diff --git a/cli/cli.ts b/cli/cli.ts
index 5ba851aea3bf..048cd36f2517 100644
--- a/cli/cli.ts
+++ b/cli/cli.ts
@@ -2954,17 +2954,17 @@ class SnippetHost implements pxt.Host {
 
         if (module.id === "this") {
             if (filename === "pxt-core.d.ts") {
-                const contents = fs.readFileSync(path.join(this.getRepoDir(), "libs", "pxt-common", "pxt-core.d.ts"), 'utf8');
+                const contents = fs.readFileSync(path.join(this.getPxtRepoDir(), "libs", "pxt-common", "pxt-core.d.ts"), 'utf8');
                 this.writeFile(module, filename, contents);
                 return contents;
             }
             else if (filename === "pxt-helpers.ts") {
-                const contents = fs.readFileSync(path.resolve(this.getRepoDir(), "libs", "pxt-common", "pxt-helpers.ts"), 'utf8');
+                const contents = fs.readFileSync(path.resolve(this.getPxtRepoDir(), "libs", "pxt-common", "pxt-helpers.ts"), 'utf8');
                 this.writeFile(module, filename, contents);
                 return contents;
             }
             else if (filename === "pxt-python.d.ts" || filename === "pxt-python-helpers.ts") {
-                const contents = fs.readFileSync(path.resolve(this.getRepoDir(), "libs", "pxt-python", filename), 'utf8');
+                const contents = fs.readFileSync(path.resolve(this.getPxtRepoDir(), "libs", "pxt-python", filename), 'utf8');
                 this.writeFile(module, filename, contents);
                 return contents;
             }
@@ -2974,10 +2974,16 @@ class SnippetHost implements pxt.Host {
         return null;
     }
 
-    private getRepoDir() {
+    private getPxtRepoDir() {
         const cwd = process.cwd();
-        const i = cwd.lastIndexOf(path.sep + "pxt" + path.sep);
-        return cwd.substr(0, i + 5);
+        let p = path.parse(cwd);
+        while (p.base) {
+            if (p.base === "pxt") {
+                return path.format(p);
+            }
+            p = path.parse(p.dir);
+        }
+        return path.join(cwd, "node_modules", "pxt-core");
     }
 
     writeFile(module: pxt.Package, filename: string, contents: string) {
@@ -4068,6 +4074,7 @@ function compilesOK(opts: pxtc.CompileOptions, fn: string, content: string) {
     return res.success
 }
 
+
 function getApiInfoAsync() {
     return prepBuildOptionsAsync(BuildOption.GenDocs)
         .then(opts => {
@@ -4874,7 +4881,7 @@ function buildCoreAsync(buildOpts: BuildCoreOptions): Promise<pxtc.CompileResult
                 console.log(compileResult.times)
 
             switch (buildOpts.mode) {
-                case BuildOption.GenDocs:
+                case BuildOption.GenDocs: {
                     const apiInfo = pxtc.getApiInfo(res.ast, compileOptions.jres)
                     // keeps apis from this module only
                     for (const infok in apiInfo.byQName) {
@@ -4905,6 +4912,7 @@ function buildCoreAsync(buildOpts: BuildCoreOptions): Promise<pxtc.CompileResult
                         }
                     }
                     return null
+                }
                 case BuildOption.Deploy:
                     if (pxt.commands.hasDeployFn())
                         return pxt.commands.deployAsync(res)
@@ -5425,6 +5433,69 @@ export async function buildShareSimJsAsync(parsed: commandParser.ParsedCommand)
     console.log(`saved prebuilt ${id} to ${outputLocation}`);
 }
 
+export async function buildCoreDeclarationFiles(parsed: commandParser.ParsedCommand) {
+    const shareId = parsed.args[0]?.trim();
+    console.log(shareId ? `Building .d.ts for ${shareId}` : `Building .d.ts for blocksprj`);
+    const cwd = process.cwd();
+    const builtFolder = path.join(cwd, "temp", shareId ? `${shareId}dts` : "dts");
+    nodeutil.mkdirP(builtFolder);
+    process.chdir(cwd);
+
+    const host = shareId ? new Host() : new SnippetHost("decl-build", { "main.ts" : "" }, { "blocksprj": "*" });
+    const mainPkg = new pxt.MainPackage(host);
+
+    if (shareId) {
+        mainPkg._verspec = `pub:${shareId}`;
+        await mainPkg.host().downloadPackageAsync(mainPkg);
+    }
+
+    console.log("installing")
+    await mainPkg.installAllAsync();
+    console.log("installed")
+    const opts = await mainPkg.getCompileOptionsAsync();
+    console.log("created compiler options")
+    opts.tsCompileOptions = opts.tsCompileOptions ?? {};
+    opts.tsCompileOptions.declaration = true;
+    await rimrafAsync(builtFolder, {});
+
+    const tsProg = pxtc.getTSProgram(opts);
+    console.log("compiled")
+
+    let combined = ""
+    const writeDts = (fileName: string, data: string) => {
+        console.log(`writing ${fileName}`);
+        const writePath = path.join(builtFolder, fileName);
+        nodeutil.mkdirP(path.parse(writePath).dir);
+        fs.writeFileSync(writePath, data);
+        combined += data + "\n\n";
+    }
+
+    // pre-created
+    for (const file of tsProg.getSourceFiles()) {
+        if (file.fileName.endsWith(".d.ts")) {
+            writeDts(
+                file.fileName,
+                file.getFullText()
+            );
+        }
+    }
+
+    // generated via build
+    tsProg.emit(
+        /** targetSourceFile **/ undefined,
+        (fileName: string, data: string) => {
+            if (!data?.trim()) return;
+            writeDts(fileName, data);
+        },
+        /** cancellation token **/ undefined,
+        /** emitOnlyDtsFiles **/ true,
+        /** customTransformers -> I believe where we should apply culling of deprecated blocks */
+    );
+
+    console.log(`writing combined.d.ts`)
+    fs.writeFileSync(path.join(builtFolder, "combined.d.ts"), combined);
+}
+
 export function gendocsAsync(parsed: commandParser.ParsedCommand) {
     const docs = !!parsed.flags["docs"];
     const locs = !!parsed.flags["locs"];
@@ -6836,6 +6907,12 @@ ${pxt.crowdin.KEY_VARIABLE} - crowdin key
         },
     }, buildShareSimJsAsync)
 
+    p.defineCommand({
+        name: "buildcoredts",
+        help: "build d.ts files for core packages",
+        argString: "<share id>",
+    }, buildCoreDeclarationFiles)
+
     simpleCmd("clean", "removes built folders", cleanAsync);
     advancedCommand("cleangen", "remove generated files", cleanGenAsync);
     simpleCmd("npminstallnative", "install native dependencies", npmInstallNativeAsync);
diff --git a/localtypings/pxtarget.d.ts b/localtypings/pxtarget.d.ts
index aa6b88b1ca8e..0bc6e75ba8d1 100644
--- a/localtypings/pxtarget.d.ts
+++ b/localtypings/pxtarget.d.ts
@@ -1076,6 +1076,7 @@ declare namespace ts.pxtc {
         clearIncrBuildAndRetryOnError?: boolean; // on error when compiling in service, try again with a full recompile.
         errorOnGreyBlocks?: boolean;
         generateSourceMap?: boolean;
+        tsCompileOptions?: pxt.Map<any>; // ts.CompilerOptions;
 
         otherMultiVariants?: ExtensionTarget[];
 
diff --git a/pxtcompiler/emitter/driver.ts b/pxtcompiler/emitter/driver.ts
index b9c11e4e5f13..16f356aa8363 100644
--- a/pxtcompiler/emitter/driver.ts
+++ b/pxtcompiler/emitter/driver.ts
@@ -25,6 +25,12 @@ namespace ts.pxtc {
         options.noImplicitAny = true;
         options.noImplicitReturns = true;
         options.allowUnreachableCode = true;
+        options.declaration = true;
+
+        for (const [key, value] of Object.entries(opts?.tsCompileOptions ?? {})) {
+            options[key] = value;
+        }
+
         return options
     }