diff --git a/parser-typechecker/src/Unison/Builtin/Decls.hs b/parser-typechecker/src/Unison/Builtin/Decls.hs
index d50279f1ad..9bfe9c9a30 100644
--- a/parser-typechecker/src/Unison/Builtin/Decls.hs
+++ b/parser-typechecker/src/Unison/Builtin/Decls.hs
@@ -107,6 +107,11 @@ constructorId ref name = do
   (_, _, dd) <- find (\(_, r, _) -> Reference.DerivedId r == ref) builtinDataDecls
   fmap fromIntegral . elemIndex name $ DD.constructorNames dd
 
+effectId :: Reference -> Text -> Maybe ConstructorId
+effectId ref name = do
+  (_, _, ed) <- find (\(_, r, _) -> Reference.DerivedId r == ref) builtinEffectDecls
+  fmap fromIntegral . elemIndex name . DD.constructorNames $ DD.toDataDecl ed
+
 noneId, someId, okConstructorId, failConstructorId, docBlobId, docLinkId, docSignatureId, docSourceId, docEvaluateId, docJoinId, linkTermId, linkTypeId, eitherRightId, eitherLeftId :: ConstructorId
 isPropagatedConstructorId, isTestConstructorId, bufferModeNoBufferingId, bufferModeLineBufferingId, bufferModeBlockBufferingId, bufferModeSizedBlockBufferingId :: ConstructorId
 seqViewEmpty, seqViewElem :: ConstructorId
@@ -164,8 +169,13 @@ seekModeAbsoluteId = Maybe.fromJust $ constructorId seekModeRef "io2.SeekMode.Ab
 seekModeRelativeId = Maybe.fromJust $ constructorId seekModeRef "io2.SeekMode.RelativeSeek"
 seekModeEndId = Maybe.fromJust $ constructorId seekModeRef "io2.SeekMode.SeekFromEnd"
 
+stdInId, stdOutId, stdErrId :: ConstructorId
+stdInId = Maybe.fromJust $ constructorId stdHandleRef "io2.StdHandle.StdIn"
+stdOutId = Maybe.fromJust $ constructorId stdHandleRef "io2.StdHandle.StdOut"
+stdErrId = Maybe.fromJust $ constructorId stdHandleRef "io2.StdHandle.StdErr"
+
 exceptionRaiseId :: ConstructorId
-exceptionRaiseId = Maybe.fromJust $ constructorId exceptionRef "Exception.raise"
+exceptionRaiseId = Maybe.fromJust $ effectId exceptionRef "Exception.raise"
 
 okConstructorReferent, failConstructorReferent :: Referent.Referent
 okConstructorReferent = Referent.Con (ConstructorReference testResultRef okConstructorId) CT.Data
diff --git a/unison-runtime/src/Unison/Runtime/Builtin.hs b/unison-runtime/src/Unison/Runtime/Builtin.hs
index 67a4d26824..cf3e0db295 100644
--- a/unison-runtime/src/Unison/Runtime/Builtin.hs
+++ b/unison-runtime/src/Unison/Runtime/Builtin.hs
@@ -1112,7 +1112,7 @@ declareUdpForeigns = do
 declareForeigns :: FDecl Symbol ()
 declareForeigns = do
   declareUdpForeigns
-  declareForeign Tracked 1 IO_openFile_impl_v3
+  declareForeign Tracked 2 IO_openFile_impl_v3
 
   declareForeign Tracked 1 IO_closeFile_impl_v3
   declareForeign Tracked 1 IO_isFileEOF_impl_v3
diff --git a/unison-runtime/src/Unison/Runtime/Foreign/Function.hs b/unison-runtime/src/Unison/Runtime/Foreign/Function.hs
index be97b6fbc1..1cbd9305d4 100644
--- a/unison-runtime/src/Unison/Runtime/Foreign/Function.hs
+++ b/unison-runtime/src/Unison/Runtime/Foreign/Function.hs
@@ -214,13 +214,8 @@ foreignCallHelper = \case
   IO_UDP_ListenSocket_sendTo_impl_v1 -> mkForeignIOF $
     \(socket :: ListenSocket, bytes :: Bytes.Bytes, addr :: ClientSockAddr) ->
       UDP.sendTo socket (Bytes.toArray bytes) addr
-  IO_openFile_impl_v3 -> mkForeignIOF $ \(fnameText :: Util.Text.Text, n :: Int) ->
+  IO_openFile_impl_v3 -> mkForeignIOF $ \(fnameText :: Util.Text.Text, mode :: IOMode) ->
     let fname = Util.Text.toString fnameText
-        mode = case n of
-          0 -> ReadMode
-          1 -> WriteMode
-          2 -> AppendMode
-          _ -> ReadWriteMode
      in openFile fname mode
   IO_closeFile_impl_v3 -> mkForeignIOF hClose
   IO_isFileEOF_impl_v3 -> mkForeignIOF hIsEOF
@@ -336,11 +331,10 @@ foreignCallHelper = \case
   IO_kill_impl_v3 -> mkForeignIOF killThread
   IO_delay_impl_v3 -> mkForeignIOF customDelay
   IO_stdHandle -> mkForeign $
-    \(n :: Int) -> case n of
-      0 -> pure SYS.stdin
-      1 -> pure SYS.stdout
-      2 -> pure SYS.stderr
-      _ -> die "IO.stdHandle: invalid input."
+    \case
+      StdIn -> pure SYS.stdin
+      StdOut -> pure SYS.stdout
+      StdErr -> pure SYS.stderr
   IO_process_call -> mkForeign $
     \(exe, map Util.Text.unpack -> args) ->
       withCreateProcess (proc exe args) $ \_ _ _ p ->
@@ -1757,8 +1751,8 @@ decodeBufferMode (Enum _ t)
   | t == noBufTag = pure NoBuffering
   | t == lineBufTag = pure LineBuffering
   | t == blockBufTag = pure $ BlockBuffering Nothing
-decodeBufferMode (Data1 _ t (IntVal i))
-  | t == sizedBlockBufTag = pure . BlockBuffering $ Just i
+decodeBufferMode (Data1 _ t (NatVal i))
+  | t == sizedBlockBufTag = pure . BlockBuffering $ Just (fromIntegral i)
 decodeBufferMode c = foreignConventionError "BufferMode" (BoxedVal c)
 
 encodeBufferMode :: BufferMode -> Closure
@@ -1837,6 +1831,34 @@ instance ForeignConvention SeekMode where
   readAtIndex stk i = bpeekOff stk i >>= decodeSeekMode
   writeBack stk sm = bpoke stk (encodeSeekMode sm)
 
+data StdHnd = StdIn | StdOut | StdErr
+
+decodeStdHnd :: Closure -> IO StdHnd
+decodeStdHnd (Enum _ t)
+  | t == stdInTag = pure StdIn
+  | t == stdOutTag = pure StdOut
+  | t == stdErrTag = pure StdErr
+decodeStdHnd c = foreignConventionError "StdHandle" (BoxedVal c)
+
+encodeStdHnd :: StdHnd -> Closure
+encodeStdHnd StdIn = std'in
+encodeStdHnd StdOut = std'out
+encodeStdHnd StdErr = std'err
+
+std'in, std'out, std'err :: Closure
+std'in = Enum Ty.stdHandleRef stdInTag
+std'out = Enum Ty.stdHandleRef stdOutTag
+std'err = Enum Ty.stdHandleRef stdErrTag
+
+instance ForeignConvention StdHnd where
+  decodeVal (BoxedVal c) = decodeStdHnd c
+  decodeVal v = foreignConventionError "StdHandle" v
+
+  encodeVal = BoxedVal . encodeStdHnd
+
+  readAtIndex stk i = bpeekOff stk i >>= decodeStdHnd
+  writeBack stk = bpoke stk . encodeStdHnd
+
 -- In reality this fixes the type to be 'RClosure', but allows us to defer
 -- the typechecker a bit and avoid a bunch of annoying type annotations.
 -- instance {-# OVERLAPPING #-} ForeignConvention [Val] where
diff --git a/unison-runtime/src/Unison/Runtime/TypeTags.hs b/unison-runtime/src/Unison/Runtime/TypeTags.hs
index d94c73b399..ea8a9236d4 100644
--- a/unison-runtime/src/Unison/Runtime/TypeTags.hs
+++ b/unison-runtime/src/Unison/Runtime/TypeTags.hs
@@ -33,6 +33,9 @@ module Unison.Runtime.TypeTags
     seekEndTag,
     exceptionTag,
     exceptionRaiseTag,
+    stdInTag,
+    stdOutTag,
+    stdErrTag,
   )
 where
 
@@ -204,6 +207,15 @@ seekAbsoluteTag, seekRelativeTag, seekEndTag :: PackedTag
           Ty.seekModeEndId ] = (at, rt, et)
   | otherwise = error "internal error: seek mode tags"
 
+stdInTag, stdOutTag, stdErrTag :: PackedTag
+(stdInTag, stdOutTag, stdErrTag)
+  | [it, ot, et] <-
+      mkTags "standard handle tags" Ty.stdHandleRef
+        [ Ty.stdInId,
+          Ty.stdOutId,
+          Ty.stdErrId ] = (it, ot, et)
+  | otherwise = error "internal error: standard handle tags"
+
 exceptionTag :: Word64
 exceptionRaiseTag :: PackedTag
 (exceptionTag, exceptionRaiseTag)