6 +318,22 @@ export function defectRes<T, E>(cause: unknown): Result<T, E> {
   }) as DefectView<T, E>;
 }
 
+/**
+ * Type guard: is `x` a {@link Result} (any of `Ok` / `Err` / `Defect`)?
+ *
+ * @remarks
+ * Unlike {@link isOk} / {@link isErr} / {@link isDefect}, which narrow a value
+ * already known to be a `Result`, this narrows from `unknown` — useful at an
+ * untyped boundary. It checks the value carries the `Result` prototype, so a
+ * look-alike plain object (`{ tag: "Ok" }`) is **not** matched. An `AsyncResult`
+ * is not a `Result` and returns `false`.
+ *
+ * @returns `true` when `x` is a `Result` produced by this library.
+ */
+export function isResult(x: unknown): x is Result<unknown, unknown> {
+  return x instanceof Res;
+}
+
 /**
  * Reuse a non-matching variant (an `Err` or `Defect`) as a differently-typed
  * `Result`, with no runtime work. Sound because the passed-through variant
@@ -517,6 +544,23 @@ export class AsyncRes<T, E> implements AsyncResult<T, E> {
     );
   }
 
+  flatTapErr<E2>(
+    f: (error: E) => Result<unknown, E2> | AsyncResult<unknown, E2>,
+  ): AsyncResult<T, E | E2> {
+    return new AsyncRes<T, E | E2>(
+      this.promise.then(async (r) => {
+        if (r.tag !== "Err") return passThrough(r);
+        try {
+          const inner = await f(r.error);
+          // Keep the original error on success; an Err/Defect from `f` wins.
+          return inner.tag === "Ok" ? passThrough(r) : passThrough(inner);
+        } catch (cause) {
+          return defectRes(cause);
+        }
+      }),
+    );
+  }
+
   recoverDefect<U, E2>(
     f: (cause: unknown) => Result<U, E2> | AsyncResult<U, E2>,
   ): AsyncResult<T | U, E | E2> {
diff --git a/packages/core/src/facade.spec.ts b/packages/core/src/facade.spec.ts
index 8d61455..f770d93 100644
--- a/packages/core/src/facade.spec.ts
+++ b/packages/core/src/facade.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { all, Err, fromNullable, isDefect, isErr, isOk, Ok, Result } from "./index.js";
+import { all, Err, fromNullable, isDefect, isErr, isOk, isResult, Ok, Result } from "./index.js";
 
 const boom = new Error("boom");
 
@@ -16,6 +16,7 @@ describe("Result facade mirrors the free functions", () => {
     expect(Result.isOk).toBe(isOk);
     expect(Result.isErr).toBe(isErr);
     expect(Result.isDefect).toBe(isDefect);
+    expect(Result.isResult).toBe(isResult);
 
     const d = Result.Ok(1).map(() => {
       throw boom;
@@ -23,6 +24,8 @@ describe("Result facade mirrors the free functions", () => {
     expect(Result.isOk(Result.Ok(1))).toBe(true);
     expect(Result.isErr(Result.Err("e"))).toBe(true);
     expect(Result.isDefect(d)).toBe(true);
+    expect(Result.isResult(Result.Ok(1))).toBe(true);
+    expect(Result.isResult(42)).toBe(false);
   });
 
   it("exposes the same interop and aggregate entry points", () => {
diff --git a/packages/core/src/facade.ts b/packages/core/src/facade.ts
index 91feb73..ad7df24 100644
--- a/packages/core/src/facade.ts
+++ b/packages/core/src/facade.ts
@@ -5,6 +5,7 @@
 // companion-object pattern. See CLAUDE.md → "Internal design".
 
 import { Err, isDefect, isErr, isOk, Ok } from "./constructors.js";
+import { isResult } from "./core.js";
 import { Defect } from "./defect.js";
 import { Do } from "./do.js";
 import {
@@ -26,7 +27,7 @@ import type { Result as ResultType } from "./types.js";
  * {@link Result.fromPromise}, {@link Result.fromSafePromise}, {@link Result.all},
  * {@link Result.allAsync}, {@link Result.allFromDict},
  * {@link Result.allFromDictAsync}, {@link Result.isOk}, {@link Result.isErr},
- * {@link Result.isDefect}.
+ * {@link Result.isDefect}, {@link Result.isResult}.
  *
  * @remarks
  * Purely additive sugar — each member **is** the corresponding free function.
@@ -56,6 +57,7 @@ export const Result = {
   isOk,
   isErr,
   isDefect,
+  isResult,
 } as const;
 
 // Re-alias the Result type into this module so a single `export { Result }`
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index a6efbba..f5218e1 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -1,5 +1,5 @@
 export { Err, isDefect, isErr, isOk, Ok } from "./constructors.js";
-export { UnwrapError } from "./core.js";
+export { isResult, UnwrapError } from "./core.js";
 export { Defect } from "./defect.js";
 export { Do } from "./do.js";
 export { Result } from "./facade.js";
diff --git a/packages/core/src/invariants.spec.ts b/packages/core/src/invariants.spec.ts
index a58c3ea..d11b3db 100644
--- a/packages/core/src/invariants.spec.ts
+++ b/packages/core/src/invariants.spec.ts
@@ -26,6 +26,7 @@ describe("Invariant 1: throw inside any combinator becomes a Defect", () => {
     expect(Err("e").orElse(t).isDefect()).toBe(true);
     expect(Err("e").recover(t).isDefect()).toBe(true);
     expect(Err("e").tapErr(t).isDefect()).toBe(true);
+    expect(Err("e").flatTapErr(t).isDefect()).toBe(true);
     expect(defectOf(boom).recoverDefect(t).isDefect()).toBe(true);
     expect(defectOf(boom).tapDefect(t).isDefect()).toBe(true);
   });
@@ -46,6 +47,7 @@ describe("Invariant 2: a Defect flows through every method except match() and re
       defectOf(boom).orElse(f),
       defectOf(boom).recover(f),
       defectOf(boom).tapErr(f),
+      defectOf(boom).flatTapErr(f),
     ];
     for (const r of passesThrough) expect(r.isDefect()).toBe(true);
     expect(f).not.toHaveBeenCalled();
diff --git a/packages/core/src/result.spec.ts b/packages/core/src/result.spec.ts
index 4ac204e..87189ed 100644
--- a/packages/core/src/result.spec.ts
+++ b/packages/core/src/result.spec.ts
@@ -277,6 +277,45 @@ describe("Result.tapErr", () => {
   });
 });
 
+describe("Result.flatTapErr", () => {
+  it("runs the failable effect on Err and keeps the original error on success", () => {
+    const seen: string[] = [];
+    const r = Err("e").flatTapErr((s) => {
+      seen.push(s);
+      return Ok("ignored");
+    });
+    expect(seen).toEqual(["e"]);
+    expect(r.unwrapErr()).toBe("e"); // original error preserved
+  });
+
+  it("threads the effect's Err", () => {
+    const r = Err("e").flatTapErr(() => Err("log_failed"));
+    expect(r.unwrapErr()).toBe("log_failed");
+  });
+
+  it("propagates a Defect from the effect", () => {
+    const r = Err("e").flatTapErr(() => defectOf(boom));
+    expect(r.isDefect()).toBe(true);
+  });
+
+  it("does not run on Ok or Defect", () => {
+    const f = vi.fn(() => Ok(1));
+    expect(Ok(1).flatTapErr(f).unwrap()).toBe(1);
+    expect(defectOf(boom).flatTapErr(f).isDefect()).toBe(true);
+    expect(f).not.toHaveBeenCalled();
+  });
+
+  it("converts a throw into a Defect", () => {
+    expect(
+      Err("e")
+        .flatTapErr(() => {
+          throw boom;
+        })
+        .isDefect(),
+    ).toBe(true);
+  });
+});
+
 describe("Result.recoverDefect (the only door to a Defect)", () => {
   it("replaces a Defect with an Ok", () => {
     expect(
diff --git a/packages/core/src/types.test-d.ts b/packages/core/src/types.test-d.ts
index d88d1ab..e741884 100644
--- a/packages/core/src/types.test-d.ts
+++ b/packages/core/src/types.test-d.ts
@@ -23,6 +23,7 @@ import {
   isDefect,
   isErr,
   isOk,
+  isResult,
   matchTags,
   Ok,
   type OkOf,
@@ -108,6 +109,10 @@ type _flatMapped = Expect<Equal<typeof flatMapped, Result<never, "e1" | "e2">>>;
 const flatTapped = r1.flatTap(() => Err<"e2">("e2"));
 type _flatTapped = Expect<Equal<typeof flatTapped, Result<number, "e1" | "e2">>>;
 
+// flatTapErr KEEPS the value type and widens the error channel (error-side mirror)
+const flatTapErred = r1.flatTapErr(() => Err<"e2">("e2"));
+type _flatTapErred = Expect<Equal<typeof flatTapErred, Result<number, "e1" | "e2">>>;
+
 // recover empties the error channel (to `never`)
 const recovered = r1.recover(() => 0);
 type _recovered = Expect<Equal<typeof recovered, Result<number, never>>>;
@@ -178,6 +183,12 @@ if (isDefect(g)) {
   type _sc = Expect<Equal<typeof g.cause, unknown>>;
 }
 
+// isResult narrows `unknown` to a Result
+declare const u: unknown;
+if (isResult(u)) {
+  type _isResult = Expect<Equal<typeof u, Result<unknown, unknown>>>;
+}
+
 // the payload is unreachable before narrowing
 // @ts-expect-error - `.value` only exists on the Ok variant
 const _noValue = g.value;
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 2a32d66..b76fdeb 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -165,6 +165,23 @@ export type ResultMethods<T, E> = {
    * @param f - the side effect (its return value is ignored).
    */
   tapErr(f: (error: E) => void): Result<T, E>;
+  /**
+   * Run a **failable** side effect on the error, keeping the original error but
+   * threading the effect's own error.
+   *
+   * @remarks
+   * The error-channel mirror of {@link ResultMethods.flatTap | flatTap}: `f`
+   * returns a `Result`, but its **success value is discarded** — on the effect's
+   * `Ok` the original `Err` flows through unchanged, while an `Err` (or `Defect`)
+   * from `f` short-circuits and threads its error (`Result<T, E | E2>`). Runs only
+   * on `Err`; `Ok` and `Defect` pass through. If `f` throws, the throw becomes a
+   * `Defect`. Use it for a failable effect _during_ error handling (e.g. writing
+   * the error to an audit log that may itself fail).
+   *
+   * @typeParam E2 - the error type the effect may introduce.
+   * @param f - the failable side effect; its `Ok` value is ignored.
+   */
+  flatTapErr<E2>(f: (error: E) => Result<unknown, E2>): Result<T, E | E2>;
 
   /**
    * Recover from a `Defect` — the **only** combinator that can touch one.
@@ -388,6 +405,15 @@ export type AsyncResult<T, E> = Awaitable<Result<T, E>> & {
   recover<U>(f: (error: E) => U): AsyncResult<T | U, never>;
   /** Asynchronous `tapErr`. `f` is synchronous; a throw becomes a `Defect`. */
   tapErr(f: (error: E) => void): AsyncResult<T, E>;
+  /**
+   * Asynchronous `flatTapErr` — a failable tap on the error that keeps the
+   * original error. `f` may return a `Result` **or** an `AsyncResult`; its `Ok`
+   * value is discarded, an `Err`/`Defect` from `f` threads through, and a throw
+   * becomes a `Defect`.
+   */
+  flatTapErr<E2>(
+    f: (error: E) => Result<unknown, E2> | AsyncResult<unknown, E2>,
+  ): AsyncResult<T, E | E2>;
 
   /** Asynchronous `recoverDefect`. `f` may return a `Result` or an `AsyncResult`. */
   recoverDefect<U, E2>(