fix(core): preserve structured error messages (#33530)

packages/core/test/process/process.test.ts

@@ -61,6 +61,7 @@ describe("AppProcess", () => {
           if (reason && reason._tag === "Fail") {
             expect(reason.error).toBeInstanceOf(AppProcess.AppProcessError)
             expect((reason.error as AppProcess.AppProcessError).exitCode).toBe(1)
+            expect((reason.error as AppProcess.AppProcessError).message).toContain("Command failed (exit 1)")
           } else {
             throw new Error("expected fail reason")
           }

packages/core/test/session-runner-model.test.ts

@@ -212,6 +212,7 @@ describe("SessionRunnerModel", () => {
         modelID: "test-model",
         variant: "unknown",
       })
+      expect(failure.message).toBe("Variant unavailable for test-provider/test-model: unknown")
     }),
   )
 
@@ -328,6 +329,7 @@ describe("SessionRunnerModel", () => {
         modelID: "test-model",
         api: "aisdk:@ai-sdk/google",
       })
+      expect(failure.message).toBe("Unsupported API for test-provider/test-model: aisdk:@ai-sdk/google")
     }),
   )
 

packages/core/test/session-runner-tool-registry.test.ts

@@ -206,6 +206,7 @@ describe("ToolRegistry", () => {
 
       expect(Exit.isFailure(exit)).toBe(true)
       if (Exit.isFailure(exit)) expect(Option.getOrUndefined(Cause.findErrorOption(exit.cause))).toBe(retentionFailure)
+      expect(retentionFailure.message).toBe("Failed to write tool output: disk full")
     }),
   )