TS strictFunctionTypes error with ChatToolRequest and ToolRequest at downstream project

[kittaakos]

Hello,

I'm running Theia 1.60.2 in a downstream project with "strict": true (in particular, strictFunctionTypes). Everything compiles fine in Theia's own build, but enabling strict mode in Theia triggers this error in
packages/ai-chat/src/common/chat-agents.ts:

diff --git a/packages/ai-chat/tsconfig.json b/packages/ai-chat/tsconfig.json
index 68f6ec825..942aad832 100644
--- a/packages/ai-chat/tsconfig.json
+++ b/packages/ai-chat/tsconfig.json
@@ -3,7 +3,8 @@
   "compilerOptions": {
     "composite": true,
     "rootDir": "src",
-    "outDir": "lib"
+    "outDir": "lib",
+    "strictFunctionTypes": true
   },
   "include": [
     "src"

TS2322
Type 'ChatToolRequest[] | undefined' is not assignable to type 'ToolRequest[] | undefined'. Type 'ChatToolRequest[]' is not assignable to type 'ToolRequest[]'. Types of property 'handler' are incompatible. Type '(arg_string: string, context: MutableChatRequestModel) => Promise<unknown>' is not assignable to type '(arg_string: string, ctx?: unknown) => Promise<unknown>'. Types of parameters 'context' and 'ctx' are incompatible. Type 'unknown' is not assignable to type 'MutableChatRequestModel'.

---

Context

• ToolRequest defines

theia/packages/ai-core/src/common/language-model.ts

Line 93 in 88ca7f4

handler: (arg_string: string, ctx?: unknown) => Promise<unknown>;

• ChatToolRequest narrows that to

theia/packages/ai-chat/src/common/chat-tool-request-service.ts

Lines 22 to 25 in 88ca7f4

	handler: (
	arg_string: string,
	context: MutableChatRequestModel,
	) => Promise<unknown>;

• Under legacy/bivariant TS this passed, but strictFunctionTypes enforces contravariance, so parameter type cannot be tightened when extending.

Questions

1. Intended TS configuration?
   Should ai-chat (maybe Theia) be compiled with strictFunctionTypes: true upstream, or does Theia expect downstream apps to disable it?
2. API change
   Would you consider widening the ChatToolRequest.handler signature to match ToolRequest and casting the context in the client code?:

   export interface ChatToolRequest extends ToolRequest {
      handler: (arg: string, ctx?: MutableChatRequestModel) => Promise<unknown>; // remove it to allow undefined and any subshape
   }

3. Best practice
   In the meantime, is it correct to ignore or cast this error in downstream Theia projects with strict compiler options?

Thank you!

[kittaakos]

Fixed via #15655