GL_INVALID_OPERATION from glVertexAttribPointer with no VAO bound in Core Profile

Direct Answers

1. Error timing: The OpenGL Core Profile specification (since 3.2) requires GL_INVALID_OPERATION to be generated immediately at the glVertexAttribPointer call (and related attribute-state calls such as glEnableVertexAttribArray, glVertexAttribDivisor, glVertexAttribFormat) when VAO name 0 is bound. The error is not deferred to a subsequent draw call.
2. Debug context effect: A debug context (GL_CONTEXT_FLAG_DEBUG_BIT / KHR_debug) does not change whether the error occurs. It only adds a synchronous glDebugMessageCallback invocation at the exact call site with source GL_DEBUG_SOURCE_API, type GL_DEBUG_TYPE_ERROR, severity GL_DEBUG_SEVERITY_HIGH, making the error easier to catch without polling glGetError.
3. Driver deviations: No current driver (NVIDIA, AMD, Intel, Mesa) permits attribute-state changes on VAO 0 in Core Profile. Historical leniency existed in early Core Profile drivers (~2010–2012), but modern implementations uniformly enforce the spec. Compatibility Profile behaves differently: VAO 0 is valid there.

Why This Happens

In Core Profile, VAO name 0 is not a valid vertex array object. The spec states that any command modifying vertex attribute state (including glVertexAttribPointer, glEnableVertexAttribArray, glVertexAttribDivisor, glVertexAttribFormat, glVertexAttribBinding, glBindVertexBuffer) generates GL_INVALID_OPERATION if the currently bound VAO is 0. Validation occurs at the API entry point; implementations that defer the error to draw time would be non-conformant.

Verification Steps

1. Create a minimal Core Profile context (e.g., GLFW: glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE), version ≥ 3.3).
2. Call glVertexAttribPointer (or any attribute-state function) before creating and binding a non-zero VAO.
3. Call glGetError() immediately — it returns GL_INVALID_OPERATION.
4. Optional: enable a debug context (GLFW_OPENGL_DEBUG_CONTEXT, GLFW_TRUE) and install glDebugMessageCallback; the callback fires synchronously at the offending call.

Common Pitfall

Many tutorials and older GLFW examples set up vertex attributes before creating a VAO. That code works only in Compatibility Profile. In Core Profile you must:

GLuint vao;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
// now safe to call glVertexAttribPointer, glEnableVertexAttribArray, etc.

After context loss/recovery, the same rule applies: recreate and bind a VAO before any attribute calls.

One Diagnostic Detail

If you are seeing the error at a draw call instead of the attribute call, confirm you are actually running a Core Profile context (check glGetString(GL_CONTEXT_PROFILE_MASK) for GL_CONTEXT_CORE_PROFILE_BIT) and not a Compatibility Profile or an ES context where rules differ.