Do not interpolate non string values automatically

.chronus/changes/fix-file-type-interpolation-2026-2-27-10-9-43.md

@@ -0,0 +1,12 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Do not interpolate non primitive values in config automatically
+  ```yaml
+      file-type: ["json", "yaml"]
+      output-file: "openapi.{file-type}"
+  ```
+  Will not be interpolated as `openapi.json,yaml` but keep the placeholder `{file-type}` intact for the emitter to handle.

packages/compiler/src/config/config-interpolation.ts

@@ -114,8 +114,13 @@ export function resolveValues<T extends Record<string, unknown>>(
       resolvingValues.delete(keyPath);
       return value;
     }
-    const replaced = value.replace(VariableInterpolationRegex, (_, expression) => {
-      return (resolveExpression(expression) as string) ?? `{${expression}}`;
+    const replaced = value.replace(VariableInterpolationRegex, (match, expression) => {
+      const resolved = resolveExpression(expression);
+      return typeof resolved === "string" ||
+        typeof resolved === "number" ||
+        typeof resolved === "boolean"
+        ? String(resolved)
+        : match;
     });
     resolvingValues.delete(keyPath);
     return replaced;

packages/compiler/test/config/config-interpolation.test.ts

@@ -133,6 +133,32 @@ describe("compiler: config interpolation", () => {
         },
       ]);
     });
+
+    it("does not interpolate variables that resolve to non-string values", () => {
+      const [resolved, diagnostics] = resolveValues({
+        "output-file": "openapi.{file-type}",
+        "file-type": ["yaml", "json"] as any,
+      });
+      expectDiagnosticEmpty(diagnostics);
+      deepStrictEqual(resolved, {
+        "output-file": "openapi.{file-type}",
+        "file-type": ["yaml", "json"],
+      });
+    });
+
+    it("interpolates number and boolean values", () => {
+      const [resolved, diagnostics] = resolveValues({
+        path: "v{version}/debug-{debug}",
+        version: 3 as any,
+        debug: true as any,
+      });
+      expectDiagnosticEmpty(diagnostics);
+      deepStrictEqual(resolved, {
+        path: "v3/debug-true",
+        version: 3,
+        debug: true,
+      });
+    });
   });
 
   describe("expandConfigVariables", () => {