diff --git a/AGENTS.md b/AGENTS.md
index 4369f52b4..6702582a4 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -180,7 +180,7 @@ source dependency-closure rule as any other migrated file: its relative runtime
 imports and JSDoc type references may target `.f.mjs` modules only. This is a
 JavaScript/JSDoc and dependency-readiness rule, not a FunctionalScript compiler
 feature gate. A proof that still imports an unmigrated helper such as
-`fjs/asserts/module.f.ts` remains `proof.f.ts` until that helper migrates. See
+`fjs/types/list/module.f.ts` remains `proof.f.ts` until that helper migrates. See
 [`fjs/fsc/README.md`](./fjs/fsc/README.md) for the migration order and the
 module-import policy it implies.
 
@@ -188,8 +188,8 @@ module-import policy it implies.
 
 Assert results in `proof` code with `assert`/`assertEq` from the current authored
 `fjs/asserts/module.f.*` source, not a hand-written `if (cond) { throw ... }`.
-During Stage 1 that helper is `.f.ts` until it migrates to `.f.mjs`; a
-`proof.f.mjs` that needs it must wait for that dependency migration.
+That helper has already migrated, so the current source is
+`fjs/asserts/module.f.mjs` and a `proof.f.mjs` may import it directly.
 
 A local `if`/`throw` in a test is itself a new branch for the coverage tool to
 track, and its failure side is normally never exercised (the test is expected to
@@ -202,7 +202,7 @@ new uncovered branch.
 
 To prove that a type resolves to what you claim, write
 `type _Name = Assert<Equal<Actual, Expected>>` — `Assert` from
-`fjs/asserts/module.f.ts`, `Equal` from `fjs/types/ts/module.f.ts`. A wrong
+`fjs/asserts/module.f.mjs`, `Equal` from `fjs/types/ts/module.f.mjs`. A wrong
 claim is then a compile error (TS2344, "Type 'false' does not satisfy the
 constraint 'true'"), and the check costs nothing at runtime.
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3de0bfb0e..d5b87cdc3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,11 @@ history.
 
 ## Unreleased
 
+- **BREAKING CHANGES:** `fjs/types/function/compare` and
+  `fjs/types/function/operator` migrate from authored TypeScript (`.f.ts`) to
+  JSDoc-typed JavaScript (`.f.mjs`) under the stage-1 TypeScript-to-mjs
+  migration — importers must use the `.f.mjs` specifier
+  [#1456](https://github.com/functionalscript/functionalscript/pull/1456)
 - **BREAKING CHANGES:** `fjs/types/array`, `fjs/types/function`,
   `fjs/types/nullable`, and `fjs/types/option` migrate from authored
   TypeScript (`.f.ts`) to JSDoc-typed JavaScript (`.f.mjs`) under the stage-1
diff --git a/fjs/asn.1/module.f.ts b/fjs/asn.1/module.f.ts
index 21461134e..caf0c7134 100644
--- a/fjs/asn.1/module.f.ts
+++ b/fjs/asn.1/module.f.ts
@@ -18,7 +18,7 @@ import {
     type Vec
 } from "../types/bit_vec/module.f.ts"
 import { identity } from "../types/function/module.f.mjs"
-import { max } from "../types/function/compare/module.f.ts"
+import { max } from "../types/function/compare/module.f.mjs"
 import { encode as b128encode, decode as b128decode } from "../basen/base128/module.f.ts"
 
 const { popFront: pop, listToVec } = msb
diff --git a/fjs/bnf/ll1/module.f.ts b/fjs/bnf/ll1/module.f.ts
index 4e57853de..0d2035f87 100644
--- a/fjs/bnf/ll1/module.f.ts
+++ b/fjs/bnf/ll1/module.f.ts
@@ -11,7 +11,7 @@
  * @module
  */
 import { type CodePoint } from '../../text/utf16/module.f.ts'
-import { strictEqual } from '../../types/function/operator/module.f.ts'
+import { strictEqual } from '../../types/function/operator/module.f.mjs'
 import { toArray } from '../../types/list/module.f.ts'
 import { rangeMap, type RangeMapArray } from '../../types/range_map/module.f.ts'
 import { contains, set, type StringSet } from '../../types/string_set/module.f.ts'
diff --git a/fjs/common/monoid/module.f.ts b/fjs/common/monoid/module.f.ts
index b8dae149a..b1dd12662 100644
--- a/fjs/common/monoid/module.f.ts
+++ b/fjs/common/monoid/module.f.ts
@@ -6,7 +6,7 @@
  *
  * @module
  */
-import type { Fold, Reduce } from '../../types/function/operator/module.f.ts'
+import type { Fold, Reduce } from '../../types/function/operator/module.f.mjs'
 import { reduce, type List } from '../../types/list/module.f.ts'
 import { flip } from '../../types/function/module.f.mjs'
 
diff --git a/fjs/crypto/secp/module.f.ts b/fjs/crypto/secp/module.f.ts
index 391af8185..c2f370035 100644
--- a/fjs/crypto/secp/module.f.ts
+++ b/fjs/crypto/secp/module.f.ts
@@ -5,7 +5,7 @@
  *
  * @module
  */
-import type { Equal, Fold, Reduce } from '../../types/function/operator/module.f.ts'
+import type { Equal, Fold, Reduce } from '../../types/function/operator/module.f.mjs'
 import { prime_field, sqrt, type PrimeField } from '../../types/prime_field/module.f.ts'
 import { repeat } from '../../common/monoid/module.f.ts'
 
diff --git a/fjs/crypto/sha2/module.f.ts b/fjs/crypto/sha2/module.f.ts
index 28a42f681..ef6ffd4da 100644
--- a/fjs/crypto/sha2/module.f.ts
+++ b/fjs/crypto/sha2/module.f.ts
@@ -14,7 +14,7 @@ import {
     uint,
     type Vec
 } from '../../types/bit_vec/module.f.ts'
-import type { Fold } from '../../types/function/operator/module.f.ts'
+import type { Fold } from '../../types/function/operator/module.f.mjs'
 import { fold, type List } from '../../types/list/module.f.ts'
 
 const { concat, front } = msb
diff --git a/fjs/djs/parser/module.f.ts b/fjs/djs/parser/module.f.ts
index 6921664ba..d702d5d59 100644
--- a/fjs/djs/parser/module.f.ts
+++ b/fjs/djs/parser/module.f.ts
@@ -5,7 +5,7 @@
  */
 import { error, ok, type Result } from '../../types/result/module.f.ts'
 import { fold, next, toArray, length, concat, type List } from '../../types/list/module.f.ts'
-import type { Fold } from '../../types/function/operator/module.f.ts'
+import type { Fold } from '../../types/function/operator/module.f.mjs'
 import type { DjsToken, DjsTokenWithMetadata } from '../tokenizer/module.f.ts'
 import { setReplace, at, type OrderedMap } from '../../types/ordered_map/module.f.ts'
 import { fromMap } from '../../types/object/module.f.ts'
diff --git a/fjs/djs/serializer/module.f.ts b/fjs/djs/serializer/module.f.ts
index e20eb9f7b..41fe45a3a 100644
--- a/fjs/djs/serializer/module.f.ts
+++ b/fjs/djs/serializer/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import type { Unknown, Object } from '../module.f.ts'
-import type { Fold } from '../../types/function/operator/module.f.ts'
+import type { Fold } from '../../types/function/operator/module.f.mjs'
 import type { Entry as ObjectEntry } from '../../types/object/module.f.ts'
 import { fold } from '../../types/list/module.f.ts'
 import { concat } from '../../types/string/module.f.ts'
diff --git a/fjs/djs/tokenizer/module.f.ts b/fjs/djs/tokenizer/module.f.ts
index bc5ef521e..6b6e0cbfd 100644
--- a/fjs/djs/tokenizer/module.f.ts
+++ b/fjs/djs/tokenizer/module.f.ts
@@ -53,7 +53,7 @@ import {
     range as asciiRange,
 } from "../../text/ascii/module.f.ts"
 import { type CodePoint, codePointListToString, stringToCodePointList } from "../../text/utf16/module.f.ts"
-import type { StateScan } from "../../types/function/operator/module.f.ts"
+import type { StateScan } from "../../types/function/operator/module.f.mjs"
 import { contains } from "../../types/range/module.f.ts"
 import { concat, empty, filter, flat, flatMap, fold, map, stateScan, toArray, type List } from "../../types/list/module.f.ts"
 import { stringifyAsTree } from "../serializer/module.f.ts"
diff --git a/fjs/emergent_testing/todo/f-mjs-test-and-coverage.md b/fjs/emergent_testing/todo/f-mjs-test-and-coverage.md
index 6b69e6051..3197475a8 100644
--- a/fjs/emergent_testing/todo/f-mjs-test-and-coverage.md
+++ b/fjs/emergent_testing/todo/f-mjs-test-and-coverage.md
@@ -31,7 +31,7 @@ may migrate from `proof.f.ts` to `proof.f.mjs` as soon as it can be expressed as
 JavaScript with JSDoc and its authored runtime and declaration-retained type
 dependencies are already `.f.mjs`; current FunctionalScript compiler support is
 not a migration gate. A proof that still depends on an unmigrated `.f.ts` helper,
-such as `fjs/asserts/module.f.ts`, remains `proof.f.ts` until that dependency
+such as `fjs/types/list/module.f.ts`, remains `proof.f.ts` until that dependency
 moves.
 
 ### Proposal
diff --git a/fjs/fsc/module.f.ts b/fjs/fsc/module.f.ts
index 6353ec011..4c75b753b 100644
--- a/fjs/fsc/module.f.ts
+++ b/fjs/fsc/module.f.ts
@@ -3,7 +3,7 @@
  *
  * @module
  */
-import { strictEqual } from '../types/function/operator/module.f.ts'
+import { strictEqual } from '../types/function/operator/module.f.mjs'
 import {
     merge as rangeMapMerge,
     fromRange,
diff --git a/fjs/fsm/module.f.ts b/fjs/fsm/module.f.ts
index fcf0e47c0..7f6f88321 100644
--- a/fjs/fsm/module.f.ts
+++ b/fjs/fsm/module.f.ts
@@ -15,7 +15,7 @@ import {
     type RangeMapArray,
     type Entry
 } from '../types/range_map/module.f.ts'
-import { type Fold, type Scan, strictEqual } from '../types/function/operator/module.f.ts'
+import { type Fold, type Scan, strictEqual } from '../types/function/operator/module.f.mjs'
 import { stringify } from '../media/json/module.f.ts'
 import { identity } from '../types/function/module.f.mjs'
 import { stringToList } from '../text/utf16/module.f.ts'
diff --git a/fjs/js/tokenizer/module.f.ts b/fjs/js/tokenizer/module.f.ts
index f36522d08..6b5bc579a 100644
--- a/fjs/js/tokenizer/module.f.ts
+++ b/fjs/js/tokenizer/module.f.ts
@@ -5,7 +5,7 @@
  *
  * @module
  */
-import { strictEqual, type Scan, type StateScan } from '../../types/function/operator/module.f.ts'
+import { strictEqual, type Scan, type StateScan } from '../../types/function/operator/module.f.mjs'
 import { merge, fromRange, get, type RangeMapArray, type RangeMerge } from '../../types/range_map/module.f.ts'
 import { empty, stateScan, flat, toArray, reduce as listReduce, scan, map as listMap, type List } from '../../types/list/module.f.ts'
 import { at, fromEntries, type Entry } from '../../types/ordered_map/module.f.ts'
diff --git a/fjs/media/json/parser/module.f.ts b/fjs/media/json/parser/module.f.ts
index 5ecc47fc9..a5f21f9f9 100644
--- a/fjs/media/json/parser/module.f.ts
+++ b/fjs/media/json/parser/module.f.ts
@@ -5,7 +5,7 @@
  */
 import { error, ok, type Result } from '../../../types/result/module.f.ts'
 import { type List, fold, next, toArray, concat } from '../../../types/list/module.f.ts'
-import { type Fold } from '../../../types/function/operator/module.f.ts'
+import { type Fold } from '../../../types/function/operator/module.f.mjs'
 import { type JsonToken } from '../tokenizer/module.f.ts'
 import { setReplace, type OrderedMap } from '../../../types/ordered_map/module.f.ts'
 import { type Unknown } from '../module.f.ts'
diff --git a/fjs/media/json/serializer/module.f.ts b/fjs/media/json/serializer/module.f.ts
index 52ddd7f9a..ec01a9ed1 100644
--- a/fjs/media/json/serializer/module.f.ts
+++ b/fjs/media/json/serializer/module.f.ts
@@ -8,7 +8,7 @@
  * @module
  */
 import { flat, map, reduce, empty, type List } from '../../../types/list/module.f.ts'
-import { type Reduce } from '../../../types/function/operator/module.f.ts'
+import { type Reduce } from '../../../types/function/operator/module.f.mjs'
 import { concat } from '../../../types/string/module.f.ts'
 import {
     codePointToString,
diff --git a/fjs/media/json/tokenizer/module.f.ts b/fjs/media/json/tokenizer/module.f.ts
index 1a9180928..04d75c88e 100644
--- a/fjs/media/json/tokenizer/module.f.ts
+++ b/fjs/media/json/tokenizer/module.f.ts
@@ -3,7 +3,7 @@
  *
  * @module
  */
-import { type StateScan } from '../../../types/function/operator/module.f.ts'
+import { type StateScan } from '../../../types/function/operator/module.f.mjs'
 import { concat, empty, flat, stateScan, type List } from '../../../types/list/module.f.ts'
 import { multiply } from '../../../types/bigfloat/module.f.ts'
 import {
diff --git a/fjs/path/module.f.ts b/fjs/path/module.f.ts
index f446cc9b7..79ab843df 100644
--- a/fjs/path/module.f.ts
+++ b/fjs/path/module.f.ts
@@ -3,7 +3,7 @@
  *
  * @module
  */
-import type { Fold, Reduce, Unary } from '../types/function/operator/module.f.ts'
+import type { Fold, Reduce, Unary } from '../types/function/operator/module.f.mjs'
 import { type List, fold, last, take, length, concat as listConcat, toArray } from '../types/list/module.f.ts'
 import { join as listJoin, concat as stringConcat } from '../types/string/module.f.ts'
 
diff --git a/fjs/sul/level/literal/module.f.ts b/fjs/sul/level/literal/module.f.ts
index 94a2489e8..2b67ed33e 100644
--- a/fjs/sul/level/literal/module.f.ts
+++ b/fjs/sul/level/literal/module.f.ts
@@ -8,7 +8,7 @@
 import { log2 } from '../../../types/bigint/module.f.ts'
 import { msb, vec, type Vec } from '../../../types/bit_vec/module.f.ts'
 import type { Func } from '../../../types/function/module.f.mjs'
-import { strictEqual, type Equal, type StateScan } from '../../../types/function/operator/module.f.ts'
+import { strictEqual, type Equal, type StateScan } from '../../../types/function/operator/module.f.mjs'
 import { equal, map, type List } from '../../../types/list/module.f.ts'
 import { join } from '../../../types/string/module.f.ts'
 
diff --git a/fjs/text/code_point/module.f.ts b/fjs/text/code_point/module.f.ts
index e25da148e..b31aefca4 100644
--- a/fjs/text/code_point/module.f.ts
+++ b/fjs/text/code_point/module.f.ts
@@ -9,7 +9,7 @@
  * @module
  */
 import { flat, type List, stateScan } from '../../types/list/module.f.ts'
-import type { StateScan } from '../../types/function/operator/module.f.ts'
+import type { StateScan } from '../../types/function/operator/module.f.mjs'
 import { contains } from '../../types/range/module.f.ts'
 
 /**
diff --git a/fjs/text/utf16/module.f.ts b/fjs/text/utf16/module.f.ts
index d7337519c..98fec3ed1 100644
--- a/fjs/text/utf16/module.f.ts
+++ b/fjs/text/utf16/module.f.ts
@@ -14,7 +14,7 @@ import {
     type Result,
     type Thunk,
 } from '../../types/list/module.f.ts'
-import { concat, type StateScan } from '../../types/function/operator/module.f.ts'
+import { concat, type StateScan } from '../../types/function/operator/module.f.mjs'
 import { contains } from '../../types/range/module.f.ts'
 import { fn } from '../../types/function/module.f.mjs'
 import {
diff --git a/fjs/text/utf8/module.f.ts b/fjs/text/utf8/module.f.ts
index 160cfa729..8272803a8 100644
--- a/fjs/text/utf8/module.f.ts
+++ b/fjs/text/utf8/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import { flatMap, toArray, type List, type Thunk } from '../../types/list/module.f.ts'
-import type { StateScan } from '../../types/function/operator/module.f.ts'
+import type { StateScan } from '../../types/function/operator/module.f.mjs'
 import type { Tuple } from '../../types/array/module.f.mjs'
 import { decoder, errorMask, isValidCodePoint } from '../code_point/module.f.ts'
 import { msb, u8List, length, type Vec } from '../../types/bit_vec/module.f.ts'
diff --git a/fjs/types/bigint/module.f.ts b/fjs/types/bigint/module.f.ts
index a6ca383f7..aac09a3b1 100644
--- a/fjs/types/bigint/module.f.ts
+++ b/fjs/types/bigint/module.f.ts
@@ -17,8 +17,8 @@
  * ```
  */
 
-import { cmp, type Sign } from '../function/compare/module.f.ts'
-import type { Unary as OpUnary, Reduce as OpReduce } from '../function/operator/module.f.ts'
+import { cmp, type Sign } from '../function/compare/module.f.mjs'
+import type { Unary as OpUnary, Reduce as OpReduce } from '../function/operator/module.f.mjs'
 import { type List } from '../list/module.f.ts'
 import { fold } from '../../common/monoid/module.f.ts'
 
diff --git a/fjs/types/bigint/proof.f.ts b/fjs/types/bigint/proof.f.ts
index d99cb9bd9..df60e7b37 100644
--- a/fjs/types/bigint/proof.f.ts
+++ b/fjs/types/bigint/proof.f.ts
@@ -16,7 +16,7 @@ import {
     roundUp8
 } from './module.f.ts'
 import { assert, assertEq } from '../../asserts/module.f.mjs'
-import { min } from '../function/compare/module.f.ts'
+import { min } from '../function/compare/module.f.mjs'
 
 const oldLog2 = (v: bigint): bigint => {
     if (v <= 0n) { return -1n }
diff --git a/fjs/types/bit_vec/module.f.ts b/fjs/types/bit_vec/module.f.ts
index 1773e9215..eec23f9c3 100644
--- a/fjs/types/bit_vec/module.f.ts
+++ b/fjs/types/bit_vec/module.f.ts
@@ -23,11 +23,11 @@
  */
 import { bitLength, divUp, mask, maxLength, xor, type Reduce as BigintReduce } from '../bigint/module.f.ts'
 import { flip, identity } from '../function/module.f.mjs'
-import type { Binary, Fold, Reduce as OpReduce } from '../function/operator/module.f.ts'
+import type { Binary, Fold, Reduce as OpReduce } from '../function/operator/module.f.mjs'
 import { map, tryFold, type Accumulator, type List, type Thunk } from '../list/module.f.ts'
 import { asBase, asNominal, type Nominal } from '../nominal/module.f.ts'
 import { repeat as mRepeat } from '../../common/monoid/module.f.ts'
-import { cmp, max, min, type Sign } from '../function/compare/module.f.ts'
+import { cmp, max, min, type Sign } from '../function/compare/module.f.mjs'
 import { mapUnwrap, type Nullable } from '../nullable/module.f.mjs'
 
 /**
diff --git a/fjs/types/bit_vec/proof.f.ts b/fjs/types/bit_vec/proof.f.ts
index 3d81fa69b..89e8c420e 100644
--- a/fjs/types/bit_vec/proof.f.ts
+++ b/fjs/types/bit_vec/proof.f.ts
@@ -1,6 +1,6 @@
 import { assert, assertEq } from '../../asserts/module.f.mjs'
 import { mask } from '../bigint/module.f.ts'
-import type { Sign } from '../function/compare/module.f.ts'
+import type { Sign } from '../function/compare/module.f.mjs'
 import { asBase, asNominal } from '../nominal/module.f.ts'
 import { length, empty, uint, type Vec, vec, lsb, msb, type BitOrder, repeat, vec8, maxLength, u8ListToVec, tryU8ListToVec, u8List, chunkList, fromSentinel } from './module.f.ts'
 import { repeat as listRepeat, toArray, type List } from '../list/module.f.ts'
diff --git a/fjs/types/btree/find/module.f.ts b/fjs/types/btree/find/module.f.ts
index 7785fcb63..432cc9bd7 100644
--- a/fjs/types/btree/find/module.f.ts
+++ b/fjs/types/btree/find/module.f.ts
@@ -5,7 +5,7 @@
  */
 import type { Leaf1, Leaf2, Branch3, Branch5, TNode } from '../types/module.f.ts'
 import type { List } from '../../list/module.f.ts'
-import { index3, index5, type Compare } from '../../function/compare/module.f.ts'
+import { index3, index5, type Compare } from '../../function/compare/module.f.mjs'
 import type { KeyOf, Index, Tuple } from "../../array/module.f.mjs";
 
 export type FirstLeaf1<T> = readonly[Index<3>, Leaf1<T>]
diff --git a/fjs/types/btree/remove/module.f.ts b/fjs/types/btree/remove/module.f.ts
index aea043854..f785d9b1d 100644
--- a/fjs/types/btree/remove/module.f.ts
+++ b/fjs/types/btree/remove/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import { collapseRoot, type Leaf1, type TNode, type Branch1, type Branch3, type Branch5, type Tree } from '../types/module.f.ts'
-import type { Compare } from '../../function/compare/module.f.ts'
+import type { Compare } from '../../function/compare/module.f.mjs'
 import { type Path, type PathItem, find } from '../find/module.f.ts'
 import { fold, concat, next } from '../../list/module.f.ts'
 import type { Tuple } from '../../array/module.f.mjs'
diff --git a/fjs/types/btree/set/module.f.ts b/fjs/types/btree/set/module.f.ts
index a577c1889..398fc3430 100644
--- a/fjs/types/btree/set/module.f.ts
+++ b/fjs/types/btree/set/module.f.ts
@@ -5,7 +5,7 @@
  */
 import { collapseRoot, type Branch1, type Branch3, type Branch5, type Branch7, type TNode, type Tree } from '../types/module.f.ts'
 import { find, type First, type PathItem, type Result } from '../find/module.f.ts'
-import type { Compare } from '../../function/compare/module.f.ts'
+import type { Compare } from '../../function/compare/module.f.mjs'
 import { fold } from '../../list/module.f.ts'
 
 type Branch1To3<T> = Branch1<T> | Branch3<T>
diff --git a/fjs/types/function/compare/module.f.mjs b/fjs/types/function/compare/module.f.mjs
new file mode 100644
index 000000000..2cab3d8ac
--- /dev/null
+++ b/fjs/types/function/compare/module.f.mjs
@@ -0,0 +1,95 @@
+/**
+ * Comparison function types and helpers.
+ *
+ * @module
+ */
+/** @import { Index, Tuple } from '../../array/module.f.mjs' */
+
+/** @typedef {-1 | 0 | 1} Sign */
+
+/**
+ * @template T
+ * @typedef {(_: T) => Sign} Compare
+ */
+
+/**
+ * @template T
+ * @typedef {(a: T) => Compare<T>} Cmp
+ */
+
+/** @type {<T>(cmp: Compare<T>) => (value: T) => Index<3>} */
+export const index3
+    = cmp => value => /** @type {Index<3>} */(cmp(value) + 1)
+
+/** @type {<T>(cmp: Compare<T>) => (v2: Tuple<2, T>) => Index<5>} */
+export const index5 = cmp => ([v0, v1]) => {
+    const _0 = cmp(v0)
+    return /** @type {Index<5>} */(_0 <= 0 ? _0 + 1 : cmp(v1) + 3)
+}
+
+/** @typedef {boolean | string | number | bigint} Cmp1 */
+
+/**
+ * @template A
+ * @template B
+ * @typedef {[A, B] extends [boolean, boolean] ? boolean :
+ *  [A, B] extends [string, string] ? string :
+ *  [A, B] extends [number, number] ? number :
+ *  [A, B] extends [bigint, bigint] ? bigint :
+ *  never
+ * } Cmp2
+ */
+
+/** @type {<A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B) => Sign} */
+export const cmp = a => b =>
+    /** @type {any} */(a) < b ? -1 : /** @type {any} */(a) > b ? 1 : 0
+
+/**
+ * Returns the smaller of two comparable values. The `Cmp2<A, B>` constraint
+ * is the same one `cmp` uses: it rejects calls that mix incompatible primitive
+ * types (e.g. `min(1)("a")`) at compile time.
+ *
+ * @type {<A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B) => A | B}
+ */
+export const min = a => b => cmp(a)(b) < 0 ? a : b
+
+/**
+ * Returns the larger of two comparable values. The `Cmp2<A, B>` constraint
+ * is the same one `cmp` uses: it rejects calls that mix incompatible primitive
+ * types (e.g. `max(1)("a")`) at compile time.
+ *
+ * @type {<A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B) => A | B}
+ */
+export const max = a => b => cmp(a)(b) > 0 ? a : b
+
+/**
+ * Binary search over `[0, len)`. `probe(mid)` returns the sign of the search
+ * key relative to the element at `mid` (`-1` before, `0` at, `1` after). On a
+ * hit it returns the matching index; on a miss it returns the converged lower
+ * bound `b` (the insertion point), which may equal `len`.
+ *
+ * `probe` must be monotonic over `[0, len)`: scanning indices left to right its
+ * result is non-increasing — a run of `1`s, then `0`s, then `-1`s. A
+ * non-monotonic probe yields an undefined position.
+ *
+ * @type {(len: number) => (probe: (mid: number) => Sign) => number}
+ */
+export const bsearch = len => probe => {
+    let b = 0
+    let e = len - 1
+    while (true) {
+        if (e < b) { return b }
+        const mid = b + (e - b >> 1)
+        switch (probe(mid)) {
+            case -1: {
+                e = mid - 1
+                break
+            }
+            case 0: { return mid }
+            case 1: {
+                b = mid + 1
+                break
+            }
+        }
+    }
+}
diff --git a/fjs/types/function/compare/module.f.ts b/fjs/types/function/compare/module.f.ts
deleted file mode 100644
index 73dc5b532..000000000
--- a/fjs/types/function/compare/module.f.ts
+++ /dev/null
@@ -1,81 +0,0 @@
-/**
- * Comparison function types and helpers.
- *
- * @module
- */
-import type { Index, Tuple } from '../../array/module.f.mjs'
-
-export type Sign = -1|0|1
-
-export type Compare<T> = (_: T) => Sign
-
-export type Cmp<T> = (a: T) => Compare<T>
-
-export const index3: <T>(cmp: Compare<T>) => (value: T) => Index<3>
-    = cmp => value => (cmp(value) + 1) as Index<3>
-
-export const index5
-    : <T>(cmp: Compare<T>) => (v2: Tuple<2, T>) => Index<5>
-    = cmp => ([v0, v1]) => {
-        const _0 = cmp(v0)
-        return (_0 <= 0 ? _0 + 1 : cmp(v1) + 3) as Index<5>
-    }
-
-export type Cmp1 = boolean | string | number | bigint
-
-export type Cmp2<A, B> =
-    [A, B] extends [boolean, boolean] ? boolean :
-    [A, B] extends [string, string] ? string :
-    [A, B] extends [number, number] ? number :
-    [A, B] extends [bigint, bigint] ? bigint :
-    never
-
-export const cmp = <A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B): Sign =>
-    a as any < b ? -1 : a as any > b ? 1 : 0
-
-/**
- * Returns the smaller of two comparable values. The `Cmp2<A, B>` constraint
- * is the same one `cmp` uses: it rejects calls that mix incompatible primitive
- * types (e.g. `min(1)("a")`) at compile time.
- */
-export const min = <A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B): A | B =>
-    cmp(a)(b) < 0 ? a : b
-
-/**
- * Returns the larger of two comparable values. The `Cmp2<A, B>` constraint
- * is the same one `cmp` uses: it rejects calls that mix incompatible primitive
- * types (e.g. `max(1)("a")`) at compile time.
- */
-export const max = <A extends Cmp1>(a: A) => <B extends Cmp2<A, B>>(b: B): A | B =>
-    cmp(a)(b) > 0 ? a : b
-
-/**
- * Binary search over `[0, len)`. `probe(mid)` returns the sign of the search
- * key relative to the element at `mid` (`-1` before, `0` at, `1` after). On a
- * hit it returns the matching index; on a miss it returns the converged lower
- * bound `b` (the insertion point), which may equal `len`.
- *
- * `probe` must be monotonic over `[0, len)`: scanning indices left to right its
- * result is non-increasing — a run of `1`s, then `0`s, then `-1`s. A
- * non-monotonic probe yields an undefined position.
- */
-export const bsearch
-    = (len: number) => (probe: (mid: number) => Sign): number => {
-        let b = 0
-        let e = len - 1
-        while (true) {
-            if (e < b) { return b }
-            const mid = b + (e - b >> 1)
-            switch (probe(mid)) {
-                case -1: {
-                    e = mid - 1
-                    break
-                }
-                case 0: { return mid }
-                case 1: {
-                    b = mid + 1
-                    break
-                }
-            }
-        }
-    }
diff --git a/fjs/types/function/compare/proof.f.ts b/fjs/types/function/compare/proof.f.ts
index 184e617a3..d3687e061 100644
--- a/fjs/types/function/compare/proof.f.ts
+++ b/fjs/types/function/compare/proof.f.ts
@@ -1,4 +1,4 @@
-import { cmp, min, max } from './module.f.ts'
+import { cmp, min, max } from './module.f.mjs'
 import { assertEq } from '../../../asserts/module.f.mjs'
 
 export const proof = () => {
diff --git a/fjs/types/function/operator/module.f.ts b/fjs/types/function/operator/module.f.mjs
similarity index 54%
rename from fjs/types/function/operator/module.f.ts
rename to fjs/types/function/operator/module.f.mjs
index 6b027ed9f..695163443 100644
--- a/fjs/types/function/operator/module.f.ts
+++ b/fjs/types/function/operator/module.f.mjs
@@ -3,27 +3,54 @@
  *
  * @module
  */
-export type Binary<A, B, R> = (a: A) => (b: B) => R
 
-export type Fold<I, O> = Binary<I, O, O>
+/**
+ * @template A
+ * @template B
+ * @template R
+ * @typedef {(a: A) => (b: B) => R} Binary
+ */
+
+/**
+ * @template I
+ * @template O
+ * @typedef {Binary<I, O, O>} Fold
+ */
 
-export const join = (separator: string): Reduce<string> => value => prior =>
+/**
+ * @template T
+ * @typedef {Fold<T, T>} Reduce
+ */
+
+/** @type {(separator: string) => Reduce<string>} */
+export const join = separator => value => prior =>
     `${prior}${separator}${value}`
 
-export const concat: Reduce<string> = i => acc =>
-    `${acc}${i}`
+/** @type {Reduce<string>} */
+export const concat = i => acc => `${acc}${i}`
 
-export type Unary<T, R> = (value: T) => R
+/**
+ * @template T
+ * @template R
+ * @typedef {(value: T) => R} Unary
+ */
 
-export const logicalNot: Unary<boolean, boolean>
-    = v => !v
+/** @type {Unary<boolean, boolean>} */
+export const logicalNot = v => !v
 
-export type Equal<T> = Binary<T, T, boolean>
+/**
+ * @template T
+ * @typedef {Binary<T, T, boolean>} Equal
+ */
 
-export const strictEqual = <T>(a: T) => (b: T): boolean =>
-    a === b
+/** @type {<T>(a: T) => (b: T) => boolean} */
+export const strictEqual = a => b => a === b
 
-export type Scan<I, O> = (input: I) => readonly[O, Scan<I,O>]
+/**
+ * @template I
+ * @template O
+ * @typedef {(input: I) => readonly[O, Scan<I,O>]} Scan
+ */
 
 /**
  * One step of a stream transducer: given an `input` symbol and the `prior`
@@ -50,28 +77,39 @@ export type Scan<I, O> = (input: I) => readonly[O, Scan<I,O>]
  * A {@link Fold} is the output-less special case (state only); driving a
  * `StateScan` over a `List` is `stateScan` in `../../list/module.f.ts`, and
  * {@link stateScanToScan} hides the state to recover a {@link Scan}.
+ *
+ * @template I
+ * @template S
+ * @template O
+ * @typedef {(input: I, prior: S) => readonly[O, S]} StateScan
  */
-export type StateScan<I, S, O> = (input: I, prior: S) => readonly[O, S]
 
-export const stateScanToScan = <I, S, O>(op: StateScan<I, S, O>) => (prior: S): Scan<I, O> => i => {
+/** @type {<I, S, O>(op: StateScan<I, S, O>) => (prior: S) => Scan<I, O>} */
+export const stateScanToScan = op => prior => i => {
     const [o, s] = op(i, prior)
     return [o, stateScanToScan(op)(s)]
 }
 
-export const foldToScan = <I, O>(fold: Fold<I, O>) => (prior: O): Scan<I, O> => i => {
+/** @type {<I, O>(fold: Fold<I, O>) => (prior: O) => Scan<I, O>} */
+export const foldToScan = fold => prior => i => {
     const result = fold(i)(prior)
     return [result, foldToScan(fold)(result)]
 }
 
-export type Reduce<T> = Fold<T, T>
-
-export const reduceToScan = <T>(op: Reduce<T>): Scan<T, T> => init =>
+/** @type {<T>(op: Reduce<T>) => Scan<T, T>} */
+export const reduceToScan = op => init =>
     [init, foldToScan(op)(init)]
 
-export const addition: Reduce<number>
-    = a => b => a + b
+/**
+ * TODO: We should have one function for `number` | `bigint` and `string`.
+ *       We can use the same approach as we use for comparing items,
+ *       see `Cmp1` and `Cmp2` types.
+ *
+ * @type {Reduce<number>}
+ */
+export const addition = a => b => a + b
 
-export const increment: (b: number) => number
-    = addition(1)
+/** @type {Unary<number, number>} */
+export const increment = addition(1)
 
 export const counter = () => increment
diff --git a/fjs/types/function/operator/proof.f.ts b/fjs/types/function/operator/proof.f.ts
index 7494ae2be..339992757 100644
--- a/fjs/types/function/operator/proof.f.ts
+++ b/fjs/types/function/operator/proof.f.ts
@@ -10,7 +10,7 @@ import {
     stateScanToScan,
     foldToScan,
     reduceToScan,
-} from './module.f.ts'
+} from './module.f.mjs'
 
 const joinTest = () => {
     const result = join(', ')('world')('hello')
diff --git a/fjs/types/function/todo/uncurry-accumulator-types.md b/fjs/types/function/todo/uncurry-accumulator-types.md
index bc1c271c2..eaa1cff9a 100644
--- a/fjs/types/function/todo/uncurry-accumulator-types.md
+++ b/fjs/types/function/todo/uncurry-accumulator-types.md
@@ -10,7 +10,7 @@
 Several sibling accumulator types still curry their data parameters, contradicting that precedent:
 
 ```ts
-// fjs/types/function/operator/module.f.ts
+// fjs/types/function/operator/module.f.mjs
 export type Fold<I, O> = Binary<I, O, O>   // (input: I) => (acc: O) => O
 export type Reduce<T>  = Fold<T, T>        // (value: T) => (acc: T) => T
 
diff --git a/fjs/types/list/module.f.ts b/fjs/types/list/module.f.ts
index 064891a58..40b6ca481 100644
--- a/fjs/types/list/module.f.ts
+++ b/fjs/types/list/module.f.ts
@@ -17,7 +17,7 @@ import {
     type Fold,
     type Reduce,
     type Equal,
-} from '../function/operator/module.f.ts'
+} from '../function/operator/module.f.mjs'
 
 export type List<T> = NotLazy<T> | Thunk<T>
 
diff --git a/fjs/types/list/proof.f.ts b/fjs/types/list/proof.f.ts
index d68f2c4da..8bb9c31d6 100644
--- a/fjs/types/list/proof.f.ts
+++ b/fjs/types/list/proof.f.ts
@@ -30,7 +30,7 @@ import {
 } from './module.f.ts'
 import { stringify, type Unknown } from '../../media/json/module.f.ts'
 import { sort } from '../object/module.f.ts'
-import { addition, strictEqual, reduceToScan } from '../function/operator/module.f.ts'
+import { addition, strictEqual, reduceToScan } from '../function/operator/module.f.mjs'
 import { assert, assertEq, assertNotNullish } from '../../asserts/module.f.mjs'
 
 const str
diff --git a/fjs/types/number/module.f.ts b/fjs/types/number/module.f.ts
index 2c6304a44..4bb2f1aaa 100644
--- a/fjs/types/number/module.f.ts
+++ b/fjs/types/number/module.f.ts
@@ -5,8 +5,8 @@
  * @module
  */
 import { reduce, type List } from '../list/module.f.ts'
-import { addition } from '../function/operator/module.f.ts'
-import { type Sign, cmp as uCmp, min as uMin, max as uMax } from '../function/compare/module.f.ts'
+import { addition } from '../function/operator/module.f.mjs'
+import { type Sign, cmp as uCmp, min as uMin, max as uMax } from '../function/compare/module.f.mjs'
 import { fold } from '../../common/monoid/module.f.ts'
 
 export const sum: (input: List<number>) => number
diff --git a/fjs/types/ordered_map/module.f.ts b/fjs/types/ordered_map/module.f.ts
index 7ad336b58..76939ef9c 100644
--- a/fjs/types/ordered_map/module.f.ts
+++ b/fjs/types/ordered_map/module.f.ts
@@ -8,10 +8,10 @@ import { value, find } from '../btree/find/module.f.ts'
 import { set } from '../btree/set/module.f.ts'
 import { remove as btreeRemove } from '../btree/remove/module.f.ts'
 import { values } from '../btree/module.f.ts'
-import type { Sign } from '../function/compare/module.f.ts'
+import type { Sign } from '../function/compare/module.f.mjs'
 import { cmp } from '../string/module.f.ts'
 import { fold, type List } from '../list/module.f.ts'
-import type { Reduce } from '../function/operator/module.f.ts'
+import type { Reduce } from '../function/operator/module.f.mjs'
 
 export type Entry<T> = readonly [string, T]
 
diff --git a/fjs/types/range_map/module.f.ts b/fjs/types/range_map/module.f.ts
index ac2d30921..0b19a202c 100644
--- a/fjs/types/range_map/module.f.ts
+++ b/fjs/types/range_map/module.f.ts
@@ -40,8 +40,8 @@ import { genericMerge, type TailReduce, type ReduceOp, type SortedList } from '.
 import { next } from '../list/module.f.ts'
 import type { Nullable } from '../nullable/module.f.mjs'
 import { cmp } from '../number/module.f.ts'
-import { bsearch } from '../function/compare/module.f.ts'
-import type { Reduce, Equal } from '../function/operator/module.f.ts'
+import { bsearch } from '../function/compare/module.f.mjs'
+import type { Reduce, Equal } from '../function/operator/module.f.mjs'
 import type { Range } from '../range/module.f.ts'
 
 export type Entry<T> = [T, number]
diff --git a/fjs/types/range_map/proof.f.ts b/fjs/types/range_map/proof.f.ts
index 970ef6c09..95c5f09b2 100644
--- a/fjs/types/range_map/proof.f.ts
+++ b/fjs/types/range_map/proof.f.ts
@@ -3,7 +3,7 @@ import { stringify, type Unknown } from '../../media/json/module.f.ts'
 import { sort } from '../object/module.f.ts'
 import { union, type SortedSet } from '../sorted_set/module.f.ts'
 import { equal, toArray } from '../list/module.f.ts'
-import { strictEqual } from '../function/operator/module.f.ts'
+import { strictEqual } from '../function/operator/module.f.mjs'
 import { cmp } from '../string/module.f.ts'
 import { assertEq } from '../../asserts/module.f.mjs'
 
diff --git a/fjs/types/sorted_list/module.f.ts b/fjs/types/sorted_list/module.f.ts
index d7779680e..f780f5b78 100644
--- a/fjs/types/sorted_list/module.f.ts
+++ b/fjs/types/sorted_list/module.f.ts
@@ -3,7 +3,7 @@
  *
  * @module
  */
-import { type Sign, type Cmp, bsearch } from '../function/compare/module.f.ts'
+import { type Sign, type Cmp, bsearch } from '../function/compare/module.f.mjs'
 import { type List, next } from '../list/module.f.ts'
 import type { Nullable } from '../nullable/module.f.mjs'
 import { identity } from '../function/module.f.mjs'
diff --git a/fjs/types/sorted_set/module.f.ts b/fjs/types/sorted_set/module.f.ts
index bf9e3e33f..46e3d4083 100644
--- a/fjs/types/sorted_set/module.f.ts
+++ b/fjs/types/sorted_set/module.f.ts
@@ -26,7 +26,7 @@
  * has(cmp)(2)(setA) // false
  * ```
  */
-import type { Cmp } from '../function/compare/module.f.ts'
+import type { Cmp } from '../function/compare/module.f.mjs'
 import { toArray } from "../list/module.f.ts"
 import { merge, intersect as listIntersect, find } from '../sorted_list/module.f.ts'
 
diff --git a/fjs/types/string/module.f.ts b/fjs/types/string/module.f.ts
index 112122f9f..dc6f0f4a3 100644
--- a/fjs/types/string/module.f.ts
+++ b/fjs/types/string/module.f.ts
@@ -17,8 +17,8 @@
  */
 import { type List, reduce as listReduce, repeat as listRepeat } from '../list/module.f.ts'
 import { compose } from '../function/module.f.mjs'
-import { type Sign, cmp as uCmp } from '../function/compare/module.f.ts'
-import { join as joinOp, type Reduce } from '../function/operator/module.f.ts'
+import { type Sign, cmp as uCmp } from '../function/compare/module.f.mjs'
+import { join as joinOp, type Reduce } from '../function/operator/module.f.mjs'
 import { fold } from '../../common/monoid/module.f.ts'
 
 // `join`'s per-separator reduction is seeded at `''` but is *not* a monoid fold:
@@ -44,4 +44,4 @@ export const cmp: (a: string) => (b: string) => Sign
     = uCmp
 
 export const splitAt = (p: number) => (v: string): readonly[string, string] =>
-    [v.substring(0, p), v.substring(p)]
\ No newline at end of file
+    [v.substring(0, p), v.substring(p)]
diff --git a/fjs/types/uint8array/proof.f.ts b/fjs/types/uint8array/proof.f.ts
index e17aca76e..67853f3e3 100644
--- a/fjs/types/uint8array/proof.f.ts
+++ b/fjs/types/uint8array/proof.f.ts
@@ -1,6 +1,6 @@
 import { maxLength, maxLengthBytes, vec } from '../bit_vec/module.f.ts'
 import { toVec, fromVec, listToVec, decodeUtf8, encodeUtf8 } from './module.f.ts'
-import { strictEqual } from '../function/operator/module.f.ts'
+import { strictEqual } from '../function/operator/module.f.mjs'
 import { equal, fromArrayLike } from '../list/module.f.ts'
 import { assert } from '../../asserts/module.f.mjs'
 
diff --git a/todo/flow.md b/todo/flow.md
index 57991a413..63be5e532 100644
--- a/todo/flow.md
+++ b/todo/flow.md
@@ -238,7 +238,7 @@ Planned engine work, each a separate change:
 
 - [fjs/types/list/module.f.ts](../fjs/types/list/module.f.ts) — the sequence
   type the naive engine binds to
-- [fjs/types/function/operator/module.f.ts](../fjs/types/function/operator/module.f.ts)
+- [fjs/types/function/operator/module.f.mjs](../fjs/types/function/operator/module.f.mjs)
   — `Scan`, `StateScan`, `Fold`: the closure-form operators `Transducer`
   generalizes (its JSDoc already frames them as Mealy machines)
 - [fjs/types/rtti](../fjs/types/rtti) — runtime type descriptions for the
diff --git a/todo/migrate-typescript-to-mjs.md b/todo/migrate-typescript-to-mjs.md
index a3170155b..5251aaf22 100644
--- a/todo/migrate-typescript-to-mjs.md
+++ b/todo/migrate-typescript-to-mjs.md
@@ -217,6 +217,11 @@ compiler-compatibility rename.
       dependencies on remaining authored TypeScript.
 - [ ] Update imports, proofs, tests, coverage globs, scripts, generated CI, and
       documentation for every migrated group.
+- [ ] Sweep prose references to already-migrated modules: `AGENTS.md`, README
+      files, and `todo/*.md` still name `.f.ts` paths that no longer exist, so
+      snippets copied from them produce broken imports and links. Per-group
+      updates miss these because they are not importers; run a repository-wide
+      `.f.ts`-reference check at least at the end of stage 1.
 - [ ] Preserve Node, Deno, Bun, proof, coverage, type-checking, declaration, and
       CI package behavior throughout the migration.
 - [ ] Add required `**BREAKING CHANGES:**` changelog entries for public runtime