fix(codemode): align promise combinators with Test262

# Conflicts:
#	packages/codemode/src/interpreter/runtime.ts
#	packages/codemode/src/tool-runtime.ts

packages/codemode/README.md

@@ -246,12 +246,12 @@ CodeMode executes a deliberately bounded JavaScript subset. It supports:
 - Regular expressions - `/literals/` and `new RegExp(...)` with `test`/`exec` (stateful `lastIndex` for `g`), plus string `match`/`matchAll`/`replace`/`replaceAll`/`split`/`search` with patterns. Match results are arrays carrying `index` and named `groups` as own properties (`input` is omitted). `replace` and `replaceAll` accept function replacers with captures, offset, input, and named groups; callbacks run sequentially, may await tool calls, and have their results coerced to strings. Invalid patterns, invalid flags, and missing-`g` calls fail with catchable errors that say what was wrong and how to fix it (escaping hints, the exact `/pattern/g` to write). Patterns run on the host engine, so pathological backtracking is bounded only by the execution timeout.
 - `Map` and `Set` - construction from entries/arrays/strings, `get`/`set`/`add`/`has`/`delete`/`clear`/`size`/`forEach`, and `keys`/`values`/`entries` returning **arrays** (not iterators).
 - URL helpers - `URL` resolution and mutation, linked `URLSearchParams`, `URL.canParse`/`URL.parse`, URI and URI-component encoding/decoding, and query parameter construction, lookup, mutation, sorting, callbacks, and materialization. URLSearchParams iteration methods return arrays, matching the Map/Set convention.
-- First-class promises - an un-awaited `tools.ns.tool(...)` is a promise value whose call starts immediately on a supervised fiber; `await` resolves it (awaiting a non-promise value is a no-op, and `return tools.ns.tool(...)` resolves like an async-function return). `Promise.all`, `Promise.allSettled`, and `Promise.race` accept any array mixing promises and plain values (built inline, beforehand, or via spread); `Promise.resolve`/`Promise.reject` construct settled promises. `Promise.allSettled` rejection reasons are the same plain `{ name?, message }` data a `catch` binding sees, and `Promise.race` interrupts its losing in-flight calls. At most 8 tool calls run concurrently. When a program completes, still-running un-awaited calls are awaited before the execution ends; a failure from a call that was never awaited surfaces as an unhandled-rejection diagnostic.
+- First-class promises - an un-awaited `tools.ns.tool(...)` is a promise value whose call starts immediately on a supervised fiber; `await` resolves it (awaiting a non-promise value is a no-op, and `return tools.ns.tool(...)` resolves like an async-function return). Promises support `then`/`catch`/`finally`, including returned-promise flattening and rejection recovery. `Promise.all`, `Promise.allSettled`, and `Promise.race` accept any array mixing promises and plain values (built inline, beforehand, or via spread); `Promise.resolve` preserves an existing promise or wraps a plain value, and `Promise.reject` creates a rejected promise. `Promise.allSettled` rejection reasons use the same values a `catch` binding sees, and `Promise.race` lets losing promises continue. At most 8 tool calls run concurrently. When a program completes, still-running un-awaited calls are awaited before the execution ends; a failure from a call that was never awaited surfaces as an unhandled-rejection diagnostic.
 - `throw value` and `throw new Error(message)` for explicit program failure. `Error` (and `TypeError`/`RangeError`/`SyntaxError`/`ReferenceError`/`EvalError`/`URIError`) are real constructors, callable with or without `new`; error values are plain `{ name, message }` data that additionally satisfy `instanceof Error` (a specific type matches itself and `Error`, as in JS). Every caught failure - thrown errors, interpreter runtime errors, and tool failures - is `instanceof Error` in a `catch` block; a thrown non-error value (`throw "text"`) is not, matching JS. Caught failures carry the `name` the equivalent real-JS failure would have - `JSON.parse` and invalid regex patterns produce a `SyntaxError` (satisfying `instanceof SyntaxError`), an unknown identifier a `ReferenceError`, assigning to a constant a `TypeError`, a bad `normalize` form a `RangeError`; failures with no specific analogue (including tool failures) are named `"Error"`. `instanceof` also recognizes `Date`, `RegExp`, `Map`, `Set`, `URL`, `URLSearchParams`, `Array`, `Object`, and `Promise`; any other right-hand side is a catchable error.
 
 Inside a program, standard-library values stay live everywhere: the internal data checkpoints (`Object.*` helpers, spread, coercion inputs) preserve the instances, so `Object.values({ d: date })[0].getTime()` and a spread copy of an object holding a Map keep working. Only at the host boundary (final result, tool arguments, `JSON.stringify`) do they serialize exactly as `JSON.stringify` would: Date and URL become strings (an invalid Date becomes `null`), while RegExp, Map, Set, and URLSearchParams become `{}`. Promise values never cross a data boundary: an un-awaited promise in a result or tool argument produces a diagnostic that says to await it, instead of serializing to `{}`.
 
-It does not expose `eval`, dynamic imports, modules, classes, generators, timers, host globals, prototype mutation, custom promise constructors (`new Promise`), promise chaining (`.then`/`.catch`/`.finally` - `await` with `try`/`catch` is the supported style), or arbitrary method calls. Unsupported syntax returns an `UnsupportedSyntax` diagnostic with a source location when available.
+It does not expose `eval`, dynamic imports, modules, classes, generators, timers, host globals, prototype mutation, custom promise constructors (`new Promise`), or arbitrary method calls. Unsupported syntax returns an `UnsupportedSyntax` diagnostic with a source location when available.
 
 CodeMode is an orchestration language, not a general JavaScript runtime.
 

packages/codemode/codemode.md

@@ -63,8 +63,10 @@ path lookup, namespace browsing, deterministic ranking, and pagination.
 ### Tool execution
 
 Calling a tool starts its Effect eagerly on a supervised fiber. The returned sandbox promise is run-once and can be
-awaited directly or through the supported `Promise` combinators. At most eight tool calls execute concurrently.
-Unfinished calls are drained before successful program completion, and an unhandled call failure becomes a diagnostic.
+awaited directly, chained with `then`/`catch`/`finally`, or passed through the supported `Promise` combinators. At most
+eight tool calls execute concurrently.
+Unfinished tracked promises are drained before successful program completion, and an unhandled rejection becomes a
+diagnostic.
 
 The public execution-policy knobs are `timeoutMs`, `maxToolCalls`, and `maxOutputBytes`. The package supplies no
 defaults because budgets are host policy. The interpreter also enforces fixed internal boundaries for tool-call
@@ -109,6 +111,62 @@ MCP tools use this canonical path: they register as grouped tools and are deferr
 output schemas are preserved in generated signatures. Direct Core tools remain direct and are not ambient globals
 inside CodeMode.
 
+## Promise Status
+
+The runtime currently provides eager, run-once promises for tool calls and async functions; `await`;
+`then`/`catch`/`finally`; and chainable `all`/`allSettled`/`race`/`resolve`/`reject`. `Promise.all` rejects promptly while
+siblings continue, and `Promise.race` leaves losers running as JavaScript does. Tracked work remains supervised in one
+execution scope, at most eight tool calls run concurrently, and ordinary success or failure drains unfinished work before
+closing. Timeout and external interruption cancel immediately instead.
+
+### Confirmed defects
+
+- [ ] Align handler callability with the values CodeMode reports as functions, or document the narrower callback
+      allowlist. For example, unsupported constructor-like callables are currently treated as absent handlers.
+- [ ] Run synchronous Promise reactions to completion. Reactions currently start in FIFO order but can interleave when
+      Effect auto-yields a long synchronous handler.
+- [ ] Give `Promise.race` settlement the same reaction turn as native promises. Its result currently settles before an
+      independently queued reaction in several Test262 ordering cases.
+- [ ] Drain queued reactions before classifying rejected sources as unhandled. A handler attached by an already-queued
+      reaction can currently lose to execution-boundary rejection reporting.
+
+### Deliberate deviations and open decisions
+
+- CodeMode drains unfinished work before ordinary success or failure closes. This keeps tool effects supervised, but a
+  race loser or fail-fast `Promise.all` sibling that never settles can hold execution open indefinitely when the host
+  supplies no timeout.
+- Promise resolution unwraps only `SandboxPromise`; arbitrary `{ then(resolve, reject) }` values remain data. Full
+  thenable assimilation requires internal callable resolver values, first-settlement arbitration, recursive adoption,
+  and cycle detection. Decide whether that machinery belongs in the bounded runtime.
+- `new Promise`, `Promise.any`, resolver APIs, subclasses/species, and the broader prototype surface are unavailable.
+  Consider `Promise.any` independently; custom constructors and subclassing are not current goals.
+- Combinators currently accept arrays plus CodeMode's spreadable strings, Maps, and Sets, while documentation and
+  diagnostics describe array inputs. Choose and document one contract.
+- `Promise.race([])` raises a clear error instead of creating a permanently pending promise.
+- Rejection tracking is execution-scoped and checked at drain time, not an ECMAScript microtask-level unhandled
+  rejection model.
+
+### Covered regressions
+
+- Nested unreturned tool calls remain alive after an async function or `then` handler settles.
+- Abandoned failing tool calls, async functions, and immediate `Promise.reject` values report unhandled rejections.
+- Ordinary program failure drains pending work and preserves the original error; a rejecting race winner drains its slow
+  loser.
+- Timeouts interrupt all in-flight promise fibers with parallel teardown, while host interruption propagates instead of
+  becoming a diagnostic.
+- Promise reactions and plain, pending, or settled `await` continuations start in deterministic FIFO order. Nested
+  reactions preserve enqueue order, while an async reaction can suspend without blocking the next queued reaction.
+- Awaiting the same promise twice settles it once.
+- Sparse combinator inputs consume holes as explicit `undefined` values.
+
+### Missing coverage
+
+- Nested unreturned work from `catch` and `finally` handlers.
+- Abandoned chained and combinator rejections.
+- External interruption while handled pending work remains.
+- Never-settling race losers and fail-fast `Promise.all` siblings under an explicit timeout.
+- Shared or duplicate promises across combinators, discarded inner chains, and detailed reaction ordering.
+
 ## Intentionally Unsupported
 
 These are product boundaries rather than DSL backlog:
@@ -130,7 +188,7 @@ represent accurately rather than guessing semantics.
 | Keep an owned tree-walking interpreter. | The product need is bounded tool orchestration, not arbitrary JavaScript. Owning the language surface keeps authority and behavior explicit. |
 | Treat schemas as the model-facing interface. | Signatures drive correct calls; Effect Schema also provides the runtime validation boundary, while JSON Schema supports adapter interoperability. |
 | Keep authority host-owned. | CodeMode can only confine programs to supplied tools. The host chooses those tools, and each tool enforces its own authorization and side-effect policy. |
-| Use progressive catalog disclosure plus search. | Large tool sets should not consume the prompt, but every namespace must remain discoverable and speculative search calls should remain valid. |
+| Use progressive catalog disclosure plus search. | Large tool sets should not consume the prompt, but every namespace must remain discoverable and speculative search calls must remain valid. |
 | Start tool promises eagerly and supervise them. | This preserves normal call-time parallelism while giving each call run-once settlement and interruption safety. |
 | Keep files outside the sandbox value space. | Models should compose structured data without routing binary payloads through generated code or context. |
 | Treat `execute` as the model-facing invocation boundary. | Nested calls are implementation details of one orchestration program. Reusing the outer context and bounding only the final result preserves complete intermediate data without inventing durable child-call identities. |
@@ -148,9 +206,6 @@ the adapter TODO. Delete entries when completed.
 The supported JavaScript subset should grow when common model-generated code improves tool orchestration. These are
 current omissions to implement, not intentional product boundaries.
 
-- [ ] Design proper multi-stage promise pipelines. Supporting `.then`, `.catch`, and `.finally` should preserve promise
-      assimilation, cancellation, failure handling, and concurrent per-item pipelines rather than adding syntax-only
-      shims. Consider `Promise.any` in the same pass.
 - [ ] Support async iteration and `for await...of`. Define behavior first for the runtime's supported promise and
       collection values, then extend it to bounded host streams when a stream boundary exists.
 - [ ] Support callback-bearing standard-library variants that models commonly generate: the mapper argument to

packages/codemode/src/interpreter/model.ts

@@ -122,7 +122,7 @@ export type DiagnosticKind =
 export const OptionalShortCircuit: unique symbol = Symbol("codemode.optional-short-circuit")
 
 export const supportedSyntaxMessage =
-  "Supported orchestration syntax: tools.* calls (they return promises - resolve them with await), data literals, destructuring, optional chaining, template literals, conditionals, switch, loops (incl. for...of and for...in over object/array/tools keys), arrow functions, spread, try/catch, array methods (map/filter/find/findIndex/some/every/reduce/flatMap/forEach/sort/slice/concat/indexOf/lastIndexOf/at/flat/reverse/includes/join), string methods (incl. match/matchAll/replace/split with regular expressions), Date/RegExp/Map/Set/URL/URLSearchParams, URI encoding helpers, Object/Math/JSON helpers, captured console.log/warn/error/dir/table, and Promise.all/allSettled/race/resolve/reject over arrays mixing promises and plain values for parallel tool calls (promise chaining with .then/.catch is not supported - use await with try/catch)."
+  "Supported orchestration syntax: tools.* calls (they return promises - resolve them with await or .then/.catch/.finally), data literals, destructuring, optional chaining, template literals, conditionals, switch, loops (incl. for...of and for...in over object/array/tools keys), arrow functions, spread, try/catch, array methods (map/filter/find/findIndex/some/every/reduce/flatMap/forEach/sort/slice/concat/indexOf/lastIndexOf/at/flat/reverse/includes/join), string methods (incl. match/matchAll/replace/split with regular expressions), Date/RegExp/Map/Set/URL/URLSearchParams, URI encoding helpers, Object/Math/JSON helpers, captured console.log/warn/error/dir/table, and Promise.all/allSettled/race/resolve/reject over arrays mixing promises and plain values for parallel tool calls."
 
 export class InterpreterRuntimeError extends Error {
   readonly node?: AstNode

packages/codemode/src/interpreter/runtime.ts

@@ -1,5 +1,5 @@
 import { parse } from "acorn"
-import { Cause, Effect, Exit, Fiber, Semaphore } from "effect"
+import { Cause, Deferred, Effect, Exit, Fiber, Queue, Scope, Semaphore } from "effect"
 import { DiagnosticCategory, ModuleKind, ScriptTarget, flattenDiagnosticMessageText, transpileModule } from "typescript"
 import {
   copyIn,
@@ -148,6 +148,11 @@ const parseProgram = (code: string): ProgramNode => {
   return parsed as ProgramNode
 }
 
+type PromiseReaction<R> = {
+  readonly effect: Effect.Effect<unknown, unknown, R>
+  readonly settlement: Deferred.Deferred<unknown, unknown>
+}
+
 const publicErrorMessage = (message: string): string =>
   message.replace(/\/(?:Users|home|private|tmp|var\/folders)\/[^\s"'`]+/g, "<redacted-path>")
 
@@ -219,7 +224,7 @@ const normalizeError = (error: unknown): Diagnostic => {
   }
 }
 
-// Shared by catch bindings, Promise.allSettled rejection reasons, and Promise.race losers.
+// Shared by catch bindings and Promise.allSettled rejection reasons.
 const caughtErrorValue = (thrown: unknown): unknown => {
   if (thrown instanceof ProgramThrow) return thrown.value
   if (thrown instanceof InterpreterRuntimeError) return createErrorValue(thrown.errorName, thrown.message)
@@ -618,9 +623,14 @@ class Interpreter<R> {
   // ToolRuntime.make like invokeTool: the interpreter never holds the tree itself.
   private readonly toolKeys: (path: ReadonlyArray<string>) => ReadonlyArray<string>
   private readonly logs: Array<string>
+  private lastValue: unknown
+  // Every promise fiber belongs to the execution rather than the async function or handler
+  // that happened to create it. The execution scope still interrupts all work on teardown.
+  private readonly promiseScope: Scope.Scope
+  private readonly reactionQueue: Queue.Queue<PromiseReaction<R>>
   // Caps how many eagerly forked tool calls run at once (the parallel-call concurrency cap).
   private readonly callPermits: Semaphore.Semaphore
-  // Fiber-backed promises whose settlement no program construct has observed yet. Successful
+  // Fiber-backed promises whose settlement no program construct has observed yet. Ordinary
   // program completion drains these (like a runtime waiting on in-flight work at exit) and
   // surfaces a never-awaited failure as an unhandled-rejection diagnostic.
   private readonly pendingSettlements: Set<SandboxPromise>
@@ -628,6 +638,8 @@ class Interpreter<R> {
   constructor(
     invokeTool: (path: ReadonlyArray<string>, args: Array<unknown>) => Effect.Effect<unknown, unknown, R>,
     toolKeys: (path: ReadonlyArray<string>) => ReadonlyArray<string>,
+    promiseScope: Scope.Scope,
+    reactionQueue: Queue.Queue<PromiseReaction<R>>,
     logs: Array<string> = [],
     shared?: { callPermits: Semaphore.Semaphore; pendingSettlements: Set<SandboxPromise> },
   ) {
@@ -636,6 +648,9 @@ class Interpreter<R> {
     this.invokeTool = invokeTool
     this.toolKeys = toolKeys
     this.logs = logs
+    this.lastValue = undefined
+    this.promiseScope = promiseScope
+    this.reactionQueue = reactionQueue
     this.callPermits = shared?.callPermits ?? Semaphore.makeUnsafe(TOOL_CALL_CONCURRENCY)
     this.pendingSettlements = shared?.pendingSettlements ?? new Set<SandboxPromise>()
     globalScope.set("tools", { mutable: false, value: new ToolReference([]) })
@@ -678,7 +693,7 @@ class Interpreter<R> {
     // top-level declarations (`let undefined = 5`, `const Object = ...`) shadow builtins like
     // JS module scope, instead of colliding with the seeded globals.
     this.pushScope()
-    return Effect.gen(function* () {
+    const evaluate = Effect.gen(function* () {
       self.hoistFunctions(program.body)
       let value: unknown = undefined
       for (const [index, statement] of program.body.entries()) {
@@ -703,31 +718,44 @@ class Interpreter<R> {
       // resolves before crossing the data boundary - `return tools.ns.tool(...)` works
       // without an explicit await, exactly as in JS.
       if (value instanceof SandboxPromise) value = yield* self.settlePromise(value)
-      yield* self.drainPendingSettlements()
       return value
+    })
+    return Effect.gen(function* () {
+      const result = yield* Effect.exit(evaluate)
+      if (Exit.isFailure(result) && Cause.hasInterruptsOnly(result.cause)) {
+        return yield* Effect.failCause(result.cause)
+      }
+
+      const drained = yield* Effect.exit(self.drainPendingSettlements())
+      if (Exit.isFailure(result)) return yield* Effect.failCause(result.cause)
+      if (Exit.isFailure(drained)) return yield* Effect.failCause(drained.cause)
+      return result.value
     }).pipe(Effect.ensuring(Effect.sync(() => self.popScope())))
   }
 
   // Awaits every fiber-backed promise the program abandoned (fire-and-forget tool calls), so
   // their work completes before the execution ends - mirroring a JS runtime waiting on
   // in-flight I/O at exit. A failure nobody could have handled becomes an unhandled-rejection
-  // diagnostic (interrupted calls, e.g. Promise.race losers, are ignored).
+  // diagnostic.
   private drainPendingSettlements(): Effect.Effect<void, unknown, never> {
     const self = this
     return Effect.gen(function* () {
+      let unhandled: InterpreterRuntimeError | undefined
       while (self.pendingSettlements.size > 0) {
         const promise = self.pendingSettlements.values().next().value
         if (promise === undefined) break
         const exit = yield* self.observePromise(promise)
-        if (Exit.isSuccess(exit) || Cause.hasInterruptsOnly(exit.cause)) continue
+        if (Exit.isSuccess(exit) || Cause.hasInterruptsOnly(exit.cause) || promise.handled) continue
+        if (unhandled !== undefined) continue
         const failure = normalizeError(Cause.squash(exit.cause))
-        throw new InterpreterRuntimeError(
+        unhandled = new InterpreterRuntimeError(
           `Unhandled rejection from an un-awaited promise: ${failure.message}`,
           undefined,
           failure.kind,
           ["Await promises so failures can be caught and handled."],
         )
       }
+      if (unhandled !== undefined) throw unhandled
     })
   }
 
@@ -743,7 +771,7 @@ class Interpreter<R> {
   }
 
   private createPromise(effect: Effect.Effect<unknown, unknown, R>): Effect.Effect<SandboxPromise, never, R> {
-    return Effect.map(Effect.forkChild(effect, { startImmediately: true }), (fiber) => {
+    return Effect.map(Effect.forkIn(effect, this.promiseScope, { startImmediately: true }), (fiber) => {
       const promise = new SandboxPromise(fiber)
       this.pendingSettlements.add(promise)
       return promise
@@ -760,28 +788,49 @@ class Interpreter<R> {
 
   // `await promise`: succeed with the fulfilled value or re-raise the failure so try/catch
   // observes it exactly like a synchronous throw at the await site.
-  private settlePromise(promise: SandboxPromise, node?: AstNode): Effect.Effect<unknown, unknown, never> {
+  private settlePromise(promise: SandboxPromise): Effect.Effect<unknown, unknown, never> {
     const self = this
-    return Effect.flatMap(this.observePromise(promise), (exit) => self.unwrapPromiseExit(promise, exit, node))
+    return Effect.flatMap(this.observePromise(promise), (exit) => self.unwrapPromiseExit(exit))
   }
 
-  private unwrapPromiseExit(
-    promise: SandboxPromise | undefined,
-    exit: Exit.Exit<unknown, unknown>,
-    node?: AstNode,
-  ): Effect.Effect<unknown, unknown> {
-    if (Exit.isSuccess(exit)) return Effect.succeed(exit.value)
-    // A call Promise.race interrupted after losing settles as a catchable program failure;
-    // any other interruption is execution teardown (timeout/host) and must keep propagating
-    // as interruption rather than becoming program-visible data.
-    if (promise?.interrupted === true && Cause.hasInterruptsOnly(exit.cause)) {
-      return Effect.fail(
-        new InterpreterRuntimeError(
-          "This tool call was interrupted because another value settled a Promise.race first.",
-          node,
+  private createReaction(
+    source: Effect.Effect<Exit.Exit<unknown, unknown>, never, R>,
+    reaction: (exit: Exit.Exit<unknown, unknown>, chained: SandboxPromise) => Effect.Effect<unknown, unknown, R>,
+  ): Effect.Effect<SandboxPromise, never, R> {
+    const settlement = Deferred.makeUnsafe<unknown, unknown>()
+    const chained = new SandboxPromise(undefined, Deferred.await(settlement))
+    this.pendingSettlements.add(chained)
+    return Effect.as(
+      Effect.forkIn(
+        Effect.flatMap(source, (exit) =>
+          Effect.sync(() =>
+            Queue.offerUnsafe(this.reactionQueue, {
+              settlement,
+              effect: Effect.suspend(() => reaction(exit, chained)),
+            }),
+          ),
         ),
-      )
-    }
+        this.promiseScope,
+        { startImmediately: true },
+      ),
+      chained,
+    )
+  }
+
+  private awaitValue(value: unknown): Effect.Effect<unknown, unknown, R> {
+    return Effect.flatMap(
+      this.createReaction(
+        value instanceof SandboxPromise
+          ? Effect.exit(this.settlePromise(value))
+          : Effect.succeed(Exit.succeed(value)),
+        (exit) => this.unwrapPromiseExit(exit),
+      ),
+      (continuation) => this.settlePromise(continuation),
+    )
+  }
+
+  private unwrapPromiseExit(exit: Exit.Exit<unknown, unknown>): Effect.Effect<unknown, unknown> {
+    if (Exit.isSuccess(exit)) return Effect.succeed(exit.value)
     return Effect.failCause(exit.cause)
   }
 
@@ -1528,12 +1577,9 @@ class Interpreter<R> {
       case "UpdateExpression":
         return this.evaluateUpdateExpression(node)
       case "AwaitExpression": {
-        // `await` resolves a promise value; awaiting anything else is a passthrough no-op,
-        // matching real JS semantics for non-thenables.
-        const self = this
-        return Effect.flatMap(this.evaluateExpression(getNode(node, "argument")), (value) =>
-          value instanceof SandboxPromise ? self.settlePromise(value, node) : Effect.succeed(value),
-        )
+        // Even an already-settled value resumes in a later reaction turn, after work that was
+        // already queued, matching JavaScript's await continuation ordering.
+        return Effect.flatMap(this.evaluateExpression(getNode(node, "argument")), (value) => this.awaitValue(value))
       }
       case "NewExpression":
         return this.evaluateNewExpression(node)
@@ -2217,14 +2263,24 @@ class Interpreter<R> {
       )
     }
     if (ref.name === "reject") {
-      return Effect.sync(() => new SandboxPromise(undefined, Effect.fail(new ProgramThrow(args[0]))))
+      return Effect.sync(() => {
+        const promise = new SandboxPromise(undefined, Effect.fail(new ProgramThrow(args[0])))
+        this.pendingSettlements.add(promise)
+        return promise
+      })
     }
 
-    const items = Array.isArray(args[0]) ? args[0] : spreadItems(args[0])
+    // Promise combinators consume arrays through their iterator, which visits holes as
+    // explicit undefined values instead of preserving sparse positions like Array.map.
+    const items = Array.isArray(args[0]) ? [...args[0]] : spreadItems(args[0])
     if (items === undefined) {
-      throw new InterpreterRuntimeError(
-        `Promise.${ref.name} expects an array of promises or plain values (e.g. Promise.${ref.name}(items.map((item) => tools.ns.tool(item)))).`,
-        node,
+      return this.createPromise(
+        Effect.fail(
+          new InterpreterRuntimeError(
+            `Promise.${ref.name} expects an array of promises or plain values (e.g. Promise.${ref.name}(items.map((item) => tools.ns.tool(item)))).`,
+            node,
+          ).as("TypeError"),
+        ),
       )
     }
 
@@ -2238,68 +2294,59 @@ class Interpreter<R> {
             ? Effect.map(this.observePromise(item), (exit) => ({ index, item, exit }))
             : Effect.succeed({ index, item: undefined, exit: Exit.succeed(item) }),
         )
-        return Effect.gen(function* () {
-          const remaining = [...observations]
-          const values: Array<unknown> = []
-          values.length = items.length
-          while (remaining.length > 0) {
-            const winner = yield* Effect.raceAll(remaining)
-            const position = remaining.indexOf(observations[winner.index])
-            if (position >= 0) remaining.splice(position, 1)
-            if (Exit.isSuccess(winner.exit)) {
-              values[winner.index] = winner.exit.value
-              continue
+        return this.createPromise(
+          Effect.gen(function* () {
+            const remaining = [...observations]
+            const values: Array<unknown> = []
+            values.length = items.length
+            while (remaining.length > 0) {
+              const winner = yield* Effect.raceAll(remaining)
+              const position = remaining.indexOf(observations[winner.index])
+              if (position >= 0) remaining.splice(position, 1)
+              if (Exit.isSuccess(winner.exit)) {
+                values[winner.index] = winner.exit.value
+                continue
+              }
+              for (const item of items) {
+                if (!(item instanceof SandboxPromise) || item === winner.item) continue
+                item.handled = true
+                self.pendingSettlements.add(item)
+              }
+              return yield* self.unwrapPromiseExit(winner.exit)
             }
-            yield* self.createPromise(
-              Effect.asVoid(
-                Effect.forEach(
-                  items,
-                  (item) => (item instanceof SandboxPromise ? self.observePromise(item) : Effect.void),
-                  { concurrency: "unbounded" },
-                ),
-              ),
-            )
-            return yield* self.unwrapPromiseExit(winner.item, winner.exit, node)
-          }
-          return values
-        })
+            return values
+          }),
+        )
       }
       case "allSettled": {
         const observations = items.map((item) =>
-          item instanceof SandboxPromise
-            ? Effect.map(this.observePromise(item), (exit) => ({ promise: item as SandboxPromise | undefined, exit }))
-            : Effect.succeed({ promise: undefined as SandboxPromise | undefined, exit: Exit.succeed(item as unknown) }),
+          item instanceof SandboxPromise ? this.observePromise(item) : Effect.succeed(Exit.succeed(item as unknown)),
         )
-        return Effect.gen(function* () {
-          const outcomes: Array<unknown> = []
-          for (const observation of observations) {
-            const { exit, promise } = yield* observation
-            if (Exit.isSuccess(exit)) {
-              outcomes.push(
-                Object.assign(Object.create(null) as SafeObject, { status: "fulfilled", value: exit.value }),
-              )
-              continue
-            }
-            const raceInterrupted = promise?.interrupted === true && Cause.hasInterruptsOnly(exit.cause)
-            if (Cause.hasInterruptsOnly(exit.cause) && !raceInterrupted) {
-              // Execution teardown (timeout/host interruption), not a program-level rejection.
-              return yield* Effect.failCause(exit.cause)
-            }
-            const thrown = raceInterrupted
-              ? new InterpreterRuntimeError(
-                  "This tool call was interrupted because another value settled a Promise.race first.",
-                  node,
+        return this.createPromise(
+          Effect.gen(function* () {
+            const outcomes: Array<unknown> = []
+            for (const observation of observations) {
+              const exit = yield* observation
+              if (Exit.isSuccess(exit)) {
+                outcomes.push(
+                  Object.assign(Object.create(null) as SafeObject, { status: "fulfilled", value: exit.value }),
                 )
-              : Cause.squash(exit.cause)
-            outcomes.push(
-              Object.assign(Object.create(null) as SafeObject, {
-                status: "rejected",
-                reason: caughtErrorValue(thrown),
-              }),
-            )
-          }
-          return outcomes
-        })
+                continue
+              }
+              if (Cause.hasInterruptsOnly(exit.cause)) {
+                // Execution teardown (timeout/host interruption), not a program-level rejection.
+                return yield* Effect.failCause(exit.cause)
+              }
+              outcomes.push(
+                Object.assign(Object.create(null) as SafeObject, {
+                  status: "rejected",
+                  reason: caughtErrorValue(Cause.squash(exit.cause)),
+                }),
+              )
+            }
+            return outcomes
+          }),
+        )
       }
       case "race": {
         if (items.length === 0) {
@@ -2313,28 +2360,26 @@ class Interpreter<R> {
             ? Effect.map(this.observePromise(item), (exit) => ({ index, exit }))
             : Effect.succeed({ index, exit: Exit.succeed(item as unknown) }),
         )
-        return Effect.gen(function* () {
-          // First settlement (fulfilled OR rejected) wins; the observations never fail, so
-          // racing them yields exactly that. Losing in-flight calls are then interrupted.
-          const winner = yield* Effect.raceAll(observations)
-          for (const [index, item] of items.entries()) {
-            if (index === winner.index || !(item instanceof SandboxPromise) || item.fiber === undefined) continue
-            item.interrupted = true
-            yield* Fiber.interrupt(item.fiber)
-          }
-          const winningItem = items[winner.index]
-          return yield* self.unwrapPromiseExit(
-            winningItem instanceof SandboxPromise ? winningItem : undefined,
-            winner.exit,
-            node,
-          )
-        })
+        return this.createPromise(
+          Effect.gen(function* () {
+            // First settlement (fulfilled OR rejected) wins. Losers continue like native
+            // promises, while a supervised drain keeps their failures handled and their work
+            // inside the execution lifetime.
+            const winner = yield* Effect.raceAll(observations)
+            for (const [index, item] of items.entries()) {
+              if (index === winner.index || !(item instanceof SandboxPromise)) continue
+              item.handled = true
+              self.pendingSettlements.add(item)
+            }
+            return yield* self.unwrapPromiseExit(winner.exit)
+          }),
+        )
       }
     }
   }
 
   private invokeFunction(fn: CodeModeFunction, args: Array<unknown>): Effect.Effect<unknown, unknown, R> {
-    const invocation = new Interpreter(this.invokeTool, this.toolKeys, this.logs, {
+    const invocation = new Interpreter(this.invokeTool, this.toolKeys, this.promiseScope, this.reactionQueue, this.logs, {
       callPermits: this.callPermits,
       pendingSettlements: this.pendingSettlements,
     })
@@ -2377,6 +2422,9 @@ class Interpreter<R> {
     args: Array<unknown>,
     node: AstNode,
   ): Effect.Effect<unknown, unknown, R> {
+    if (ref.receiver instanceof SandboxPromise) {
+      return this.invokePromisePrototypeMethod(ref.receiver, ref.name, args, node)
+    }
     if (typeof ref.receiver === "string") {
       if (
         (ref.name === "replace" || ref.name === "replaceAll") &&
@@ -2413,6 +2461,86 @@ class Interpreter<R> {
     throw new InterpreterRuntimeError(`Method '${ref.name}' is not available in CodeMode.`, node)
   }
 
+  private invokePromisePrototypeMethod(
+    promise: SandboxPromise,
+    name: string,
+    args: Array<unknown>,
+    node: AstNode,
+  ): Effect.Effect<SandboxPromise, never, R> {
+    const callback = (value: unknown) => {
+      if (
+        !(value instanceof ToolReference && value.path.length > 0) &&
+        !(value instanceof PromiseMethodReference) &&
+        !(value instanceof CodeModeFunction) &&
+        !(value instanceof IntrinsicReference) &&
+        !(value instanceof GlobalMethodReference) &&
+        !(value instanceof CoercionFunction) &&
+        !(value instanceof UriFunction) &&
+        !(value instanceof ErrorConstructorReference)
+      )
+        return undefined
+      return (callbackArgs: Array<unknown>, chained: SandboxPromise) =>
+        Effect.flatMap(
+          value instanceof ToolReference
+            ? this.createToolCallPromise(value.path, callbackArgs)
+            : value instanceof PromiseMethodReference
+              ? this.invokePromiseMethod(value, callbackArgs, node)
+              : value instanceof CodeModeFunction
+                ? this.invokeFunction(value, callbackArgs)
+                : value instanceof IntrinsicReference
+                  ? this.invokeIntrinsic(value, callbackArgs, node)
+                  : value instanceof GlobalMethodReference
+                    ? Effect.sync(() => {
+                        if (value.namespace === "console") return this.invokeConsole(value.name, callbackArgs, node)
+                        if (
+                          value.namespace === "Object" &&
+                          callbackArgs[0] instanceof ToolReference &&
+                          !objectMethodsPreservingIdentity.has(value.name)
+                        )
+                          return this.invokeObjectMethodOnTools(value.name, callbackArgs[0], node)
+                        return invokeGlobalMethod(value, callbackArgs, node)
+                      })
+                    : value instanceof CoercionFunction
+                      ? Effect.succeed(invokeCoercion(value, callbackArgs, node))
+                      : value instanceof UriFunction
+                        ? Effect.succeed(invokeUriFunction(value, callbackArgs, node))
+                        : Effect.succeed(
+                            createErrorValue(
+                              value.name,
+                              callbackArgs[0] === undefined ? "" : coerceToString(callbackArgs[0]),
+                            ),
+                          ),
+          (result) => {
+            if (result === chained) {
+              return Effect.fail(
+                new InterpreterRuntimeError("Chaining cycle detected for promise.", node).as("TypeError"),
+              )
+            }
+            return result instanceof SandboxPromise ? this.settlePromise(result) : Effect.succeed(result)
+          },
+        )
+    }
+    const onFulfilled = name === "then" ? callback(args[0]) : undefined
+    const onRejected = name === "then" ? callback(args[1]) : name === "catch" ? callback(args[0]) : undefined
+    const onFinally = name === "finally" ? callback(args[0]) : undefined
+    const settlement = Effect.exit(this.settlePromise(promise))
+
+    return this.createReaction(
+      settlement,
+      (exit, chained) =>
+        Effect.gen(function* () {
+          if (onFinally !== undefined) yield* onFinally([], chained)
+          if (Exit.isSuccess(exit)) {
+            if (onFulfilled === undefined) return exit.value
+            return yield* onFulfilled([exit.value], chained)
+          }
+          if (onRejected !== undefined)
+            return yield* onRejected([caughtErrorValue(Cause.squash(exit.cause))], chained)
+          return yield* Effect.failCause(exit.cause)
+        }),
+    )
+  }
+
   private invokeStringReplacer(
     value: string,
     name: "replace" | "replaceAll",
@@ -3206,17 +3334,9 @@ class Interpreter<R> {
         return new ComputedValue(undefined)
       }
 
-      // Any property access on a promise is a confused program (`p.then(...)`, `p.value`);
-      // reading `undefined` here would hide the missing await, so both paths get an explicit,
-      // await-hinting error instead of the forgiving unknown-property fallthrough.
       if (objectValue instanceof SandboxPromise) {
         if (key === "then" || key === "catch" || key === "finally") {
-          throw new InterpreterRuntimeError(
-            `Promise.prototype.${String(key)} is not supported in CodeMode; use await instead (with try/catch to handle failures) - e.g. \`const result = await tools.ns.tool(...)\`.`,
-            propertyNode,
-            "UnsupportedSyntax",
-            [supportedSyntaxMessage],
-          )
+          return new IntrinsicReference(objectValue, key)
         }
         throw new InterpreterRuntimeError(
           "This value is an un-awaited Promise and has no readable properties; await it first - e.g. `const result = await tools.ns.tool(...)`.",
@@ -3505,18 +3625,33 @@ export const executeWithLimits = <const Tools extends Record<string, unknown>>(
     })
   }
 
-  const operation = Effect.gen(function* () {
-    const program = parseProgram(options.code)
-    const interpreter = new Interpreter<Services<Tools>>(tools.invoke, tools.keys, logs)
-    const value = yield* interpreter.run(program)
-    const result = copyOut(copyIn(value, "Execution result"), true) as DataValue
-    return {
-      ok: true,
-      value: result,
-      ...logged(),
-      toolCalls: tools.calls,
-    } satisfies Result
-  }).pipe((program) => {
+  const operation = Effect.scoped(
+    Effect.gen(function* () {
+      const scope = yield* Effect.acquireRelease(Scope.make("parallel"), (scope, exit) => Scope.close(scope, exit))
+      const reactionQueue = yield* Queue.unbounded<PromiseReaction<Services<Tools>>>()
+      yield* Effect.forever(
+        Effect.gen(function* () {
+          const reaction = yield* Queue.take(reactionQueue)
+          yield* Effect.yieldNow
+          yield* Effect.forkIn(
+            Effect.flatMap(Effect.exit(reaction.effect), (exit) => Deferred.done(reaction.settlement, exit)),
+            scope,
+            { startImmediately: true },
+          )
+        }),
+      ).pipe(Effect.forkIn(scope, { startImmediately: true }))
+      const program = parseProgram(options.code)
+      const interpreter = new Interpreter<Services<Tools>>(tools.invoke, tools.keys, scope, reactionQueue, logs)
+      const value = yield* interpreter.run(program)
+      const result = copyOut(copyIn(value, "Execution result"), true) as DataValue
+      return {
+        ok: true,
+        value: result,
+        ...logged(),
+        toolCalls: tools.calls,
+      } satisfies Result
+    }),
+  ).pipe((program) => {
     const timeoutMs = limits.timeoutMs
     if (timeoutMs === undefined) return program
     return program.pipe(

packages/codemode/src/tool-runtime.ts

@@ -616,7 +616,7 @@ export const prepare = <R>(tools: HostTools<R>, catalogBudget = defaultCatalogBu
     "## Language",
     "",
     "Use common JavaScript data operations, functions, control flow, selected standard-library methods, and awaited tool calls. Built-ins include Date, RegExp, Map, Set, URL, URLSearchParams, and URI encoding helpers.",
-    "Modules/imports, classes, generators, timers, fetch, eval, prototype access, unlisted methods, and promise chaining are unavailable. Use Code Mode tools for external operations. Use await with try/catch.",
+    "Modules/imports, classes, generators, timers, fetch, eval, prototype access, and unlisted methods are unavailable. Use Code Mode tools for external operations. Use await with try/catch.",
     "Prefer explicit `return`; otherwise only the final top-level expression becomes the result.",
     "Dates and URLs serialize to strings at data boundaries; Map/Set/RegExp/URLSearchParams serialize to `{}`.",
   ]

packages/codemode/src/values.ts

@@ -1,7 +1,7 @@
 import type { Effect, Fiber } from "effect"
 
 export class SandboxPromise {
-  interrupted = false
+  handled = false
   constructor(
     readonly fiber: Fiber.Fiber<unknown, unknown> | undefined,
     readonly immediate?: Effect.Effect<unknown, unknown>,

packages/codemode/test/codemode.test.ts

@@ -652,9 +652,12 @@ describe("CodeMode public contract", () => {
     expect(instructions).toContain("not a general-purpose runtime")
     expect(instructions).not.toContain("Standard modern JavaScript works")
     expect(instructions).not.toContain("TypeScript type annotations")
-    for (const missing of ["Modules/imports", "classes", "generators", "fetch", "promise chaining"]) {
+    for (const missing of ["Modules/imports", "classes", "generators", "fetch"]) {
       expect(instructions).toContain(missing)
     }
+    expect(instructions).toContain("selected standard-library methods, and awaited tool calls")
+    expect(instructions).toContain("Use await with try/catch")
+    expect(instructions).not.toContain("promise chaining are unavailable")
     expect(instructions).toContain("URL, URLSearchParams, and URI encoding helpers")
     expect(instructions).not.toContain("host globals")
     expect(instructions).toContain("Use Code Mode tools for external operations")

packages/codemode/test/promise-test262.test.ts

@@ -0,0 +1,334 @@
+/*
+ * Portions adapted from Test262 at revision 250f204f23a9249ff204be2baec29600faae7b75:
+ * - test/built-ins/Promise/all/S25.4.4.1_A1.1_T1.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A2.1_T1.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A2.3_T1.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A2.3_T2.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A2.3_T3.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A7.1_T1.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A8.2_T1.js
+ * - test/built-ins/Promise/all/S25.4.4.1_A8.2_T2.js
+ * - test/built-ins/Promise/all/iter-arg-is-number-reject.js
+ * - test/built-ins/Promise/all/resolve-non-thenable.js
+ * - test/built-ins/Promise/allSettled/is-function.js
+ * - test/built-ins/Promise/allSettled/returns-promise.js
+ * - test/built-ins/Promise/allSettled/resolves-empty-array.js
+ * - test/built-ins/Promise/allSettled/resolves-to-array.js
+ * - test/built-ins/Promise/allSettled/resolved-all-fulfilled.js
+ * - test/built-ins/Promise/allSettled/resolved-all-mixed.js
+ * - test/built-ins/Promise/allSettled/resolved-all-rejected.js
+ * - test/built-ins/Promise/allSettled/iter-arg-is-number-reject.js
+ * - test/built-ins/Promise/allSettled/resolve-non-thenable.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A1.1_T1.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.1_T1.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.1_T2.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.1_T3.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.2_T1.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.3_T1.js
+ * - test/built-ins/Promise/race/S25.4.4.3_A7.3_T2.js
+ * - test/built-ins/Promise/race/iter-arg-is-number-reject.js
+ * - test/built-ins/Promise/resolve/S25.4.4.5_A1.1_T1.js
+ * - test/built-ins/Promise/resolve/S25.4.4.5_A2.1_T1.js
+ * - test/built-ins/Promise/resolve/resolve-non-obj.js
+ * - test/built-ins/Promise/resolve/resolve-non-thenable.js
+ * - test/built-ins/Promise/reject/S25.4.4.4_A1.1_T1.js
+ * - test/built-ins/Promise/reject/S25.4.4.4_A2.1_T1.js
+ * - test/built-ins/Array/from/from-array.js
+ * - test/language/statements/async-function/declaration-returns-promise.js
+ * - test/language/statements/async-function/evaluation-body-that-returns.js
+ * - test/language/statements/async-function/evaluation-body-that-returns-after-await.js
+ * - test/language/statements/async-function/evaluation-body-that-throws.js
+ * - test/language/statements/async-function/evaluation-body-that-throws-after-await.js
+ *
+ * Copyright 2014 Cubane Canada, Inc.  All rights reserved.
+ * Copyright (C) 2016 the V8 project authors. All rights reserved.
+ * Copyright (C) 2019 Leo Balter. All rights reserved.
+ * Copyright (C) 2018 Rick Waldron. All rights reserved.
+ * Copyright 2015 Microsoft Corporation. All rights reserved.
+ * Copyright 2016 Microsoft, Inc. All rights reserved.
+ * Test262 portions are governed by the BSD license in LICENSE.test262.
+ */
+import { describe, expect, test } from "bun:test"
+import { Effect } from "effect"
+import { CodeMode } from "../src/index.js"
+
+const value = async (code: string) => {
+  const result = await Effect.runPromise(CodeMode.execute({ code, tools: {} }))
+  if (!result.ok) throw new Error(`expected success, got ${result.error.kind}: ${result.error.message}`)
+  return result.value
+}
+
+describe("Test262 Promise static adaptations", () => {
+  test("Promise statics are callable", async () => {
+    expect(
+      await value(
+        `return [typeof Promise.all, typeof Promise.allSettled, typeof Promise.race, typeof Promise.resolve, typeof Promise.reject]`,
+      ),
+    ).toEqual(["function", "function", "function", "function", "function"])
+  })
+
+  test("invalid combinator inputs return rejected TypeError promises", async () => {
+    expect(
+      await value(`
+        const observe = async (run) => {
+          let returned = false
+          try {
+            const promise = run()
+            returned = promise instanceof Promise
+            await promise
+            return [returned, "fulfilled"]
+          } catch (error) {
+            return [returned, error.name]
+          }
+        }
+        return await Promise.all([
+          observe(() => Promise.all(42)),
+          observe(() => Promise.allSettled(42)),
+          observe(() => Promise.race(42)),
+        ])
+      `),
+    ).toEqual([
+      [true, "TypeError"],
+      [true, "TypeError"],
+      [true, "TypeError"],
+    ])
+  })
+
+  test("Promise.all returns a promise and creates a fresh empty array", async () => {
+    expect(
+      await value(`
+        const input = []
+        const promise = Promise.all(input)
+        const result = await promise
+        return [promise instanceof Promise, result instanceof Array, result.length, result !== input]
+      `),
+    ).toEqual([true, true, 0, true])
+  })
+
+  test("Promise.all preserves values and input order", async () => {
+    expect(
+      await value(`
+        const first = { id: 1 }
+        const second = { id: 2 }
+        const result = await Promise.all([Promise.resolve(3), first, Promise.resolve(second)])
+        return [result.length, result[0], result[1] === first, result[2] === second]
+      `),
+    ).toEqual([3, 3, true, true])
+  })
+
+  test("Promise.all adopts rejection from either input position", async () => {
+    expect(
+      await value(`
+        const observe = async (promise) => {
+          try { await promise; return "fulfilled" } catch (reason) { return reason }
+        }
+        return await Promise.all([
+          observe(Promise.all([Promise.reject(1), Promise.resolve(2)])),
+          observe(Promise.all([Promise.resolve(1), Promise.reject(2)])),
+        ])
+      `),
+    ).toEqual([1, 2])
+  })
+
+  test("Promise.all treats sparse positions as undefined values", async () => {
+    expect(
+      await value(`
+        const input = []
+        input[1] = 1
+        const result = await Promise.all(input)
+        return [result.length, result[0] === undefined, result[1]]
+      `),
+    ).toEqual([2, true, 1])
+  })
+
+  test("Promise.allSettled returns a promise and a fresh array", async () => {
+    expect(
+      await value(`
+        const input = []
+        const promise = Promise.allSettled(input)
+        const result = await promise
+        return [promise instanceof Promise, result instanceof Array, result.length, result !== input]
+      `),
+    ).toEqual([true, true, 0, true])
+  })
+
+  test("Promise.allSettled preserves order and settlement shapes", async () => {
+    expect(
+      await value(`
+        const reason = { id: 4 }
+        const result = await Promise.allSettled([
+          Promise.resolve(1),
+          Promise.reject(2),
+          3,
+          Promise.reject(reason),
+        ])
+        return [result, result.map((item) => Object.keys(item))]
+      `),
+    ).toEqual([
+      [
+        { status: "fulfilled", value: 1 },
+        { status: "rejected", reason: 2 },
+        { status: "fulfilled", value: 3 },
+        { status: "rejected", reason: { id: 4 } },
+      ],
+      [
+        ["status", "value"],
+        ["status", "reason"],
+        ["status", "value"],
+        ["status", "reason"],
+      ],
+    ])
+  })
+
+  test("Promise.allSettled preserves input order across settlement timing", async () => {
+    expect(
+      await value(`
+        const slow = async () => { await Promise.resolve(); return "slow" }
+        return await Promise.allSettled([slow(), Promise.resolve("fast")])
+      `),
+    ).toEqual([
+      { status: "fulfilled", value: "slow" },
+      { status: "fulfilled", value: "fast" },
+    ])
+  })
+
+  test("Promise.allSettled adopts a non-thenable object", async () => {
+    expect(
+      await value(`
+        const object = { id: 1 }
+        const result = await Promise.allSettled([object])
+        return [result, result[0].value === object]
+      `),
+    ).toEqual([[{ status: "fulfilled", value: { id: 1 } }], true])
+  })
+
+  test("Promise.allSettled treats sparse positions as undefined values", async () => {
+    expect(
+      await value(`
+        const input = []
+        input[1] = 1
+        const result = await Promise.allSettled(input)
+        return [result.length, result[0].status, result[0].value === undefined, result[1]]
+      `),
+    ).toEqual([2, "fulfilled", true, { status: "fulfilled", value: 1 }])
+  })
+
+  test("Promise.race returns a promise and settles from its first input", async () => {
+    expect(
+      await value(`
+        const rejected = Promise.reject(2)
+        const fulfilled = Promise.resolve(1)
+        const promise = Promise.race([fulfilled, rejected])
+        return [promise instanceof Promise, await promise]
+      `),
+    ).toEqual([true, 1])
+  })
+
+  test("Promise.race preserves primitive fulfillment and rejection", async () => {
+    expect(
+      await value(`
+        const fulfilled = await Promise.race([23])
+        let rejected
+        try { await Promise.race([Promise.reject(7)]) } catch (reason) { rejected = reason }
+        return [fulfilled, rejected]
+      `),
+    ).toEqual([23, 7])
+  })
+
+  test("Promise.race observes later fulfillment and rejection", async () => {
+    expect(
+      await value(`
+        const slow = async () => { await Promise.resolve(); await Promise.resolve(); return 1 }
+        const rejectSoon = async () => { await Promise.resolve(); throw 2 }
+        try { await Promise.race([slow(), rejectSoon()]); return "fulfilled" } catch (reason) { return reason }
+      `),
+    ).toBe(2)
+  })
+
+  test("Promise.race selects fulfilled and rejected inputs by settlement order", async () => {
+    expect(
+      await value(`
+        const delayed = async (value) => { await Promise.resolve(); return value }
+        const first = await Promise.race([Promise.resolve(1), Promise.resolve(2)])
+        const second = await Promise.race([Promise.resolve(1), delayed(9)])
+        const third = await Promise.race([delayed(9), Promise.resolve(2)])
+        let fourth
+        try { await Promise.race([Promise.reject(1), Promise.resolve(2)]) } catch (reason) { fourth = reason }
+        return [first, second, third, fourth]
+      `),
+    ).toEqual([1, 1, 2, 1])
+  })
+
+  test("Promise.race treats a sparse first position as undefined", async () => {
+    expect(
+      await value(`
+        const input = []
+        input[1] = 1
+        return (await Promise.race(input)) === undefined
+      `),
+    ).toBe(true)
+  })
+
+  test("Promise.resolve adopts values and nested promises", async () => {
+    expect(
+      await value(`
+        const object = { id: 1 }
+        return [
+          await Promise.resolve(42),
+          await Promise.resolve(Promise.resolve("nested")),
+          (await Promise.resolve(object)) === object,
+        ]
+      `),
+    ).toEqual([42, "nested", true])
+  })
+
+  test("Promise.resolve preserves promise identity", async () => {
+    expect(
+      await value(`
+        const promise = Promise.resolve(1)
+        const identities = new Map([[promise, "same"]])
+        return identities.get(Promise.resolve(promise))
+      `),
+    ).toBe("same")
+  })
+
+  test("Promise.reject preserves primitive and object reasons", async () => {
+    expect(
+      await value(`
+        const object = { reason: true }
+        const reasons = [undefined, null, false, true, 0, "", 42, object]
+        const observe = async (reason) => {
+          try { await Promise.reject(reason); return false } catch (caught) { return caught === reason }
+        }
+        return await Promise.all(reasons.map(observe))
+      `),
+    ).toEqual([true, true, true, true, true, true, true, true])
+  })
+})
+
+describe("Test262 async function adaptations", () => {
+  test("async functions return promises and adopt returned values", async () => {
+    expect(
+      await value(`
+        const plain = async () => 42
+        const afterAwait = async () => { await Promise.resolve(); return 43 }
+        const first = plain()
+        const second = afterAwait()
+        return [first instanceof Promise, second instanceof Promise, await first, await second]
+      `),
+    ).toEqual([true, true, 42, 43])
+  })
+
+  test("async functions reject for throws before and after await", async () => {
+    expect(
+      await value(`
+        const throwsBefore = async () => { throw 1 }
+        const throwsAfter = async () => { await Promise.resolve(); throw 2 }
+        const observe = async (promise) => {
+          try { await promise; return "fulfilled" } catch (reason) { return reason }
+        }
+        return await Promise.all([observe(throwsBefore()), observe(throwsAfter())])
+      `),
+    ).toEqual([1, 2])
+  })
+})

packages/codemode/test/promise.test.ts

@@ -92,33 +92,14 @@ describe("first-class promise values", () => {
         }
         const first = load(1)
         const second = load(2)
-        return [first instanceof Promise, second instanceof Promise, await Promise.all([first, second])]
+        return await Promise.all([first, second])
       `),
     ).toEqual([
-      true,
-      true,
-      [
-        [1, 1],
-        [2, 2],
-      ],
+      [1, 1],
+      [2, 2],
     ])
   })
 
-  test("async function errors reject instead of throwing at the call site", async () => {
-    expect(
-      await value(`
-        const fail = async () => { throw new Error("boom") }
-        const promise = fail()
-        try {
-          await promise
-          return "no"
-        } catch (error) {
-          return error.message
-        }
-      `),
-    ).toBe("boom")
-  })
-
   test("an un-awaited tool call starts eagerly, in call order, before any await", async () => {
     const trace = makeTrace()
     const result = await value(
@@ -212,6 +193,22 @@ describe("first-class promise values", () => {
     expect(diagnostic.suggestions?.join(" ")).toContain("Await promises")
   })
 
+  test("an unhandled rejection does not stop later work from draining", async () => {
+    const trace = makeTrace()
+    const diagnostic = await error(
+      `
+        tools.host.fail({})
+        tools.host.sleepy({ id: 1, ms: 30 })
+        return "done"
+      `,
+      { trace },
+    )
+    expect(diagnostic.kind).toBe("ToolFailure")
+    expect(diagnostic.message).toContain("Unhandled rejection from an un-awaited promise")
+    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+  })
+
   test("a never-awaited failing async function surfaces as an unhandled promise rejection", async () => {
     const diagnostic = await error(`
       const fail = async () => { throw new Error("boom") }
@@ -235,6 +232,58 @@ describe("first-class promise values", () => {
     expect(diagnostic.kind).toBe("ToolFailure")
     expect(diagnostic.message).toContain("Lookup refused")
   })
+
+  test("keeps unreturned calls alive after their async function settles", async () => {
+    const trace = makeTrace()
+    expect(
+      await value(
+        `
+          const start = async () => {
+            tools.host.sleepy({ id: 1, ms: 30 })
+            return "started"
+          }
+          await start()
+          return "done"
+        `,
+        { trace },
+      ),
+    ).toBe("done")
+    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+  })
+
+  test("keeps unreturned calls alive after their promise handler settles", async () => {
+    const trace = makeTrace()
+    expect(
+      await value(
+        `
+          await Promise.resolve().then(() => {
+            tools.host.sleepy({ id: 1, ms: 30 })
+          })
+          return "done"
+        `,
+        { trace },
+      ),
+    ).toBe("done")
+    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+  })
+
+  test("drains pending work after program failure and preserves the original failure", async () => {
+    const trace = makeTrace()
+    const diagnostic = await error(
+      `
+        tools.host.fail({})
+        tools.host.sleepy({ id: 1, ms: 30 })
+        throw new Error("original")
+      `,
+      { trace },
+    )
+    expect(diagnostic.kind).toBe("ExecutionFailure")
+    expect(diagnostic.message).toBe("Uncaught: original")
+    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+  })
 })
 
 describe("promises at data boundaries", () => {
@@ -335,10 +384,6 @@ describe("Promise.all over arbitrary arrays", () => {
     expect(trace.maxActive).toBeLessThanOrEqual(8)
   })
 
-  test("resolves the empty array", async () => {
-    expect(await value(`return await Promise.all([])`)).toEqual([])
-  })
-
   test("rejects with the first failure, catchable in-program", async () => {
     expect(
       await value(`
@@ -418,45 +463,36 @@ describe("Promise.allSettled", () => {
 })
 
 describe("Promise.race", () => {
-  test("first settlement wins and losers are interrupted", async () => {
+  test("first settlement wins and losers continue", async () => {
     const trace = makeTrace()
     const result = await value(
       `
         const fast = tools.host.sleepy({ id: 1, ms: 10 })
-        const slow = tools.host.sleepy({ id: 2, ms: 5000 })
+        const slow = tools.host.sleepy({ id: 2, ms: 30 })
         return await Promise.race([fast, slow])
       `,
       { trace },
     )
     expect(result).toBe(1)
-    expect(trace.interrupted).toBe(1)
-    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+    expect(trace.completed).toBe(2)
   })
 
-  test("awaiting an interrupted loser afterwards is a catchable program failure", async () => {
+  test("a losing chain continues and remains awaitable", async () => {
     expect(
       await value(`
-      const fast = tools.host.sleepy({ id: 1, ms: 10 })
-      const slow = tools.host.sleepy({ id: 2, ms: 5000 })
-      const winner = await Promise.race([fast, slow])
-      try {
-        await slow
-        return "no"
-      } catch (e) {
-        return { winner, caught: e.message }
-      }
+      const slow = tools.host.sleepy({ id: 2, ms: 30 }).then((id) => id * 2)
+      const winner = await Promise.race([slow, "fast"])
+      return [winner, await slow]
     `),
-    ).toEqual({
-      winner: 1,
-      caught: "This tool call was interrupted because another value settled a Promise.race first.",
-    })
+    ).toEqual(["fast", 4])
   })
 
   test("a rejection can win the race", async () => {
     expect(
       await value(`
       try {
-        await Promise.race([tools.host.fail({}), tools.host.sleepy({ id: 1, ms: 5000 })])
+        await Promise.race([tools.host.fail({}), tools.host.sleepy({ id: 1, ms: 30 })])
         return "no"
       } catch (e) {
         return e.message
@@ -465,12 +501,37 @@ describe("Promise.race", () => {
     ).toBe("Lookup refused")
   })
 
+  test("a rejecting winner still drains its slow loser", async () => {
+    const trace = makeTrace()
+    const diagnostic = await error(
+      `return await Promise.race([tools.host.fail({}), tools.host.sleepy({ id: 1, ms: 30 })])`,
+      { trace },
+    )
+    expect(diagnostic.kind).toBe("ToolFailure")
+    expect(diagnostic.message).toBe("Lookup refused")
+    expect(trace.completed).toBe(1)
+    expect(trace.interrupted).toBe(0)
+  })
+
   test("a plain value wins over pending promises", async () => {
     const trace = makeTrace()
     expect(
-      await value(`return await Promise.race([tools.host.sleepy({ id: 1, ms: 5000 }), "immediate"])`, { trace }),
+      await value(`return await Promise.race([tools.host.sleepy({ id: 1, ms: 30 }), "immediate"])`, { trace }),
     ).toBe("immediate")
-    expect(trace.interrupted).toBe(1)
+    expect(trace.interrupted).toBe(0)
+    expect(trace.completed).toBe(1)
+  })
+
+  test("a losing rejection remains handled", async () => {
+    expect(
+      await value(`
+        const rejectLater = async () => {
+          await tools.host.sleepy({ id: 1, ms: 20 })
+          throw new Error("late")
+        }
+        return await Promise.race([rejectLater(), "winner"])
+      `),
+    ).toBe("winner")
   })
 
   test("an empty race is a clear error instead of hanging", async () => {
@@ -480,23 +541,35 @@ describe("Promise.race", () => {
 })
 
 describe("Promise.resolve / Promise.reject", () => {
-  test("resolve wraps plain values and passes promises through", async () => {
-    expect(await value(`return await Promise.resolve(42)`)).toBe(42)
-    expect(await value(`return await Promise.resolve(Promise.resolve("nested"))`)).toBe("nested")
+  test("resolve adopts a tool-call promise", async () => {
     expect(await value(`return await Promise.resolve(tools.host.sleepy({ id: 3 }))`)).toBe(3)
   })
 
-  test("reject produces a promise whose await throws the reason", async () => {
+  test("an abandoned rejected promise surfaces as an unhandled rejection", async () => {
+    const diagnostic = await error(`
+      Promise.reject(new Error("boom"))
+      return "done"
+    `)
+    expect(diagnostic.kind).toBe("ExecutionFailure")
+    expect(diagnostic.message).toContain("Unhandled rejection from an un-awaited promise")
+    expect(diagnostic.message).toContain("boom")
+  })
+})
+
+describe("Promise combinator values", () => {
+  test("all, allSettled, and race return chainable promises", async () => {
     expect(
       await value(`
-      try {
-        await Promise.reject("nope")
-        return "no"
-      } catch (e) {
-        return e
-      }
-    `),
-    ).toBe("nope")
+        const all = Promise.all([Promise.resolve(1), 2])
+        const settled = Promise.allSettled([Promise.resolve(3)])
+        const race = Promise.race([Promise.resolve(4)])
+        return [
+          await all.then((values) => values.join(",")),
+          await settled.then((values) => values[0].value),
+          await race.then((value) => value + 1),
+        ]
+      `),
+    ).toEqual(["1,2", 3, 5])
   })
 })
 
@@ -531,18 +604,185 @@ describe("timeout interruption of forked calls", () => {
     expect(result.error.kind).toBe("TimeoutExceeded")
     expect(trace.interrupted).toBe(2)
   })
+
+  test("interrupts promise fibers concurrently during scope teardown", async () => {
+    const cleanup = { active: 0, overlapped: false }
+    const tool = Tool.make({
+      description: "Wait until interrupted",
+      input: Schema.Struct({}),
+      output: Schema.Never,
+      run: () =>
+        Effect.never.pipe(
+          Effect.onInterrupt(() =>
+            Effect.gen(function* () {
+              cleanup.active += 1
+              yield* Effect.sleep(20)
+              cleanup.overlapped ||= cleanup.active > 1
+              cleanup.active -= 1
+            }),
+          ),
+        ),
+    })
+    const result = await Effect.runPromise(
+      CodeMode.execute({
+        tools: { host: { first: tool, second: tool } },
+        code: `
+          tools.host.first({})
+          tools.host.second({})
+          return await Promise.resolve("waiting")
+        `,
+        limits: { timeoutMs: 50 },
+      }),
+    )
+    expect(result.ok).toBe(false)
+    if (result.ok) return
+    expect(result.error.kind).toBe("TimeoutExceeded")
+    expect(cleanup.overlapped).toBe(true)
+  })
+})
+
+describe("promise chaining", () => {
+  test("then transforms values and flattens returned promises", async () => {
+    expect(
+      await value(`
+        return tools.host.sleepy({ id: 2 })
+          .then((id) => tools.host.sleepy({ id: id + 1 }))
+          .then((id) => id * 2)
+      `),
+    ).toBe(6)
+  })
+
+  test("handlers run after synchronous statements", async () => {
+    expect(
+      await value(`
+        const order = []
+        const chained = Promise.resolve().then(() => order.push("then"))
+        order.push("sync")
+        await chained
+        return order
+      `),
+    ).toEqual(["sync", "then"])
+  })
+
+  test("nested reactions run before downstream reactions queued later", async () => {
+    expect(
+      await value(`
+        const order = []
+        await Promise.resolve()
+          .then(() => {
+            order.push(1)
+            Promise.resolve().then(() => order.push(2))
+          })
+          .then(() => order.push(3))
+        return order
+      `),
+    ).toEqual([1, 2, 3])
+  })
+
+  test("plain and settled await resume after reactions that are already queued", async () => {
+    expect(
+      await value(`
+        const order = []
+        Promise.resolve().then(() => order.push(1))
+        await 0
+        order.push(2)
+        Promise.resolve().then(() => order.push(3))
+        await Promise.resolve()
+        order.push(4)
+        return order
+      `),
+    ).toEqual([1, 2, 3, 4])
+  })
+
+  test("reactions registered on the same pending promise preserve order", async () => {
+    expect(
+      await value(`
+        const order = []
+        const pending = tools.host.sleepy({ id: 1 })
+        pending.then(() => order.push(1))
+        pending.then(() => order.push(2))
+        await pending
+        return order
+      `),
+    ).toEqual([1, 2])
+  })
+
+  test("an async reaction does not block the next queued reaction", async () => {
+    expect(
+      await value(`
+        const order = []
+        const first = Promise.resolve().then(async () => {
+          order.push(1)
+          await tools.host.sleepy({ id: 1 })
+          order.push(3)
+        })
+        Promise.resolve().then(() => order.push(2))
+        await first
+        return order
+      `),
+    ).toEqual([1, 2, 3])
+  })
+
+  test("catch receives normalized errors and recovers the chain", async () => {
+    expect(await value(`return tools.host.fail({}).catch((error) => error.message)`)).toBe("Lookup refused")
+    expect(
+      await value(`
+        return Promise.resolve(1)
+          .then(() => { throw new Error("boom") })
+          .catch((error) => error.message)
+      `),
+    ).toBe("boom")
+  })
+
+  test("then rejection handlers and omitted handlers pass through settlement", async () => {
+    expect(await value(`return Promise.resolve(4).then(undefined).catch(undefined)`)).toBe(4)
+    expect(await value(`return Promise.reject("nope").then(undefined, (reason) => reason + "!")`)).toBe("nope!")
+  })
+
+  test("supported builtin callables can be handlers", async () => {
+    expect(await value(`return Promise.resolve({ a: 1 }).then(JSON.stringify)`)).toBe('{"a":1}')
+    expect(await value(`return Promise.resolve(4).then(Promise.resolve)`)).toBe(4)
+  })
+
+  test("finally awaits its callback and preserves the original settlement", async () => {
+    expect(
+      await value(`
+        let cleanup = 0
+        const result = await Promise.resolve(7).finally(async () => {
+          await tools.host.sleepy({ id: 1 })
+          cleanup = 1
+          return 99
+        })
+        return [result, cleanup]
+      `),
+    ).toEqual([7, 1])
+    expect(
+      await value(`return Promise.reject(new Error("original")).finally(() => 99).catch((error) => error.message)`),
+    ).toBe("original")
+  })
+
+  test("a rejected finally callback replaces the original settlement", async () => {
+    expect(
+      await value(`
+        return Promise.resolve(1)
+          .finally(() => Promise.reject(new Error("cleanup")))
+          .catch((error) => error.message)
+      `),
+    ).toBe("cleanup")
+  })
+
+  test("a self-resolving chain rejects instead of deadlocking", async () => {
+    expect(
+      await value(`
+        let chained
+        chained = Promise.resolve().then(() => chained)
+        return chained.catch((error) => [error.name, error.message])
+      `),
+    ).toEqual(["TypeError", "Chaining cycle detected for promise."])
+  })
 })
 
 describe("unsupported promise surface", () => {
-  test(".then/.catch/.finally give a clear await-instead error", async () => {
-    for (const method of ["then", "catch", "finally"]) {
-      const diagnostic = await error(`return tools.host.sleepy({ id: 1 }).${method}((x) => x)`)
-      expect(diagnostic.kind).toBe("UnsupportedSyntax")
-      expect(diagnostic.message).toContain(`Promise.prototype.${method} is not supported`)
-      expect(diagnostic.message).toContain("await")
-    }
-  })
-
   test("other property reads on a promise hint at the missing await", async () => {
     const diagnostic = await error(`return tools.host.sleepy({ id: 1 }).value`)
     expect(diagnostic.kind).toBe("InvalidDataValue")

packages/codemode/test/test262-promise.md

@@ -0,0 +1,40 @@
+# Test262 Promise Coverage
+
+The Promise tests adapt Test262 at revision `250f204f23a9249ff204be2baec29600faae7b75`. The upstream Promise tree
+contains 729 files; 459 are under the eight API directories corresponding to CodeMode's five Promise statics and three
+chaining methods. The executable suite currently cites 39 distinct Promise, Array, and async-function sources.
+
+This is coverage of CodeMode's confined Promise surface, not a claim of ECMAScript or Test262 conformance. One upstream
+file may contain both adapted and inapplicable assertions, so a cited source means only that the represented assertions
+were adapted. `LICENSE.test262` contains the upstream BSD terms.
+
+## Covered Surface
+
+- `Promise.all`, `Promise.allSettled`, and `Promise.race` result types, ordering, sparse positions, mixed values, and
+  rejection propagation.
+- `Promise.resolve` value adoption and promise identity.
+- `Promise.reject` reason preservation.
+- Async-function promise creation, returned-value adoption, and throws before and after `await`.
+
+The Promise PR's handwritten tests remain authoritative for tool-call concurrency, execution-scoped lifetime,
+cancellation, output boundaries, model-safe tool failures, unhandled-rejection diagnostics, and chaining behavior.
+
+## Exclusions
+
+- `new Promise`, externally captured resolving functions, and executor semantics.
+- Promise subclasses, constructors, species, realms, proxies, property descriptors, and function metadata.
+- Custom thenables, poisoned `then` accessors, and arbitrary iterable or iterator-closing behavior.
+- `Promise.any`, `Promise.try`, `Promise.withResolvers`, `Promise.allKeyed`, and `Promise.allSettledKeyed`.
+- Exact native microtask behavior for `.then`, `.catch`, and `.finally`; those are exercised separately by the Promise
+  PR's chaining tests and audit.
+- Exact native unhandled-rejection timing. CodeMode drains execution-owned work and reports safe diagnostics at its
+  execution boundary.
+
+## Observed Differences
+
+- Promise combinators accept arrays plus CodeMode's supported spreadable collections. The model guidance currently
+  describes arrays, so this remains a contract decision rather than conformance coverage.
+- `Promise.race([])` returns an actionable error instead of a permanently pending promise, which cannot usefully finish
+  a bounded CodeMode execution.
+- Promise identity is preserved, but CodeMode intentionally rejects general binary operators over Promise references;
+  identity is tested through `Map` keys instead of `===`.

packages/core/test/mcp.test.ts

@@ -544,6 +544,7 @@ it.effect("advertises MCP output schemas to Code Mode", () =>
     const execute = (yield* toolDefinitions(registry)).find((tool) => tool.name === "execute")
 
     expect(execute?.description).toContain("tools.demo.search(input: {}): Promise<{\n  ok: boolean,\n}>")
+    expect(execute?.description).not.toContain("promise chaining are unavailable")
   }),
 )