diff --git a/CHANGELOG.md b/CHANGELOG.md
index 51c6ebb43..87611de93 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,22 @@ history.
 
 ## Unreleased
 
+- **BREAKING CHANGES:** `fjs/cli` migrates from authored TypeScript
+  (`.f.ts`) to JSDoc-typed JavaScript (`.f.mjs`), splitting its
+  type-level API into a sibling `types.ts` — importers must use the
+  `.f.mjs` specifier for runtime values and the `types.ts` specifier
+  for types. `proof.f.ts` stays TypeScript for now
+  [#1491](https://github.com/functionalscript/functionalscript/pull/1491)
+- **BREAKING CHANGES:** `fjs/website` migrates from authored
+  TypeScript (`.f.ts`) to JSDoc-typed JavaScript (`.f.mjs`) — importers
+  must use the `.f.mjs` specifier. `proof.f.ts` stays TypeScript for
+  now
+  [#1491](https://github.com/functionalscript/functionalscript/pull/1491)
+- **BREAKING CHANGES:** `fjs/dev/update` migrates from authored
+  TypeScript (`.f.ts`) to JSDoc-typed JavaScript (`.f.mjs`) — importers
+  must use the `.f.mjs` specifier. `proof.f.ts` stays TypeScript for
+  now
+  [#1491](https://github.com/functionalscript/functionalscript/pull/1491)
 - **BREAKING CHANGES:** `fjs/text/sgr` migrates from authored
   TypeScript (`.f.ts`) to JSDoc-typed JavaScript (`.f.mjs`), splitting
   its type-level API into a sibling `types.ts` — importers must use
diff --git a/fjs/README.md b/fjs/README.md
index 67da724f2..94a10c281 100644
--- a/fjs/README.md
+++ b/fjs/README.md
@@ -106,5 +106,5 @@ fjs run ./my-tool.f.ts foo bar   # options.args === ['foo', 'bar']
 ```
 fjs/module.ts          — Node.js entry point (runs main via the node runner)
 fjs/module.f.ts        — FunctionalScript command dispatcher (Commands list + dispatch)
-fjs/cli/module.f.ts    — generic Command/Commands/dispatch primitives
+fjs/cli/module.f.mjs   — generic dispatch primitive (Command/Commands types in fjs/cli/types.ts)
 ```
diff --git a/fjs/cas/cli/module.f.ts b/fjs/cas/cli/module.f.ts
index 613e54b1a..e6463c62c 100644
--- a/fjs/cas/cli/module.f.ts
+++ b/fjs/cas/cli/module.f.ts
@@ -8,7 +8,8 @@ import { cBase32ToVec, vecToCBase32 } from '../../basen/cbase32/module.f.mjs'
 import { forEachStep, pure, step } from '../../effects/module.f.mjs'
 import { errorExit, log, writeFromStream } from '../../effects/node/module.f.mjs'
 import type { All, Read, Write, WriteFile } from '../../effects/node/types.ts'
-import { dispatch, type Commands } from '../../cli/module.f.ts'
+import { dispatch } from '../../cli/module.f.mjs'
+import type { Commands } from '../../cli/types.ts'
 import type { MemOp } from '../../effects/memory/types.ts'
 import { casAddFile, fileCas, type FileCasOperation } from '../module.f.ts'
 
diff --git a/fjs/cas/cli/proof.f.ts b/fjs/cas/cli/proof.f.ts
index db5e5babf..8b3cb926b 100644
--- a/fjs/cas/cli/proof.f.ts
+++ b/fjs/cas/cli/proof.f.ts
@@ -3,7 +3,7 @@ import { computeSync, sha256 } from '../../crypto/sha2/module.f.mjs'
 import { maxLength, vec, vec8 } from '../../types/bit_vec/module.f.mjs'
 import { defaultNodeProgramOptions, emptyState, virtual } from '../../effects/node/virtual/module.f.ts'
 import type { NodeProgramOptions } from '../../effects/node/types.ts'
-import { dispatch } from '../../cli/module.f.ts'
+import { dispatch } from '../../cli/module.f.mjs'
 import { vecToCBase32 } from '../../basen/cbase32/module.f.mjs'
 import { assert, assertEq } from '../../asserts/module.f.mjs'
 
diff --git a/fjs/cli/module.f.ts b/fjs/cli/module.f.mjs
similarity index 71%
rename from fjs/cli/module.f.ts
rename to fjs/cli/module.f.mjs
index 80e59b75c..fc476ea75 100644
--- a/fjs/cli/module.f.ts
+++ b/fjs/cli/module.f.mjs
@@ -1,22 +1,22 @@
+/**
+ * CLI command dispatch table.
+ *
+ * See `./types.ts` for the type-level API.
+ *
+ * @module
+ */
+
 import { errorExit, log } from '../effects/node/module.f.mjs'
-import type { NodeOp, NodeProgramOptions, Write } from '../effects/node/types.ts'
+/** @import { NodeOp, NodeProgramOptions, Write } from '../effects/node/types.ts' */
 import { pure, step } from '../effects/module.f.mjs'
-import type { Effect } from '../effects/types.ts'
+/** @import { Effect } from '../effects/types.ts' */
 import { at, fromEntries } from '../types/object/module.f.mjs'
-
-type Handler<O extends NodeOp> = (options: NodeProgramOptions) => Effect<O, number>
-
-export type Command<O extends NodeOp> = {
-    readonly names: readonly string[]
-    readonly description: string
-    readonly handler: Handler<O> | Commands<O>
-}
-
-export type Commands<O extends NodeOp> = readonly Command<O>[]
+/** @import { Commands } from './types.ts' */
 
 const helpMeta = { names: ['help', 'h', '?'], description: 'Print this help message' }
 
-export const dispatch = <O extends NodeOp>(commands: Commands<O>) => (options: NodeProgramOptions): Effect<O | Write, number> => {
+/** @type {<O extends NodeOp>(commands: Commands<O>) => (options: NodeProgramOptions) => Effect<O | Write, number>} */
+export const dispatch = commands => options => {
     const [cmd, ...rest] = options.args
     const rows = [...commands, helpMeta]
     const nameCol = rows.map(({names}) => names.join(', '))
@@ -25,7 +25,7 @@ export const dispatch = <O extends NodeOp>(commands: Commands<O>) => (options: N
         'Available commands:',
         ...rows.map(({description}, i) => `  ${nameCol[i].padEnd(width)}  ${description}`)
     ].join('\n')
-    const map = fromEntries(commands.flatMap(c => c.names.map(n => [n, c] as const)))
+    const map = fromEntries(commands.flatMap(c => c.names.map(n => /** @type {const} */ ([n, c]))))
     if (cmd === undefined) {
         return errorExit(`Error: command is required.\n${helpText}`)
     }
diff --git a/fjs/cli/proof.f.ts b/fjs/cli/proof.f.ts
index 9adbeef00..8a917b8d6 100644
--- a/fjs/cli/proof.f.ts
+++ b/fjs/cli/proof.f.ts
@@ -1,7 +1,8 @@
 import { pure } from '../effects/module.f.mjs'
 import type { NodeOp, NodeProgramOptions } from '../effects/node/types.ts'
 import { defaultNodeProgramOptions, emptyState, virtual } from '../effects/node/virtual/module.f.ts'
-import { dispatch, type Commands } from './module.f.ts'
+import { dispatch } from './module.f.mjs'
+import type { Commands } from './types.ts'
 import { assert, assertEq } from '../asserts/module.f.mjs'
 
 const makeOptions = (args: readonly string[]): NodeProgramOptions =>
diff --git a/fjs/cli/todo/dispatch-help-rendering.md b/fjs/cli/todo/dispatch-help-rendering.md
index c63022390..1f29e93a6 100644
--- a/fjs/cli/todo/dispatch-help-rendering.md
+++ b/fjs/cli/todo/dispatch-help-rendering.md
@@ -5,7 +5,7 @@
 
 ### Problem
 
-`dispatch` (`fjs/cli/module.f.ts:17-46`) interleaves three distinct concerns —
+`dispatch` (`fjs/cli/module.f.mjs:18-50`) interleaves three distinct concerns —
 building the name→command lookup map, rendering the aligned help table, and
 routing — and computes the first two eagerly at the top of every call:
 
@@ -22,9 +22,9 @@ const map = fromEntries(commands.flatMap(c => c.names.map(n => [n, c] as const))
 ```
 
 `helpText` — the column-measuring and padding work — is consumed only by the
-three error/help branches (`:28`, `:38`, `:42`); on the common success path
+three error/help branches (`:29`, `:39-41`, `:45`); on the common success path
 it is dead work. And because `dispatch` recurses for nested command groups
-(`:35`, `:45`), the table and the map are rebuilt at every level of the
+(`:36`, `:48`), the table and the map are rebuilt at every level of the
 descent even when neither is used.
 
 Per AGENTS.md, separation of concerns is "always appropriate" even with a
@@ -34,7 +34,7 @@ its own named, independently testable function.
 
 ### Proposal
 
-Extract a module-scope pure helper in `fjs/cli/module.f.ts` (the layer that
+Extract a module-scope pure helper in `fjs/cli/module.f.mjs` (the layer that
 owns `Command`/`Commands`):
 
 ```ts
@@ -64,6 +64,6 @@ help rendering is the clear win.
 
 ### Related
 
-- `fjs/cli/module.f.ts:17-46` — current `dispatch`.
+- `fjs/cli/module.f.mjs:18-50` — current `dispatch`.
 - [positional-arity-check](./positional-arity-check.md) — separate concern
   (argument validation), independent of this change.
diff --git a/fjs/cli/todo/options-edsl.md b/fjs/cli/todo/options-edsl.md
index 232e4439d..bd5d66a91 100644
--- a/fjs/cli/todo/options-edsl.md
+++ b/fjs/cli/todo/options-edsl.md
@@ -5,7 +5,7 @@
 
 ### Problem
 
-`Command` (`fjs/cli/module.f.ts`) hands every handler a raw
+`Command` (`fjs/cli/types.ts`) hands every handler a raw
 `args: readonly string[]` and has no concept of a named option. Any command
 that wants a flag has to hand-parse it:
 
diff --git a/fjs/cli/todo/positional-arity-check.md b/fjs/cli/todo/positional-arity-check.md
index ef3380ff1..3996eab36 100644
--- a/fjs/cli/todo/positional-arity-check.md
+++ b/fjs/cli/todo/positional-arity-check.md
@@ -25,8 +25,8 @@ handler: ({ home, args: [hashCBase32, path, ...rest] }) => {
 
 ### Proposal
 
-A combinator in `fjs/cli/module.f.ts`, the layer that already owns
-`Command`/`dispatch`:
+A combinator in `fjs/cli/module.f.mjs`, the layer that already owns
+`dispatch` (`Command` is defined in `fjs/cli/types.ts`):
 
 ```ts
 export const exact = <O extends NodeOp>(name: string, n: number) =>
diff --git a/fjs/cli/types.ts b/fjs/cli/types.ts
new file mode 100644
index 000000000..eae90dedb
--- /dev/null
+++ b/fjs/cli/types.ts
@@ -0,0 +1,18 @@
+/**
+ * Types for the CLI command dispatch table.
+ *
+ * @module
+ */
+
+import type { NodeOp, NodeProgramOptions } from '../effects/node/types.ts'
+import type { Effect } from '../effects/types.ts'
+
+type Handler<O extends NodeOp> = (options: NodeProgramOptions) => Effect<O, number>
+
+export type Command<O extends NodeOp> = {
+    readonly names: readonly string[]
+    readonly description: string
+    readonly handler: Handler<O> | Commands<O>
+}
+
+export type Commands<O extends NodeOp> = readonly Command<O>[]
diff --git a/fjs/dev/update/module.f.ts b/fjs/dev/update/module.f.mjs
similarity index 51%
rename from fjs/dev/update/module.f.ts
rename to fjs/dev/update/module.f.mjs
index e9b704a21..d49727c9f 100644
--- a/fjs/dev/update/module.f.ts
+++ b/fjs/dev/update/module.f.mjs
@@ -3,18 +3,23 @@
  *
  * @module
  */
+
 import { history, historyStep, mapStep, step } from '../../effects/module.f.mjs'
-import type { Effect } from '../../effects/types.ts'
+/** @import { Effect } from '../../effects/types.ts' */
 import { mkdir, readUtf8File, writeUtf8File } from '../../effects/node/module.f.mjs'
-import type { Mkdir, NodeProgram, ReadFile, WriteFile } from '../../effects/node/types.ts'
+/** @import { Mkdir, NodeProgram, ReadFile, WriteFile } from '../../effects/node/types.ts' */
 import { unwrap } from '../../types/result/module.f.mjs'
 
-const source = '.copilot/mcp.json' as const
-const targetDirectory = '.vscode' as const
-const target = '.vscode/mcp.json' as const
+const source = /** @type {const} */ ('.copilot/mcp.json')
+const targetDirectory = /** @type {const} */ ('.vscode')
+const target = /** @type {const} */ ('.vscode/mcp.json')
 
-/** Regenerates VS Code's local MCP configuration from the canonical Copilot configuration. */
-export const syncMcp = (): Effect<Mkdir | ReadFile | WriteFile, void> => {
+/**
+ * Regenerates VS Code's local MCP configuration from the canonical Copilot configuration.
+ *
+ * @type {() => Effect<Mkdir | ReadFile | WriteFile, void>}
+ */
+export const syncMcp = () => {
     const sourceText = history(mapStep(readUtf8File(source), unwrap))
     const targetDirectoryReady = historyStep(
         sourceText,
@@ -23,5 +28,9 @@ export const syncMcp = (): Effect<Mkdir | ReadFile | WriteFile, void> => {
     return mapStep(targetWritten, unwrap)
 }
 
-/** Runs all local development configuration generators. */
-export const main: NodeProgram = () => mapStep(syncMcp(), () => 0)
+/**
+ * Runs all local development configuration generators.
+ *
+ * @type {NodeProgram}
+ */
+export const main = () => mapStep(syncMcp(), () => 0)
diff --git a/fjs/dev/update/proof.f.ts b/fjs/dev/update/proof.f.ts
index c64d4ef0b..cdcfcc01d 100644
--- a/fjs/dev/update/proof.f.ts
+++ b/fjs/dev/update/proof.f.ts
@@ -7,7 +7,7 @@ import { assert, assertEq } from '../../asserts/module.f.mjs'
 import { utf8 } from '../../text/module.f.mjs'
 import { readUtf8File } from '../../effects/node/module.f.mjs'
 import { defaultNodeProgramOptions, emptyState, virtual } from '../../effects/node/virtual/module.f.ts'
-import { main, syncMcp } from './module.f.ts'
+import { main, syncMcp } from './module.f.mjs'
 import { step } from '../../effects/module.f.mjs'
 
 const mcp = '{"servers":{}}' as const
diff --git a/fjs/effects/todo/map-step-combinator.md b/fjs/effects/todo/map-step-combinator.md
index ee3030b63..db321be0e 100644
--- a/fjs/effects/todo/map-step-combinator.md
+++ b/fjs/effects/todo/map-step-combinator.md
@@ -39,13 +39,13 @@ Also `fjs/dev/module.f.ts`, `fjs/cas/evo/module.f.ts`,
 yield an exit code" shape of a `NodeProgram`:
 
 ```ts
-// fjs/cli/module.f.ts:38-40
+// fjs/cli/module.f.mjs:39-41
 return step(
     log(helpText),
     () => pure(0))
 
-// fjs/website/module.f.ts:17-19
-const program: Effect<WriteFile, number> = step(
+// fjs/website/module.f.mjs:19-21
+const program = step(
     writeFile('index.html', html),
     () => pure(0))
 ```
diff --git a/fjs/module.f.ts b/fjs/module.f.ts
index 4872ab465..9d0333b8a 100644
--- a/fjs/module.f.ts
+++ b/fjs/module.f.ts
@@ -9,7 +9,8 @@ import { commands as casCommands } from './cas/cli/module.f.ts'
 import { main as ciMain } from './ci/module.f.ts'
 import { import_ } from './effects/node/module.f.mjs'
 import type { NodeOp, NodeProgram } from './effects/node/types.ts'
-import { dispatch, type Commands } from './cli/module.f.ts'
+import { dispatch } from './cli/module.f.mjs'
+import type { Commands } from './cli/types.ts'
 import { casMcpServer } from './mcp/module.f.ts'
 import { pure, step } from './effects/module.f.mjs'
 import { unwrap } from './types/result/module.f.mjs'
diff --git a/fjs/todo/66g-fjs-run-commands.md b/fjs/todo/66g-fjs-run-commands.md
index 17939d94e..a8db0bf63 100644
--- a/fjs/todo/66g-fjs-run-commands.md
+++ b/fjs/todo/66g-fjs-run-commands.md
@@ -25,7 +25,7 @@ runtime. Every downstream caller (`fjs run`, bin scripts) already goes through
 export type NodeMain = NodeProgram | Commands<NodeOp>
 ```
 
-Widen `dispatch` in `fjs/cli/module.f.ts` to accept either a `Commands` array
+Widen `dispatch` in `fjs/cli/module.f.mjs` to accept either a `Commands` array
 or a `Program` function, and short-circuit to the function when it receives one:
 
 ```ts
@@ -61,4 +61,5 @@ wrapper in `fjs/cas/module.f.ts` simplifies to `export const main = commands`.
 - `fjs/module.f.ts` — the `run` handler at line 39.
 - `fjs/cas/module.f.ts` — the `main = dispatch(commands)` boilerplate this issue
   eliminates.
-- `fjs/cli/module.f.ts` — `dispatch` and `Commands` used by the new branch.
+- `fjs/cli/module.f.mjs` — `dispatch` used by the new branch; `Commands`
+  is defined in `fjs/cli/types.ts`.
diff --git a/fjs/website/module.f.ts b/fjs/website/module.f.mjs
similarity index 65%
rename from fjs/website/module.f.ts
rename to fjs/website/module.f.mjs
index 4759c381f..81e40f76c 100644
--- a/fjs/website/module.f.ts
+++ b/fjs/website/module.f.mjs
@@ -3,20 +3,21 @@
  *
  * @module
  */
+
 import { htmlUtf8 } from '../media/html/module.f.mjs'
 import { writeFile } from '../effects/node/module.f.mjs'
-import type { WriteFile } from '../effects/node/types.ts'
+/** @import { WriteFile } from '../effects/node/types.ts' */
 import { pure, step } from '../effects/module.f.mjs'
-import type { Effect } from '../effects/types.ts'
-import type { Vec } from '../types/bit_vec/types.ts'
+/** @import { Effect } from '../effects/types.ts' */
 
-const html: Vec = htmlUtf8()(
+const html = htmlUtf8()(
     ['a',
         { href: 'https://github.com/functionalscript/functionalscript' },
         'GitHub Repository'
     ])
 
-const program: Effect<WriteFile, number> = step(
+/** @type {Effect<WriteFile, number>} */
+const program = step(
     writeFile('index.html', html),
     () => pure(0))
 
diff --git a/fjs/website/proof.f.ts b/fjs/website/proof.f.ts
index 8346e01cd..218c4023f 100644
--- a/fjs/website/proof.f.ts
+++ b/fjs/website/proof.f.ts
@@ -1,4 +1,4 @@
-import { main } from './module.f.ts'
+import { main } from './module.f.mjs'
 import { emptyState, virtual } from '../effects/node/virtual/module.f.ts'
 import { assertEq, assertNotNullish } from '../asserts/module.f.mjs'
 
diff --git a/package.json b/package.json
index e2bf73233..cfd6266cd 100644
--- a/package.json
+++ b/package.json
@@ -15,9 +15,9 @@
     "cov": "node --test --experimental-test-coverage --test-coverage-include=**/module.f.ts --test-coverage-include=**/module.f.mjs",
     "start": "node ./fjs/module.ts",
     "ci-update": "node ./fjs/module.ts ci",
-    "dev-update": "node ./fjs/module.ts r ./fjs/dev/update/module.f.ts",
+    "dev-update": "node ./fjs/module.ts r ./fjs/dev/update/module.f.mjs",
     "update": "npm run ci-update && npm install && deno install && bun install",
-    "index-html": "node ./fjs/module.ts r ./fjs/website/module.f.ts",
+    "index-html": "node ./fjs/module.ts r ./fjs/website/module.f.mjs",
     "website": "npm run prepack && npm run index-html"
   },
   "engines": {