From f6c7a4722073afbaf06b5eea24866af11793f71f Mon Sep 17 00:00:00 2001
From: Alessio Attilio <attilio.alessio@protonmail.com>
Date: Mon, 12 Jan 2026 00:55:02 +0100
Subject: [PATCH] fix(engine): fallback to package scripts for missing tasks

---
 crates/turborepo-engine/src/builder.rs        | 29 ++++++++++++-------
 .../repro-10516/package.json                  | 10 +++++++
 .../task_dependencies/repro-10516/turbo.json  |  9 ++++++
 .../repro-10516/workspace-a/package.json      |  9 ++++++
 .../repro-10516/workspace-b/package.json      |  6 ++++
 .../tests/task-dependencies/repro-10516.t     | 15 ++++++++++
 6 files changed, 68 insertions(+), 10 deletions(-)
 create mode 100644 turborepo-tests/integration/fixtures/task_dependencies/repro-10516/package.json
 create mode 100644 turborepo-tests/integration/fixtures/task_dependencies/repro-10516/turbo.json
 create mode 100644 turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-a/package.json
 create mode 100644 turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-b/package.json
 create mode 100644 turborepo-tests/integration/tests/task-dependencies/repro-10516.t

diff --git a/crates/turborepo-engine/src/builder.rs b/crates/turborepo-engine/src/builder.rs
index 9841a6d6e2907..7dcf344cac595 100644
--- a/crates/turborepo-engine/src/builder.rs
+++ b/crates/turborepo-engine/src/builder.rs
@@ -667,16 +667,25 @@ impl<'a, L: TurboJsonLoader> EngineBuilder<'a, L> {
             }
         }
 
-        if task_definitions.is_empty() && self.should_validate_engine {
-            let (span, text) = task_id.span_and_text("turbo.json");
-            return Err(BuilderError::MissingPackageTask(Box::new(
-                MissingPackageTaskError {
-                    span,
-                    text,
-                    task_id: task_id.to_string(),
-                    task_name: task_name.to_string(),
-                },
-            )));
+        if task_definitions.is_empty() {
+            let has_script_in_package_json = self
+                .package_graph
+                .package_json(&package_name)
+                .is_some_and(|pkg_json| pkg_json.scripts.contains_key(task_name.task()));
+
+            if has_script_in_package_json {
+                task_definitions.push(ProcessedTaskDefinition::default());
+            } else if self.should_validate_engine {
+                let (span, text) = task_id.span_and_text("turbo.json");
+                return Err(BuilderError::MissingPackageTask(Box::new(
+                    MissingPackageTaskError {
+                        span,
+                        text,
+                        task_id: task_id.to_string(),
+                        task_name: task_name.to_string(),
+                    },
+                )));
+            }
         }
 
         Ok(task_definitions)
diff --git a/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/package.json b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/package.json
new file mode 100644
index 0000000000000..3951359aed3bd
--- /dev/null
+++ b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/package.json
@@ -0,0 +1,10 @@
+{
+  "name": "root-tasks",
+  "scripts": {
+    "test": "echo 'root test'"
+  },
+  "workspaces": [
+    "workspace-a",
+    "workspace-b"
+  ]
+}
diff --git a/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/turbo.json b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/turbo.json
new file mode 100644
index 0000000000000..2f05525e8fec4
--- /dev/null
+++ b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/turbo.json
@@ -0,0 +1,9 @@
+{
+  "tasks": {
+    "dev": {
+      "dependsOn": [
+        "workspace-a#build:openapi"
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-a/package.json b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-a/package.json
new file mode 100644
index 0000000000000..6843b766191fa
--- /dev/null
+++ b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-a/package.json
@@ -0,0 +1,9 @@
+{
+  "name": "workspace-a",
+  "scripts": {
+    "build:openapi": "echo 'build:openapi workspace-a'"
+  },
+  "dependencies": {
+    "workspace-b": "*"
+  }
+}
\ No newline at end of file
diff --git a/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-b/package.json b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-b/package.json
new file mode 100644
index 0000000000000..558ea00a54201
--- /dev/null
+++ b/turborepo-tests/integration/fixtures/task_dependencies/repro-10516/workspace-b/package.json
@@ -0,0 +1,6 @@
+{
+  "name": "workspace-b",
+  "scripts": {
+    "build": "echo 'build workspace-b'"
+  }
+}
diff --git a/turborepo-tests/integration/tests/task-dependencies/repro-10516.t b/turborepo-tests/integration/tests/task-dependencies/repro-10516.t
new file mode 100644
index 0000000000000..a50282473e90d
--- /dev/null
+++ b/turborepo-tests/integration/tests/task-dependencies/repro-10516.t
@@ -0,0 +1,15 @@
+Setup
+  $ . ${TESTDIR}/../../../helpers/setup_integration_test.sh task_dependencies/repro-10516
+
+Test reproduction of issue 10516
+  $ ${TURBO} run dev --graph
+  
+  digraph {
+	compound = "true"
+	newrank = "true"
+	subgraph "root" {
+		" workspace-a#build:openapi" -> " ___ROOT___"
+		" workspace-a#dev" -> " workspace-a#build:openapi"
+		" workspace-b#dev" -> " workspace-a#build:openapi"
+	}
+  }